ANALYSIS OF WELLBORE INSTABILITY WHILE DRILLING

ANALYSIS OF WELLBORE INSTABILITY WHILE DRILLING EXPLORATORY

WELLS IN BANGLADESH

EVANA TINNY

MASTER OF ENGINEERING IN PETROLEUM ENGINEERING

DEPARTMENT OF PETROLEUM AND MINERAL RESOURCES ENGINEERING

BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY

DHAKA-1000, BANGLADESH

MARCH 2018

ANALYSIS OF WELLBORE INSTABILITY WHILE DRILLING EXPLORATORY WELLS IN BANGLADESH

A PROJECT

BY

EVANA TINNY

SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE

DEGREE OF MASTER OF ENGINEERING IN PETROLEUM ENGINEERING

DEPARTMENT OF PETROLEUM & MINERAL RESOURCES ENGINEERING

BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY

DHAKA-1000, BANGLADESH

MARCH 2018

RECOMMENDATION OF THE BOARD OF EXAMINERS

Ths tmdersiglred certify that they have read and recommended to the department of Peroleum

sxl MiEier*I Rssources Engine€riag {PMRE} for asceptance of the prsjcct $titl€d *ANALYSIS

+F 11-ELL8*RE I3{ST,{BILITY \\{HII.E trRITLINS EXPLORATORY WELLS I}iBANGLADESH" Submitted by EVANA TINNY, Reg No. 0413132004; in partial fulfilment oftle requirements for the degree of Master of Engineering in Petroleum Engineering.

{kairuala{Sryrrv'iwr} 'WF-t*LShahriar MahmudAssistant Professor

Dryrnrent c,fPetrolqrui & Mineral Eesoirces gnsisseriry

{Ph{RE}Bangladesh University of Engineering and Technolo gy ( BUET).Dhaka

L.Mer

Depatrc* of Petraleu$ & Minsal Resources fasisffii&g{PHRE}Bangladesh University of Engineering and Technoiogy {BUET),Dhaka

h@ 'lMW,k. Moharnmed Mahbubur Rahman

Associate Professor

Deparrae*t ofPeo'oleutrr & Mineral Xesources Engisffiicg{PMRE}Bangladesh University of Engineering and Technology (BUET),

Dh*a

Dste: tr3 March 2018.

DECLARATION

It is hereby declared that this project titled “ANALYSIS OF WELLBORE INSTABILITY WHILE DRILLIG EXPLORATORY WELLS IN BANGLADESH” or any part of it has not been submitted elsewhere for the award of any degree or diploma.

Evana Tinny

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ABSTRACT

Wellbore instability is one of the most prevalent downhole challenges since oil well drilling began. The difficulties become more profound and unpredictable when the wells are exploratory. Wellbore Instability issues related to exploratory wells causes significant amount of non-productive time (NPT). These problems could lead to fishing operation and sidetrack which may cause greater financial loss. In Bangladesh wellbore related issues especially stuck pipe cases were experienced from time to time. This work analyzes the wellbore instability incidents of two exploratory wells, Mubarakpur-1 and Sunetro-1.

Based on available data like Geological Technical Order (GTO), Daily Drilling Reports, Well Logging Reports, Mud Reports, XRD (X-ray Diffraction) reports, Cavings samples with pictures, Well Completion Report etc. this work presents the sequence of events leading to the stuck pipe incident on Mubarakpur-1 and Sunetro-1 wells. Based on available information the root cause(s) of wellbore instability issues of the said wells have been analyzed and suggestions have been made on how to drill similar wells while tackling such issues in future.

Mubarakpur-1 and Sunetro-1 are different in geological aspects. Formation pressure and wellbore stresses were higher in Mubarakpur-1 than Sunetro-1. Shale formation showed non-reactive character in Mubarakpur-1, whereas Sunetro-1 suffered from shale swelling effect. Both wells encountered stuck pipe incidents but the sequences of events were different. Mubarakpur-1 faced compressive failure due to stress concentration around the wellbore. And Sunetro-1 crossed the reactive shale instability which resulted in hole pack-off. Some operational mishaps were also responsible for both stuck pipe incidents. The analysis showed that focus should be given on understanding the in-situ stress condition while drilling, identifying formation type. So that appropriate mud weight selection, BHA design can be implemented for trouble free operation.

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ACKNOWLEDGEMENTS

The thesis is submitted to Petroleum and Mineral Resources Engineering department in order to partially fulfill the requirements for the degree of Master of Engineering (M.Engg.). This thesis work has been carried out under the department of Petroleum and Mineral Resources Engineering (PMRE), Bangladesh University of Engineering and Technology (BUET), Bangladesh.

I would like to thank Petrobangla (Bangladesh Oil, Gas and Mineral Corporation) and BAPEX (Bangladesh Petroleum Exploration and Production Company Limited), to allow me to study on Mubarakpur-1 well and Sunetro-1 by providing required data.

I would also like to take this opportunity to thank several people that have contributed in the work of this project. I am highly indebted to my supervisor Shahriar Mahmud, Assistant professor, Department of PMRE, BUET, Bangladesh. His immense support and guidance throughout the time of the project is highly appreciated. Most importantly, I am grateful for the motivation and positivity towards me from beginning to the very end. My special thanks to Hazzaz Bin Yousuf, Assistant professor, Department of PMRE, BUET, Bangladesh, for his helpful instructions and assistance in fleshing out some questions regarding my study. My deepest gratitude goes to Howleder Ohidul Islam, Deputy General Manager, Bangladesh Petroleum Exploration and Production Company Limited (BAPEX), Bangladesh. Along the way he provided me with excellent assistance and shared some of his deep knowledge on subjects within the thesis as well as other aspects of the industry.

Thanks must be given to Muhammad Josim Uddin Shak, Manager, BAPEX, Bangladesh. His helpful attitude during my stressful period completing my thesis works will never be forgotten. And last but not least, I would like to thank Md Kamrul Hasan Shovon, Manager, BAPEX, Bangladesh for supporting to work on this project.

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ABBREVIATION

BHA Bottom Hole Assembly

DC Drill Collar

ECD Equivalent Circulating Density

EMW Effective Mud Weight

Gpm Gallons per minute

HVP High Viscous Pill

HWDC Heavy Weight Drill Pipe

KCL Potassium Chloride

LCM Lost Circulation Material

LOT Leak Off Test

LSND Low Solids Non-Dispersed

MD Measured Depth

MW Mud Weight

NPT Non Productive Time

OD Outer Diameter

PDC Polycrystalline Diamond Compact

POOH Pull out of Hole

RIH Run in Hole

ROP Rate of Penetration

Sg Specific Gravity

SPM Stroke Per Minute

SPP Surface Pump Pressure

TD Target Depth

WOB Weight on Bit

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TABLE OF CONTENTS

ABSTRACT ............................................................................................................................... i

ACKNOWLEDGEMENTS........................................................................................................ ii

ABBREVIATION ..................................................................................................................... iii

LIST OF FIGURES .................................................................................................................. vi

LIST OF TABLES .................................................................................................................. viii

CHAPTER 1: INTRODUCTION ................................................................................................1

1.0 Introduction .......................................................................................................................1

1.1 Mubarakpur exploratory well-1 .........................................................................................1

1.2 Sunetro Exploratory Well-1 ..............................................................................................3

1.3 Objectives .........................................................................................................................5

CHAPTER 2: LITERATURE REVIEW .....................................................................................6

2.0 Wellbore instability ...........................................................................................................6

2.1 Wellbore Stresses (Zoback, 2007)......................................................................................6

2.1.1 Principle Stress ....................................................................................................................... 6

2.1.2 Relative magnitudes of the principal stresses in the earth ......................................................... 7

2.1.3 Effective Stress ....................................................................................................................... 8

2.1.4 Stress concentration around a vertical wellbore ....................................................................... 9

2.1.5 Failure Types on wellbore wall.............................................................................................. 11

2.2 Causes of Wellbore instability ......................................................................................... 13

2.2.1 Uncontrollable factors ........................................................................................................... 15

2.2.2 Controllable factors ............................................................................................................... 19

2.3 Types of Wellbore instability ........................................................................................... 21

2.3.1 Stuck Pipe ............................................................................................................................. 21

2.3.2 Lost Circulation .................................................................................................................... 31

2.3.3 Hole Deviation ...................................................................................................................... 32

2.3.4 Pipe Failures ......................................................................................................................... 33

2.3.5 Tight Hole ............................................................................................................................. 34

2.3.6 Hole Washout ....................................................................................................................... 35

2.3.7 Mud Contamination .............................................................................................................. 35

2.3.8 Excessive Cavings at surface ................................................................................................. 36

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2.3.9 Hydrogen-Sulfide-Bearing Zones and Shallow Gas ............................................................... 38

2.3.10 General Equipment & Personnel .......................................................................................... 38

CHAPTER 3: WELLBORE INSTABILITY ISSUES WHILE DRILLING ............................... 39

3.1 Wellbore instability issues while drilling Mubarakpur-1 .................................................. 39

3.2 Wellbore Instability Issues while drilling Sunetro -1 ........................................................ 43

CHAPTER 4: DISCUSSIONS ................................................................................................. 48

4.1 Mubarakpur-1 .................................................................................................................. 48

4.2 Sunetro-1 ......................................................................................................................... 60

CHAPTER 5: CONCLUSIONS AND RECOMMENDATIONS ............................................... 67

5.1 Conclusions ..................................................................................................................... 67

5.1.1 Mubarakpur-1 ....................................................................................................................... 67

5.1.2 Sunetro-1 .............................................................................................................................. 68

5.2 Recommendations ........................................................................................................... 70

5.2.1 Mubarakpur-1 ....................................................................................................................... 70

5.2.2 Sunetro-1 .............................................................................................................................. 71

REFERENCES ......................................................................................................................... 72

APPENDIX A: Tight spots and Overpulls found in Mubarakpur -1 (WCR, 2017; DDR, 2016) .............................................................................................................................................. 75

APPENDIX B: Tight spots and Overpulls found in Sunetro -1(WCR, 2013; DDR, 2012). .... 78

APPENDIX C: Deviation Survey Data of Sunetro-1.............................................................. 79

APPENDIX D: Non-Reactive Character of Illite ................................................................... 80

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LIST OF FIGURES

Figure 1.1: Location of Mubarakpur Exploratory well -1 (GTO, 2014). .......................................2 Figure 1.2: Location Map of Sunetro Exploratory Well-1 on Regional Tectonic Map ..................4 (GTO, 2012) ...............................................................................................................................4 Figure 2.1: Wellbore stresses (Osisanya, 2011) ...........................................................................6 Figure 2.2: The vertical and horizontal maximum and minimum stresses (Petrowiki, 2016). .......7 Figure 2.3: Three faulting states based on Andersonian faulting theory (Zoback, 2007) ...............8 Figure 2.4: Principal stress trajectories around a cylindrical opening in a bi-axial stress field (Zoback, 2007) ............................................................................................................................9 Figure 2.5: Variation of effective hoop stress (Zoback, 2007) .................................................... 10 Figure 2.6: Wellbore stress concentration for a vertical well (Petrowiki, 2016) .......................... 12 Figure 2.7: Effects of mud weight on wellbore stress concentration (Zoback, 2007) .................. 13 Figure 2.8: Causes of wellbore instability (McLellan, 1994b; Bowes, 1997; Chen, 1998; Mohiuddin, 2001) ..................................................................................................................... 14 Figure 2.9: Drilling through naturally fractured or faulted formations (Pasic, 2007) .................. 15 Figure 2.10: Drilling through tectonically stressed formations (Pasic, 2007) .............................. 16 Figure 2.11: Drilling through mobile formations (Pasic, 2007) .................................................. 17 Figure 2.12: Drilling through unconsolidated formations (Pasic, 2007) ..................................... 17 Figure 2.13: Drilling through naturally over-pressured shale (Bowes, 1997) .............................. 18 Figure 2.14: Drilling through induced over-pressured shale (Bowes, 1997) ............................... 18 Figure 2.15: Effect of mud weight on the stress in wellbore wall (Pasic, 2007) .......................... 19 Figure 2.16: Effect of the well depth (a) and the hole inclination (b) on wellbore stability ......... 20 (Pasic, 2007) ............................................................................................................................. 20 Figure 2.17: Differential sticking (Mitchell, 2011) ................................................................... 22 Figure 2.18: Settled cuttings due to poor hole cleaning (Rabia, 2001). ....................................... 24 Figure 2.19: Safe Mud Weights Envelope (Rabia, 2001). .......................................................... 26 Figure 2.20: Swelling of reactive shales (Rabia, 2001) .............................................................. 27 Figure 2.23: Types of lost circulation (Mitchell, 2011). ............................................................. 31 Figure 2.24: Example of Hole deviation (Petrowiki, 2015). ....................................................... 32 Figure 2.25: Twist off (Petrowiki, 2015) ................................................................................... 33 Figure 2.26: Fatigue cracks (Bert, 2009) .................................................................................... 34 Figure 2.27: Tight hole phenomena (Bjerke, 2013). ................................................................... 35 Figure 2.28: Blocky and angular shaped cavings (HXR, 2018). ................................................. 36 Figure 2.29: Platy and splintery shaped cavings (HXR, 2018). .................................................. 37 Figure 2.30: Splintered Cavings (HXR, 2018). .......................................................................... 37 Figure 3.1: Well design of Mubarakpur-1 well (WCR, 2017) .................................................... 40 Figure 3.2: Cavings found in Mubarakpur-1 while drilling at 3505-3530m. ............................... 41 Figure 3.3: Schematic diagram of Sunetro well no#1 (WCR, 2012) ........................................... 44 Figure 3.4 (a): Cavings observed after 2460m. .......................................................................... 45 Figure 3.4 (b): Cavings observed after 3550m. .......................................................................... 45

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Figure 3.5: Eroded Stabilizer after drilling 2463m. .................................................................... 46 Figure 3.6: Bit condition after drilling 4660m............................................................................ 46 Figure 4.1: Interval Velocity Vs Depth from VSP data .............................................................. 49 Figure 4.2 a. D Exponent data of Mubarakpur Exploratory Well #1........................................... 50 Figure 4.2b: D Exponent data of Mubarakpur Exploratory Well #1 (Side Track). ...................... 51 Figure 4.3a: Wire Line Log curve comparison with Gamma based Lithology ............................ 52 Figure 4.3b: Wire Line Log curve comparison with Gamma based Lithology (Side Track). ...... 53 Figure 4.3c: Wire Line Log curve below 4250m. ....................................................................... 54 Figure 4.4a: Flow Line Temperature of Mubarakpur -1 ............................................................. 55 Figure 4.4b: Flow Line Temperature of Mubarakpur -1 (Side Track). ........................................ 56 Figure 4.5: Stuck scenario in Mubarakpur-1 well. ..................................................................... 58 Figure 4.6: Comparison between Planned and Actual MW in Mubarakpur -1 well. ................... 59 Figure 4.7: Mud loss profile of Sunetro-1. ................................................................................. 63 Figure 4.8: Planned and Actual MW in Sunetro -1 well. ............................................................ 64 Figure 4.9: ROP variation curve with depth. .............................................................................. 65 Figure 5.1: Rig time analysis for Mubarakpur-1 well. ................................................................ 68 Figure 5.2: Rig time analysis for Sunetro-1 well. ....................................................................... 69 Figure (Appendix D): Non swelling clay, Illite structure. .......................................................... 80

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LIST OF TABLES

Table 2.1: Definitions Of S1 And S3 For Andersonian Faulting Classifications ............................7

Table 2.2: Pipe Sticking Causes (Rabia, 2001) .......................................................................... 21

Table 2.3: Minimum flow rate of effective hole cleaning (Wiper trip, 2015).............................. 24

Table 3.1: Leak Off Test (LOT) data of Mubarakpur Exploratory Well#1 ................................. 41

Table 4.1: Cation Exchange Capacity (CEC) from Cavings at Various Depths (Source: XRD Report) ...................................................................................................................................... 57

Table 4.2: Flow rates of Mubarakpur-1 well in different hole sections (Wiper Trip, 2015). ....... 60

Table 4.3: BHA design from 1405m-1706m .............................................................................. 61

Table 4.4: BHA design from 3503m-4499.42m ......................................................................... 61

Table 4.5: BHA design from Sidetrack depth to 4342.85m ........................................................ 62

Table 4.6: Flow rates of Sunetro-1 well in different hole sections (Wiper Trip, 2015)................ 65

Table (Appendix C): Deviation Survey Record of Sunetro-1 ..................................................... 79

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CHAPTER 1: INTRODUCTION

1.0 Introduction

Drilling exploratory well involves dealing with unexpected or unknown behavior of formation rock matrix, especially when historical data from nearby wells are not available. It often results in momentous amount of Non Productive Time (NPT) and cost. In some cases fishing operations are necessary which also may take long time and even force the company to terminate the well. Such operations cost even approximately 40% of the total well cost (Elahi, 2013). The Drilling Industry in Bangladesh has also experienced wellbore related issues in new structures and sometimes the sequence of these issues lead to stuck pipe cases. While the problem is quite common all around the world, there is a lack of in-depth analysis of these incidents in Bangladesh. Exploratory wells Mubarakpur-1 and Sunetro-1 both are these kinds of cases in Bangladesh. No considerable offset data were available during planning and executing the wells. As there was not much prior geological knowledge of these regions available before the drilling program commenced, the degree of uncertainty was higher than usual (WCR, 2017; WCR, 2013; GTO, 2014; GTO, 2012).

1.1 Mubarakpur exploratory well-1

It is located in Santhia Upazilla of Pabna district in Bangladesh. The well lies southeast of Singra-1 and southwest of Hazipur x-1(Figure 1.1). On completion of a five-year joint seismic survey by Petrobangla (Bangladesh Oil, Gas & Mineral Corporation) and German company Prakla Seismos in 1984, Mubarakpur was identified as a prospective site. In Geological aspect Mubarakpur area is quite different from others gas field area. Along with the fact that no offset data were available, Pressure prognoses from Hazipur #1 and Singra#1 wells as well as considering Kamta well #1 were considered. According to nearby well data, normal hydrostatic pressure was expected in Mubarakpur area upto 3500m. The tectonic framework of Bangladesh may be broadly divided into two main units. First one is stable platform in the northwest and another one is Deep basin to the southeast. There is also a narrow northeast-southwest trending zone called “Hinge zone” separates the above two units diagonally almost through the middle of the country. The geology of Mubarakpur area is associated with stable to transitional (Hinge zone) tectonic setting. The zone is about 25km wide northeast-southwest zone that separates Precambrian platform in the northwest from deep basin to the southeast. There is no surface expression of this unit but it is marked by the sudden increase of dip in subsurface sedimentary layers as shown strongly by the seismic marker (seismic line PK-01, 8426, 8316, 8424, 8425, HC-01) at the top of the Sylhet limestone unit of

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Eocene age. The hinge zone is not a tectonic hinge but represents Eocene shelf edge / slope break i.e. a paleocontinental slope. It extends more than 500 km from Calcutta, India, north-eastward to Bangladesh’s northern border. Its equivalent continues into the Assam basin of India for an additional 300 km, defining the eastern limit of the Indian continent during the Eocene. It may be noted that the western part of Bengal Basin is marked by a series of migrating shelf break with the most prominent being the Eocene shelf edge. That is why it is known as Eocene hinge zone (GTO, 2014) The exploratory well Mubarakpur-1, was spudded on August 2014 by BAPEX (Bangladesh Petroleum Exploration and Production Company Limited). Expected reservoirs of the site are Stratigraphic Trap (delineated as onlap closure), onlap features (a bunch of sand beds) pinching out against the slope channel sequences towards the northwest. But the crew experienced major problems like excessive tight spots and overpull, cavings, high torque and drag, logging problem, poor cementation and finally the drill pipe got stuck (target depth 4700 m). Numerous attempts were made to recover the string including the BHA (Bottom Hole Assembly). Ultimately the BHA with 09 stands was lost. Later, the well was sidetracked and reached target depth which was a costly process (WCR, 2017; GTO, 2014).

Figure 1.1: Location of Mubarakpur Exploratory well -1 (GTO, 2014).

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1.2 Sunetro Exploratory Well-1

The well was spudded on 10 August 2012. The location of the well falls in the village Gabhi of Selborash Union which belongs to Dharmapasha Upazilla of Sunamgonj District (Figure 1.2). There was no well in close proximity of Sunetro structure. Chattak and Bibiyana were the surrounding fields which were about 60 km and 70 km distance from Sunetra exploratory well -1 respectively. Well behavior of surrounding well like Chattak and Bibiyana was considered understanding the Sunetro Structure. A number of prospects were obtained from the Gravity Survey carried out over this area back in 1960s. Later on during late 1980s aeromagnetic survey also confirmed the presence of Sunetro prospect.

In 2008-09 BAPEX did 136 lkm of survey in two phases and 126 lkm of infill survey were conducted in 2009-10 field session. After thorough interpretation of several prospective horizons and well-tie with neighboring Bibiyana Gas Field, the outcome was a closed low amplitude structure which was presumed to be very promising in terms of hydrocarbon occurrence. This structure has extent in the Dharmapasha Upazilla of Sunamgonj district and Barhatta & Mohangonj Upazilla of Netrokona district.

Drilling of Sunetro-1 well went smoothly till 4000 m MD. After reaching 4499 m the string got stuck at 4347 m while tripping-out. The string was recovered by fishing operation, leaving the 31.91m BHA in the hole. Second section was side-tracked from 4265 m to drill ahead. After drilling to 4683 m the pipe stuck again. This time also the string was recovered leaving the second fish downhole. Problems like tight spots and overpull, mud loss, cavings, high torque and drag, ROP dropped, hole deviation, logging problem were also encountered throughout the drilling period (WCR, 2013; GTO, 2012).

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Figure 1.2: Location Map of Sunetro Exploratory Well-1 on Regional Tectonic Map (GTO, 2012)

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1.3 Objectives The objectives of the study are:

To analyze the wellbore instability issues of these two wells To investigate the root causes of the incidents To make recommendations for safe, effective and trouble free operations for future

drilling programs within these areas To provide further suggestions to improve the quality of operations overall by following

best practices.

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CHAPTER 2: LITERATURE REVIEW

2.0 Wellbore instability

Wellbore instability is a major concern during drilling operations. It is recognized when the hole diameter is markedly different from the bit size and the hole does not maintain its structural integrity. Before drilling, rocks are in a state of equilibrium. When a hole is drilled, the surrounding rock deforms slightly because of the stress relief induced by the cavity and results in a new set of stresses known as wellbore stresses like axial, tangential and radial (Figure 2.1). If the redistributed stress-state mismatch rock strength, instability occur (Osisanya, 2011). It manifests itself in different ways like hole closure, hole enlargement, hole deviation, excessive cavings, logging problem, excessive reaming, high torque and drag etc and sometimes leading to stuck pipe that may require plugging and side tracking which potentially adding millions of dollars to the well cost.

Figure 2.1: Wellbore stresses (Osisanya, 2011)

2.1 Wellbore Stresses (Zoback, 2007)

2.1.1 Principle Stress

In easiest terms, stress is defined as force acting over a particular area. At each point there is a particular stress axes orientation for which all shear stress components are zero, the directions of which are referred to as the "principal stress directions". The stresses acting along the principal stress axes are called principal stresses. The magnitudes of the principal stresses are S1, S2, and S3, corresponding to the greatest principal stress, the intermediate principal stress, and the least principal stress, respectively. It has been found in most parts of the world, at depths within reach of the drill bit, that the stress acting vertically on a horizontal plane (defined as the vertical stress, Sv) is a principle stress. This requires that the other two principal stresses act in a horizontal

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direction. Because these horizontal stresses almost always have different magnitudes, they are referred to as the greatest horizontal stress, SHmax, and the least horizontal stress, SHmin. (Figure 2.2)

Figure 2.2: The vertical and horizontal maximum and minimum stresses (Petrowiki, 2016).

2.1.2 Relative magnitudes of the principal stresses in the earth

The vertical stress can be the greatest, the intermediate, or the least principal stress. In 1924, Anderson developed a classification scheme to describe these three possibilities, based on the type of faulting that would occur in each case (Table 2.1 and Figure 2.3). A normal faulting regime is one in which the vertical stress is the greatest stress. When the vertical stress is the intermediate stress, a strike-slip regime is indicated. If the vertical stress is the least stress the regime is defined to be reverse. The horizontal stresses at a given depth will be smallest in a normal faulting regime, larger in a strike-slip regime, and greatest in a reverse faulting regime. In general, vertical wells will be progressively less stable as the regime changes from normal to strike-slip to reverse, and consequently will require higher mud weights to drill.

Table 2.1: Definitions Of S1 And S3 For Andersonian Faulting Classifications

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Figure 2.3: Three faulting states based on Andersonian faulting theory (Zoback, 2007)

2.1.3 Effective Stress

The concept of effective stress is based on the pioneering work in soil mechanics by Terzaghi (1923) who noted that the behavior of a soil (or a saturated rock) will be controlled by the effective stresses, the differences between externally applied stresses and internal pore pressure. Implicitly, the pressure exerted by mud is taken up by both rock matrix and pore pressure. The so-called “simple” or Terzaghi definition of effective stress is

σi j = Si j − δi j Pp where σij is the effective stress, Pp is the pore pressure, δij is the Kronecker delta (δij = 1, if i = j, δij = 0 otherwise), and Sij represents the total stresses, which are defined without reference to pore pressure. So, the principle stresses can be written in terms of effective stress as:

σ1= S1-Pp σ2= S2-Pp σ3= S3-Pp

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2.1.4 Stress concentration around a vertical wellbore

The stress concentration around a vertical well drilled parallel to the vertical principal stress, Sv, in an isotropic, elastic medium is described by the Kirsch equations Figure 2.4 (taken from Kirsch’s original paper), the creation of a cylindrical opening (like a wellbore) causes the stress trajectories to bend in such a way as to be parallel and perpendicular to the wellbore wall because it is a free surface which cannot sustain shear traction. Moreover, as the material removed is no longer available to support far-field stresses, there is a stress concentration around the well.

Figure 2.4: Principal stress trajectories around a cylindrical opening in a bi-axial stress field

(Zoback, 2007)

Mathematically, the effective stresses around a vertical wellbore of radius R are described in terms of a cylindrical coordinate system by the following:

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Where θ is measured from the azimuth of SHmax and ΔP is the difference between the mud weight in the wellbore and the pore pressure, P0. σΔT represents thermal stresses arising from the difference between the mud temperature and formation temperature(ΔT). The stress concentration varies strongly as a function of position around the wellbore and distance from the wellbore wall. Also, the stress concentration is symmetric with respect to the direction of the horizontal principal stresses. For an east–west direction of SHmax, Figure 2.5 shows that σθθ (the so-called effective hoop stress) is strongly compressive to the north and south, the azimuth of Shmin, or 90◦ from the direction of SHmax.

Figure 2.5: Variation of effective hoop stress (Zoback, 2007)

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2.1.5 Failure Types on wellbore wall

Two major failure modes

Compressive Failure Tensile Failure

Compressive and tensile wellbore failure is a direct result of the stress concentration around the wellbore that results from drilling a well into an already-stressed rock mass. In a homogeneous and isotropic elastic material in which one principal stress acts parallel to the wellbore axis, the effective hoop stress and radial stress at the wall of a cylindrical, vertical wellbore (overburden stress, Sv is a principal stress acting parallel to the wellbore axis) is given by the following equation:

where θ is an angle measured from the azimuth of the maximum horizontal stress, Shmax, SHmin is the minimum horizontal stress, P0 is the pore pressure, ΔP is the difference between the wellbore pressure (mud weight) and the pore pressure, and σΔT is the thermal stress induced by the cooling of the wellbore by ΔT. The effective stress acting parallel to the wellbore axis is:

Where ν is Poisson’s ratio. At the point of minimum compression around the wellbore (i.e., at θ = 0, parallel to SHmax), above equations reduce to

At the point of maximum stress concentration around the wellbore (i.e., at θ = 90°, parallel to SHmin),

The equations for σθθ; and σzz are illustrated in Figure 2.6. At the point of maximum compression around the wellbore, the maximum principal horizontal stress is amplified appreciably. If the stress concentration is high enough, it can exceed the rock strength, and the rock will fail in compression. Compressive failures that form in the region around the wellbore

http://petrowiki.org/File:Vol2_page_0021_eq_001.png

http://petrowiki.org/File:Vol2_page_0021_eq_002.png

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where the stress concentration is greatest are commonly called stress-induced wellbore breakouts. In a vertical well, the zone of compressive failure is centered at the azimuth of minimum horizontal far-field compression as this is where the compressive hoop stress is greatest.

Figure 2.6: Wellbore stress concentration for a vertical well (Petrowiki, 2016)

The mud acts first against the pressure of the pore fluid, and any excess pressure is then applied to the rock. This assumes a reasonably efficient mud cake, and can be modified to account for its absence. If the mud weight is increased, it results in an increase in σrr and a decrease in σθθ and σzz; this usually inhibits breakout formation. On the other hand, elevated mud pressures increase the likelihood of drilling-induced tensile wall fractures. In Figure 2.8 When Pmud = Pp, σrr = ΔP = 0, possibly leading to large amounts of failure in weak rock. When the mud weight is increased, it increases the radial stress on the wellbore wall and decreases the circumferential stress. This shrinks the Mohr circle without changing its midpoint, leading to a decreased risk of wellbore failure. The increase in effective strength can be as large in weak rock as it is in strong rock, and increases with mud weight at a rate defined by the internal friction.

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Figure 2.7: Effects of mud weight on wellbore stress concentration (Zoback, 2007)

Drilling-induced tensile fractures form when the mud weight is comparable, or slightly greater than the pore pressure (but not comparable to S3). This only occurs for certain stress states and well orientations. Drilling-induced tensile fractures are limited to the wellbore wall because the fracture does not propagate into the formation; drilling induced tensile fractures are not associated with lost circulation or drilling problems.

2.2 Causes of Wellbore instability

Drilling a dissimilar diameter hole is a interplay of uncontrollable factors and controllable factors. These factors are shown in Figure 2.8

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Figure 2.8: Causes of wellbore instability (McLellan, 1994b; Bowes, 1997; Chen, 1998; Mohiuddin, 2001)

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2.2.1 Uncontrollable factors

Naturally fractured or faulted formations

A natural fracture system in the rock can often be found near faults. Rock near faults can be broken into large or small pieces. If they are loose they can fall into the wellbore and jam the string in the hole (Nguyen, 2007). Even if the pieces are bonded together, impacts from the BHA due to drill string vibrations can cause the formation to fall into the wellbore. This type of sticking is particularly unusual in that stuck pipe can occur while drilling. Figure 2.9 shows possible problems as result drilling a naturally fractured or faulted system. This mechanism can occur in tectonically active zones, in prognoses fractured limestone, and as the formation is drilled. Drill string vibrations have to be minimized to help stabilize these formations (Bowes, 1997). Hole collapse problems may become quite severe if weak bedding planes intersect a wellbore at unfavorable angles. Such fractures in shales may provide a pathway for mud or fluid invasion that can lead to time-depended strength degradation, softening and ultimately to hole collapse. The relationship between hole size and the fracture spacing will be important in such formations.

Figure 2.9: Drilling through naturally fractured or faulted formations (Pasic, 2007)

Tectonically Stressed Formations

Wellbore instability is caused when highly stressed formations are drilled and if exists a significant difference between the near wellbore stress and the restraining pressure provided by the drilling fluid density. Tectonic stresses build up in areas where rock is being compressed or stretched due movement of the earth´s crust. The rock in these areas is being buckled by the pressure of the moving tectonic plates. When a hole is drilled in an area of high tectonic stresses the rock around the wellbore will collapse into the wellbore and produce splintery cavings similar to those produced by over-pressured shale (Figure 2.10). In the tectonic stress case the hydrostatic pressure required to stabilize the wellbore may be much higher than the fracture pressure of the other exposed formations (Bowes, 1997). This mechanism usually occurs in or

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near mountainous regions. Planning to case off these formations as quickly as possible and maintaining adequate drilling fluid weight can help to stabilize these formations.

Figure 2.10: Drilling through tectonically stressed formations (Pasic, 2007)

High in-situ stresses

Anomaliously high in-situ stresses, such as may be found in the vicinity of salt domes, near faults, or in the inner limbs of folds may give rise to wellbore instability. Stress concentrations may also occur in particularly stiff rocks such as quartzose sandstones or conglomerates. Only a few case histories have been described in the literature for drilling problems caused by local stress concentrations, mainly because of the difficulty in measuring or estimating such in situ stresses.

Mobile formations

The mobile formation squeezes into the wellbore because it is being compressed by the overburden forces. Mobile formations behave in a plastic manner, deforming under pressure. The deformation results in a decrease in the wellbore size, causing problems of running BHA´s, logging tools and casing (Figure 2.11). A deformation occurs because the mud weight is not sufficient to prevent the formation squeezing into the wellbore (Bowes, 1997). This mechanism normally occurs while drilling salt. An appropriate drilling fluid and maintaining sufficient drilling fluid weight are required to help stabilize these formations.

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Figure 2.11: Drilling through mobile formations (Pasic, 2007)

Unconsolidated formations

An unconsolidated formation falls into the wellbore because it is loosely packed with little or no bonding between particles, pebbles or boulders. The collapse of formations is caused by removing the supporting rock as the well is drilled (Figure 2.12). It happens in a wellbore when little or no filter cake is present. The un-bonded formation (sand, gravel, etc.) cannot be supported by hydrostatic overbalance as the fluid simply flows into the formations. Sand or gravel then falls into the hole and packs off the drill string. The effect can be a gradual increase in drag over a number of meters, or can be sudden (Bowes, 1997). This mechanism is normally associated with shallow formation. An adequate filter cake is required to help stabilize these formations.

Figure 2.12: Drilling through unconsolidated formations (Pasic, 2007)

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Naturally Over-Pressured Shale Collapse

Naturally over-pressured shale is the one with a natural pore pressure greater than the normal hydrostatic pressure gradient. Naturally over-pressured shales are most commonly caused by geological phenomena such as under-compaction, naturally removed overburden and uplift (Figure 2.13). Using insufficient mud weight in these formations will cause the hole to become unstable and collapse (Bowes, 1997; Tan, 1999). This mechanism normally occurs in prognosed rapid depositional shale sequences. The short time hole exposure and an adequate drilling fluid weight can help to stabilize these formations.

Figure 2.13: Drilling through naturally over-pressured shale (Bowes, 1997)

Induced Over-Pressured Shale Collapse

Induced over-pressured shale collapse occurs when the shale assumes the hydrostatic pressure of the wellbore fluids after number of days exposures to that pressure. When this is followed by no increase or a reduction in hydrostatic pressure in the wellbore, the shale, which now has a higher internal pressure than the wellbore, collapses in a similar manner to naturally over pressured shale (Figure 2.14) (Bowes, 1997). This mechanism normally occurs in water based drilling fluids, after a reduction in drilling fluid weight or after a long exposure time during which the drilling fluid was unchanged.

Figure 2.14: Drilling through induced over-pressured shale (Bowes, 1997)

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2.2.2 Controllable factors

Bottom hole pressure (mud density)

Depending upon the application, either the bottom hole pressure, the mud density or the equivalent circulating density (ECD), is usually the most important determinant of whether an open wellbore is stable (Figures 2.15 and 2.16) (Hawkes, 1997; Gaurina 1998). The supporting pressure offered by the static or dynamic fluid pressure during either drilling, stimulating, working over or producing of a well, will determine the stress concentration present in the near wellbore vicinity. Because rock failure is dependent on the effective stress the consequence for stability is highly dependent on whether and how rapidly fluid pressure penetrate the wellbore wall. That is not to say however, that high mud densities or bottom hole pressures are always optimal for avoiding instability in a given well. In the absence of an efficient filter cake, such as in fractured formations, a rise in a bottom hole pressure may be detrimental to stability and can compromise other criteria, e.g., formation damage, differential sticking risk, mud properties, or hydraulics (McLellan, 1994b; Mohiuddin, 2001; Tan, 1993).

Figure 2.15: Effect of mud weight on the stress in wellbore wall (Pasic, 2007)

Well Inclination and Azimuth

Inclination and azimuthally orientation of a well with respect to the principal in-situ stresses can be an important factor affecting the risk of collapse and/or fracture breakdown occurring (Figure 2.15). This is particularly true for estimating the fracture breakdown pressure in tectonically stressed regions where there is strong stress anisotropy (McLellan, 1994b).

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Figure 2.16: Effect of the well depth (a) and the hole inclination (b) on wellbore stability (Pasic, 2007)

Transient wellbore pressures

Transient wellbore pressures, such as swab and surge effects during drilling, may cause wellbore enlargement (Hawkes, 1997). Tensile stress can occur when the wellbore pressure across an interval is rapidly reduced by the swabbing action of the drill string for instance. If the formation has a sufficiently low tensile strength or is pre-fractured, the imbalance between the pore pressures in the rock and the wellbore can literally pull loose rock off the wall. Surge pressures can also cause rapid pore pressures increases in the near-wellbore area sometimes causing an immediate loss in rock strength which may ultimately lead to collapse. Other pore pressure penetration-related phenomena may help to initially stabilize wellbores, e.g. filter cake efficiency in permeable formations, capillary threshold pressures for oil-based mud and transient pore pressure penetration effects (McLellan, 1994b).

Physical/chemical fluid-rock interaction

There are many physical/chemical fluid-rock interaction phenomena which modify the near wellbore rock strength or stress. These include hydration, osmotic pressures, swelling, rock softening and strength changes, and dispersion. The significance of these effects depend on a complex interaction of many factors including the nature of the formation (mineralogy, stiffness, strength, pore water composition, stress history, temperature), the presence of a filter cake or permeability barrier is present, the properties and chemical composition of the wellbore fluid, and the extent of any damage near the wellbore (McLellan, 1994b).

Drill string vibrations (during drilling)

Drill string vibrations can enlarge holes in some circumstances. Optimal bottom hole assembly (BHA) design with respect to the hole geometry, inclination, and formations to be drilled can sometimes eliminate this potential contribution to wellbore collapse. Some authors claim that

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hole erosion may be caused due to a too high annular circulating velocity. This may be most significant in a yielded formation, a naturally fractured formation, or an unconsolidated or soft, dispersive sediment. The problem may be difficult to diagnose and fix in an inclined or horizontal well where high circulating rates are often desirable to ensure adequate hole cleaning (McLellan, 1994b).

Drilling fluid temperature

Drilling fluid temperatures, and to some extent, bottom hole producing temperatures can give rise to thermal concentration or expansion stresses which may be detrimental to wellbore stability. The reduced mud temperature causes a reduction in the near-wellbore stress concentration, thus preventing the stresses in the rock from reaching their limiting strength (McLellan, 1994b).

2.3 Types of Wellbore instability

2.3.1 Stuck Pipe

When dill pipe cannot pull up or cannot go down or cannot rotate, the phenomenon is called stuck pipe. The causes of pipe sticking are more numerous. They can be divided among differential sticking and mechanical sticking. Differential sticking occurs in permeable zones when drill collars, drill pipe or casing get embedded in mud cake and pinned to the borehole wall by the difference between the mud’s hydrostatic pressure and a lower formation pressure. Mechanical sticking covers various causes including formation related sticking (Bailey, 1991). Pipe sticking Mechanism and Causes can be summarized in Table 2.2.

Table 2.2: Pipe Sticking Causes (Rabia, 2001)

Pipe Sticking Causes Differential Sticking Mechanical Sticking

Hole Pack Off/Bridge Formation & BHA (Wellbore

Geometry)

Differential Force

Settled Cuttings Key Seating

Shale Instability Mobile Formation

Unconsolidated formations Under gauge Hole

Fractured and faulted formation

Micro Doglegs and Ledges

Cement Blocks Drilling Into Magma

Junk

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Differential sticking

During all drilling operations the drilling fluid hydrostatic pressure is designed and maintained at a level which exceeds the formation pore pressure by usually 200 psi. In a permeable formation, this pressure differential (overbalance) results in the flow of drilling fluid filtrates from the well to the formation. As the filtrate enters the formation the solids in the mud are screened out and a filter cake is deposited on the walls of the hole. The pressure differential across the filter cake will be equal to the overbalance (Rabia, 2001). When the drill string comes into contact with the filter cake, the portion of the pipe which becomes embedded in the filter cake is subjected to a lower pressure than the part which remains in contact with the drilling fluid. As a result, further embedding into the filter cake is induced. The drill string will become differentially stuck if the overbalance and therefore the side loading on the pipe is high enough and acts over a large area of the drill string. This is shown in Figure 2.17 (Rabia, 2001).

Figure 2.17: Differential sticking (Mitchell, 2011)

Mathematically, the differential sticking force depends on the magnitude of the overbalance and the area of contact between the drill pipe and porous zone. Hence, Differential force = (mud hydrostatic – formation pressure) x area of contact (Rabia, 2001). The signs of differential sticking are:

The pipe can neither be moved up and down nor rotated; Circulation is unaffected (Awili, 2015).

Differential sticking may be prevented by:

Maintaining lowest continuous fluid loss; Keeping circulating mud free of drilled solids;

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Keeping a very low differential pressure with allowance for swab and surge; Using a mud system that yields smooth mud cake (low friction co-efficient); Maintaining drill string rotation at all times; Using grooved or spiral drill collars; Minimizing length of drill collars and Bottom Hole Assembly (BHA) (Awili, 2015).

If differential sticking occurs, the following solutions are mostly used:

Immediate working/jarring of the string downwards; Reducing drilling fluid hydrostatic pressure by gasifying with air or by diluting the fluid.

Close attention must be paid to kick indicators while reducing hydrostatic pressure; Oil spotting around stuck portion of string; Washing over the stuck pipe (Awili, 2015).

Mechanical sticking

Mechanical sticking is a “catch-all” category for sticking problems. Any drilling or completion tool can become mechanically stuck. In this case the pipe is usually completely stuck with little or no circulation (Mitchell, 2011; Rabia 2001). Mechanical sticking can occur as result of the hole packing off (or bridging) or due to formation & BHA (wellbore geometry).

Hole pack off/Bridging

Formation solids (cuttings, cavings) settle around the drill string and pack off the annulus resulting in stuck pipe is called hole pack off. But medium to large pieces of hard formation, cement or junk falls into the wellbore and jams the drill string resulting in stuck pipe is called hole bridging (Amoco, 1996). Hole pack off /bridging can be caused by any one or a combination of the following processes:

Settled cuttings due to inadequate hole cleaning Shale instability Unconsolidated formations Fractured and faulted formations Cement blocks Junk falling in the well

Settled cuttings

Settled cuttings due to inadequate hole cleaning (Figure 2.18) is one of the major causes of stuck pipe. Best hole cleaning occurs around large OD pipe such as drill collars, while cuttings beds can form higher up the hole where the pipe OD is smaller. The problem of settled cuttings is particularly severe in horizontal and high directional wells. In these wells, when the pipe is

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moved upwards, the cuttings may be compacted around the BHA. This can result in complete packing off of the drill string and eventual pipe sticking (Rabia, 2001). With increasing deviation of the wellbore, drilling fluid parameters, drilling practices and hydraulics should be optimized in order to effectively clean the hole (Rabia, 2001).

Figure 2.18: Settled cuttings due to poor hole cleaning (Rabia, 2001).

In vertical wells, good hole cleaning is achieved by the selection and maintenance of suitable mud parameters and ensuring that the circulation rate selected results in an annular velocity (around 100-120 ft/min) which is greater than the slip velocity of the cuttings (Rabia, 2001). For effective hole cleaning in vertical and horizontal wells the flow rate must exceed the minimum values listed in Table 2.3.

Table 2.3: Minimum flow rate of effective hole cleaning (Wiper trip, 2015)

Hole Size Recommended Flow Rate (gpm)

171/2” / 16” 900-1000 12¼” 800-900 8½” 400-450

61/8” / 6” 250-300

Highly inclined wells are particularly difficult to clean due to the tendency of drilled cuttings to fall to the low side of the hole. In a highly deviated well the cuttings have only a small distance to fall before they settle on the low side of the hole and form a cuttings bed. Cuttings beds develop in boreholes with inclinations of 30 degrees or greater, depending on the flow rates and suspension properties of the drilling fluid. Complete removal of cuttings beds by circulation may be impossible (Rabia, 2001).

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Once cuttings beds have formed, there is always a risk that on pulling the pipe up the hole, the cuttings are dragged from the low side of the hole forming a cuttings pile (Figure 2.18). If this pile accumulates around the BHA, it may plug the hole and cause the pipe to mechanically stuck (Rabia, 2001). Besides causing stuck pipe, settled cuttings can also cause:

Formation breakdown due to increased Equivalent Circulating Density; Slow rate of penetration; Excessive over pull on trips; Increased torque (Rabia, 2001).

Hole cleaning is one of the main solutions to preventing this stuck pipe problem and can be controlled by:

Good mud rheology especially yield point and gel strength; Controlling drill rate to ensure hole is clean; Checking volume of cuttings coming over to shale shaker; Controlling annular velocities; Recognizing increased over pull; Reciprocating and rotating pipe while circulating; Using viscous sweeps; Recognizing low side section of deviated holes; Regular Wiper trips (Rabia, 2001).

If sticking occurs, then:

Attempt to establish circulation; Simultaneously apply downwards force gradually until circulation starts; Once circulation starts, rotate the string; In low angles holes, a weighted viscous pill should be used to ‘float out’ the cuttings; In high angle holes, a low viscous pill should be used to disturb the cuttings bed followed

by weighted pills to carry cuttings out of hole (Awili, 2015).

Shale Instability

Shale represents 70% of the rocks encountered whilst drilling oil and gas wells. Also shale instability is by far the most common type of wellbore instability (Awili, 2015). The shale formation become unstable due to chemical stressed and mechanical stressed. Chemical stressed shales are classified as being swelling type which belongs to reactive formations and Mechanical stress shales are classified as being brittle type which belongs to tectonic formation, Naturally/Induced pressured formation etc (Amoco, 1996).

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Brittle Shales

Instability in brittle shales is caused mainly by tangential stresses around the wellbore which are induced as a result of the well being drilled. The induced stresses depend on the magnitude of the in-situ stresses, wellbore pressure, rock strength and hole angle and direction. Formation dip may also be a contributory factor to brittle shale failure. A safe mud envelope may be established which can be used to determine the safe mud weights to prevent either tensile failure or collapse (compressive) failure. Brittleshalestendtofailbybreakingintopiecesandsloughingintothehole.Rigindications of brittle shale failure include:

large amounts of angular, splintery cavings when circulating the well drag on trips large amounts of hole fill

The Figure 2.19 shown shows the Inner Drucker-Prager criterion for predicting safe mud weights.

Figure 2.19: Safe Mud Weights Envelope (Rabia, 2001).

Swelling Shales

Shales swelling (Figure 2.20) can be caused by hydrational processes or by the osmotic potential which develops between the pore fluid of the shale and drilling fluid salinity. The swelling of shales (Figure 2.20) is controlled by several complex factors including:

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Clay content Type of clay minerals (i.e hydratable or inert) Pore water content and composition Porosity In-situ stresses Temperature

Figure 2.20: Swelling of reactive shales (Rabia, 2001)

The degree of clay hydration depends on the clay type and the cation exchange capacity (CEC) of the clay content. The greater the CEC, the more hydratable is the clay. In drilling operations the following clay types are encountered: Smectite with CEC of 80-150 meq/100g. Most of the hydratable shales (termed gumbos) belong to this group. Bentonite clays belong to the smectite group.

Illite with CEC of 10-40 meq/100g. Chlorite with CEC of 10-40meq/100g. Kaolinite with CEC of 3-10meq/100g (Rabia, 2001).

Sticking to the fluids program can prevent effects of formation instability. Other solutions include making use of a well program that isolates a potential troublesome formation and speeding up the drilling process to cut down time of drilling sensitive formations. Formation instability can be identified by the following:

Large amounts of angular or splintery cuttings when circulating; Drag on trips; Large amounts of hole fill.

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Formation instability will cause material to fall inside the hole creating caves or contract the wellbore and might cause sticking. Sloughing and caving are also due to formation instability. If these occur, then the solution is establishing circulation, then working the drill string preferably downwards; when the string is freed, circulate all material out before changing the mud properties to continue drilling (Awili, 2015).

Unconsolidated formations:

Unconsolidated formations are usually encountered near the surface and include: loose sands, gravel and silts. Unconsolidated formations have low cohesive strengths and will therefore collapse easily (Figure 2.12) and flow into the wellbore in lumps and pack off the drill string. Surface rig indications of an impending stuck pipe situation near top hole are: increasing torque, drag and pump pressure while drilling. Other signs include increased ROP and large fill on bottom. A common remedial action is to use a mud system with an impermeable filter cake to reduce fluid invasion into the rock. Reduction of flow rate, and in turn annular velocity, will reduce erosion of the hole and removal of the filter cake (Rabia, 2001).

Fractured and Faulted formations

Symptoms of fractured and faulted formation include:

Large and irregular rock fragments at shale shakers; Increased torque, drag and rate of penetration; Small amount of lost circulation.

The fractured formation falls into the well due to stresses originally holding the formation together being relieved by drilling of the hole. Excessive vibration might also cause the drill string to whip down hole and dislodge the fractured rocks. The problem can be prevented by:

Reducing drill string vibration; Minimizing surge pressures; Sufficient hole cleaning to reduce hole pack off.

If sticking occurs, jar the string. If this is not successful, an inhibited Hydrochloric acid pill may be spotted around the stuck zone to break down the material surrounding the pipe (Rabia, 2001).

Cement Blocks

Cement blocks from the rat hole might fall into the well bore and cause sticking. This can be prevented by minimizing the rat hole to a maximum of 5 feet and ensuring good tail cement at the casing shoe. If sticking occurs due to cement blocks, jar the string or inject acid solution down hole to dissolve the cement. Green cement is improperly set cement. Green cement can occur after setting a cement plug inside casing or open hole. If the drill string is run too fast into top of cement and the cement is still green, then the cement can flash set around the pipe and cause permanent sticking. Flash setting is phenomenon that is not very well understood but a

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possible explanation is that the energy release while circulating and rotating could be sufficient to cause it. A good practice to prevent this is starting circulation 2 or 3 stands above expected top of cement and also keeping a low weight on bit (Awili, 2015).

Junk

Several recorded incidents of pipe sticking occurred as a result of junk falling into the hole. This includes junk falling into the wellbore from the surface or from upper parts of the hole. Typical junks dropped from surface include pipe wrenches, spanners, broken metal, hard hats etc. This problem can be minimized by keeping the hole covered when no tools are run in the hole. Junks can also fall from within the well including broken packer elements, liner hanger slips and metal swarf from milling operation (Rabia, 2001).

Formation and BHA (well geometry) causes

The formation & BHA (wellbore geometry) can also cause mechanical sticking as follows:

Ledges and micro doglegs Key seating Mobile formations Under gauge hole

Ledges and micro doglegs

Micro doglegs and ledges develop when drilling formations of varying strengths or dipping formations. A gauge hole is drilled in the harder zone and an oversized hole, caused by fluid erosion, is drilled in the softer zone. This oversized hole causes the bit and the BHA to be deflected to the low side of the hole causing a small dogleg when the next hard section is drilled. The drilling of several successive layers of varying strengths result in both hole ledges and micro -doglegs to develop (Rabia, 2001).

Key seating

Key seating (Figure 2.21) is caused by the rotating drill string coming into contact with soft, easily drillable formations. The rotational action causes the tool joint to erode a narrow groove in the formation which is approximately equal to the diameter of the drill pipe tool joint. The created groove or slot is smaller in size than the larger BHA components below. When pulling out of hole (POOH), the BHA may be pulled into the narrow -sized key seat resulting in BHA being stuck. Key seats are often seen in soft formations or in wells with ledges and doglegs. The doglegs and ledges allow the drill string to bend and provide points of contact between the tool joint and the walls of the hole. Key seats may also develop in casing shoes in highly deviated wells.

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Pipe stuck in a key seat can be recognized by the following symptoms;

Circulation is free when the pipe is stuck Hole is tight on tripping out only. Tight hole position can be correlated with the positions of large OD members of the

BHA. Tight hole will occur at the same depth on trips.

Pipe stuck in a key seat must be worked and jarred downwards (and only downwards) until free movement and rotation is established. Once the drill string is free in a downwards direction, the string should be slowly pulled past the key seat using minimum tension and slow rotation (Rabia, 2001).

a. Mechanism b. Formation of Key seat

Figure 2.21: Key Seating (Rabia, 2001).

Undergauge hole

The drilling of abrasive formations such as sandstones can result in bit and stabilizer gauge wears. This loss of gauge (diameter) causes an undergauged hole to be drilled. Development of undergauge hole is to be avoided as it results in costly reaming operations on the subsequent bit run. In addition, incidence of stuck pipe has occurred as a result of running full gauge bits and stabilizers into the undergauge section (Figure 2.22). Extra caution should always be exercised when tripping in the hole after pulling an undergauge bit (Rabia, 2001).

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Figure 2.22: Undergauge hole (Rabia, 2001).

2.3.2 Lost Circulation

Loss circulation is the loss of mud or cement to the formation during drilling. This can be either a partial lost, some returns to the surface or a complete loss with no returns to the surface. It is an expensive problem while drilling and often accounts for more than 20% of the costs in exploratory wells and developing fields (Marbun, 2011).Lost circulation invariably results in higher costs for materials, services, and additional rig time. Depending on the timing and severity of its occurrence, it can lead to the loss of formation-evaluation data because the information normally obtained from mud returns and drilled cuttings are no longer available (Mitchell, 2011).Mud losses can be experienced as a result of permeable zones (Figure 2.23 marked as A), natural fractures (Figure 2.23 marked as C), induced fractures (Figure 2.23 marked as D), caverns (Figure 2.23 marked as B).

Figure 2.23: Types of lost circulation (Mitchell, 2011).

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Complete prevention of lost circulation is not possible. When lost circulation occurs, sealing the zone is necessary. Normally LCM is mixed with mud to seal loss zones (Azar, 2007).

2.3.3 Hole Deviation

When the hole deviates from the vertical or planned course, the bit tends to walk while drilling. Formation dip and rock properties can influence the path of the bit. This can cause both technical and legal problems.

Figure 2.24: Example of Hole deviation (Petrowiki, 2015).

It is not exactly known what causes a drill bit to deviate from its intended path. It is, however, generally agreed that one or a combination of several of the factors may be responsible for the deviation like Heterogeneous nature of formation and dip angle, Drill string characteristics, specifically the bottom hole assembly (BHA) makeup, Stabilizers (location, number, and clearances), Applied weight on bit (WOB), Hole-inclination angle from vertical, Drill-bit type and its basic mechanical design, Hydraulics at the bit, Improper hole cleaning etc. It is known that some resultant force acting on a drill bit causes hole deviation to occur. The mechanics of this resultant force is complex and is governed mainly by the mechanics of the BHA, rock/bit interaction, bit operating conditions, and, to some lesser extent, by the drilling-fluid hydraulics. The forces imparted to the drill bit because of the BHA are directly related to the makeup of the BHA, i.e.:

Stiffness Stabilizers Reamers

http://petrowiki.org/Hole_cleaning

http://petrowiki.org/Drilling_fluids


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The BHA is a flexible, elastic structural member that can buckle under compressive loads. The buckled shape of a given designed BHA depends on the amount of applied WOB. The significance of the BHA buckling is that it causes the axis of the drill bit to misalign with the axis of the intended hole path, thus causing the deviation. Pipe stiffness and length and the number of stabilizers (their location and clearances from the wall of the wellbore) are two major parameters that govern BHA buckling behavior. Actions that can minimize the buckling tendency of the BHA include reducing WOB and using stabilizers with outside diameters that are almost in gauge with the wall of the borehole (Petrowiki, 2015).

2.3.4 Pipe Failures

Drill pipe failures are a prevalent drilling problem. It can be put into one of the following categories: twist-off caused by excessive torque; parting because of excessive tension; burst or collapse because of excessive internal pressure or external pressure, respectively; or fatigue as a result of mechanical cyclic loads with or without corrosion.

Twist off Pipe failure as a result of twist off occurs when the induced shearing stress caused by high torque exceeds the pipe-material ultimate shear stress. In vertical-well drilling, excessive torques are not generally encountered under normal drilling practices. In directional and extended-reach drilling, however, torques in excess of 80,000 lbf-ft is common and easily can cause twist off to improperly selected drill string components (Petrowiki, 2015).

Figure 2.25: Twist off (Petrowiki, 2015)

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Parting: Pipe-parting failure occurs when the induced tensile stress exceeds the pipe-material ultimate tensile stress. This condition may arise when pipe sticking occurs and an overpull is applied in addition to the effective weight of suspended pipe in the hole above the stuck point (Petrowiki, 2015). Burst or collapse: Pipe failure as a result of collapse or burst is rare; however, under extreme conditions of high mud weight and complete loss of circulation, pipe burst may occur (Petrowiki, 2015). Fatigue: Fatigue is a dynamic phenomenon that may be defined as the initiation of micro-cracks and their propagation into macro-cracks as a result of repeated applications of stresses. It is a process of localized progressive structural fractures in material under the action of dynamic stresses. It is well established that a structural member that may not fail under a single application of static load may very easily fail under the same load if it is applied repeatedly. Failure under cyclic (repeated) loads is called fatigue failure (Petrowiki, 2015).

Figure 2.26: Fatigue cracks (Bert, 2009)

2.3.5 Tight Hole

A hole is said to be tight when the downward force restrict string movement above normal operating conditions (Amoco, 1996). When the force is upward, it is called overpull. In these phenomena formations are slowly moving into the wellbore and if the wellbore stays open for a sufficient period of time the drill string may meet restrictions during tripping, hence the string may not be able to pass anymore (Bjerke, 2013).Tight hole happened in the wellbore that caused by shale or clay formation become weakened by adsorption of water into the day of the wellbore rock (Marbun, 2011).

http://petrowiki.org/Lost_circulation

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Figure 2.27: Tight hole phenomena (Bjerke, 2013).

2.3.6 Hole Washout

A hole washout occurs when the diameter of the hole drilled is greater than the bit size used to drill the hole. Hole erosion and washout occur across weak and soft formations as a result of using large flow rates resulting in excessive mud annular velocities. Washouts also occur across reactive shales which slough into the hole when contacting uninhibited water-based mud. Signs of washouts at surface include:

Increased volume of cuttings on shale shakers Large cuttings Difficulty in running into the hole Bottoms up time increase

Hole washouts cause several problems including difficulty in cleaning the hole, poor directional control, difficulty in running into the hole and most importantly result in very poor cement jobs. A number of casing buckling problems have been observed across severely washed-out holes which could not be cemented to the critical height required to prevent buckling. Preventing washouts should be planned ahead of drilling the well. In the field, if washouts are suspected then mud inhibition should be increased, lifting capacity of mud improved by increasing the mud yield point (YP) and annular velocities reduced to the absolute minimum consistent with effective hole cleaning (Rabia, 2001).

2.3.7 Mud Contamination

A mud is said to be contaminated when a foreign material enters the mud system and causes undesirable changes in mud properties, such as density, viscosity, and filtration. Generally, water-based mud systems are the most susceptible to contamination. Mud contamination can result from overtreatment of the mud system with additives or from material entering the mud during drilling (Petrowiki, 2015)


http://petrowiki.org/Drilling_fluid_types

http://petrowiki.org/Drilling_fluid_types#Water-based_fluids

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2.3.8 Excessive Cavings at surface

Drill cuttings are considered representative of the liothology being drilled in a wellbore. However, the fragments that are two to three times larger and/or having odd shapes compared to the regular cuttings are commonly understood as cavings from the wall of the borehole and they are seldom of any help in compilation of a lithology. In fact, cutting description manual recommends ignoring cutting sizes greater than half an inch. Nevertheless, these cavings carry critical information that needs expert decoding concerning impending or happening wellbore instability, formation overpressure and overall well behavior evaluation. Cavings can be produced due to several mechanisms, such as underbalance drilling, stress relief, pre-existing planes of weakness or simply by mechanical action of the drilling process and/or drilling tools. A simplified approach is made to describe the cavings morphology and its interpretation in terms of wellbore stability. The relative amount of cavings in the bulk sample is also an indication of the degree of instability of the borehole walls. Cavings are the first and foremost indicator of wellbore deterioration, and the correct interpretation or knowledge of cavings can help save millions of dollars by using appropriate prevention/remedial actions. The most noticeable and predictive cavings for wellbore stability and formation pressure are those of clay and shale. Collectively size, shape, appearance and relative percentage of the cavings compared to the total load of what is coming at the shale-shaker versus time are necessary to keep track of the health of the wellbore. Hence, listening to the wellbore by monitoring continuously of what is coming at the shaker by an expert set of eyes during any drilling fluid circulation pre-drilling, while-drilling or post-drilling operation is essential(Kumar, 2012) A key parameter to manage wellbore instability in real time: provides an early warning of wellbore instability. Monitoring cavings rate: Cavings volume with respect to time indicates which drilling practices destabilize the wellbore. Performing cavings morphology: Types of cavings like platy, angular, or splintered.

Figure 2.28: Blocky and angular shaped cavings (HXR, 2018).

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Reactive Shales react with water-based mud and swell which can result in gummy cavings and bit balling (Figure 2.28). .

Figure 2.29: Platy and splintery shaped cavings (HXR, 2018).

Weak bedding is evidenced by platy shaped cavings (Figure 2.29). Drilling problems most often associated with splintery and platy shaped cavings are tight hole, pack-off and stuck pipe.

Figure 2.30: Splintered Cavings (HXR, 2018).

Many areas have high overpressure, especially at depth. Mud windows become very narrow when overpressure is high, insufficient MW will cause the rock to break off the wellbore wall in splintery shapes (Figure 2.30). When splintery shaped cavings are observed, it is a good indication to increase MW (HXR, 2018).

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2.3.9 Hydrogen-Sulfide-Bearing Zones and Shallow Gas

Drilling H2S-bearing formations poses one of the most difficult and dangerous problems to humans and equipment. If it is known or anticipated, there are very specific requirements to abide by in accordance with Intl. Assn. of Drilling Contractors rules and regulations. Shallow gas may be encountered at any time in any region of the world. The only way to combat this problem is to never shut in the well; divert the gas flow through a diverter system instead. High-pressure shallow gas can be encountered at depths as low as a few hundred feet where the formation-fracture gradient is very low. The danger is that if the well is shut in, formation fracturing is more likely to occur, which may result in the most severe blowout problem, like underground blow (Petrowiki, 2017).

2.3.10 General Equipment & Personnel

Equipment

The integrity of drilling equipment and its maintenance are major factors in minimizing drilling problems. Proper rig hydraulics (pump power) for efficient bottom and annular hole cleaning, proper hoisting power for efficient tripping out, proper derrick design loads and drilling line tension load to allow safe overpull in case of a sticking problem, and well-control systems (ram preventers, annular preventers, internal preventers) that allow kick control under any kick situation are all necessary for reducing drilling problems. Proper monitoring and recording systems that monitor trend changes in all drilling parameters and can retrieve drilling data at a later date, proper tubular hardware specifically suited to accommodate all anticipated drilling conditions, and effective mud-handling and maintenance equipment that will ensure that the mud properties are designed for their intended functions are also necessary (Petrowiki, 2017).

Personnel

Given equal conditions during drilling/completion operations, personnel are the key to the success or failure of those operations. Overall well costs as a result of any drilling/completion problem can be extremely high; therefore, continuing education and training for personnel directly or indirectly involved is essential to successful drilling/completion practices (Petrowiki, 2017).

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CHAPTER 3: WELLBORE INSTABILITY ISSUES WHILE DRILLING

Mubarakpur -1 and Sunetro-1 both experienced the most expensive instability issue, the pipe stuck incident. However the root causes were different.

3.1 Wellbore instability issues while drilling Mubarakpur-1

Mubarakpur-1 well was designed in five stages, upto 5” liner (Figure 3.1). The target depth was 4700m. But after drilling upto 4379m, pipe got stuck at 4175m and sidetracked from 3469m. The drilling activities was smooth before entering 17 ½” hole section. The first tight spot was found at 937m while tripping in. Reaming down was enough to continue drilling. More overpull was found while drilling this section (Appendix A). Wireline logging completed after reaching section TD. The 12 ¼” hole section was full of instability issues. More than 15 tight spots and overpulls upto 15-70klbs were found within 2060m-3030m (Appendix A). Drilling continued with reaming. After 2688m multiple reaming were needed after every stand down. And below 2962m it changed to after every half single stand down. Mud conditioning, hole cleaning circulation, spot pill pumping also followed to make the drill path smooth. After reaching to target depth 3469m of this section the well was idle for almost 5 days due to absence of logging team. But no problem was encountered during wireline logging. In this section they changed mud system to LSND from gel polymer after 1621m. Mud weight range, 1.09sg – 1.15sg, was according to planned program. With new PDC bit 81/2” section started to drill from 3479m. After drilling 3670m cavings findings started and mud weight increased from 1.15sg to 1.18sg (Figure 4.6). More than 15 tight spots and overpulls upto 50klbs observed in this section. Below 4066m during POOH 50 klbs overpull observed which needed 30 minutes reaming and back reaming to make the hole clear. While drilling ahead wiper trip with reaming was done as a routine. After 4147m to RIH every stand reamed down two times and back reamed one time. But the hole was not smooth enough. Huge cavings of average size 1-1.5” continued (Figure 3.2). Within 4147m-4379m mud weight increased from 1.26sg to 1.29sg, high viscous pill pumped and also wiper trip in short and long depth. Drilling continued upto 4379m. For wireline logging string POOH, drill bit was found worned out, not balled up. Twenty five diamonds out of 46 were broken. Hole cleaning, mud conditioning circulation was given. It could not make the way free for wireline logging attempt failed below 3475m. Two centralizers and caliper tools added to increase dead load, but no progress occurred. Repeatedly reaming up and down was done below the casing shoe depth from 3469m to 3501m to log the section again.

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Figure 3.1: Well design of Mubarakpur-1 well (WCR, 2017)

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After POOH the bit was found 0-50% (14 bit cutter) and 50-80%( 13 bit cutter) broken out of total 47. Again about ten times attempted to run logging tools but failed. Mud treatment was changed to KCL based mud from LSND. Cavings and overpull problems could not be prevented. Mud weight increased gradually from 1.30sg to 1.33sg. Reaming up and down continued with increased standpipe pressure (from 2100 psi to 2650 psi). An attempt was made to go down without rotation and without circulation. This attempt was faster. 10klbs tight spot and 40klbs overpull were observed. The pump was started but SPP increased to 2700 psi, circulation was not coming back. Tried to rotate the string but resulted in excessive torque. Pipe got stuck at 4175m. Started pump with gradually higher SPM and tried to pull up with little rotation but SPP abnormally increased to 3380 psi. Jar was not working, generator shut down, pump pressure bleed off from 3380psi to 1080 psi. Continuous tried to string up and down with higher pump rate. SPP increased to 3870psi with 78 SPM. It crossed the LOT pressure which was 3425 psi (Table 3.1). About 29m3 mud loss was recorded within 30 minutes. But string could not be released. Decision was taken to back-off for locating free point and it was located at 4087m. The string recovered by leaving 358.93m fish [09 stands (261.45m) + BHA (97.48m)] in the hole (WCR, 2017; DDR, 2016).

Table 3.1: Leak Off Test (LOT) data of Mubarakpur Exploratory Well#1

Last Casing

Hole Depth

Mud Wt. sg

Applied Pressure

Hydrostatic Pressure

EMW Remarks

20" 261m 1.03 100 381 1.30 LOT 133/8" 1601 m 1.09 1500 2480 1.75 LOT 9 5/8" 3470 m 1.15 3425 5670 1.844 LOT

7” 4283mMD 1.63 1450 9921 1.86 LOT

Figure 3.2: Cavings found in Mubarakpur-1 while drilling at 3505-3530m.

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Sidetrack

Before beginning sidetrack, survey was done at every 500m. Within depth 63m to 2720m 0.15o to 1.05o inclination recorded. Sidetrack started from 3495m, continued with sliding upto 3510.5m. New mud system clay seal polymer introduced. Tight spots and overpull problems started from 3674m (Appendix A) and continued in various depth. To clean the hole high viscous pill pumped several times in several depths while drilling. After drilling upto 4152m string POOH and found the bit balled up. While reaming drill pipe got stuck at 3754m and got free within 10 minutes through jarring. Cavings were observed at shale shaker and drilling continued with low ROP upto 4213m. Tight spots and overpull were also common in this section. While reaming higher torque was found at 4239m-4241m, 4247m4249m. Wieline logging tools were run upto 3475m. Attempt to run centralizer failed. Cavings observation continued. After reaching 4280m high torque was noticed within 4274m-4276m depth while reaming and back reaming. Even rotation of sting stopped several times. Attempt to run 7” liner failed and found 14 blades, 6 rings, one stop collar of the centralizer lost in the hole when POOH was complete. During POOH, at 4175m, SPP rose to 3440 psi immediately with a low SPM of 8 on both pumps. Gradually the pressure bled off to zero. Tried to pull the string up but about 25 klbs overpull was got, while tried to go down, got 40 klbs tight spot. Pumping out with rotation and got cleared. This was the stuck point of the previous hole. Mud weight was increased from 1.24sg to 1.70sg through this section. After reaching 1.70sg mud weight hole condition improved. Cavings also decreased. Continuous HVP pumping, reaming, back reaming was done and finally set the liner at 4280m. Wireline logging completed.

The 61/2” section was comparatively smoother than previous sections. Cavings were observed. After drilling upto 4515m cavings size was large. With increased mud weight from 1.63sg to 1.70sg hole became stable and wireline logging completed (WCR, 2017; DDR, 2016).

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3.2 Wellbore Instability Issues while drilling Sunetro -1

This well encountered two pipe stuck incidents. Drilling went smoothly till the programmed TD of 4000 m. Then decision was taken to continue drilling in deeper horizons. The architecture of this well is shown in Figure 3.3 The first wellbore instability issue was noticed in 12 ¼’’ hole section. It only had one degree inclination at 1420m which was negligible. From 1433m partial mud loss 2.5m3-17m3 continued. After 1565m, tight spot and overpull continued upto236klbshookload (Appendix B). Reaming, setting jar after 1706m helped to recover little bit. ROP decreased drastically from 30m/hr to 1.8m/hr below 2438m. Cavings were noticed at shale shaker (Figure 3.4a). Observed 11.5m3

hole enlargement from carbide bomb circulation and 14m3 mud loss while drilling 2442m-2450.16m3 within 19 hours. After POOH found that string stabilizer teeth eroded (Figure 3.5), 23 bit cutters lost/label and 14 were partially broken. With the addition of stabilizer drilling continued upto 2870m. Again ROP dropped and mud loss was observed 11.5m3. Cavings were observed at shale shaker after 3140m. Lithology changed to shale from sand in this section. Wireline logging conducted without any obligation. In 8 ½” section one more stabilizer was added in the start of drilling 3204m. Drilling continued to 3503m. From 3526m mud loss reduced. But ROP was very low and after POOH maximum bit teeth found eroded and broken. With the progress of drilling with new bit no change in ROP was observed. That time the bit was found to be in good condition after POOH from 3689m. Tight spot and overpull was not much noticeable above this depth. Hole inclination reached to 1.25 degrees. Cavings continued within 3550m to 3775m (Figure 3.4 b), mud weight increased from 1.11sg-1.14sg within this depth, high viscous pill was pumped. ROP dropped gradually about to 0.39m/hr. After drilling 3810.61m, bit balling was observed but no mud loss was noticed. At 3928m huge bit bumping with torque was encountered. WOB decreased from 8.5 tons to 5 tons to stop bit bumping. At 3950m inclination was found 6 degrees which is in countable range for a vertical well. 3 joint DC subtracted to drop the inclination and recorded 3 degrees at 4029m. Overpull and tight spot continued through the depth 4007m to 4268m. Jarring, single by single stand reaming, back reaming was done and finally reached to 4499m. After hole clean up circulation POOH was started and at 4355m found over pull several times up to 179 klbs hook load and pipe got stuck. Crew tried to free the string by jarring, pumping saltex pill, but no progress. During free point location logging free point was observed above 4325m.

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Figure 3.3: Schematic diagram of Sunetro well no#1 (WCR, 2012)

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Figure 3.4 (a): Cavings observed after 2460m.

Figure 3.4 (b): Cavings observed after 3550m.

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Figure 3.5: Eroded Stabilizer after drilling 2463m.

Figure 3.6: Bit condition after drilling 4660m.

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The string was successfully backed-off from the Joint of DC & HWDC at the depth of 4306.90m. 01 Bit, 02 stabilizers and 03 DC’s were left (total length 31.91m out of 134.08m of BHA) as fish in the hole. Gamma ray and the Resistivity log (Multi array induction log) could run. But with these caliper failed to run.RIH started with the same BHA. Observed tight spot in depth 3557 to 3567m and ream it down. Wireline logging started again and finished without any problem.

Side Track

After cement plugging, kick-off point was from 4245m for sidetracking by sliding and continued to build up angle upto 4307m, almost the depth of the previous fishing top. Then rotation started. Side track operation started from 4264m and continued to 4303.50m. Drilling rate was very poor(0.5m/hr) from 4297.5m to 4303.50m. Set cement plug and continued side tracking from 4246.7m to 4257.26m with 0.25m/hr drilling rate. Drilling continued upto 4342.85m.After drilling 4660.88m, drill string was POOH and bit balling was observed. A total of 20 PDC teeth were found damaged (Figure 3.6), hole deviation recorded 4.5 degree. With new bit string was RIH and continued drilling to 4683m .Tight spots and overpull were observed at several depth within 4267m-4681m. During POOH overpull was observed while reaming from 4672m-4681m.The crew tried to go down but tight spot was found. Continued jarring, pumped high viscous pill, spotting fluid, but no progress. Pipe got stuck at 4675m. Drill string recovered from 4642m and 34.54m fish left in the hole.

Wireline logging could not proceed down to 4430m due to tight spot (WCR, 2013; DDR, 2012).

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CHAPTER 4: DISCUSSIONS

Pipe stuck incident cannot occur suddenly. It comes with lots of alarming signs. Mubarakpur-1 and Sunetro-1 both crossed various hole instability signals like tight spots, overpulls, cavings, mud loss, hole enlargement, hole inclination, broken and eroded bit etc. The total effect of all these difficulties results in pipe stuck incident.

4.1 Mubarakpur-1

The beginning of wellbore instability issues in Mubarakpur-1 well was observed in 17 ½”hole section. The formation at problematic depth of 17 ½” section was shale. From mud log report presence of soft, sticky, reactive shales was found. The swelling effect of shales created the hole tight at some depth (Appendix A). Wiper trip with reaming was enough to continue drilling activities. The mud system was changed to Low Solids Non Dispersed (LSND) from gel polymer in 12¼” section to prevent swelling effect of shales. And in real time the mud weight was increased from 1.09sg to 1.15sg through the whole section. As the cuttings were blocky to sub-blocky, swelling appearance of shale can be the reason of tight hole. Continuation of excessive tight spot and overpull was not effected much after changing mud system. It was also the sign of insufficient mud weight. The stress concentration around the wellbore could not be prevented much by the mud weight and the break off from wellbore wall caused the tight hole situation. The maximum flow rate for this section was 763 gpm which did not exceed the minimum value of effective hole cleaning (Wiper trip, 2015). In this circumstance lack of proper hole cleaning was also the reason of wellbore restriction. The 8½” section was evaluated for pressure deviation by monitoring the parameters like interval velocity, D exponent, wireline log, flow line temperature. Interval velocity The interval velocity trend line with respect to depth (Figure 4.1). Sudden decrease of interval velocity was observed between 3400m-3500m and with the increment of depth it continued to increase again upto 4400m. Then the trend moved to decreased mode continuously. According to above, high pressure may be started from 4450m or 4500m and continued to 5500m. After that pressure trend may be changed to normal. D-Exponent D-Exponent value has been recorded and the trend-line was monitored to get indication of any geo-pressure transition. From the illustrated pressure plots, D-exponent exhibits few distinct trends all indicating a normal trend. Shifting of trend lines however, not all coinciding with Bit change or Formation change, change in lithological sequence. Frequent

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changing in drilling parameters, was causing difficulty in analyzing pressure indicators however, from the D-Exponent trend it indicates that the pressure is normal up to 4450m. After that transition zone starts. Also second pressure trend changed after 4600m (Figure 4.2 a). The D-Exponent trend of sidetrack section was shown in Figure 4.2 b.

Figure 4.1: Interval Velocity Vs Depth from VSP data

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Figure 4.2 a. D Exponent data of Mubarakpur Exploratory Well #1.

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Figure 4.2b: D Exponent data of Mubarakpur Exploratory Well #1 (Side Track).

Wireline log After drilling of each section, wireline logs were conducted. Relevant wireline logs data like Resistivity, Density and Sonic were then incorporated into the pressure plots to evaluate their trend. Wireline log plots also show normal pressure trends up to 4500 m after that pressure trend may be changed. According to Sonic, Resistivity and Neutron log a major change occurred after 4600m (Figure 4.3a). So, high pressure zone may be started from 4600m. The wireline log from sidetrack section was shown in Figure 4.3b and Figure 4.3c.

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Figure 4.3a: Wire Line Log curve comparison with Gamma based Lithology

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Figure 4.3b: Wire Line Log curve comparison with Gamma based Lithology (Side Track).

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Figure 4.3c: Wire Line Log curve below 4250m.

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Flow line temperature To evaluate the pressure formation from the Flow Line Temperature it should keep in mind that the discontinuity of drilling such as tripping, new section drilling and any time gap of mud circulation can shift the trend line of temperature increment. Adding new mud also has taken into consideration to analyze the temperature trend. Considering all circumustances, it seems that the normal increament of the formation temperature sustain up to TD (Figure 4.4a and Figure 4.4b).

Figure 4.4a: Flow Line Temperature of Mubarakpur -1

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Figure 4.4b: Flow Line Temperature of Mubarakpur -1 (Side Track).

From the pressure evaluation parameters it was found that over-pressured zone started from almost 4200m which was nearby the ending depth of the 8 ½” section. But instability issues like tight spots, overpulls and excessive cavings were observed from the beginning of drilling this section. The cavings were observed at shale shaker after 3500m was drilled with splintered type shape (Figure 3.2). Insufficient MW causes the rock to break off the wellbore wall in splintery shapes (HXR, 2018). When splintery shaped cavings are observed, it was necessary to increase MW. From the mud log report the lithology of the shales through the 81/2” section was non-reactive. From XRD report, predominantly existence in illite layers was found (Table 4.1).Illite is different in nature. It does not reactive with water base mud (Appendix D). Most values are below 20. So, the change of mud system from LSND to Potassium KCL system could not improve hole condition.

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Table 4.1: Cation Exchange Capacity (CEC) from Cavings at Various Depths (Source: XRD Report)

Sample No CEC (%)

S1 10 S2 8 S3 10 S4 7 S5 4 S6 5

When the well was drilled through the over-pressured zone, significant amount or rock volume was removed, which caused stress concentration around the wellbore. Due to low mud weight the compressive stress broke out the formation which continued splintered type cavings (Figure 3.2). Excessive cavings restricted the path of drilling and tight spots, overpulls found. It also reduced the ROP. In the mud log report the shale and sand section showed calcareous behavior. So, the continuous drag between calcareous grains and bit due to excessive wiper trip with reaming and back reaming can be the reason of bit broken. Damaged bit was also the reason of low ROP. Excessive cavings lead to washout at different depth of formation. During cementing, the linear flow was not able to remove mud from the washout area. That's why at certain point, poor or bad cementation was found. For this reason, turbulent flow was needed, but it wasn’t available. Moreover, good hole condition is the precondition of good cementing. If there is a lack of good cement in between water sand and gas sand, it causes cross flow after some production due to pressure depletion and hampers the production. The crew ran the string in the hole without rotation and circulation in faster mode (Figure 4.5) that action was wrong. Circulation resists settling cuttings and rotation enhances the rate of circulation. In this case absence of both, the compressive stress around the wellbore and cuttings in the hole packed off round the drill string. Then sudden start of circulation could not put back the packed cuttings from settling well. SPP crossed the LOT pressure, that fractured the formation and 29 m3 mud loss occurred within 30 minutes. Finally pipe got stuck and could not be recovered without leaving fish.

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Figure 4.5: Stuck scenario in Mubarakpur-1 well.

The sidetrack section was totally in over-pressured zone. The shale zone showed non-reactive character in large percentage in mud log report. But small amount of bit balling was found. It could be the effect of changed mud system which could not prevent short percentage of swelling. The stress concentration effect was same through sidetrack section as the before 81/2” section. The tight spots, overpull and cavings phenomena was continued as above 8½” section. Mud weight increased in large margin to prevent formation break down and it worked after reaching 1.70sg. And the drilling was completed considering equivalent circulating density (ECD). It walked in at distance from the planned mud weight scenario (Figure 4.6)

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Figure 4.6: Comparison between Planned and Actual MW in Mubarakpur -1 well.

Now it can be established that return of circulation through the stuck event is very important. In the Mubarakpur-1 well circulation was restricted through the stuck pipe incident. So, it was a mechanical sticking. Above discussion indicates to the inadequate hole cleaning as circulation flow rate did not exceed minimum flow rate for effective hole cleaning (Table 4.2). So, settled cutting is possible reason.

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Table 4.2: Flow rates of Mubarakpur-1 well in different hole sections (Wiper Trip, 2015).

Hole Size Minimum Recommended Flow Rate (gpm)

Flow rate in Mubarakpur-1 well (gpm).

171/2” 900-1000 383-809 12¼” 800-900 571-763 8½” 400-450 402-472

383-567 (Sidetrack)

61/2” 250-300 251-295 From the XRD report the shales through the pipe stuck zone were not reactive with water based mud system. So, reactive formation or swelling effect can be denied. According to geological background, the formation was not tectonically active formation. Over-pressured zone was noticed but below the stuck point. As stuck pipe occurred after drilling some depth of the transition zone, effect of over-pressured is possible. Through the whole drilling operation it was observed that only increased mud weight helped to prevent all instability of issues. So, it can be strongly agreed that due to low mud weight stress concentration around the wellbore caused the compressive failure. That’s the vital reason of shale instability. Stuck pipe depth was 4175m and it was not a unconsolidated formation. The formation was consolidated, compact. No sign of lost circulation and low ROP was the sign of absence of fractured rocks. Cement blocks and junk related problems were also not recorded through the drilling operation. Formation and BHA related problems like key seating, micro doglegs and ledges, under gauge hole, mobile formation etc were not found through the drilling operation (WCR, 2017; DDR, 2016).

4.2 Sunetro-1

Sunetro-1 well had various wellbore instability issues like hole deviation, mud loss, Bit teeth broken, cavings, tight spots and overpulls, low ROP, finally two times stuck pipe incidents.

Hole Deviation: The deviation started at 1420m with 1 degree. There was no stabilizer put in BHA design for drilling section 1413-1433m. This could be the reason for the initial 1 degree unplanned hole deviation. With the addition of two stabilizers the deviation kept in hold mode (Table 4.3). The 2 degree deviation at 3040m raised through the change from rock bit to PDC bit. The single stabilizer was removed at first and added back again. The maximum deviation was recorded as six degree at 3850m which returned to 3 degree at 4029m. The BHA design shows sudden addition of three drill collars which caused the string sliding and inclination built within that depth (Table 4.4). After keeping the string in drop mode, the deviation was reduced (Appendix C). In the beginning of sidetrack, mud motor added and Stabilizer was not included

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in the BHA design (Table 4.5). It could be the reason of angle build in. The only survey data was4.5o at 4660m.

Table 4.3: BHA design from 1405m-1706m

BHA No. 1 2 3 Depth In 1405 1413 1433 Depth Out 1413 1433 1706

- No. Item Length (m) No. Item Length

(m) No. Item Length (m)

BHA

1 Bit 12 1/4 0.32 1 Bit 12 1/4 0.32 1 Bit 12

1/4 0.32

1 Bit Sub 0.93 1 Bit Sub 0.93 1 NB Stb. 1.60 3 9 ½” DC 28.13 3 9 ½” DC 28.13 1 9 ½” DC 9.43 1 X-O-S 1.88 1 X-O-S 1.88 1 S. Stb. 1.62 10 HWDP 92.89 10 HWDP 92.89 2 9 ½” DC 18.70

1 X-O-S 1.88 10 HWDP 92.89 Total Length 124.15 124.15 126.44 Remarks Drilling Drilling Drilling

Table 4.4: BHA design from 3503m-4499.42m

BHA No. 4 5 6 Depth In 3503m 3689.41m 3810.61m

Depth Out 3689.41m 3810.61 3887 m



BHA

1 Bit 8 1/2 0.25 1 Bit 8 1/2 0.25 1 Bit 8 1/2 0.24 1 NB STb 1.82 1 NB STb 1.82 1 NB STb 1.82 1 6 3/4” DC 9.45 1 6 3/4” DC 9.45 1 6 3/4” DC 9.45 1 STb 1.71 1 STb 1.71 1 STb 1.71 2 6 3/4” DC 18.68 2 6 3/4” DC 18.68 5 6 3/4” DC 47.00

3 5” HWDP 27.86 3 5” HWDP 27.86 3 5” HWDP 27.86

1 Jar 9.28 1 Jar 9.28 1 Jar 9.28 7 5” HWDP 65.03 7 5” HWDP 65.03 7 5” HWDP 65.03

Total Length

134.08 134.08 162.39

Remarks Drilling Drilling Drilling

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BHA No. 7 8 9 Depth In 3887 m 3960 m 4039m Depth Out 3960 4039m 4499.42



BHA

1 Bit 8 1/2 0.25 1 Bit 8 1/2 0.28 1 Bit 8 1/2 0.28 1 NB STb 1.82 1 NB STb 1.82 1 NB STb 1.82 1 6 3/4”

DC 9.45 4 6 3/4” DC 37.77 1 6 3/4” DC 9.42 1 STb 1.71 1 STb 1.71 1 STb 1.71 5 6 3/4”

DC 47.00 2 6 3/4” DC 18.68 2 6 3/4” DC 18.68

3 5” HWDP 27.86 3 5” HWDP 27.86 3 5” HWDP 27.86

1 Jar 9.28 1 Jar 9.28 1 Jar 9.28 7 5”

HWDP 65.03 7 5” HWDP 65.03 7 5” HWDP 65.03

Total Length 162.40

162.43 134.08

Remarks Drilling Drilling Drilling

Table 4.5: BHA design from Sidetrack depth to 4342.85m

BHA No. 1 2 3 Depth In 4245 Depth Out 4342.85



BHA

1 Bit 8 1/2 0.28 1 Bit 8 1/2 0.25 1 Bit 8 1/2 0.29

1 Mud Motor 8.90 1 Bit Sub 0.91 1 Mud

Motor 8.90

3 6 3/4” DC 28.31 1 X-O-S 0.81 3 6 3/4” DC 28.13 2 5” HWDP 18.63 3 6 3/4” DC 28.13 1 5” HWDP 9.30 1 Jar 9.28 1 5” HWDP 9.30 1 Jar 9.28

7 5” HWDP 65.03 1 Jar 9.28 8 5” HWDP 74.36 8 5” HWDP 74.36 Total Length 130.43 123.04 130.26

Remarks Side Tracking Cement Drilling Side Track & Formation Drilling

Mud loss and Hole enlargement Mud loss was very common from 1433m-2840m (Figure 4.7). Inside this depth, 2m3 mud loss was in every single hour at 2422m-2449m and 7m3 mud loss were noticeable within 3 hours at 2459.43m-2463m. From mud log report the presence of unconsolidated or loosely consolidated formation was found. The moderately hard shale section started after 3090m. It was recorded that after 3526m mud loss was not remarkable and further

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no mud loss was found by dropping carbide at 3871m and 3897.77m, the moderately hard and compact formation.

After dropping carbide 11.5m3, hole enlargement was found below 2449m which was the result of washout of loose formation.

Figure 4.7: Mud loss profile of Sunetro-1.

Bit teeth lost/broken While drilling Sunetro-1 a number of bit got damaged (Figure 3.6). The presence of calcareous cementing material and the formation of angular to sub angular shaped grain can be the reason of bit teeth lost/broken. In this case, bit selection was not perfect according to formation.

Cavings Maximum tabular and angular type cavings were observed at shale section (Figure 3.4a & Figure 3.4b). No splintered type cavings were noticeable. So, there was no over pressured zone. Swelling effect of thin laminated shale was the reason of cavings. In the total drilling operation rock bit was used which was replaced after every 90m-100m drilling. Moving drill string several times through the loose and swelled formation and also several correction of hole inclination, made the hole wall breakage. Inadequate mud weight was also responsible for cavings. Through the cavings presence in shale shaker mud weight was not increased enough to prevent sloughing.

Tight spots and overpulls In Sunetro-1 tight spots and over pull was limited within certain depth. In 17 ½” section the depth was within 1410-1671m and 3680m-4350m in 12¼” section. Breakdown of loose formation and swelling effect of shale section restricted the path of hole and resulted in tight spots and overpulls. The minimum flow rate for effective hole cleaning was not

0

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Mud loss in various depth

64 | P a g e

exceeded in 17½” and 12¼” section. As there was several changes in hole inclination, during wiper trip tight spots and subsequent overpull were due to irregular hole diameter. From mud report it was found that mud system changed to LSND polymer from gel polymer at 1407m and mud weight range was 1.06sg-1.21sg through the hole drilling operation. And depth from 1407-2571m continued with a stable mid weight 1.7sg (Figure 4.8). Low mud weight could be another reason of hole breakout through unconsolidated formation.

Figure 4.8: Planned and Actual MW in Sunetro -1 well.

ROP declination From Figure 4.9 ROP declination can be understood. Hole inclination, broken bit teeth, wrong bit selection, formation break down all are the reasons of ROP declination. With the start of inclination ROP drop started and continued with the poor hole condition.

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.

Figure 4.9: ROP variation curve with depth.

Stuck Pipe Sunetro-1 well was encountered two times pipe stuck incidents. Both events have been analyzed here separately with possible reasons of stuck pipe. First time the circulation was restricted during pipe stuck incident. It was mechanical sticking. Cavings were blocky and angular shape. It was the indication of reactive formation. Absence of splintered type cavings also ruled out the presence of tectonic and over-pressured formation. So, swelling effect can be the reason of shale instability. Mud weight was not increased with the indication of tight spot, cavings, and mud loss over unconsolidated formation. Low hydrostatic pressure allowed to concentrating stress around the wellbore. Hole cleaning rate did not exceed minimum hole cleaning flow rate in 12¼” and 17½” section (Table 4.6). Settled cuttings could have led to reduced hole diameter.

Table 4.6: Flow rates of Sunetro-1 well in different hole sections (Wiper Trip, 2015).

Hole Size Minimum Recommended Flow Rate (gpm)

Flow rate in Sunetro-1 well (gpm).

171/2” 900-1000 343-461 12¼” 800-900 397-560 8½” 400-450 375-441

425-431 (Sidetrack)

Mud loss was noticeable, but ROP was following a declining trend. The mud log report represents the presence of loose and unconsolidated formation. So, fractured formation was not present over there.

0

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10

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20

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719

719-

100

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97

1397

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.5-1

413

1413

-14

33

1433

-17

06

1706

-24

63

2463

-25

86

2586

-30

48

3048

-31

59

3159

-31

65

3165

-32

04

3204

-35

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3503

-36

89.4

3689

.4-3

810

3810

-38

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3887

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60

3960

-40

39

4039

-44

99.4

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RO

P a

vg. (

m/h

r)

Depth

ROP in various depth

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Cement blocks, junks was not recorded throughout the whole operation. Hole inclination was corrected several times. But the continuous deviation survey was not done. BHA design was not appropriate. Addition and subtraction of jar, drill collar and stabilizer in BHA from time to time caused hole inclination. Due to several correction of inclination of the well, possibility to happening micro doglegs cannot be ignored. Second pipe stuck event occurred while sidetracking. It was also mechanical sticking as circulation was restricted. The hole cleaning rate was above minimum effective hole cleaning rate. So, settled cuttings possibility is low. Bit balling indicates the sign of reactive shale. Swelling effect was present while sidetracking. Cement blocks and junks were not listed. Hole inclination was noticed and survey was not done except one depth; 4029m. So possibility of micro doglegs was there. The formation of sidetracking zone was not unconsolidated. It was consolidated shale section (WCR, 2013; DDR, 2012).

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CHAPTER 5: CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

Mubarakpur-1 and Sunetro-1 both encountered stuck pipe incidents in different manners for different reasons. It is tough to indicate to a specific cause of stuck pipe incident as it is linked with various parameters. Each incident can occur for different reasons; hence the mitigation measures should be taken accordingly.

5.1.1 Mubarakpur-1

Mubarakpur-1 was different from other gas fields in geological manner. Non reactive shale is less common. From available data and investigation after the stuck pipe event it can be determined that tight spots, overpulls, cavings, wireline logging problem all instability issues occurred in a chain and finally pipe stuck happened. Inadequate hole cleaning and shale instability due to insufficient mud weight was the prime reason of hole pack off. Some bad drilling practices were also responsible for instable hole condition. The crew needed to run the string with rotation and proper circulation. The nearby well Hazipur-1 used 1.38sg to 1.68sg mud weight to drill 1981m-3816m. But for Mubarapur-1 the plan was to drill only with 1.30sg mud to 4700m. Pressure profile was already calculated before drilling. Drilling mud system was needed to plan according to that. Proper planning could have prevented the instability issues.

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Figure 5.1: Rig time analysis for Mubarakpur-1 well (WCR, 2017).

5.1.2 Sunetro-1

Sunetro-1 was spudded in 2012. It was planned to drill upto 3700m. Overall, the operation lacked use of modern drilling tools (e.g. lack of directional survey, no PDC bit used). It was further decided to continue drilling and encountered stuck pipe scenario twice. Exploratory well always need better planning than development ones. In case of drilling this kind of well below the target depth is more risky and extra preparatory arrangements, monitoring are required. In

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Rig Time Analysis

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both stuck pipe cases similar indications were there for this to happen. But during drilling sidetrack no significant precautionary measures were taken. According to analysis the swelling effect of shale, inadequate hole cleaning, inadequate mud weight could be the main reasons of pipe stuck. BHA design was not proper in some depth. Due elongated the duration of operation in six months 28 days with huge loss of money (Figure 5.2) (GTO, 2012; WCR, 2013)

Figure 5.2: Rig time analysis for Sunetro-1 well (WCR, 2013).

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5.2 Recommendations

Each well drilling teaches us lots of aspects of drilling related issues. Mubarakpur-1 and Sunetro-1 both faced lots of instability issues and could be used as learnings for uture operation; especially as there is a plan for drilling Mubarakpur-2 in near future. Solutions can be different for different wells facing same problem. So, its not easy to recommend something specifically. But to generalize, some recommendations were made for future operrations based on learnings from drilling these two wells.

5.2.1 Mubarakpur-1

A safe mud envelope may be established before drilling which canbe used to determine the safe mud weight to prevent tensile or compressive failure. Near most wells data and LOT data can be subsidary for this estimation. Redundant methodologies can be put in place to confirm these data and thus reduce degree of uncertainity

Non Invasive Fluid (NIF) can be recommended to reduce above issues. It can seal heterogeneous permeable formations (including micro-fractured shale formation), increase fracture pressure of the formation in a certain degree, widen the window of drilling fluid density, drill formations with different pressures using the same fluid system and remarkably reduce downhole troubles and formation damage. Its application is found worldwide like Assam field wells in India, Shengke 1, Bogu 1, Bogu 2, Bo 930 , Pai 2-7, Pai 2-10,Pai 2-14, Pai 2-Ping 41 wells in China.(Borah, 2009; Yu-zhi, 2010; SLDTI, 2014)

For effective hole cleaning minimum recommended flow rate must be followed. Both low-viscous and hi-viscous pill can be applied successively to remove settled cuttings.

When drilling operation stopped for maintenance work or other purposes and hole exposed to formation, continuous circulation is needed to keep the hole free from being stuck

To prevent time depend factors which are the causes of unstable hole problem, it is better to minimize the time for which an open hole containing a shale sction is left uncovered.

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5.2.2 Sunetro-1

Effective BHA design is needed for smooth drilling operation Proper mud system and mud weight selection needed to avoid mud loss and cavings Bit selection must be done according to formation characteristics. Surveys should be taken on a more frequent basis in order to keep a close eye on well

trajectory. Lack of adequate directional surveys may lead to a tortuous well and even failure to target the reservoir

For effective hole cleaning minimum recommended flow rate must be followed. Both low-viscous and hi-viscous pill can be applied successively to remove settled cuttings.

Optimized wiper trip practice with or without reaming and back-reaming is needed along unconsolidated formation.

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REFERENCES

Amoco Training Manual, (1996), Third Edition, Training to Reduce Unscheduled Events. Awili, B.O., (2015). Analysis of Stuck Pipe Incidents in Menengai, Fourtieth Workshop on

Geothermal Reservoir Engineering, Stanford University, Stanford, California, SGP-TR-204.

Azar, J.J., Samuel, G.R., (2007), Drilling Engineering, Publisher Pennwell Corporation, USA. Bailey, L., Jones, T., Belaskie, J., Orban, J., Sheppard, M., Houwen, O., Jardine, S., McCann, D.,

(1991), Causes, Detection and Prevention, Oilfield review, Schlumberger Cambridge Research, England.

Bert, D., Zheng, N.,Storaune, A.,(2009), Case study of Drill string Failure Analysis and New

Deep-well Guidelines Lead to Success,110708-PA SPE. Bjerke, H.(2013). Revealing Causes of Restrictions by Signatures in Real-Time Hook Load

Signals.(Master’s Thesis), Norwegian University of Science and Technology(NTNU), Trondheim, Norway.

Borah, K.K., and Mishra, S.K. , .(2009). Deep drilling Challenges in Oil’s Assam Field- a Case

Study.SPE/IADC 125337. Bowes, C., Procter, R. (1997): Drillers Stuck Pipe Handbook, 1997 Guidelines & Drillers

Handbook Credits, Schlumberger, Ballater, Scotland. Chen, X., Tan, C.P., Haberfield, C.M. (1998): A Comprehensive Practical Approach for

Wellbore Instability Management, paper SPE 48898 presented at the 1998 SPE International Conference and Exhibition in China, 2-6 November, Bejing.

Daily Drilling Report (DDR) of Sunetro-1.(2012).BAPEX. Daily Drilling Report (DDR) of Mubarakpur-1.(2016).BAPEX. Elahi, N.M., Peyman,E., Saeid, J.(2013). Prediction of drilling pipe sticking by active learning

method (ALM). Gaurina-Međimurec, N. (1998): Horizontal Well Drill-In Fluids, Rudarsko-geološko-

naftnizbornik, Vol. 10, Zagreb. Geological Technical Order (GTO) of Sunetro-1.( 2012).BAPEX. Geological Technical Order (GTO) of Mubarakpur-1.(2014).BAPEX.

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Hawkes, C.D., McLellan, P.J. (1997): A New Model for Predicting Time- Dependent Failure of Shales: Theory and Application, paper 97-131 presented at the 48th Annual Technical Meeting of The Petroleum Society, 8-11 June, Calgary, Canada.

HXR drilling services.(2018).Retrived from:

http://www.hxrdrillingservices.com/articles/geomechanics-around-world-series-1. Kumar, D., Ansari, S., Wang, S., Yiming, J., Ahmed, S., Povstyanva, M., Tivhelaar,

B., (2012).Real-time Wellbore Stability Analysis: An observation from Cavings at shale shaker.

Marbun, B., Zulkhifly, S., Hariz, I., and K. Dita.(2011). Geothermal Drilling –

An overview Proceeding 35th Annual Convention & Exhibition, Indonesian Petroleum Association (IPA).

McLellan, P.J., Wang, Y., (1994b): Predicting the Effects of Pore Pressure Penetration on the

Extent of Wellbore Instability: Application of a Versatile Poro-Elastoplastic Model, paper SPE/ISRM 28053 prepared for presentation at the 1994 Eurock SPE/ISRM Rock Mechanics in Petroleum Engineering, Delft, The Netherlands, 29- 31 August 1994.

Mitchell, R., Miska, S. (2011).Fundamentals of Drilling Engineering.Published by SPE, USA. Mohiuddin, M.A., Awal, M.R., Abdulraheem, A., Khan, K. (2001): A New Diagnostic Approach

to Identify the Causes of Borehole Instability Problems in an Offshore Arabian Field, paper SPE 68095 presented at the 2001 SPE Middle East Oil Show, 17-20 March, Bahrain.

Nguyen, V., Abousleiman, Y., Hoang, S. (2007): Analyses of Wellbore Instability in Drilling

Through Chemically Active Fractured Rock Formations: Nahr Umr Shale, paper SPE 105383 presented at the 15th SPE Middle East Oil & Gas Show and Conference, 11-14 March, Bahrain International Exhibition Centre, Kingdom of Bahrein.

Osisanya, S., (2011).Practical Approach to Solving Wellbore Instability Problems.SPE, USA. Pasic, B., Gaurina-Medimurec, N., Matanovic, D., (2007): Wellbore Instability: Causes and

Consequesces, University of Zagreb, Faculty of mining, Geology and petroleum engineering, Croatia.

Petrowiki.(2015).Hole Deviation. Retrieved from: http://www.petrowiki.org/Hole-deviation.

Published by SPE International. Petrowiki.(2015).Drillpipe Failures. Retrieved from:

http://www.petrowiki.org/Drillpipe_failures. Published by SPE International. Petrowiki.(2015).Mud Contamination. Retrieved from:

http://www.petrowiki.org/Mud_contamination. Published by SPE International.

http://www.petrowiki.org/Hole-deviation

http://www.petrowiki.org/Drillpipe_failures

http://www.petrowiki.org/Mud_contamination

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Petrowiki.(2016).Subsurface stress and pore pressure. Retrieved from: http://petrowiki.org/Subsurface_stress_and_pore_pressure. Published by SPE International

Petrowiki.(2016).Compressive wellbore failure. Retrieved from:

http://petrowiki.org/Elastic_wellbore_stress_concentration#Compressive_wellbore_failure.Published by SPE International

Petrowiki.(2017). Equipment and Personnel Related Problems. Retrieved from:

http://petrowiki.org/PEH:Drilling_Problems_and_Solutions#Equipment_and_Personnel-Related_Problems. Published by SPE International.

Petrowiki.(2017).Hydrogen Sulfide Bearing Zones and Shallow Gas.Retrieved from:

http://petrowiki.org/PEH:Drilling_Problems_and_Solutions# Hydrogen-Sulfide-Bearing Zones and Shallow Gas.Published by SPE International.

Rabia, H.(2001).Well engineering and construction (e-book). Entrac Consulting, UK. SLDTI.(2014).Drilling Fluid and Fomation Damage Control Technologies.Retrieved from:

http://www.sldti.com/eng/TechnologiesandServices/DrillingFluidandFormationDamageControlTechnologies.aspx. Published by Shengli Drilling Technology Reasearch Institute of Sinopec.

Tan, C.P., Willoughby, D.R. (1993): Critical Mud Weight and Risk Countour Plots for

Designing Inclined Wells, paper SPE 26325 presented at the 68th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, 3-6 October, Houston, Texas.

Tan, C.P., Chen, X., Willoughby, D.R., Choi, S.K., Wu, B., Rahman, S.S., Mody, F.K. (1999): A

´Keep It Simple´: Simple Approach for Managing Shale Instability˝, paper SPE/IADC 52866 presented at the 1999 SPE/IADC Drilling Conference, 9-11 March, Amsterdam, Holland.

Well Completion Report (WCR) of Sunetro-1 well.(2013).BAPEX. Well Completion Report (WCR) of Mubarakpur-1 well.(2017). BAPEX

Wiper trip.(2015). Hydraulics optimization holecleaning.Retrieved from: http://www.wipertrip.com/drilling-fluids/miscellaneous/349-hydraulics-optimization-holecleaning.html

Yu-zhi,X., Gong-rang, L., Bao-fang, L., and Jinghui, Z.,.(2010). Application of super high

densitydrilling fluid under ultra high temperature on well Shengke-1.SPE 131189. Zoback, M. D.,ReservoirGeomechanics, (2007).

http://petrowiki.org/Subsurface_stress_and_pore_pressure

http://petrowiki.org/PEH:Drilling_Problems_and_Solutions#Equipment_and_Personnel-Related_Problems

http://petrowiki.org/PEH:Drilling_Problems_and_Solutions#Equipment_and_Personnel-Related_Problems

http://petrowiki.org/PEH:Drilling_Problems_and_Solutions

http://www.sldti.com/eng/TechnologiesandServices/DrillingFluidandFormationDamageControlTechnologies.aspx

http://www.sldti.com/eng/TechnologiesandServices/DrillingFluidandFormationDamageControlTechnologies.aspx

http://www.wipertrip.com/drilling-fluids/miscellaneous/349-hydraulics-optimization-hole

http://www.wipertrip.com/drilling-fluids/miscellaneous/349-hydraulics-optimization-hole

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APPENDIX A: Tight spots and Overpulls found in Mubarakpur -1 (WCR, 2017; DDR, 2016) Hole Section Depth (m) What Happened When How

rcovered Remarks

17 ½” 937 Tight spot Tripping in

Wiper Trip

1203-1040 Overpull Tripping out

Wiper Trip

1279-1260 Overpull Tripping out

Wiper Trip

1193-1180 Overpull Trippng out

Wiper Trip

12 ¼” 2150-2119 Overpull(45 klbs) Wiper trip

Reaming up & down

2090-2060 Overpull(35 klbs) Wiper trip

Reaming up & down

2177-2225 Tightspot(15-20 klbs) RIH for wiper trip

Reaming down


Reaming down


Reaming down

2470-2480 Tight spot(40 klbs) Wiper trip with reaming

Reaming down

2370-2372 Tight spot (15-20 klbs)

RIH Wiper trip MW increased, Multiple reaming after stand down

2369-2375 Tight spot (15-20 klbs)

RIH

2462-2465 Tight spot (15-20 klbs)

RIH

2647-2651 Tight spot (15-20 klbs)

RIH

2890-2880 Overpull (20-25 klbs) Wiper trip

Reaming Down

2710-2690 Overpull (70 klbs) Wiper trip

Reaming down

2704-2720 Tight spot Wiper trip

Reaming down

3030-3000 Overpull (upto 40 klbs)

POOH Multiple reaming

2860-2880 Tight spot (70 klbs) Tripping In

Multiple reaming

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Hole Section Depth (m) What Happened When How rcovered

Remarks

8 ½” Several depth after 3675m

Both overpull and Tight spots

Wiper trip

Reaming up & down

3800 Overpull(50 klbs) POOH Reaming up & down for 30 minutes

3492 Tight spot RIH Multiple reaming

Huge cavings observed (1” – 1 ½” avg)

4190 Pipe stuck Drlling Get free by downward jarring

4196 Overpull Wier trip with back reaming

Reaming up & down for 20 minutes

Several depth upto 4295m

Overpull Wiper tripWiper trip

Back reaming, Wiper trip upto 10 stands

MW increased, HVP pumped

After drilling below casing shoe , 4379

Tight spot RIH for Wiper trip

Reaming down

Drilling rate decreased to 0.35 m/hr

3749-3786 Tight spot RIH with reaming down

Reaming down

Huge cavings, MW increased

3665-3693 Overpull (35 klbs) Wiper trip

Reaming

3938 Tight spot(45 klbs) Wiper trip

Reaming

3938 Overpull( 20 klbs) Wiper trip with reaming up

Multiple reaming

MW increased, Tight spots and overpull were very common at every stand

4172-4195 Tight spot(10 klbs) RIH Reaming 4195-4172 Overpull( 40 klbs) POOH Reaming 4175 Pipe Stuck Recover with

358.93m fish

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Hole Section Depth (m) What Happened When How rcovered

Remarks

8 ½” (Sidetrack)

3552 Tight spot RIH Reaming down

3754 Pipe stuck RIH with reaming

Reaing down, MW increased

Get free after 10 minutes, Cavings observed.

4280-3464 Overpull and Tight spot (25-50 klbs)Tight spot an overpull (10-15 klbs)

Wiper trip


Below casing shoe of 9 5/8” casing, 3569

Overpull (24 klbs) Wiper trip


4142 Overpull (31 klbs) POOH Reaming up, MW increased

After POOH liner, 14 blades, 6 rings, 1 stop collar of the centralizer lost in the hole.







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APPENDIX B: Tight spots and Overpulls found in Sunetro -1(WCR, 2013; DDR, 2012). Hole Section

Depth (m) What happened When How recovered

Remarks

12 ¼” 1565 Overpull (234 klbs) Wiper Trip Reamed up and down

Setting jar with the BHA

1671 Overpull (236klbs) Wiper Trip Reamed up and down

1571 Tight spot (129 klbs) Wiper Trip Reamed up and down



1410 Tight spot (112 klbs) RIH Drilling continued with very low dilling rate 1 m/hr till 1890m

8 ½” 3167-3204 Tight spot RIH 3689 Overpull (100 klbs) Wiper trip MW increased 4350-4100 Overpull POOH 4040-4036 Overpull POOH 3786 Tight spot While

tripping in Jarring first, then reaming

4096 Tight sot Trippning in

4118 Tight spot Tripping in 4273 Tight spot Tripping in 4280 Tight spot Tripping in 4355 Overpull (upto 179

klbs) POOH Jarring

4355 Pipe stuck POOH Recovered with 31.91m fishing length

8 ½” (Side track)

4267 Tight spot RIH 4573 Tight spot RIH 4509 Tight spot Wiper trip 4619 Overpull Wiper trip 4654 Tight spot While

drilling

4683 Overpull POOH 4679 overpull Wiper trip Reaming 4675 Pipe Stuck POOH Recovered

with 34.54m fishing length

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APPENDIX C: Deviation Survey Data of Sunetro-1

Sunetro Exploratory Well # 1 was drilled as a vertical well. To monitor the course of the well deviation survey has been conducted by using Single-shot inclination-only survey tools (Totco) at different depth while drilling. By the incident of stuck pipe at 4344 m, a side track was made from 4245 m (successful at 4265m).

Since no professional directional service was utilized, deviation data is not available in regular interval for the side track well. The available deviation surveys as conducted at different depth intervals using Totco are presented below:

Table (Appendix C): Deviation Survey Record of Sunetro-1

Deviation Survey

Depth

mRKB Deviation Angle

1 1420 1°

2 2458 10'

3 3040 2° 15'

4 3146 1° 30'

5 3204 1°

6 3804 1°15"

7 3950 6°

8 4029 3°

9 4660 4.5°(Side Track)

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APPENDIX D: Non-Reactive Character of Illite

Illite clay

In Geological aspect Mubarakpur area was quite different from others gas field area. The problematic shale zones were Illites which do not show interlayer swelling. The compensating cations of illite are primarily the potassium ion (K+) [Figure (Appendix D)]. Ca and Mg can also sometimes substitute for K. The ionic diameter allows the K+ to fit snugly between unit layers forming a bond that prevents swelling in the presence of water. Mixed layers of illite and Smectite often cause various problems in borehole stability and drilling fluid maintenance. The troublesome nature of these clay minerals can be related to the weakly-bonded interlayer cations and weak layer charges that lead to swelling and dispersion upon contact with water. With increasing burial depths, the smectite gradually converts into illite/smectite mixed-layer clays and finally to illite and mica. As a result, shale formations generally become less swelling but more dispersive in water with increasing depth (Source MI SWACO Drilling Fluid Manual).

Figure (Appendix D): Non swelling clay, Illite structure.

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Documents

ANALYSIS OF WELLBORE INSTABILITY WHILE DRILLING