The Pennsylvania State University

The Graduate School

Department of Energy and Mineral Engineering

CUTTING TRANSPORT ANALYSIS IN HORIZONTAL WELLS:

A COMPUTATIONAL APPROACH

A Thesis in

Energy and Mineral Engineering

by

Erhamah Alsuwaidi

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Master of Science

May 2018

ii

The thesis of Erhamah Alsuwaidi was reviewed and approved* by the following:

John Yilin Wang

Associate Professor of Petroleum and Natural Gas Engineering

Thesis Advisor

Amin Mehrabian

Assistant Professor of Petroleum and Natural Gas Engineering

Virendra Puri

• Distinguished Professor of Agricultural and Biological Engineering

•

Luis Ayala

Professor of Petroleum and Natural Gas Engineering

Associate Department Head for Graduate Education

*Signatures are on file in the Graduate School

iii

ABSTRACT

Horizontal well is one of the most important methods that enhances the well production.

On the other hand, drilling a successful horizontal well needs engineering attentions and

optimizations. Clean wellbores is very an essential engineering parameter, where the key is to

understand cuttings transport in different geological conditions and with different drilling

parameters. During the past decades, substantial amount of lab work has been done to study the

parameters that influences the cuttings transport performance. In this work, a numerical model

was created to investigate the effect of parameters on the cuttings transport systematically. A

parametric study was carried out to investigate the effects of cuttings size, cuttings density, mud

weight, drill pipe rotation, drill pipe eccentricity, drilling rate, and circulation rate on the

transportation of cuttings.

The results of the numerical model showed a match with what other researchers have

done in the lab. The results of the parametric study showed that the effect of altering the

parameters was negligible when the well is in a vertical orientation. For high angles of

inclination, simulating cuttings with low densities resulted with lower volume fractions of settled

cuttings when compared with the result of simulating higher density cuttings. Also, the

parametric study showed that smaller cuttings were easy to transport through the annulus.

Therefore, the drilling engineer must try to break the lithology as small as possible to ensure

better cuttings transport. Increasing the mud weight showed a significant improvement in the

cuttings transport. But, on the other hand, the drilling engineering must be careful when using

muds with high weights. Drill pipe rotation resulted with cleaner wellbore when it was

implemented at high inclination angles. Negative drill pipe eccentricity results with significantly

high cuttings’ volume fraction which can lead to catastrophic failures. Increasing the drilling rate

iv

helps with decreasing the drilling time, but it also results with high cuttings accumulation. The

circulation rate showed an inverse relationship the cuttings volume fraction.

v

TABLE OF CONTENTS

List of Figures .......................................................................................................................... vi

List of Tables ........................................................................................................................... ix

Acknowledgements .................................................................................................................. x

Chapter 1 INTRODUCTION ................................................................................................... 1

Chapter 2 LITERATURE REVIEW ........................................................................................ 5

Chapter 3 PROBLEM STATEMENT ..................................................................................... 13

Chapter 4 PARAMETERS EFFECTING CUTTINGS TRANSPORT ................................... 14

4.1 Drag coefficient, Reynold’s number, and cuttings surface condition ........................ 14 4.2 Cuttings size ............................................................................................................... 19 4.3 Cuttings density .......................................................................................................... 19 4.4 Drilling fluid density .................................................................................................. 20 4.6 Eccentricity ................................................................................................................ 23 4.7 Drill string rotation and hole angle ............................................................................ 24 4.8 Cuttings transport physics .......................................................................................... 25 4.9 Transport velocity, fluid velocity, transport ratio ...................................................... 31

Chapter 5 COMPUTATIONAL CUTTINGS TRANSPORT MODEL .................................. 33

5.1 Overview of the CFD Processes ................................................................................ 33 5.2 Geometry .................................................................................................................... 35 5.3 Mesh ........................................................................................................................... 35 5.4 CFD Model ................................................................................................................ 36 5.5 CFD model validation ................................................................................................ 46

Chapter 6 COMPUTATIONAL PARAMETRIC STUDY...................................................... 49

6.1 Cuttings density effect................................................................................................ 49 6.2 Cuttings size effect ..................................................................................................... 54 6.3 Mud weight effect ...................................................................................................... 58 6.4 Drill pipe rotation effect ............................................................................................. 63 6.5 Drill Pipe Eccentricity effect ...................................................................................... 68 6.6 Drilling rate effect ...................................................................................................... 74 6.7 Circulation rate effect ................................................................................................. 80

Chapter 7 CONCLUSIONS ..................................................................................................... 85

References ........................................................................................................................ 88

vi

LIST OF FIGURES

Figure 1-1. Cuttings transport is well drilling (Amanna and Movaghar, 2016)....................... 2

Figure 1-2. Vertical well illustrating the cuttings distribution around the annulus (Wei et

al., 2013). ......................................................................................................................... 3

Figure 1-3. Parameters effecting the cuttings transport ability in horizontal wells(Adari, et

al., 2000). ......................................................................................................................... 4

Figure 4-1. Relationships between C_D and N_Re (Chien 1994) ........................................... 16

Figure 4-2. Heywood definition of length, L, Breadth, B, and thickness, T (Nayland and

Stanley-Wood 1983). ....................................................................................................... 18

Figure 4-3. Fluids Rheology (Petrowiki) ................................................................................. 22

Figure 4-4. On the left is a representation of concentric annulus and in the right is a

representation of eccentric annulus (Iyoho and Azar 1981)............................................ 24

Figure 4-5. Schematic of forces acting on a cutting ................................................................ 26

Figure 4-6. Shows the longitudinal and cross-sectional views of the forces and the

velocity profiles acting of the cuttings particle (Cayeux et al 2013.)............................... 26

Figure 5-1. Simulation workflow ............................................................................................. 34

Figure 5-2. Front and side view of the investigated annulus ................................................... 35

Figure 5-3. Assigned mesh at the inlet/outlet .......................................................................... 36

Figure 5-4. Volume fraction of cuttings at y = 0 ft (inlet) ....................................................... 43

Figure 5-5. Volume fraction of cuttings at y = 12.5 ft (midpoint) ........................................... 44

Figure 5-6. Volume fraction of cuttings at y = 12.5 ft (midpoint) ........................................... 44

Figure 5-7. Drilling fluid velocity profile (base model) .......................................................... 45

Figure 5-8. Cuttings velocity profile (base model) .................................................................. 45

Figure 5-9. Validation geometry .............................................................................................. 46

Figure 6-1. Average volume fraction of cuttings in the annulus (cuttings density) ................. 50

Figure 6-2. Volume fraction of cuttings at the middle of the annulus (cuttings density) ........ 51

Figure 6-3. Cuttings transport ratio (cuttings density) ............................................................. 52

Figure 6-4. Cuttings mass flow rate ratio (cuttings density) .................................................... 53

vii

Figure 6-5. Average volume fraction of cuttings in the annulus (cuttings size) ...................... 55

Figure 6-6. Volume fraction of cuttings at the middle of the annulus (cuttings size) .............. 56

Figure 6-8. Cuttings mass flow rate ratio (cuttings size) ......................................................... 57

Figure 6-9. Average volume fraction of cuttings in the annulus (drilling mud) ...................... 60

Figure 6-10. Volume fraction of cuttings at the middle of the annulus (drilling mud) ............ 60

Figure 6-11. Cuttings transport ratio (drilling mud) ................................................................ 61

Figure 6-12. Cuttings mass flow rate ratio (drilling mud) ....................................................... 62

Figure 6-13. Average volume fraction of cuttings in the annulus (drill pipe rotation) ............ 64

Figure 6-14. Volume fraction of cuttings at the middle of the annulus (drill pipe rotation) .... 65

Figure 6-15. Cuttings transport ratio (drill pipe rotation) ........................................................ 66

Figure 6-16. Cuttings mass flow rate ratio (drill pipe rotation) ............................................... 67

Figure 6-17. Average volume fraction of cuttings in the annulus (drill pipe eccentricity) ..... 69

Figure 6-18. Volume fraction of cuttings at the middle of the annulus (drill pipe

eccentricity) ...................................................................................................................... 70

Figure 6-19. Cuttings transport ratio (drill pipe eccentricity) .................................................. 71

Figure 6-20. Cuttings mass flow rate ratio (drill pipe eccentricity) ......................................... 72

Figure 6-21. Volume fraction of cuttings at the middle of the annulus with drill pipe

rotation (drill pipe eccentricity) ....................................................................................... 74

Figure 6-22. Average volume fraction of cuttings in the annulus (drilling rate) ..................... 76

Figure 6-23. Volume fraction of cuttings at the middle of the annulus (Drilling rate) ............ 76

Figure 6-24. Cuttings transport ratio (Drilling rate) ................................................................ 77

Figure 6-25. Cuttings mass flow rate ratio (Drilling rate) ....................................................... 78

Figure 6-26. Volume fraction of cuttings at the middle of the annulus with drill pipe

rotation (Drilling rate) ...................................................................................................... 79

Figure 6-27. Average volume fraction of cuttings in the annulus (circulation rate) ................ 81

Figure 6-28. Volume fraction of cuttings at the middle of the annulus (circulation rate) ....... 82

Figure 6-29. Cuttings transport ratio (circulation rate) ............................................................ 83

viii

Figure 6-30. Cuttings mass flow rate ratio (circulation rate) ................................................... 83

ix

LIST OF TABLES

Table 4-1. Shape coefficients for some geometrical and irregular shapes (Nayland and

Stanley-Wood 1983) ........................................................................................................ 18

Table 5-1. Base model drilling conditions ............................................................................... 42

Table 5-2. Validation parameters ............................................................................................. 47

Table 5-3. Average cuttings concentration comparison ........................................................... 47

Table 6-1. Investigated parameters .......................................................................................... 49

Table 6-2. Investigated parameters (cuttings density) ............................................................. 50

Table 6-3. Investigated parameters (cuttings size) ................................................................... 55

Table 6-4. Investigated parameters (drilling mud) ................................................................... 58

Table 6-5. Investigated parameters (drill pipe rotation) .......................................................... 63

Table 6-6. Investigated parameters (drill pipe eccentricity) .................................................... 68

Table 6-7. Investigated parameters (Drilling rate) ................................................................... 75

Table 6-8. Investigated parameters (circulation rate) .............................................................. 80

Table 7-1. Effect of parameters on cuttings transport performance ......................................... 86

x

ACKNOWLEDGEMENTS

All praise is due to the creator and the lord of the world, Allah. Without his support and

guidance nothing is possible. I ask his forgiveness and seek his aid during the rest of my life.

I would thank my family for their support during my studies. Without their support and

wise words, I would not reach this point. I would like to thank my wife for her support, love, and

patience. She went through a lot for me and never complained. From her, I was blessed with a

beautiful daughter during my studies. This event gave me more courage to continue my

education.

I owe a deep gratitude to Dr. Wang for his support, trust and constant encouragement.

Also, for his patient guidance throughout my research. I would like to extend my sincere thanks

to Dr. Virendra Puri for his guidance and kind support.

I would like to thank all my friends for their support during this long journey. Their

existence in my life is a blessing

1

Chapter 1

INTRODUCTION

Drilling horizontal wells is more complicated than vertical wells. Horizontal wells are

drilled for many reasons:

1. Have larger contact with the pay zone. This increases the productivity of the well at

least 3 to 5 times compared to vertical wells

2. Can be used to reached reservoirs that cannot be reached using vertical wells

3. Can reduce the number of drilling pad, and therefore reducing the footprint

4. Can be used to enhance productivity in naturally fractured reservoirs, such as chalk,

CBM, and shale

Figure 1-1 illustrates the process of circulating cuttings in well drilling. During the

drilling process, the drill bit cut the rocks into small pieces called cuttings. Cuttings need to be

removed away from the bit to enable continued and efficient drilling, and to be transported up to

the surface to avoid any stuck pipe problems. This is achieved by circulating drilling fluids

through the drill string and then the annulus to the surface. The whole process is called hole

cleaning.

2

Figure 1-1. Cuttings transport is well drilling (Amanna and Movaghar, 2016).

Poor hole cleaning can lead to a number of problems that can delay or even stop the

drilling operations. These problems can lead to huge economic losses for the operating

companies. Inadequate hole cleaning can lead to a lot of drilling problems, such as (PetroWiki

2015):

• Pipe sticking

• Premature bit wear

• Slow drilling

• Excessive torque and drag on drill string

• Logging and cementing problems

3

• Difficulties in casing landing

Figure 1-2. Vertical well illustrating the cuttings distribution around the annulus (Wei et al., 2013).

In order to understand the hole cleaning process more comprehensively, we need to outline

the parameters that affect the migration of cuttings from around the drilling bit to the surface.

Cuttings transport in vertical wells is less of a problem, as the created cuttings are distributed

evenly around the wellbore, which prevents any chance of accumulation (Clark and Bickham,

1994).In vertical drilling, the right drilling fluid is usually needed to transport the cuttings out of

the wellbore. On the other hand, cuttings transport is more complicated in horizontal drilling.

Cuttings tend to settle in the lower side of the wellbore and accumulate to form a bed. This

accumulation is the result of the attractive force of gravity of the earth.

4

The key variables that controls the cuttings transport ability are illustrated in figure 1-3.

Figure 1-3 shows the relationship between the control we have on the cuttings transport and the

influence of each parameter on the cuttings transport efficiency

Figure 1-3. Parameters effecting the cuttings transport ability in horizontal wells(Adari, et al.,

2000).

The goal of this research is to perform a parametric study to investigate the influence of

the above parameters on the hole cleaning process.

5

Chapter 2

LITERATURE REVIEW

Cutting transport phenomenon has been a subject of interest since 1970 and up to this

point. Understanding the physics behind the transportation of cuttings would make the design of

the drilling operation easier. Investigators started to test this phenomenon by identifying the

parameters that influences the transportation of cuttings. These parameters were tested initially by

carrying out experimental procedures. Others investigated the parameters using some

mathematical models.

Zeidler conducted an experimental investigation of the settling velocity of drilled

particles in Newtonian fluids using an apparatus that he developed. Also, he investigated the

transportation of cuttings in an annulus using both mud and water as drilling fluids. He used two

apparatus to do his investigation: (1) settling velocity apparatus and (2) cuttings transport annular

section. The settling velocity apparatus was 15 ft long and 3 in. ID glass tubing. He used four

measuring station along the length of the tube to read the time a cutting needs to reach one of the

four stations. Electronic timers were used in the four stations. The cuttings transport test was

carried out using a 65 ft long wellbore annulus. The inner and outer radii were 4 ½ and 8 ½ in.

respectively. Zeidler made five conclusions from his experiment: (1) the pipe rotation has a

significant effect on the recovery of cuttings, (2) viscosity has a minor effect on the recovery of

cuttings, (3) correlations were developed for the settling velocity and the recovery fraction of

cuttings subjected to turbulent flow of water, (4) over the range of Reynolds numbers 2 ≤

𝑁𝑅𝑒,𝑝 ≤ 1500 in Newtonian fluids, a good prediction of the settling velocity of cuttings can be

made using the following equation:

6

𝑣𝑠 = 𝑐𝑜𝑛𝑠𝑡. (4

3𝑔𝑐)

𝑚

∗ 𝜌𝑓𝑚−1

∗ (𝜌𝑝 − 𝜌𝑓)𝑚deq3𝑚−1∗

1

𝜇𝑒2𝑚−1

(2.1)

where 𝑔𝑐 is the gravitational acceleration, 𝜌𝑙 and 𝜌𝑠 are the fluid and cuttings densities

respectively. 𝑚, 𝜇 and deq are the slope of Reynolds vs drag coefficient curve, fluid viscosity and

equivalent sphere diameter respectively, and (5) in the laminar flow, the physics behind cuttings

transportation is not fully understood, in both Newtonian and non-Newtonian fluids (Zeidler

1972).

Hussain and Azar carried out an experimental procedure to study the cuttings transport

using different drilling fluids. They studied the effect of particle size, flow rate, apparent

viscosity, and yield point to plastic viscosity ratio on fluid carrying capacity. They also tested the

applicability of a semi empirical transport model developed by Zeidler. They made four

conclusions from their experiment: (1) in vertical annuli, fluid annular velocity plays an important

role on the carrying capacity of the drilling fluid, while other parameters have an effect on the

carrying capacity of the drilling fluid only at low to medium annular fluid velocity, (2) Zeidler’s

semi empirical formulations for the prediction of cuttings settling velocity and recovery fractions

were valid with some limitations, (3) as the ratio of yield point to plastic viscosity increases, the

carrying capacity increases, and (4) at low to medium fluid annular velocities; apparent viscosity,

yield point, and initial gel strength have a significant effect on the carrying capacity of the drilling

fluid. These parameters have a negligible effect on the carrying capacity at high fluid annular

velocity (Hussaini and Azar 1983).

Tomren et al. conducted a large-scale study of cuttings transport in horizontal wells using

realistic scenarios, unlike other investigators. Other investigators used very high fluid velocities

7

or/and short pipe sections. From his investigation, he concluded the following: (1) higher annular

velocities are required for effective hole cleaning in horizontal wells than in vertical wells, (2) for

deviation of 10 degrees from the vertical axis, cuttings evacuation are similar to the vertical

situation, (3) at angles greater than 40 degrees, formation of a bed of cuttings occur even at high

fluid velocities, (4) in horizontal wells, the most effective parameters in the hole cleaning process

are the drilling fluid velocity, hole inclination, and mud rheological properties, (5) for a given

flowrate, cuttings accumulation increases with deviation up to an angle where it becomes

independent of the deviation angle, (6) cuttings accumulation is strongly effected by drill pipe

eccentricity, but only moderately effected by fluid viscosity, (7) downward sliding occur at hole

angles 40 to 50 degrees, and (8) high viscosity drilling fluids have higher carrying capacity than

low viscosity fluids (Tomren et al. 1986).

Peden et al. investigated the cuttings transport phenomenon in inclined wellbores

experimentally. He built a 21 ft borehole simulator to investigate the effects of different

parameters on the circulation rate required to ensure that the cuttings in horizontal wells are

efficiently transported to the surface. He concluded the following: (1) he identified two forms of

transportation of cuttings to the surface: Rolling/sliding and Suspension; (2) each form responds

differently to variations in drilling parameters, (3) pipe rotation shows little effect on cuttings

transport when water is used as a drilling fluid, (4) pipe rotation have a significant effect on

cuttings transport when medium to high viscous fluids are used as drilling fluids, (5) annular fluid

velocity required to transport the cuttings are very sensitive to variations of angles, (6) for the two

forms of transportation, the relationship between the hole inclination and the annular fluid

velocity is different, (7) the flow regime plays an important role on the removing of cuttings from

the wellbore (Peden et al. 1990).

Sifferman and Becker tested experimentally ten variables that has a direct effect on the

cuttings transport process. He tested the variables over a range of angles. They used a test section

8

of 60 ft long with an inner diameter and outer diameter of 4.5 in and 8 in respectively. They

concluded the following: (1) cuttings were difficult to remove once a bed had built up on the low

side of the wellbore, (2) mud velocity and density have the greatest effect on the hole cleaning,

(3) moreover, hole inclination and drill pipe rotation have less effect on the cleaning process than

the two mentioned above, (4) small increases in the mud weight decreases the height of the

cuttings bed significantly, (5) bed sliding occur at angles between 45 and 60 degrees, and (6) bed

sliding is little at angles between 60 and 90 degrees (Sifferman and Becker 1992).

Martins and Costapinto Santana carried out a dimensionless analysis to evaluate the

cuttings transport in horizontal and near horizontal wells. They came up with a mechanistic

model that consists of two layers: (1) the top layer is a heterogeneous suspension and (2) the

bottom one is a compacted bed of solids. The authors used a modified Lockhart-Martinelli

dimensionless parameter for solid-liquid flows to evaluate the cuttings transport phenomena. The

outcomes of their research are: (1) a modified two layer model to analysis cuttings transport, (2)

from the model, a computer simulator was generated for designing field operations, (3) larger

diameter drill pipes, high density fluids, and high flow rates are the most effective actions for

cuttings transport problems; (4) they came up with a new procedure for friction loss calculations

in highly horizontal annuli, and (5) they developed dimensionless diagrams in order to evaluate

the well cleaning as a function of the modified Lockhart and Martinelli parameter and the flow

regime of the upper layer (Martins and Costapinto Santana 1992).

Clark and Bickham examined the transportation of cuttings from the bit to the surface.

They concluded that transportation of cuttings comes in different mechanisms depending on the

angle of inclination. They developed a model that relates the transportation of cuttings with the

fluid mechanics. They concluded also: (1) mechanisms governing the transportation of cuttings

are: settling, lifting, and rolling; (2) each transportation mechanism is associated with a certain

range of inclination angles, (3) the following parameters were implemented in the model to

9

analyze the cuttings transports: operation conditions, mud properties, well configuration, and

cuttings properties; and (4) results from the models were in good agreement with the results

obtained experimentally for flow rates below critical conditions (Clark and Bickham 1994).

Chien carried out an experimental investigation to introduce new correlation that is able

to predict the settling velocity of irregularly shaped cuttings in both Newtonian and non-

Newtonian fluids at all flow regimes. He developed the correlation by collecting data on the drag

coefficients and Reynolds number of irregularly shaped cuttings. He concluded the following: (1)

parameters effecting the developed correlation are: size, surface condition, density of cutting, and

fluid properties; (2) there exists an effective viscosity at a specific shear rate that it’s used to

calculate the settling velocity, (3) the settling shear rate ranges from 0.1 to 50 𝑠𝑒𝑐𝑜𝑛𝑑𝑠−1 for

most drilling and fracturing operations, (4) he presented the mathematical expression for the

effective viscosity for most rheological models, (5) the effective viscosity depends on the settling

velocity for non-Newtonian fluids. From there, a trial and error approach or numerical iteration

method can be used to calculate the settling velocity; (6) there are no relationship between the

viscosity and the shear rate for Newtonian fluids, and (7) fluid rheology has a negligible effect on

the settling velocity in the turbulent regime. In the turbulent regime, the settling velocity is

mainly influenced by the cuttings density, surface characteristics and fluid density (Chien 1994).

Nguyen and Rahman preformed a laboratory procedure to develop a mathematical model

that is able to predict the transportation modes that occur in horizontal wells. The mathematical

model consists of three components: (1) a bed of cuttings with uniform concentration, (2) a

dispersed region with varying concentration, and (3) a clear fluid or a suspension region. A

computer simulator was developed by relating the operation conditions, well configuration,

cuttings properties, and drilling fluid properties with the transportation modes. The simulator

showed good agreement with the results observed in the lab (Nguyen and Rahman 1998).

10

Hemphill and Larsen studied the capabilities of water-based and oil-based drilling fluids

to carry the cuttings from the bit to the surface. Unlike previous researchers, they carried his

investigation experimentally by testing both fluid types under the same conditions. Previous

researchers tested both fluid types by measuring the accumulation of cuttings in the annulus, but

not by measuring the velocity. They compared the efficiency of water-based and oil-based

drilling fluids in cleaning the cuttings in horizontal wellbores. They concluded that both water-

based and oil-based drilling fluids have similar capabilities in cleaning the annulus when

operating at the same conditions. Also, They concluded that the most dominant parameter on the

degree of hole cleaning is the fluid velocity (Hemphill and Larsen 1996).

Larsen et al. carried out an experimental study to develop new correlations that would

help engineers design proper drilling operations in high angles of inclination. His model helps

engineers estimate the critical transport fluid velocity, the average cuttings travel velocity, and the

annular cuttings concentration. He used an annulus with an inner and outer radii of 2.375 in. and

5 in. respectively. The annulus was 35 ft long. Using his correlation, he performed a parametric

study on the cuttings transportation for horizontal wells (Larsen et al. 1997).

Philip et al. tried to verify the role of Taylor Vortices in the transport of drill cuttings.

Computerized software was able to verify that Taylor Vortices form in real drilling conditions.

Taylor Vortices form when the rpm of the drill pipe reach critical values. Chenevert provided

equations that can calculate the critical rpm for both Newtonian and non-Newtonian fluids. The

author conducted an experimental procedure to test the critical rpm on a range of Newtonian and

non-Newtonian fluids. The results from the experimental analysis were in agreement with the

results from the computational analysis. From his experiment, he concluded that for Newtonian

fluids, the carrying capabilities increases as the viscosity increase. Unlike Newtonian fluids, Non-

Newtonian fluids experience an inverse relationship between the carrying capabilities and the

11

viscosity. Increasing the polymer content of non-Newtonian fluids lowers the consistency index

of the fluid, and therefore lowers the velocities close to stationary walls (Philip et al. 1998).

Kamp and Rivero reviewed the mechanics behind cuttings transport for the sake of

providing a state-of-the-art modeling approach. Having a comprehensive modeling approach is

essential for planning horizontal wells drilling. Also, it helps with the prediction of cuttings

transport in horizontal wells. A combination of two modeling strategies are used: (1) layers-

model for flow in the wellbore and (2) full numerical solutions in given cross-sectional areas.

Rivero developed a layers model that consists of two layers: (1) a moving bed of packed cuttings

and (2) a mixture of drilling fluid and cuttings. Using this model, computational simulations can

be performed to predict variables such as cuttings bed height and transport velocities (Kamp and

Rivero 1999).

Li and Kuru conducted a numerical study on the mechanics governing the cuttings

transport using foam as a drilling fluid in vertical wells. They developed a one-dimensional

unsteady-state two-phase model to carry out this investigation. Their model was used to make

prediction on the optimum flow rate and rheological properties of the drilling fluids that would

achieve high hole-cleaning efficiencies in vertical wellbores. They carried out a sensitivity

analysis using the model they developed to see if the obtained results where similar to the results

obtained from the field data. They concluded the following: (1) for vertical wells, high

concentrations of cuttings always exist at the bottom of the hole, (2) as the apparent viscosity

increases, the foam quality increases linearly at a given share rate and, therefore, increases the

carrying capacity, (3) extreme decreases in the viscosity of foam is observed when its quality

exceeds 98%, resulting in lower carrying capacities, (4) the influx of water from the reservoir

decreases the foam quality. This decrease in the quality of the foam increases the flowing bottom

hole pressure. And (5) the influx of gas from the reservoir increases the foam quality. This

12

increase in the quality of the foam decreases the flowing bottom hole pressure (Li and Kuru

2003).

Yu et al. proposed a new technology that would increase the efficiency of cuttings

transport in horizontal and horizontal wellbores. In horizontal and horizontal wellbores, gravity is

the main reason behind the settling of cuttings in the lower side of the wellbore. The researchers

of this work suggested that if gas bubbles were able to get attached to cutting particles, it will

counteract the effect of gravity because of its buoyancy feature. At the same time, the attachment

of gas bubbles would increase the surface area exposed to the fluid flow, and therefore, increase

the drag force applied on the cuttings/bubble system. Increasing drag force on the cuttings means

more efficient hole-cleaning. The researchers also carried out experimental tests to determine the

effect of chemical surfactants, pH , and cuttings size on the attachment of gas bubbles to the

cutting particles. Researchers concluded the following: (1) to achieve optimal attachment, two

chemical surfactants were necessary, one to attach bubbles to the cuttings and the second to

strengthen the bubble/cuttings attachment, and (2) the optimal attachment occur with surfactant

chemicals with pH ranging from 9 to 11 (Yu et al. 2004).

13

Chapter 3

PROBLEM STATEMENT

The investigating process of the hole cleaning phenomenon is a very complex task. A

substantial amount of research was done on the topic since the introduction of the first horizontal

well. Several researchers carried out lab work to investigate the effect of the parameters in figure

1-3 in the hole cleaning phenomenon, others used computer modeling approach to create a more

realistic and meaningful scenario for horizontal wells being cleaned or washed from the cuttings.

In this research, investigation of the cuttings transport through the annulus using

numerical modelling will be done. The procedure of this research is:

1. To conduct a complete literature review

2. To characterize the drilling cuttings and drilling fluid

3. To build a numerical model capable of simulating cuttings transport

4. To validate the numerical model

5. To carry out a parametric study

6. To analyze results of the parametric study

7. To document new findings in a thesis

14

Chapter 4

PARAMETERS EFFECTING CUTTINGS TRANSPORT

The understanding of the cuttings transport phenomenon is very essential for a successful

horizontal drilling operation. In this chapter, the parameters that effect the cuttings transport

performance are outlined. Also a comprehensive explanation of the physics of particle transport is

included.

4.1 Drag coefficient, Reynold’s number, and cuttings surface condition

Drag forces plays an important role in the transportation of cuttings along the wellbore.

Analyzing the drag coefficient and the Reynolds number is important to understand the settling

phenomenon of cuttings inside the wellbore. The drag coefficient represent the amount of kinetic

energy, induced by the settling velocity, needed to overcome the drag force on the particle (Chien

1994). For a particle with an equivalent diameter,𝑑𝑒𝑞, the drag coefficient in the settling process

is defined as (Fjar et al. 2002)

𝐶𝐷 ≈

{

24

𝑁𝑅𝑒 𝑓𝑜𝑟 𝑁𝑅𝑒 < 1

24

𝑁𝑅𝑒√1 + 0.2𝑁𝑅𝑒 + 0.0003𝑁𝑅𝑒 𝑓𝑜𝑟 1 < 𝑁𝑅𝑒 < 10

5

0.1 𝑓𝑜𝑟 𝑁𝑅𝑒 > 105

(4.1)

Where 𝐶𝐷, 𝜌𝑝, and 𝜌𝑓 are the drag coefficient, particle density, and fluid density, respectively. 𝑣𝑠,

is the settling velocity of the cuttings. The Reynolds number represent the ratio between the

inertial forces and the viscous forces (Chien 1994).

15

𝑁𝑅𝑒 =𝑑𝑒𝑞𝑣𝑠𝜌𝑓

𝜇𝑓

(4.2)

Where 𝑁𝑅𝑒 and 𝜇𝑓 are the Reynolds number and the fluid viscosity respectively.

During the drilling operation, as the drilling bit penetrates rocks, the created cuttings has

no shape trend. The cuttings come is different irregular shapes. In the laminar flow regime

(𝑁𝑅𝑒 < 10), surface condition effect on the drag coefficient is minor, and therefore, ignored. In

the turbulent flow regime (𝑁𝑅𝑒 > 100), the irregular shapes of the cuttings has an effect on the

drag coefficient, and therefore, affecting the settling velocity (Chien 1994).

The sphericity of a particle is quantified using the following equation (Wadell 1934)

𝜓 =𝐴𝑠′

𝐴𝑠

(4.3)

Where 𝐴𝑠′ is the surface area of a sphere that has the same volume as the investigated cutting and

𝐴𝑠 is the surface area of the cutting. Cuttings samples taken from the fields showed that the

average sphericity is around 0.8 (Chien 1994). For irregularly shaped cuttings, the drag

coefficient in the turbulent regime is defined as (Chien 1994)

𝐶𝐷𝑡 =67.289

𝑒5.030𝜓

(4.4)

Using the average sphericity of cuttings

(4.5)

16

𝐶𝐷𝑡 = 1.250, 𝜓 = 0.8

The drag coefficient can be related to the Reynolds number for irregular shaped particles

by the following expression (Chien 1994)

𝐶𝐷 =30

𝑁𝑅𝑒+67.289

𝑒5.030𝜓, 𝑓𝑜𝑟 0.2 ≤ 𝜓 ≤ 1.0

(4.6)

The above expression is true for Reynold’s numbers ranging from 0.001 to 10,000.

Figure 4-1 shows a graphical representation of the Eq. 4.6 when plotted against the particle’s

Reynold’s number.

Figure 4-1. Relationships between C_D and N_Re (Chien 1994)

17

Another approach of calculating the sphericity was developed by Nayland et al. (1983).

Nayland evaluated the sphericity of a particle using the following equation

𝜓 = 4.38𝛼𝑜

23

𝛼𝑠

(4.7)

Where 𝛼𝑜 is the Heywood equidimensional shape factor and can be determined experimentally,

and 𝛼𝑠 is the particle surface shape factor. The surface shape factor expression of an irregular

particle was developed by Heywood and adopted by Nayland, et al. (1983) as follows

𝛼𝑠 = 1.57 + 𝐶 [𝛼𝑜𝑚]

43(𝑛 + 1

𝑛)

(4.8)

Where 𝐶 is a constant that can be determined experimentally, and 𝑚 and 𝑛 are the flatness and

the elongation respectively. The flatness and the elongation can be calculated using the following

expression (Nayland and Stanley-Wood 1983)

𝑚 =𝐵

𝑇

(4.9)

𝑛 =𝐿

𝐵

(4.10)

18

Where 𝐵, 𝐿, and 𝑇 are the breadth, length, and thickness.

Figure 4-2. Heywood definition of length, L, Breadth, B, and thickness, T (Nayland and Stanley-

Wood 1983).

Nayland provided a table that contains the values of the Heywood equidimensional shape factor

and the 𝐶 parameter that were determined experimentally.

Table 4-1. Shape coefficients for some geometrical and irregular shapes (Nayland and Stanley-

Wood 1983)

Shape group 𝜶𝒐 𝑪

Geometric

Form

Tetrahedral 0.328 4.36

Cubical 0.696 2.55

Spherical 0.524 1.86

Approximate

form

Angular Tetrahedral 0.38 3.3

Prismoidal 0.47 3.0

Sub-angular 0.51 2.6

Rounded 0.54 2.1

19

4.2 Cuttings size

During the drilling operation, the drill pipe travels through different lithological units to

reach the designated target. Because of the different physical properties of each lithology, they

break into different cutting sizes. It is very difficult to estimate the size of the cuttings, therefore,

researchers took samples from drilling site to estimate a size range that will be used to carry out

their tests. Looking through previous work done in this topic, most of the researchers tested

cuttings with sizes falling between 0.085-0.275 in. In this work, sizes within the mentioned range

will be tested.

Previously, investigators made some contradicting conclusions on the effect of cuttings

size on the transportation capabilities. In the investigation carried out by Larsen et al. (1997), they

found out that transportation of small cuttings (0.09 in) was more difficult compared to larger

cuttings at high angles. On the other hand, Rubiandini (1999) made an opposite conclusion.

During his analysis, he found out that larger cuttings needed higher velocities to be transported

out of the wellbore.

4.3 Cuttings density

A sensitivity analysis will be carried out to investigate the effect of cuttings density in the

transportation capabilities. Three types of rocks will be considered: (1) sandstone, (2) limestone,

and (3) dolomite. The densities of sandstone, limestone, and dolomite are 2.65, 2.71, and 2.87

gm/cc, respectively.

20

4.4 Drilling fluid density

The mud weight is a very important parameter in the drilling operation. The mud weight

can be adjusted based on the surrounding lithological environment. Lithological units with low

fracture gradients can experience fluid losses if the mud weight is high. On the other hand, if the

mud weight is low, the borehole might collapse.

In this work, the influence of the mud weight selection on the cuttings transport will be

examined. Mud weight ranges from 8 ppg to 20 ppg. For the sake of this work, mud weights of

8.33, 15, and 20 ppg will be considered.

Drilling fluid types

In oil and gas drilling operations, the drilling mud is one of most important factors that

influences the cuttings evacuation. Drilling muds has much more functions beside the

transportation of cuttings. It serves (1) as a cooling fluid for the bit, (2) to clean the bit, (3) as a

friction reducer between the drilling string and the surrounding, (4) to create a low-permeability

filter cake that seals the pores and other openings in the formation penetrated by the bit, and (5) to

maintain stability in the uncased sections of the wellbore (Apaleke et al., 2012). The drilling

muds are classified into three main categories: (1) Water based drilling muds, (2) Oil based

drilling muds, and (3) Pneumatic drilling. When choosing a drilling fluid, engineers look at the

specific weight, viscosity, fluid loss, and reactivity that are suitable with the treated formation. In

the following sections, the rheological models and some details of the drilling fluids will be

provided.

21

Water based drilling muds

Water based drilling fluids or muds are used in most of the world’s drilling operations

(Caenn and Chillingar 1996). Fresh water, brines, and sea water are the main fluids that make up

water based muds. Salt is added to fresh water based drilling fluids to achieve shale stability or to

drill through salt zones. Main advantages of using water based drilling fluids are (Apaleke et al.

2012):

1. Low cost

2. Environmentally friendly

3. Abundantly available

4. It can achieve fast rates of penetration

Oil based drilling muds

Oil based drilling fluids were used to overcome some of the major issues with the use of

water based drilling muds. In the formulation of oil based drilling muds, the industry uses diesel,

mineral oil, or paraffins. Oil based drilling muds are used because (Caenn and chillingar 1996):

1. It does not react with water sensitive formations

2. Its lubricant nature helps with eliminating stuck-pipe, torque, and drag problems

3. It has greater temperature stability

On the other hand, oil based drilling fluids has some limitations such as:

1. High cost

2. Environmental concerns

3. Special handling

22

Pneumatic drilling

Pneumatic drilling fluids uses different base fluids: (1) dry gas, (2) mist, (3) foam, and (4)

gel foam. Pneumatic drilling fluids are mainly used for relatively shallow formations. The main

advantages of using pneumatic drilling are (Apaleke et al. 2012):

1. Formation damage is minimized

2. High rates of penetration

3. Low cost

4. Minimization of loss of circulation

5. Improved bit performance

6. Abundantly available

Limitations on the use of pneumatic drilling fluids:

1. High risk of explosions

2. Gas may be corrosive

3. It cannot drill through water bearing zones

Rheological models

Figure 4-3. Fluids Rheology (Petrowiki)

23

Choosing the appropriate rheological model helps researchers analyze and model the

cuttings transport in a more accurate manner. In the petroleum industry, non-Newtonian fluids are

usually used as drilling fluids (PetroWiki). Viscosity of non-Newtonian fluids is a function the

shear stress (PetroWiki). Non-Newtonian fluids are modelled in the oil industry using (1) Yield

Power law model, (2) Power law model, and, most widely using, (3) Bingham Plastic model.

4.6 Eccentricity

During the drilling operation of inclined wells, the drill-pipe tend to deviate from the

center of the annulus due to its weight. This deviation increase as the angle of inclination increase

until the drill-pipe lay against the walls of the hole. This deviation or decentralization is called

eccentricity. Eccentricity can be calculated using the following equation (Iyoho and Azar 1981)

∈ = 2 ∗𝑒

𝑑ℎ

(4.11)

𝑑ℎ = 𝑑𝑜 − 𝑑𝑖 (4.12)

Where 𝑒 is the offset between the centers of the inner and the outer pipes and 𝑑ℎis the

hydraulic diameter, which is the difference between the outer diameter and the inner diameter of

the pipes. For fully eccentric annulus, ∈ = 1 and 𝑒 = 𝑟𝑜 − 𝑟𝑖, where 𝑟𝑜 and 𝑟𝑖 are the radii of the

outer pipe and inner pipe respectively.

24

Figure 4-4. On the left is a representation of concentric annulus and in the right is a representation

of eccentric annulus (Iyoho and Azar 1981)

Iyoho and Azar found out that with increasing eccentricity, velocity values decrease for

the reduced section of the annulus. On the other hand, the increased section of the annulus

experienced an increase of velocity.

4.7 Drill string rotation and hole angle

A study was done by Peden et al. (1990) to investigate the effect of drill string rotation

and hole angle on the transport capability for inclined wells by calculating the minimum required

transport velocity to initiate rolling or suspension. The minimum transport velocity is defined as a

velocity a which the drill cuttings a either in full suspension or rolling. The experiment was

carried out at different eccentricity levels and different hole size. It was found that for concentric

annuli:

1. Initially, as the hole angle increases, the minimum transport velocity increase

until it reaches a maximum value (at the critical angle). The minimum transport

velocity decreases from that maximum value at higher hole angles.

2. The largest minimum transport velocity appears when the hole angle is between

40𝑜 and 60𝑜.

25

3. Smaller concentric annuli are easier to clean than larger once.

4. For small concentric annulus, the minimum transport velocity decrease with

increasing rotary speed.

5. Rotary speed has a small effect on the minimum transport velocity for concentric

annulus with large clearance.

6. It is easy to transport large cuttings at low hole angles (0𝑜to 500)

The rheology of the fluid, rotary speed, and cutting size are all parameters that affect the point

where the critical angle appears.

For eccentric annulus, Peden et al. (1990) found the following:

1. At all hole angles and no rotation, the minimum transport velocity was lower for

-50% and 50% eccentricity than the case where there was no eccentricity.

2. With rotation, the minimum transport velocity decreased further for the case of

50% eccentricity, and no noticeable effect on the minimum transport velocity for

the case of -50% eccentricity.

4.8 Cuttings transport physics

An efficient wellbore cleaning process is very important for the success of the drilling

operation. Cuttings transport becomes problematic in inclined and horizontal wells. Due to

gravitational force, cuttings tend to settle in inclined and horizontal wells. The settling of the

cuttings creates a bed of accumulated cuttings. This bed can cause a lot of costly problems such

as bit wear, stuck pipes, and high torque and drag.

For a single cutting, there are five forces that governs its movement: (1) Gravity 𝐹𝑔,

buoyancy 𝐹𝑏, Drag 𝐹𝑑, lift 𝐹𝐿, van der Waals 𝐹𝑣. The gravity and buoyancy forces are dependent

26

Gravity force

on the cutting’s and fluid’s properties. The drag and lift forces depends on the fluid flow

properties and the Van der Waals force depends on the neighboring cuttings.

Figure 4-5. Schematic of forces acting on a cutting

Figure 4-6. Shows the longitudinal and cross-sectional views of the forces and the velocity profiles

acting of the cuttings particle (Cayeux et al 2013.).

Lift force

Drag force

Buoyancy force

Van der Waal force

Fluid flow

27

Cuttings settling velocity/ Terminal velocity

Suspension

The settling/terminal velocity is a parameter that depends in the size, geometry, and

density of the cuttings. It also depends on the rheological properties of the carrying fluid.

Depending on the flow region, the settling velocity can be represented in three forms: (1) laminar

settling velocity, (2) transitional settling velocity, and (3) Newtonian settling velocity.

The settling velocity of a particle can be obtained by balancing the drag force with the

submerged force (Ramadan et al. 2003)

𝐹𝐷 =𝐶𝐷𝜋

8∗ 𝑑𝑒𝑞

2 𝜌𝑓 ∗ (𝑣𝑠2) (4.13)

𝐹𝐺 =𝜋

6𝑑𝑒𝑞3 (𝜌𝑝 − 𝜌𝑓) ∗ 𝑔 (4.14)

𝑣𝑠 = √4 ∗ 𝑔 ∗𝑑𝑝(𝜌𝑝 − 𝜌𝑓)

3𝜌𝑓𝐶𝐷 (4.15)

The above settling velocity equation represents the falling or settling of a single cutting in

a viscous fluid. In cuttings transport, the fluid is carrying a group of cuttings, not just a single

cutting. Therefore, the settling velocity should be corrected to take into account neighboring

cuttings. Richardson and Zaki (1954) developed relations between the slurry concentration and

the settling velocities by introducing the slurry porosity parameter. Slurry porosity is the

volumetric ratio of the slurry (Chih-Young Chang 2016). For the three regions mentioned above,

the corrected settling velocity is given by (Richardson and Zaki 1954)

28

𝑣𝑠,𝑐𝑟𝑡 = 𝑣𝑠 ∗ (1 −𝑐𝑠𝜌𝑝)

5.5

, 𝑁𝑅𝑒 ≤ 2

(4.16)

𝑣𝑠,𝑐𝑟𝑡 = 𝑣𝑠 ∗ (1 −𝑐𝑠𝜌𝑝)

3.5

, 2 < 𝑁𝑅𝑒 < 500

(4.17)

𝑣𝑠,𝑐𝑟𝑡 = 𝑣𝑠 ∗ (1 −𝑐𝑠𝜌𝑠)2

, 𝑁𝑅𝑒 ≥ 500

(4.18)

Where 𝑐𝑠 is the cuttings concentration.

Bedload Transport

Saltation

In the interface between the settled bank and the suspension, cuttings continuously move

between the two regions. Cutting, initially on the surface of the settle bank, can suddenly jump

and reenter the main suspension stream or just resettle on the main bank again. This process is

called saltation and sometimes “intermittent suspension” Mack et al. (2014). In order for a cutting

to experience saltation, there must be enough velocity to overcome the sum of the gravitational

force and the buoyancy force. Again, this velocity can just be enough to make it bounce and land

again on the main bank.

29

The dimensionless shear stress represents the ratio between the shear stress given by the

fluid flow on the settled bank to the apparent weight. The apparent weight expression was found

in the paper by Mack et al. (2014)

𝑎𝑝𝑝𝑎𝑟𝑒𝑛𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 = 𝑎𝑝𝑤 = (𝜌𝑝 − 𝜌𝑓)𝑔𝑑𝑒𝑞

(4.19)

𝑠ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑠𝑠 = 𝜏 = 𝜇𝑓𝛾

(4.20)

Given the above expressions, the dimensionless shear stress or Shields number can be

calculated using the following expression (Shields 1936)

𝑆 = 𝜏∗ =𝜏

𝑎𝑝𝑤=

𝜇𝑓𝛾

(𝜌𝑝 − 𝜌𝑓)𝑔𝑑𝑒𝑞

(4.21)

Where 𝛾 is the shear rate.

The shields number is used to quantify the initiation of movement of particles in the

interface between the main stream and the settled bank (Shields 1936). The movement of

particles can be either by rolling, lifting or a combination of two. In order for a particle to move,

it must be subjected to drag, lift, or both forces. The lift force can be expressed by the following

expressions (Chih-Young Chang 2016)

𝐹𝐿 = 𝐶𝐿 ∗𝜋

8𝑑𝑒𝑞2 𝜌𝑓(𝑣𝑠

2) (4.22)

30

𝐶𝐿 =12.92

𝜋𝜀

(4.24)

𝐶𝐿 is the lift coefficient. 𝜀 represents the ratio of the square root of the shear Reynolds

number to the particle Reynolds number.

A dimensionless constant was developed by Nakagawa and Tsujimoto (1980) that

quantifies the rate of particles picked up by shear

𝑃𝑠∗ =

𝑃𝑆√𝑑𝑒𝑞

√(𝜌𝑐𝜌𝑓− 1) ∗ 𝑔

= 𝐹0𝜏∗ (1 −

𝜏𝑐𝑟∗

𝜏∗)3

(4.21)

𝐹0 was found experimentally by Nakagawa and it is equal to 0.03. 𝑃𝑠∗, 𝑃𝑆, and 𝜏𝑐𝑟

∗ are the

dimensionless pick up rate, dimensional pick up rate, and the critical Shields number respectively.

Coefficient of restitution

Particles dropped from a specific height usually bounce to a height less than the initial

one. This after bouncing height was quantified by the following equation (Mack et al. 2014) and

(ANSYS user’s guide 2009)

(4.21)

31

𝐶𝑜𝑅 =𝑣𝑎𝑓𝑡𝑒𝑟

𝑣𝑖𝑛𝑖𝑡𝑖𝑎𝑙= √

ℎ𝑏𝑜𝑢𝑛𝑐𝑒ℎ𝑖𝑛𝑖𝑡𝑖𝑎𝑙

The above ratio is called the coefficient of restitution. A Particle landing on the settled

bank has the chance of reentering the main stream again if the CoR of the particle is large. Elastic

materials tend to bounce again to their initial height, which means that CoR for elastic materials

is equal to 1. But inelastic materials, like all cuttings, can never re-bounce to their initial height.

Making the CoR less than 1 for all cuttings. Because of the law of conservation of energy, when

inelastic materials collide with the settled bank, energy dissipates from them to the settled bank.

Since the settled bank is made up of cuttings, those cuttings tend to move around the location of

the collision. And sometimes, those cuttings can receive enough energy to bounce from the

settled bank and again land on it.

4.9 Transport velocity, fluid velocity, transport ratio

The transport of cuttings occurs when the submerged force is overcome with the velocity

of the fluid. In order to understand this process, a clear understanding of the fluid velocity and

general expressions of the transport velocities are needed. The fluid velocity is basically equal to

the flow rate divided by the open cross-sectional area in the wellbore. The transport velocity is

the difference between the fluid velocity and the settling velocity of the cutting (Bourgoyne et al.,

1991).

𝑉𝑓𝑙𝑢𝑖𝑑 =𝑄

𝐴𝑓

(4.38)

32

𝑉𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡 =𝑄

𝐴𝑓− 𝑣𝑠

(4.39)

𝐹𝑇 =𝑉𝑡𝑟𝑎𝑠𝑝𝑜𝑟𝑡𝑉𝑓𝑙𝑢𝑖𝑑

(4.40)

Where 𝑉𝑓𝑙𝑢𝑖𝑑, 𝑄, 𝑉𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡, and 𝐹𝑇 are the fluid velocity, flow rate, transport velocity, and

transport ratio respectively.

33

Chapter 5 COMPUTATIONAL CUTTINGS TRANSPORT MODEL

In this chapter, a computational cuttings transport model was built. The details of the

governing equations and simulation inputs of the base model is provided in this chapter. To

validate the computational model, a comparison with an experimental results was made.

5.1 Overview of the CFD Processes

The program used to conduct this computational simulation was ANSYS FLUENT 18. In

the provided package, there are programs used to create the geometry, the mesh, and post-

processing results. The license used in this work was the academic license. In order to perform a

proper computational job, there are several steps that must be made. Figure 5-1 illustrates the

workflow used to perform all the computational work in this thesis. In the following sections,

details of each step are provided.

34

Figure 5-1. Simulation workflow

Define Geometry

Assign Mesh

Define Flow Model

Assign Fluid/Solid types

Define Boundary conditions

Define initial conditions

Assign the solution method

Initialize solution

Transient solution enabled

Show Results

?

35

5.2 Geometry

The drilling fluid travels through the drill pipe to the drill bet to carry the accumulated

cuttings. After that, the cuttings travel through the annulus to the surface. Because of that,

modeling of the annulus is sufficient to study the behavior of the cuttings inside the annulus. This

practice also saves a lot of time and computational power.

The investigated annulus has an inner and outer radii of 5.00 in. and 8.50 in, respectively.

The length of the test section is 25 ft. Figure 5-2 shows a front and side view of the investigated

annulus.

Figure 5-2. Front and side view of the investigated annulus

5.3 Mesh

The mesh job plays an important role on the degree of accuracy of the simulation results.

After defining the geometry, we need to assign an appropriate meshing for the multi-phase fluid

flow. Zhou and Shaw (2003) suggested that for fluid flow inside a pipe, finer cells near the walls

are recommended for better convergence and simulation speed.

36

For the base model, and all future models, an inflation feature was assigned for the inner

and outer edges of the annulus. This way, finer elements will be created close to the edges. A

smooth transition was specified as the appropriate inflation option. The transition ratio was kept

as 0.3 with a maximum number of layers of 8. The growth rate was 1.2. The resultant mesh is

illustrated in figure 5-3.

Figure 5-3. Assigned mesh at the inlet/outlet

The size function was set as curvature with a fine mesh size. The resultant mesh has

472,140 nodes and 445,788 elements.

5.4 CFD Model

In this section, a detailed process of defining the base model will be provided. The same

process will be done for future simulations. In order to define a proper multi-phase flow, a

comprehensive understanding of the capabilities and limitations of the available models must be

37

achieved. In this section, the appropriate multi-phase flow model, the turbulence model, the base

model inputs and results are provided.

CFD Solvers

In ANSYS FLUENT, there are two available solvers that can be used to solve the multi-

phase model: (1) the pressure-based solver and (2) the density-based solver. In the pressure-based

solver, using the continuity and momentum equations, the pressure equation can be derived and

solved (ANSYS Theory 2009). On the other hand, in the density-based solver, the density field

can be found from the continuity equation and the pressure field can be found from the equation

of state (ANSYS Theory 2009). When comparing both solvers, the pressure-based approach is

better for low speed incompressible flows, while the density-based approach is better for high-

speed compressible flows (ANSYS Theory 2009). In this work, the pressure-based approach will

be used.

Moreover, the velocity formulation is another important input in the CFD job. There are

two options available: (1) the absolute velocity formulation and (2) the relative velocity

formulation. The absolute velocity formulation is preferred in fluid flow where the main domain

is not rotating, and some of the fluid is not moving (ANSYS User’s Guide 2009). Whereas the

relative velocity formulation is better when some of the domain is rotating and all the fluid is

moving (ANSYS User’s Guide 2009). For this work, when the inner pipe is not rotating, the

absolute velocity formulation will be used. When the rotation effect is tested, the relative

formulation will be used.

The solver was kept at as a transient solver and a gravity of 32.1 ft/s2 was enabled and

specified on the negative z-direction.

38

CFD Models

In ANSYS FLUENT, there are two approaches to solve multi-phase models: (1) the

Euler-Lagrange approach and the Euler-Euler approach. In the Euler-Lagrange approach, “the

fluid phase is treated as a continuum by solving the time-averaged Navier-Stokes equations, while

the dispersed phase is solved by tracking a large number of particles through the calculated flow

fluid” (ANSYS Theory 2009). In the Euler-Lagrange approach, the dispersed phase is assumed to

be much diluted with low volume fractions.

In the Euler-Euler approach, all “phases are treated mathematically as interpenetrating

continua” (ANSYS Theory 2009). Where the sum of the volume fraction of all phases is equal to

one (ANSYS Theory 2009). In the Eulerian model, the momentum and continuity equations are

solved for each phase, and the “coupling between the phases is achieved through the pressure and

interphase exchange coefficients” (ANSYS Theory 2009).

Between the two approaches, the Euler-Euler model is more appropriate because it is

more capable of modeling complex multi-phase flows with a wide range of particles

concentrations or volume fractions.

Turbulence models

Accounting for turbulence is very important in multi-phase flow simulations. Turbulence

plays an important effect on the transport of cuttings inside the annulus. ANSYS FLUENTS

offers two methods to account for the turbulence in the model: (1) the κ-ε turbulence model and

(2) the Reynolds Stress Model. Both models have comparable results, but the Reynolds stress

model is the better option when we are dealing with a complex flow and rotating boundaries. The

Reynolds stress model calculates seven transport equations which adds to the accuracy of the

results. In this work, the Reynolds stress model will be implemented (ANSYS Theory 2009).

39

In the Reynolds stress model, the linear pressure-strain model and standard wall functions

were enabled. The eleven model constants were kept as default and the dispersed model was

enabled. The dispersed model is appropriate in our case because most of the turbulence effect the

suspending cuttings and not the settled once (ANSYS Theory 2009).

Phase definitions

In the Eulerian model, it is important to specify the primary phase and the secondary

phase. In the base model, water was the primary phase and sand as the secondary phase. There is

much details in defining the secondary phase. The secondary phase was defined as a granular

phase with a diameter of 0.25 in. For the granular viscosity, I used the gidaspow model to account

for the viscosity of the suspension. For the bulk viscosity of the granular phase, the only available

model in FLUENT is the Lun et al. model. Therefore, it was used in the simulation of the base

model (ANSYS 2013b).

After the accumulation of the cuttings in the lower side of the annulus, the frictional

viscosity becomes important to account for the frictional interaction between the cuttings.

Therefore, it was enabled. All other properties and models of the secondary phase was kept as

default, as no difference was observed in the results when they were altered.

Phase interactions

For a proper multi-phase flow modeling, the interaction between the two phases must be

defined carefully. The drag function must be enabled because it is used to calculate the

momentum exchange coefficients (ANSYS user’s guide 2009). In previous works done by

Bakker et al. (2004) and Gohel et al. (2012) in stirred tanks, the Gidaspow model with Brucato

correction factor generated good results when compared with experimental work. Therefore, this

setup is used in the base model and all future models.

40

In the base model, the turbulent dispersion was enabled to account for diffusion.

Therefore, the effect was enabled for the base model and all future models. Any of the available

models can be used, as they gave similar results when altered.

The collision between particles must be accounted for in granular flow simulation.

FLUENT accounts for the collision using the restitution coefficient. The default value of 0.9 was

used for the base model and all future models (ANSYS user's guide 2009).

Definition of drilling fluid and cuttings

The drilling fluid serves as the primary phase and the driving force that carries the

secondary phase (cuttings) from the drill bet to the surface. In the base model, fresh water was

used. The density of the fresh water is 62.3 lb/ft3 and its viscosity is 6.74x10-4 lb/ft-s.

As mentioned above, the diameter of the cuttings used in the base model was specified as

0.175in. and its density is 2.71 g/cc.

Boundary conditions

In this section, a brief description of the initial conditions of the boundaries will be

provided. In the base model, we have an inlet, outlet, drill pipe (inner wall) and casing (outer

wall).

Inlet

The inlet of the base model can be modified in several methods. The method chosen for

the base model and all future models is a mass flow inlet. The direction is orthogonal to the inlet

in the positive y-direction. For the base model, the initial gauge pressure was kept as zero at the

inlet. A constant mass flow rate of 50 lb/s (350 gpm) was assigned for the drilling fluid (water) at

41

the inlet. A cuttings mass flow rate of 17 lb/min was assigned at the inlet. The turbulent intensity

and viscosity ratio was kept as default, 5% and 10 respectively.

Outlet

The outlet was defined as a pressure outlet with zero gauge pressure orthogonal to the

boundary. The backflow turbulent intensity and viscosity ration were 5% and 10 respectively.

Drill Pipe (inner wall) and Casing (outer wall)

For the sake of the base model, the inner was assumed to be stationary with no slip

condition to both phases. The roughness height was kept at zero and the roughness constant was

specified as 0.5. The outer wall was defined the same way as the inner wall.

Solution settings

FLUENT has different solvers that can be used in multi-phase flow calculations. The

results from all solves where similar. Therefore, the SIMPLE algorithm was used for the base

model and all future models to solve for the velocity and pressure. For the gradient, momentum,

volume fraction, turbulent kinetic energy, turbulent dissipation rate, and the Reynolds stresses;

the default options were used (ANSYS user’s guide 2009).

42

Results of the base model

Table 5-1. Base model drilling conditions

Parameters Values Cuttings density (g/cc) Limestone (2.71 g/cc)

Cuttings size (in) 0.175 Drilling Mud (ppg) Water

Rotary speed (RPM) 0 Eccentricity 0

Drilling rate (ft/hr) 45 Circulation rate (gpm) 350 Inclination (degrees) 60

Table 5-1 outlines the values of the drilling parameters used in the base model. After

simulating the base model, it is expected to observe similar trends between the base model and

previously done experiments in the literature. The results of the base model was compared to the

results of Tomren et al. (1986). In Tomren’s experiment, similar setup was used to obtain the

results, except the used length. Tomren was able to investigate a case with a length of 40 ft.

Because of the available ANSYS license, the maximum number of elements is 520,000, this

means that large models will have very coarse mesh and will result with very inaccurate

outcomes. Nevertheless, we are expecting to see similar trends using the computational approach.

Volume fraction

The contour plots of the volume fractions of the cuttings can be shown in figures 5-4, 5-

5, and 5-6. The cuttings behaved in a similar manner to what was observed in Tomren’s

experiment. Near the inlet of the annulus, for both cases, the cuttings settled to form a dune. After

that, the height profile decreased gradually for the rest of the model. Therefore, we can conclude

that the highest dunes occur at locations close to the inlet. With respect to the cuttings volume

fractions values, lower volume fractions were observed in Tomren’s experiment, and that is

43

because of the different lengths between the experiment and the computational model. But

overall, both approaches should similar trends.

Figure 5-4. Volume fraction of cuttings at y = 0 ft (inlet)

Cuttings volume

fraction of 1%

Cuttings volume

fraction of 1%

Cuttings volume

fraction of 35%

44

Figure 5-5. Volume fraction of cuttings at y = 12.5 ft (midpoint)

Figure 5-6. Volume fraction of cuttings at y = 12.5 ft (midpoint)

Velocity profiles of the drilling fluid and the cuttings

A viewing window was created in the middle of the annulus using the Post-CFD program

offered by ANSYS 18. In the window, the velocity profiles of the drilling fluid and cuttings were

generated. The location of the viewing window is where the highest cuttings bed was observed.

Figure 5-7 and figure 5-8 illustrates the drilling fluid and the cuttings velocity profiles

respectively. The observed velocity profiles has high velocities at the top of the annulus because

of the small flowing area at that location. Also, the drilling fluid and the cuttings velocities are

zero at the location of the settled bed.

Cuttings volume

fraction of 7%

Cuttings volume

fraction of 20%

45

Figure 5-7. Drilling fluid velocity profile (base model)

Figure 5-8. Cuttings velocity profile (base model)

Water velocity:

5.26 ft/sec

315.6 ft/sec

3.59 miles/hour

Cuttings velocity:

4.72 ft/sec

283.2 ft/sec

3.22 miles/hour

Cuttings velocity:

1.42 ft/sec

85.2 ft/sec

0.97 miles/hour

Water velocity:

1.58 ft/sec

94.8 ft/sec

1.08 miles/hour

46

5.5 CFD model validation

In this section, the CFD model created in previous sections will be validated. Tomren et

al. conducted an experimental test to examine the cuttings transport capabilities in directional

wells (Tomren et al. 1986). In order to compare his results to the CFD model, a new geometry

matching the geometry used by Tomren is needed. In his experiment, he had an annulus that has

an inner and outer diameters of 1.9 in. and 5 in. respectively. The length of his annulus was 40 ft.

Using the same geometry, a CFD model was built as shown below

Figure 5-9. Validation geometry

The above geometry was meshed using the same steps described in section 5.3. Table 5-1

summarizes the drilling parameters used in Tomren’s experiment

40 ft.

47

Table 5-2. Validation parameters

Parameters Test 1 Test 2 Test 3 Cuttings size (in) 0.25 0.25 0.25

Cuttings density (g/cc) Sandstone (2.65) Sandstone (2.65) Sandstone (2.65)

Drilling Mud (ppg) Water Water Water

Drilling rate (ft/hr) 53.3 53.3 53.3

Circulation rate (gpm) 225 200 175

Rotary speed (RPM) 0 0 0

Eccentricity 0 0 0

Inclination (degrees) 90 90 90

In Tomren’s experiment, the mass flow rate of the cuttings was specified as 20 lb/min.

Using the cuttings density and the size of the drill bit, the calculated drilling rate used in

Tomren’s experiment was 53.3 ft/hr. Using the model definitions specified in section 5.4 and the

drilling parameters in table 5-1, the CFD validation was carried out. To compare the results from

Tomren’s experiments to the CFD results, the average cuttings concentration was used. Their

results can be seen in table 5-2.

Table 5-3. Average cuttings concentration comparison

Experimental

results of Tomren

et al. (1986)

Simulation results

of Alsuwaidi (2017)

Test number Average cuttings concentration (percentage) Percent

difference

1 2.5 2.31 7.60%

2 3.7 3.40 8.12%

3 6.9 6.33 8.26%

The percent difference between my numerical results and Tomren’s experimental results

reached up to 8.26%. There are several reasons behind this difference. First, in the experiment

done by Tomren, he used actual asymmetric cuttings. Real-life cuttings have irregular shapes.

Second, the boundary conditions are very perfect in CFD modelling, which is not the case in real-

life drilling operations.

48

Even though there are some difference in the results obtained by CFD simulation, the

obtained trends from the CFD matched the trends observed in experimental procedures.

Therefore, the CFD results can be used to assess the cuttings transport performance.

49

Chapter 6 COMPUTATIONAL PARAMETRIC STUDY

In this chapter, a parametric study will be performed to examine the effect of eight

drilling parameter on the cuttings transport. Some of the examined parameter can be controlled,

such as circulation and drilling rates, and others we cannot control. Table 6-1 shows the

parameters that will be tested in this chapter.

Table 6-1. Investigated parameters

Parameters Values Cuttings density (g/cc) Sandstone (2.65), Limestone (2.71 g/cc),

Dolomite (2.87 g/cc) Cuttings size (in) 0.085, 0.175, 0.25

Drilling fluid (ppg) Water, 15, 20 Rotary speed (RPM) 0, 40, 60, 100

Eccentricity 0.5, 0, -0.25, -0.5 Drilling rate (ft/hr) 35, 45, 55

Circulation rate (gpm) 150, 200, 350, 400, 450 Inclination (degrees) 0, 30, 60, 90

6.1 Cuttings density effect

In this section, the effect of cuttings density on the cuttings transport performance will be

investigated. Three types of cuttings were considered: (1) Sandstone (2.65 g/cc), (2)

Limestone (2.71 g/cc), and (3) Dolomite (2.87 g/cc). All other parameters were kept

constant. The effect of cuttings density was tested over a range of angles. Table 6-2

shows the drilling conditions used to simulate this job.

50

Table 6-2. Investigated parameters (cuttings density)

Parameters Values Cuttings density (g/cc) Sandstone (2.65), Limestone (2.71 g/cc),

Dolomite (2.87 g/cc) Cuttings size (in) 0.175

Drilling Mud (ppg) Water Rotary speed (RPM) 0

Eccentricity 0 Drilling rate (ft/hr) 45

Circulation rate (gpm) 350 Inclination (degrees) 0, 30, 60, 90

The average volume fraction in the annulus, volume fraction at the middle of the annulus,

transport ratio, and mass flow rate ratio are used to evaluate the cuttings transport performance

for three different densities. Figures 6-1 and 6-2 shows the average volume fraction in the annulus

and volume fraction at the middle of the annulus.

Figure 6-1. Average volume fraction of cuttings in the annulus (cuttings density)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 10 20 30 40 50 60 70 80 90 100

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

AVERAGE VOLUME FRACTION OF CUTTINGS IN ANNULUS

2.65 gcc 2.71 gcc 2.87 gcc

51

Figure 6-2. Volume fraction of cuttings at the middle of the annulus (cuttings density)

Figures 6-2 and 6-3 show that heavier cuttings are harder to transport than lighter cuttings

over the whole range of angles. At low inclination angles, the density effect on the cuttings

transport performance is not significant, as the cuttings are not experiencing forces that pulls

them to the walls of the annulus. Thus, at low inclination angles (vertical wells), cuttings are

somewhat distributed evenly in annulus. At high angles (+30 degrees), the cuttings start to

experience the pull of the gravity toward the lower side of the annulus. This pull makes the

transport of cuttings harder and forces some of the cuttings to settle, or even slide back down the

hole.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30 40 50 60 70 80 90 100

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

VOLUME FRACTION OF CUTTINGS AT MIDDLE OF THE ANNULUS

2.65 gcc 2.71 gcc 2.87 gcc

52

To support the above claim, the cuttings transport ratio and the cuttings’ mass flow rate

ratio were calculated over the range of angles. Equation 4.40 was used to calculate the transport

ratio at every angle for the three types of cuttings. The mass flow rate ratio can be calculated

easily by dividing the cuttings’ mass flow rate at the outlet over the cuttings’ mass flow rate at the

inlet. Figures 6-3 and 6-4 shows the cuttings transport ratio and the cuttings’ mass flow rate ratio

respectively.

Figure 6-3. Cuttings transport ratio (cuttings density)

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 10 20 30 40 50 60 70 80 90 100

TRA

NSP

OR

TR

ATIO

ANGLE OF INCLINATION

TRANSPORT RATIO2.65 gcc 2.71 gcc 2.87 gcc

53

Figure 6-4. Cuttings mass flow rate ratio (cuttings density)

In figure 6-3, we can see that heavier cuttings have low transport ratios compared to

lighter cuttings. Also, we can tell that at angle 60 degrees, we are experiencing the worst cuttings

transport situation. Similar conclusions can be made by looking at figure 6-4. The highest cuttings

accumulation occurs at angle 60 degrees. From equation 4.18, the submerged force, we can

conclude that the submerged force is influenced by the cuttings density. For a constant drilling

fluid circulation rate, higher cuttings density increases the strength of the submerged force.

Therefore, the cuttings tend to settle faster. The contrary is true for low density cuttings.

From figure 6-4, we can conclude that the effect of higher cuttings density on the

transport performance becomes more significant as the angle of inclination increase. For

horizontal wells, 84 % of cuttings with density of 2.65 g/cc were transported to the outlet.

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

0 10 20 30 40 50 60 70 80 90 100

MA

SS F

LOW

RAT

IO

ANGLE OF INCLINATION

MASS FLOW RATE RATIO (OUTLET/INLET)

2.65 gcc 2.71 gcc 2.87 gcc

54

Whereas only 67% of cuttings with density of 2.87 g/cc were transported to the outlet. The goal is

to increase the mass flow rate ratio as much as possible and make it approach 1. This can be

achieved by increases the circulation rate of the drilling fluid. This increases the drag force

exerted on the cuttings and helps overcoming the submerged force. Keeping in mind that there are

serious consequences of increasing the circulation rate to extremely high values. The

consequences are discussed later in this chapter.

Drilling engineers can optimize the cuttings transport performance by investigating the

types of lithology expected while drilling the well. Doing so helps with make engineering

decisions regarding the drilling process. For example, if the drill bit is expected to penetrate a

lithology that has a high density, the drilling engineer can enhance the recovery process by

altering some of the drilling parameters. Like for example, drill pipe rotary speed of circulation

speed in a way it does not affect the formation.

Jiimaa carried out a simulation to investigate the influence to drilling parameters in

cuttings transport. Jiimaa used WellPlanTM to carry out the analysis. Six cuttings densities were

investigated: (1) Coal (1.2-1.5 g/cc), (2) consolidated shale (2-2.8 g/cc), (3) carbonate with 10%

porosity (2.56 g/cc), (4) sandstone (2.56 g/cc), (5) carbonate (2.71 g/cc), and (6) ingenious (2.9

g/cc). From his analysis, he concluded that cuttings with higher density requires higher

circulation rates in order to maintain a clean wellbore. Also, he concluded that at high inclination

angles, cuttings transport will become harder when compared to the transportation at low

inclination angles (Jiimaa 2013). This conclusion agrees with the result obtained in this section.

6.2 Cuttings size effect

In this section, the effect of cuttings size on the cuttings transport performance will be

investigated. Three cuttings sizes were considered: (1) 0.085 in., (2) 0.175 in., and (3) 0.25 in.

55

All other parameters were kept constant. The effect of cuttings size was tested over a

range of angles. Table 6-3 shows the drilling conditions used to simulate this job.

Table 6-3. Investigated parameters (cuttings size)

Parameters Values Cuttings density (g/cc) Limestone (2.71 g/cc)

Cuttings size (in) 0.085, 0.175, 0.25 Drilling Mud (ppg) Water

Rotary speed (RPM) 0 Eccentricity 0

Drilling rate (ft/hr) 45 Circulation rate (gpm) 350 Inclination (degrees) 0, 30, 60, 90

Figures 6-5 and 6-6 shows the average volume fraction in the annulus and volume

fraction at the middle of the annulus.

Figure 6-5. Average volume fraction of cuttings in the annulus (cuttings size)

0

0.05

0.1

0.15

0.2

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

AVERAGE VOLUME FRACTION OF CUTTINGS IN ANNULUS

0.085 in 0.175 in 0.25 in

56

Figure 6-6. Volume fraction of cuttings at the middle of the annulus (cuttings size)

Cuttings size effect on the transport performance can be seen in figures 6-5 and 6-6. From

the figures we can conclude that cuttings with larger sizes are harder to transport than cuttings

with smaller sizes. The cuttings size effect is significant for the range of angles between 30 to 60

degrees. At low angles, the cuttings size has a small effect on the cuttings transport capabilities.

If the density of the cuttings is kept constant, increasing the size of the cuttings will result

with increasing mass. And therefore, cuttings with greater mass will be heavier and harder to

transport.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

VOLUME FRACTION OF CUTTINGS AT MIDDLE OF THE ANNULUS

0.085 in 0.175 in 0.25 in

57

Figure 6-7. Cuttings transport ratio (cuttings size)

Figure 6-8. Cuttings mass flow rate ratio (cuttings size)

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

TRA

NSP

OR

T R

ATIO

ANGLE OF INCLINATION

TRANSPORT RATIO

0.085 in 0.175 in 0.25 in

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

MA

SS F

LOW

RAT

IO

ANGLE OF INCLINATION

MASS FLOW RATE RATIO

0.085 in 0.175 in 0.25 in

58

Figures 6-7 and 6-8 shows the cuttings transport ratio and the cuttings mass flow rate

ratio for the three sizes considered. We can tell from figure 6-7 that smaller cuttings are easier to

transport for the whole range of angles. Also smaller cuttings are less effected to the increase of

angle compared to larger cuttings. With a constant circulation rate and density, it is easier to keep

smaller cuttings in suspension than larger cuttings. Moreover, it is easier to initiate saltation for

smaller cuttings than larger once.

It is very hard to predict the sizes of cuttings before drilling the well. This challenge can

be eliminated by looking at created samples from previously drilled wells. The drilling engineer

can look at the created cuttings from previously drilled wells that had the same drilling design as

the proposed well. Using this information, an optimization process can be done to achieve the

best cuttings transport performance as possible. Moreover, to obtain a better cuttings transport,

the drilling engineer should try to break the cuttings as small as possible in order to minimize the

settling process of cuttings.

6.3 Mud weight effect

In this section, different mud weights are going to be used to examine the cuttings

transport performance. For this simulation job, three mud weights were considered: (1) water

(8.33), (2) mud 2 (15 ppg), and (3) mud 3 (20 ppg). All other parameters were kept constant.

The effect of mud weight was tested over a range of angles. Table 6-4 shows the drilling

conditions used to simulate this job.

Table 6-4. Investigated parameters (drilling mud)

Parameters Values Cuttings density (g/cc) Limestone (2.71 g/cc)

Cuttings size (in) 0.175 Drilling Mud (ppg) Water (8.33), mud 2 (15), mud 3 (20)

59

Rotary speed (RPM) 0 Eccentricity 0

Drilling rate (ft/hr) 45 Circulation rate (gpm) 350 Inclination (degrees) 0, 30, 60, 90

The rheological properties for the three mud systems were kept constant and equal to the

rheological properties of water. The only altered fluid property is the density.

The selection of the correct mud weight to be used in the drilling operation is very

important. Choosing the wrong drilling mud might cause some catastrophic problems. For

example, if a very low mud weight is chosen, there might be some difficulty transporting cuttings

to the surface and therefore result with high accumulations. High cuttings accumulation can cause

the drill pipe to stick, and can also create excessive torque and drag on the drill pipe. Such

problems hinders the drilling operation and can cost a lot of money. On the other hand, choosing

a high weight drilling mud can also result with a catastrophic problem. High weight drilling mud

can break into the formation, and therefore, the drilling fluid will be lost in the created formation

fractures. Loss of drilling mud can also create an imbalance between the mud pressure gradient

and the formation pore pressure gradient and this can force the formation fluid to travel up the

annulus and cause a blowout.

A numerical analysis is carried out to investigate the effect of drill mud weight on the

transport of cuttings. Figures 6-9 and 6-10 shows the average volume fraction of cuttings in the

annulus and the volume fraction of cuttings in the middle of the annulus for three types of drilling

muds.

60

Figure 6-9. Average volume fraction of cuttings in the annulus (drilling mud)

Figure 6-10. Volume fraction of cuttings at the middle of the annulus (drilling mud)

0

0.02

0.04

0.06

0.08

0.1

0.12

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

AVERAGE VOLUME FRACTION OF CUTTINGS IN ANNULUS

water 15 ppg 20 ppg

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

VOLUME FRACTION OF CUTTINGS AT MIDDLE OF THE ANNULUS

water 15 ppg 20 ppg

61

Looking at the figures 6-9 and 6-10 we can confidently say that the effect of increasing

the mud weight increases the transport performance of cuttings significantly. Looking at figure 6-

10, for angle 60 degrees (worst hole cleaning case), changing the mud from water to mud 2 (15

ppg) decreases the volume fraction of cuttings by 33.3%. Also, changing from mud 2 to mud 3

decreases the volume fraction by 47%. Even though higher density drilling mud is performing

better than lower one, the drilling engineer needs to keep in mind the catastrophic failures that

might occur from choosing the wrong mud system.

Beside the volume fraction figures, the transport ratio and the cuttings mass flow rate

ratio were calculated and plotted for the investigated range of angles. The results can be seen in

figures 6-11 and 6-12.

Figure 6-11. Cuttings transport ratio (drilling mud)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

TRANSPORT RATIO

water 15 ppg 20 ppg

62

Figure 6-12. Cuttings mass flow rate ratio (drilling mud)

We can conclude from figures 6-11 and 6-12 that for high mud weight, the cuttings

transport performance is always good and slightly effect by the change of angle of inclination.

Lower mud weights systems have higher response to angle change than higher mud weights.

Even though higher mud weight system has better cuttings transport capabilities, we need

to keep in mind the huge consequences that can happen if very high mud weight is used during

the drilling operation. The mud weight selection needs to be optimized, and this numerical model

can help the drilling engineer get an insight of what to expect of altering the mud weight. As

mentioned before, using prior knowledge of the lithological units, the drilling engineers can

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

0 10 20 30 40 50 60 70 80 90 100

MA

SS F

LOW

RAT

IO

ANGLE OF INCLINATION

MASS FLOW RATE RATIO

water 15 ppg 20 ppg

63

enhance the cuttings transport performance by choosing the right mud system every time a new

lithological unit is drilled.

Sifferman and Baker carried out an experimental investigation to examine the effect of

mud weight on cuttings transport. The found out that small increases in the mud weight will

improve the cuttings transport significantly (Sifferman and baker 1992). This agrees with what

was observed in this section.

6.4 Drill pipe rotation effect

In this section, different drill pipe rotation speeds are going to be used to examine the

cuttings transport performance. For this simulation job, four rotary speeds were considered: (1) 0

RPM, (2) 40 RPM, (3) 60 RPM, and (4) 100 RPM. All other parameters were kept constant.

The effect of drill pipe rotation was tested over a range of angles. Table 6-5 shows the

drilling conditions used to simulate this job.

Table 6-5. Investigated parameters (drill pipe rotation)

Parameters Values Cuttings density (g/cc) Limestone (2.71 g/cc)

Cuttings size (in) 0.175 Drilling Mud (ppg) Water (8.33)

Rotary speed (RPM) 0, 40, 60, 100 Eccentricity 0

Drilling rate (ft/hr) 45 Circulation rate (gpm) 350 Inclination (degrees) 0, 30, 60, 90

The drill pipe rotation is a very important parameter that influences the cuttings transport

performance. It is expected that the drill pipe rotation effect is most significant at high angles. In

64

figures 6-13 and 6-14, the average volume fraction of cuttings in the annulus and the volume

fraction of cuttings at the middle of the annulus are plotted.

Figure 6-13. Average volume fraction of cuttings in the annulus (drill pipe rotation)

0

0.02

0.04

0.06

0.08

0.1

0.12

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

AVERAGE VOLUME FRACTION OF CUTTINGS IN THE ANNULUS

0 rpm 40 rpm 60 rpm 100 rpm

65

Figure 6-14. Volume fraction of cuttings at the middle of the annulus (drill pipe rotation)

As expected, it can be observed from figures 6-13 and 6-14 that the drill pipe rotation

effect is most significant at high angles. With drill pipe rotation, the effect of the roughness of

drill pipe becomes greater as the inclination angle increase. This effect makes the frictional

component created by the drill pipe rotation strong. The frictional component helps with the

eroding of settled cuttings and carries them to higher drilling fluid velocity regions. From figure

6-14, at 100 rpm, the worst cuttings transport situation occurs between angle 25 to 40 degrees.

Whereas, at 0 rpm, the worst cuttings transport situation occurs between angle 50 to 70 degrees.

Figure 6-13 also supports the above conclusion.

It is clear from figures 6-13 and 6-14 that at very low angles of inclination, the increasing

the drill pipe rotation has a negligible effect on the cuttings transport performance. Drilling

engineers can increase the rotary speed as the angle of inclination becomes larger

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 10 20 30 40 50 60 70 80 90 100

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

VOLUME FRACTION OF CUTTINGS AT MIDDLE OF THE ANNULUS

0 rpm 40 rpm 60 rpm 100 rpm

66

The transport ratio and the cuttings mass flow ratio are illustrated in figures 6-15 and 6-

16.

Figure 6-15. Cuttings transport ratio (drill pipe rotation)

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0 10 20 30 40 50 60 70 80 90 100

TRA

NSP

OR

T R

ATIO

ANGLE OF INCLINATION

TRANSPORT RATIO

0 rpm 40 rpm 60 rpm 100 rpm

67

Figure 6-16. Cuttings mass flow rate ratio (drill pipe rotation)

Also, from figures 6-15 and 6-16, the same conclusion can be made on the relationship

between the effect of drill pipe rotation and the angle of inclination. When there is no drill pipe

rotation, the cuttings bed height increases as the inclination angle increases. With drill pipe

rotation, the accumulation is hindered because of the created turbulence around in the annulus.

The turbulence helps the cuttings stay in high velocity regions, and also helps with picking up

settled cuttings. Altering the rheological properties of the drilling fluid can make the effect of drill

pipe rotation more significant. If the drill pipe rotation was implemented with drilling muds that

have high viscosity, it is expected that the cuttings transport performance will improve

significantly. Increased viscosity with drill pipe rotation helps cuttings stay in suspension. This

can be examined in the future.

0.5

0.6

0.7

0.8

0.9

1

1.1

0 20 40 60 80 100

MA

SS F

LOW

RAT

E R

ATIO

ANGLE OF INCLINATION

MASS FLOW RATE RATIO

0 rpm 40 rpm 60 rpm 100 rpm

68

The effect of drill pipe rotation was investigated by Sanchez et al. experimentally. He

found out that the effect of drill pipe rotation was most significant for directional well drilling

(Sanchez et al. 1999)

6.5 Drill Pipe Eccentricity effect

In this section, the effect of drill pipe eccentricity is going to be examined. For this

simulation job, four eccentricity degrees were considered: (1) 0.5, (2) 0, (3) -0.25, and (4) -0.5.

All other parameters were kept constant. The effect of drill pipe eccentricity was tested

over a range of angles. Table 6-6 shows the drilling conditions used to simulate this job.

Table 6-6. Investigated parameters (drill pipe eccentricity)

Parameters Values Cuttings density (g/cc) Limestone (2.71 g/cc)

Cuttings size (in) 0.175 Drilling Mud (ppg) Water (8.33)

Rotary speed (RPM) 0 Eccentricity 0.5, 0, -0.25, -0.5

Drilling rate (ft/hr) 45 Circulation rate (gpm) 350 Inclination (degrees) 0, 30, 60, 90

Eccentricity has a significant effect on the cuttings transport performance. It is believed

that cuttings transport becomes hard as the degree of eccentricity increase in the negative

direction. Looking at figure 4-4, by definition, eccentricity value becomes negative as the drill

pipe moves closer the lower side of the casing. Positive eccentricity occurs when the drill pipe

69

moves to the upper side of the casing. In reality, positive never happens, but it will be considered

for the sake of comparison.

Figures 6-17 and 6-18 shows the average volume fraction of cuttings in the annulus and

the volume fraction of cuttings at the middle of the annulus respectively.

Figure 6-17. Average volume fraction of cuttings in the annulus (drill pipe eccentricity)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 20 40 60 80 100

VO

LUM

EFR

AC

TIO

N

ANGLE OF INCLINATION

AVERAGE VOLUME FRACTION OF CUTTINGS IN THE ANNULUS

0.5 0 -0.25 -0.5

70

Figure 6-18. Volume fraction of cuttings at the middle of the annulus (drill pipe eccentricity)

The maximum packing limit of spheres with equal radii is 0.63. Therefore, ANSYS

cannot model simulations were cuttings volume fraction reaches above 0.63. For the -0.5

eccentricity, ANSYS cannot calculate the occurring high volume fractions for the small available

space between the drill pipe and the casing. The program crashes for angles 60 and 90 degrees.

For angle 30, we can see in figure 6-18 that the volume fraction is going to increase significantly

as we increase the angle. The reason behind this significantly high accumulation is the low fluid

velocity profile around the drill pipe. This low fluid velocity profile is not enough to keep the

cuttings in suspension. The -0.25 eccentricity case was added to help with the comparison with

the concentric and positively eccentric cases.

In the positively eccentric case, 0.5, the cuttings volume fraction values were lower than

what was found in the concentric case. That is because in the positively eccentric case, the high

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100

VO

LUM

EFR

AC

TIO

N

ANGLE OF INCLINATION

VOLUME FRACTION OF CUTTINGS AT MIDDLE OF THE ANNULUS

0.5 0 -0.25 -0.5

71

fluid velocity region was seen directly above the settled cuttings bed. This high fluid velocity

region helps with the initiation of saltation and helps keep the cuttings in suspension. Therefore,

improve the cuttings transport performance.

Figures 6-19 and 6-20 shows the cuttings transport ratio and the cuttings mass flow rate

ratio respectively.

Figure 6-19. Cuttings transport ratio (drill pipe eccentricity)

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100

TRA

NSP

OR

T R

ATI

O

ANGLE OF INCLINATION

TRANSPORT RATIO

0.5 0 -0.25

72

Figure 6-20. Cuttings mass flow rate ratio (drill pipe eccentricity)

The drill pipe eccentricity effects the velocity profiles of both the cuttings and the drilling

fluid. As mentioned previously, in positively eccentric drill pipe, the high velocity region of the

drilling fluid exist directly above the settled cuttings bed. This increases the transport velocity,

and therefore, increases the transport ratio. This can be observed in figure 6-19. Higher transport

ratio means better cuttings transport performance. We can see from figure 6-20 that the positive

eccentric case has a better cuttings mass flow rate ratio for all angles, and that’s because of the

high drilling fluid velocity exerting more drag on the cuttings.

At very low inclination angles (0 to 15 degrees), the gravity force subjected on the

cuttings is just pulling the cuttings against the force of drag. Therefore, eccentric drill pipes have

a negligible effect on the cuttings transport performance. The cuttings are not pulled toward low

0.5

0.6

0.7

0.8

0.9

1

1.1

0 20 40 60 80 100

MA

SS F

LOW

RA

TE R

ATI

O

ANGLE OF INCLINATION

MASS FLOW RATE RATIO

0.5 0 -0.25

73

velocity regions and the right circulation rate is sufficient for a great cuttings transport

performance.

At inclination angles of 50 to 70 degrees, the gravity force and the drag force are not

aligned. Therefore, the wellbore eccentricity creates a region of low drilling fluid velocity at the

lower side. Therefore, cuttings are not experiencing enough drag to stay in suspension and a

settled bed of cuttings is created. In such cases, increasing the circulation rate would increase the

drag force on the cuttings and help them stay in suspension. Keeping in mind that very high

circulation rate would result with unwanted consequences. More details will be provided in

section 6.7.

In 1986, Tomren et al. investigated the effect of drill pipe eccentricity on the cuttings

transport. The negative eccentric case yield with the highest cuttings accumulation (Tomren et al.

1986).

Drill Pipe Eccentricity with 100 RPM rotation

In this section, drill pipe rotation was implemented with some negative eccentricity to

measure the degree of improvement in the cuttings transport performance. The -0.25 eccentricity

case was taken from the above section with the same drilling conditions plus 100 RPM rotary

speed. The cuttings volume fraction at the middle of the annulus can be seen in figure 6-21.

74

Figure 6-21. Volume fraction of cuttings at the middle of the annulus with drill pipe rotation (drill

pipe eccentricity)

Adding drill pipe rotation to the eccentric case improves the cuttings performance

significantly. Also, as the angle of inclination increase, the cuttings transport performance

improvements also increase. At angles 30, 60, and 90 degrees, the improvements were 25.5%,

35.3%, and 40.9% respectively. The drill pipe rotation helps with eroding the settled cuttings.

This eroding effect becomes stronger as the angle of inclination increases.

6.6 Drilling rate effect

In this section, different drilling rates are going to be used to examine the cuttings

transport performance. For this simulation job, five drilling rates were considered: (1) 5 ft/hr, (2)

35 ft/hr, (3) 45 ft/hr, (4) 55 ft/hr, and (5) 100 ft/hr. All other parameters were kept constant.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

VOLUME FRACTION OF CUTTINGS AT

MIDDLE OF THE ANNULUS

-0.25 with 100 rpm rotation -0.25

75

The effect of drill pipe rotation was tested over a range of angles. Table 6-6 shows the

drilling conditions used to simulate this job.

Table 6-7. Investigated parameters (Drilling rate)

Parameters Values Cuttings density (g/cc) Limestone (2.71 g/cc)

Cuttings size (in) 0.175 Drilling Mud (ppg) Water (8.33)

Rotary speed (RPM) 0 Eccentricity 0

Drilling rate (ft/hr) 5, 35, 45, 55, 100 Circulation rate (gpm) 350 Inclination (degrees) 0, 30, 60, 90

In experimental works, drilling rate showed moderate effect on the cuttings transport

performance. The cuttings transport performance becomes worse with increasing drilling rate.

Figures 6-22 and 6-23 shows the cuttings average volume fraction in the annulus and the cuttings

volume fraction at the middle of the annulus respectively.

0

0.05

0.1

0.15

0.2

0.25

0 20 40 60 80 100

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

AVERAGE VOLUME FRACTION OF CUTTINGS IN THE ANNULUS

5 35 45 55 100

76

Figure 6-22. Average volume fraction of cuttings in the annulus (drilling rate)

Figure 6-23. Volume fraction of cuttings at the middle of the annulus (Drilling rate)

If all other drilling conditions are conditions are constant, the cuttings volume fraction

will increase with increasing drilling rate. Looking at figures 6-22 and 6-23, we can see that the

effect of changing the drilling rate is moderate.

Even though the effect of the drilling rate on the cuttings transport performance is

moderate, the problem of increasing volume fraction of cuttings needs to be treated. Such

problem can be eliminated by increasing the circulation rate or the rotary speed can enhance the

cuttings transport performance.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

VOLUME FRACTION OF CUTTINGS AT MIDDLE OF THE ANNULUS

5 35 45 55 100

77

Figures 6-24 and 6-25 shows the transport ratio and the cuttings mass flow rate ratio

respectively.

Figure 6-24. Cuttings transport ratio (Drilling rate)

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100

TRA

NSP

OR

T R

ATIO

ANGLE OF INCLINATION

TRANSPORT RATIO

5 35 45 55 100

78

Figure 6-25. Cuttings mass flow rate ratio (Drilling rate)

From the two figures above, we can see that as the angle of inclination increase from 0 to

60 degrees, the effect of increasing the drilling rate becomes more significant. Also, we can

conclude from the above figures that the effect of increasing the drilling rate becomes less

significant between angles 60 to 100 degrees. That is because of the exerted forces on the

cuttings. At very low angles, gravity is pulling the cuttings down the hole. Having the right

circulation rate can provide enough drag in the cuttings to overcome the gravity force, and

improve the cuttings transport performance. As the inclination angle increases between 50 to 70

degrees, the gravity force is pulling the cuttings toward the walls of the casing and down the hole.

The accumulated cuttings on the walls can slide back into the hole as the result of the gravity’s

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 20 40 60 80 100

MA

SS F

LOW

RAT

E R

ATIO

ANGLE OF INCLINATION

MASS FLOW RATE RATIO

5 35 45 55 100

79

component. That is why worst cuttings transport performance is experience between those angles

of inclination. At 90 degrees inclination, the gravity is just forcing the cuttings to settle down in

the casing. A combination of higher circulation rates and drill pipe rotation can improve the

cuttings transport performance in this case.

Drilling rate effect with 100 RPM rotation

In this section, drill pipe rotation is implemented with the process of increasing the

drilling rate to measure the degree of improvement in the cuttings transport performance. For this

investigation, the 5 ft/hr, 45 ft/hr, and the 100 ft/hr drilling rates plus 100 RPM rotary speed is

considered. The cuttings volume fraction at the middle of the annulus can be seen in figure 6-26.

Figure 6-26. Volume fraction of cuttings at the middle of the annulus with drill pipe rotation

(Drilling rate)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30 40 50 60 70 80 90 100

VO

LUM

EFR

AC

TIO

N

ANGLE OF INCLINATION

VOLUME FRACTION OF CUTTINGS AT MIDDLE OF THE ANNULUS

5 ft/hr (100 rpm) 45 ft/hr (100 rpm) 100 ft/hr (100 rpm)

5 ft/hr 45 ft/hr 100 ft/hr

80

In figure 6-26, we can observe the significant improvement in the cuttings transport

performance with drill pipe rotation is implemented. For 60 degrees inclination and 100 ft/hr

drilling rate, the cuttings volume fraction improvement was 33.3%. For 60 degrees inclination

and 45 ft/hr drilling rate, the cuttings volume fraction improvement was 48%. For 60 degrees

inclination and 5 ft/hr drilling rate, the cuttings volume fraction improvement was 64%. We can

conclude from the above that the improvement from drill pipe rotation is most significant when it

is implemented with low drilling rates. Also, the worst cuttings transport performance case occurs

between the angles of 25 to 35 degrees when the drill pipe rotation is implemented. That is

because the drill pipe rotation effect becomes more significant at high angles.

6.7 Circulation rate effect

In this section, different drilling rates are going to be used to examine the cuttings

transport performance. For this simulation job, five drilling rates were considered: (1) 5 ft/hr, (2)

35 ft/hr, (3) 45 ft/hr, (4) 55 ft/hr, and (5) 100 ft/hr. All other parameters were kept constant.

The effect of drill pipe rotation was tested over a range of angles. Table 6-8 shows the

drilling conditions used to simulate this job.

Table 6-8. Investigated parameters (circulation rate)

Parameters Values Cuttings density (g/cc) Limestone (2.71 g/cc)

Cuttings size (in) 0.175 Drilling Mud (ppg) Water (8.33)

Rotary speed (RPM) 0 Eccentricity 0

Drilling rate (ft/hr) 45 Circulation rate (gpm) 150, 200, 350, 400, 450 Inclination (degrees) 0, 30, 60, 90

81

The circulation rate is very essential in the drilling operation. It is the main driving force

that carried cuttings and picks up settled once from the accumulated cuttings bed. Increasing the

circulation rate improves the cuttings transport performance. Figures 6-27 and 6-28 shows the

cuttings average volume fraction in the annulus and the cuttings volume fraction at the middle of

the annulus respectively.

Figure 6-27. Average volume fraction of cuttings in the annulus (circulation rate)

0

0.05

0.1

0.15

0.2

0 20 40 60 80 100

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

AVERAGE VOLUME FRACTION OF CUTTINGS IN THE ANNULUS

150 200 350 400 450

82

Figure 6-28. Volume fraction of cuttings at the middle of the annulus (circulation rate)

As expected, increasing the circulation rate improves the cuttings transport performance.

But, increasing the circulation rate might not always be beneficial, as it can cause loss of

circulation. Loss of circulation happens when the drilling mud breaks into the formation. This

happens because of the excess pressure caused by high circulation rate. On the other hand, very

low circulation rate can cause the borehole to collapse. Therefore, the drilling engineer must be

very careful when choosing the appropriate circulation rate.

Figures 6-29 and 6-30 shows the transport ratio and the cuttings mass flow rate ratio

respectively.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100

VO

LUM

E FR

AC

TIO

N

ANGLE OF INCLINATION

VOLUME FRACTION OF CUTTINGS AT MIDDLE OF THE ANNULUS

150 200 350 400 450

83

Figure 6-29. Cuttings transport ratio (circulation rate)

Figure 6-30. Cuttings mass flow rate ratio (circulation rate)

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 20 40 60 80 100

MA

SS F

LOW

RA

TE R

ATI

O

ANGLE OF INCLINATION

MASS FLOW RATE RATIO

150 200 350 400 450

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0 20 40 60 80 100

TRA

NSP

OR

T R

ATI

O

ANGLE OF INCLINATION

TRANSPORT RATIO

150 200 350 400 450

84

From equation 6.39, we can confidently say that as the circulation rate increase, the

transport velocity increase. Higher transports velocity result with higher transport ratio, which

means better cuttings transport performance. We can see the effect of increasing the circulation

rate on the transport ratio in figure 6-29. The effect of increasing the circulation rate is most

significant at angle 60 degrees. Figure 6-30 shows that increasing the circulation rate result with a

higher cuttings mass flow rate ratio, and therefore, better cuttings transport performance.

At very low inclination angles, increasing the circulation rate will not improve the

cuttings transport performance significantly. Therefore, drilling engineers can use lower

circulation rates for angles between 0 and 15 degrees. Since the worst cuttings transport

performance occurs between angles 50 to 70 degrees, increasing the circulation rate would

enhance the cuttings transport performance significantly. That because at those angles, the

cuttings tend to be pulled by gravity toward the lower side of the wellbore and back down the

hole. Higher drilling fluid velocity helps the cuttings to stay in suspension; and if the velocity is

higher enough, some settled cuttings might start to bounce and enter the high velocity stream

again. For the angle ranges between 15 to 50 degrees and 70 to 90 degrees, cuttings transport

performance is somehow similar. Therefore, similar circulation rate can be used for both ranges

of angles.

85

Chapter 7 CONCLUSIONS

Cuttings transport behavior is a very complex concept. Understanding the behavior of

cuttings when subjected to gravity and drag forces is very important for the success of the drilling

operation. A lot of lab work was done in the past that examine some of the main parameters that

effect the cuttings transport performance. In this research, a numerical model was built using

ANSYS FLUENT 18 to perform a parametric study.

From the parametric study, the following conclusions were made:

1. For all parameters investigated, the effect on the cuttings transport performance

from the implication of the parameters is very negligible for the vertical well

orientation.

2. For all cases, angle 60 degrees results with the worst cuttings transport

performance. Angles 30 degrees and 90 degrees had similar cuttings transport

performance.

3. For the following parameters: (1) Cuttings density, (2) Cuttings size, and (3)

Drilling rate; a direct relationship with volume fraction of cuttings exist.

4. For the following parameters: (1) Mud weight, (2) rotary speed, and (3)

circulation rate; an inverse relationship with the volume fraction of cuttings exist.

5. Negative eccentricity results with significantly high volume fractions. It is very

hard to model negative eccentricity cases numerically and further work is needed.

6. Positive eccentricity enhances the cuttings transport performance, even though it

never happens in real-life.

The degree of effect each parameters has on the cuttings transport performance is

different. Table 7-1 shows the parameters and their effect on the cuttings transport performance.

86

Table 7-1. Effect of parameters on cuttings transport performance

Parameters Effect

Cuttings density Moderate

Cuttings size Moderate

Mud weight High

Drill pipe rotation High at high angles of inclination

Drill pipe eccentricity High

Drilling rate High

Circulation rate High

Drilling engineers can use this numerical model to get an insight of the expected cuttings

transport performance. Also, the cuttings transport performance can be optimized using such

model. For example, if the type of lithology and the angle of inclination are known, the drilling

engineer can choose the right mud system, rotary speed, drilling rate, and circulation rate that

ensures a good cuttings transport performance. If a horizontal well is considered and the drilling

is taking place in limestone, the following drilling parameters will be sufficient for a good

cuttings transport performance:

• Mud weight: 15~17 ppg

• Rotary speed: 60 rpm

• Drilling rate: 35 ft/hr

• Circulation rate: 450 gpm

Using the above parameters will result with a mass flow rate ratio of 0.9. Even though the

above process looks straightforward, the drilling engineer must keep in mind the consequences of

choosing the wrong mud system or the wrong circulation rate. Such errors can create an

87

unbalance of pressures between the wellbore and the formation and result with catastrophic

failure of the well.

More research is needed for the implantation of the shape factor in the numerical model.

The shape factor influences the drag coefficient and therefore effects the cuttings transport

performance. Another factor that can improve the results from the numerical model is

implementing a mixture of cuttings sizes. This way the numerical model can be more realistic.

Also, further examination can be done by altering two drilling parameters instead of once to

observe more conclusions

88

References

[1] ANSYS Inc, I.N.C. 2009. ANSYS FLUENT 12.0 User’s Guide

[2] ANSYS Inc, 2009, ANSYS FLUENT 12.0 Theory Guide

[3] Yu, M., Melcher, D., Ahmed, R. et al. 2004. A New Approach to Improve Cuttings Transport

in Horizontal and Inclined Wells. Presented at the SPE Annual Technical Conference and

Exhibition, Houston, Texas, 26-29 September. Paper SPE 90529-MS

[4] Amanna, B. and Movaghar, M. 2016. Cuttings transport behavior in directional drilling using

computational fluid dynamics (CFD). Journal of Natural Gas Science and Engineering 34:

670—679.

[5] Apaleke, A., Al-Majed, A., and Hossain, M. 2012. Drilling Fluid: State of The Art and Future

Trend. Presented at the North Africa Technical Conference and Exhibition, Cairo, Egypt,

20—22 February. SPE-149555

[6] Bourgoyne, A., Millheim, K., and Chenevert , M.E. 1991. Applied Drilling Engineering, Vol.

2. Richardson, Texas: Textbook Series, SPE.

[7] Caenn, R. and Chillingar, G. 1996. Drilling Fluids: State of the Art. Journal of Petroleum

Science and Engineering 14 (3): 221—230.

[8] Cayeux, E., Tanripada, S., and Mesagan, T. 2013. Real-Time Evaluation of Hole Cleaning

Conditions Using a Transient Cuttings Transport Model. Presented at the SPE/IADC Drilling

Conference, Amesterdam, Netherlands, 5—7 March. SPE-163492-MS

[9] Chien, S. 1994. Settling Velocity of Irregularly Shaped Particle. SPE Drilling &

Completion 9 (4): 281—289. SPE-26121-PA

[10] Clark, R.K. and Bickham, K.L. 1994. A Mechanistic Model for Cuttings Transport.

Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana,

25—28 September. SPE-28306-MS

89

[11] Hemphill, T. and Larsen, T. 1996. Hole-Cleaning Capabilities of Water - and Oil – Based

Drilling Fluids: A Comparative Experimental Study. SPE Drilling & Completion 11 (4):

201—207. SPE-26328-PA

[12] Hussaini, S. and Azar, J. 1983. Experimental Study of Drilled Cutting Transport Using

Common Drilling Muds. SPE Journal 23 (1): 11—20. SPE-10674-PA

[13] Iyoho, A.W. and Azar, J. 1981. An Accurate Slot-Flow Model for Non-Newtonian Fluid

Flow through Eccentric Annuli. SPE Journal 21 (5): 565—572. SPE-9447-PA

[14] Kamp, A.M. and Rivero, M. 1999. Layer Modeling for Cuttings Transport in Highly

Inclined Wellbores. Presented at the Latin American and Caribbean Petroleum Engineering

Conference, Caracas, Venezuela, Bolivarian Republic of, 21—23 April. SPE-53942-MS

[15] Larsen, T.I., Azar, J.J., and Pilehvari, A.A. 1997. Development of a New Cuttings-

Transport Model for High-Angle Wellbores Including Horizontal WellS. SPE Drilling &

Completion 12 (2): 129—136. SPE-25872-PA

[16] Li, Y. and Kuru, E. 2003. Numerical Modeling of Cuttings Transport with Foam in

Vertical Wells. Presented at the Canadian International Petroleum Conference, Calgary,

Canada, 10—12 June. PETSOC-2003-066

[17] Martins, A.L. and Costapinto Santana, C. 1992. Evaluation of Cuttings Transport in

Horizontal and Near Horizontal Wells. Presented at the SPE Latin America Petroleum

Engineering Conference, Caracas, Venezuela, and Bolivarian Republic of, 8—11 March.

SPE-23643-MS

[18] Rahman, S.S. and Nguyen, D. 1998. A Three-Layer Hydraulic Program for Effective

Cuttings Transport and Hole Cleaning in Highly Deviated and Horizontal Wells . SPE

Journal. SPE-51320-MS

90

[19] Peden, J.M., Ford, J.T., Oyeneyin, M.B. et al. 1990. Experimental Investigation of

Drilled Cuttings Transport in Inclined Boreholes. Presented at the SPE Annual Technical

Conference and Exhibition, New Orleans, Louisiana, 23—26 September. SPE-20421-MS

[20] Philip, Z., Sharma, M.M., and Chenevert, M.E. 1998. The Role of Taylor Vortices in the

Transport of Drill Cuttings. Presented at the SPE India Oil and Gas Conference and

Exhibition, New Delhi, India, 17—19 February. SPE-39504-MS

[21] Sifferman, T.R. and Becker, T.E. 1992. Hole Cleaning in Full-Scale Inclined

Wellbores. SPE Drilling & Engineering 7 (2): 115—120. SPE-20422-PA

[22] Tomren, P.H., Iyoho, A.W., and Azar, J.J. 1986. Experimental Study of Cuttings

Transport in Directional Wells. SPE Drilling & Engineering 1 (1). SPE-12123-PA

[23] Wei, N., Yingfeng , M., and Gao , L. 2013. Cuttings Transport Models and Experimental

Visualization of Underbalanced Horizontal Drilling. Mathematical Problems in

Engineering 2013. Article ID 764782

[24] Yu, M., Melcher, D., and Takach, N. 2004. A New Approach to Improve Cuttings

Transport in Horizontal and Inclined Wells. Presented at the SPE Annual Technical

Conference and Exhibition, Houston, Texas, 26—29 September. SPE-90529-MS

[25] Zeidler, H. 1972. An Experimental Analysis of the Transport of Drilled Particles . SPE

Journal 12 (1). SPE-3064-PA

[26] Richardson, J.F. and Zaki, W.N., 1954, “Sedimentation and Fluidisation”, Trans. Inst.

Chem. Eng., 32, pp. 35-53

[27] Shields, A. (1936), Anwendung der Ahnlichkeitsmechanik und Turbulenz-forschung auf

die Geschiebebewegung, Mitt. Preuss. Vers. Wasserbau Schiffbau, 26, 5–24

[28] Nakagawa, H. & T. Tsujimoto. 1980. Sand bed instability due to bed load motion.

Journal of the Hydraulics Division, 106(12), pp. 2029-2051

91

[29] Fjar, E., Risnes, R., and Holt, R. 2002. Petroleum Related Rock Mechanics, second

edition. Elsevier Science

[30] Wadell, H. 1934. The Coefficient of Resistance as a Function of Reynolds Number for

Solids of Various Shapes. Journal of the Franklin Institute 217 (4): 459—490

[31] Mack, M., Sun, J., and Khadilar, C., 2014, “Quantifying Proppant Transport in Thin

Fluids-Theory and Experiments”, Paper SPE 39428 presented at the SPE Hydraulic

Fracturing Technology Conference and Exhibition, Woodland, Texas, 4-6 Febuary

[32] Ramadan, A., Skalle, P., and Johansen, S.T. 2003. A mechanistic model to determine the

critical low velocity required to initiate the movement of spherical bed particles in inclined

channels. Chemical Engineering Science 58 (10): 2153—2163

[33] Chih-Young Chang, O. 2016. Physics of proppant transport. In INTEGRATED 3-

DIMENSIONAL MODELING OF PROPPANT TRANSPORT THROUGH HYDRAULIC

FRACTURE NETWORK IN SHALE GAS RESERVOIR Chapt. 4. Penn State University

[34] Zhou, Y. and Shaw, S.N. 2003. Fluid Flow in Coiled Tubing: CFD Simulation. Presented

at the Canadian International Petroleum Conference, Calgary, Canada, 10—12 June.

PETSOC-2003-212

[35] FLUENT, FLUENT INCORPORATED. Choosing the Relative or Absolute Velocity

Formulation (25 January 2003 revision)

[36] International, S.P.E. Petrowiki. Hole Cleaning

[37] International, S.P.E PetroWiki. Fluid Friction

[38] Nayland, G. and Stanley-Wood, . 1983. Enlargement and Compaction of Particulate

Solids, first edition. Butterworth-Heinemann.

[39] Sanchez, R.H., Azar, J.J., and Bassal, A.A. 1999. Effect of Drillpipe Rotation on Hole

Cleaning During Directional-Well Drilling. Journal of Canadian Petroleum Technology 4 (2):

101—108. SPE-56406