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Copyright 2007, Society of Petroleum Engineers

This paper was prepared for presentation at the 15th

SPE Middle East Oil & Gas Show andConference held in Bahrain International Exhibition Centre, Kingdom of Bahrain, 1114 March2007.

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SPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paperfor commercial purposes without the written consent of the Society of Petroleum Engineers isprohibited. Permission to reproduce in print is restricted to an abstract of not more than300 words; illustrations may not be copied. The abstract must contain conspicuousacknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

AbstractUnderbalanced drilling techniques (UBD) are evolving at a

fast pace as oil companies learn how to use the technology tomaximize well production and lower finding costs.

Although there was some air drilling through surface drilling

activities in Iran for the past decade, the full package of UBD

equipment and other UBD techniques were not introduced

until National Iranian Oil Company (NIOC) supplied the

package for a pilot project in southern Iran. The first well

selected to be drilled UBD was GS 333, the reason for

selecting that particular well was because the Gachsaran field

is depleted and sever mud losses is experienced while

conventional drilling, also, the UBD technique is expected to

increase productivity of the well by decreasing formation

damage resulted from drilling fluid losses.

The drill pipe injection technique was proposed to be utilized

in this well because of its suitability for the well conditions

where it is not expected to drill through the gas cap and the

well will be completed in 8 open hole.

This paper discusses the well design, UBD design parameters,

Underbalanced modeling considerations and the lessons

learned from this job which all led to the success of this

project. It is also describe the process used to address HSE

issues of the project in the face of the potential presence of

H2S while drilling. The drilling and production results that

were achieved were significant to the future operationalpotential and viability of this mature field. Of equal

importance was the success of the operation from a health,

safety and environmental (HSE) perspective in an

environment in which the industry has been traditionally

reluctant to use UBD techniques.

Introduction

NIOC GS 333 was the 1

st

well of the NIOC UBD campaign.The well was drilled from 2,630 m (9 5/8 shoe depth) to a

total depth of 2,938 m MD (2567m TVD). The underbalanced

section of this well was drilled in approximately 99 hrs.

The key performance indicators included:

1. To drill the deviated reservoir section from 2,630 m MDto 2,938 m TD eliminating/minimizing lost circulation

such that no drilling time is lost curing losses or

associated with drilling problems.2. To eliminate, as far as realistically possible, formation

damage caused by the loss of conventional drilling fluid

to the formation while drilling overbalanced.

3. To eliminate, as far as realistically possible, impairmentof the reservoir formation by damaging fluid or material.

4. To evaluate and characterize the reservoir productionrates along the well path.

To achieve these key performance indicators,

NIOC/Weatherford UBS was enlisted to provide

underbalanced drilling services along with RIG 78,

Schlumberger (MWD, motor and directional services). GS-

333 was drilled using a nitrified Diesel circulating system

employing the drill string injection UBD technique.

Procedures were put in place for the maintenance of constant

bottomhole pressures and reduction of pressure transients.UBD on this well experienced some typical logistical and start

up problems associated with a steep learning curve, this being

the first such operation in Iran. Despite all the problems

encountered the well GS-333, was:

Drilled to 308 m of total open hole depth.

No loss circulation was encountered while drilling.

Successfully implemented UBD technology.

No QHSE incidents were recorded.

Objective

The primary objectives of this underbalanced drilling project

are to:

Minimize drilling induced formation damage

Eliminate drilling fluid losses

Improve drilling performance

Well planGS 333 is a new drill, underbalanced deviated well. The well

plan is as outlined in Figure 1 [2].

Geological and Reservoir DescriptionThe target reservoir for the GS 333 is Asmari formation, the

formation is fractured carbonated formation. The reservoir

SPE 105536

First Application of Underbalanced Drilling in Fractured Carbonate Formations ofIranian Oilfields Leads to Operational Success and Cost SavingsA. Hooshmandkoochi, M. Zaferanieh & A. Malekzadeh, National Iranian Oil Company

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drive mechanism is Gas Cap. Shale strings are not expected in

this formation.

Expected reservoir pressure and temperature are 2622 psi and

141 F, respectively. Reservoir fluid is oil with API gravity of

25, GOR 564 SCF/STB, H2S concentration of 240 ppm. The

permeability of the reservoir is 0.1 1000 md with a porosity

of 9% [2].

Underbalanced Drilling Design ParametersCasing Design- 9 5/8 casing design was checked for

underbalanced drilling operations and the result shows there is

no need for any casing modifications.

Rig Modification- There are no essential modifications to be

made on the rig to suite UBD operations. The substructure has

to be high enough to allow RCH to be installed on top of the

Hydril.

BHA and Drill String Design- The plan is to use a 5 DP and

5 HWDP on 6 BHA. The BHA consists of 6 Mud

Motor and MWD to drill 8 hole.

Drilling Fluid Selection- The drilling fluid selection is one of

the most critical decisions in planning an underbalanced well.The right fluid(s) selection will not only lead to suitable BHCP

but will also minimize pressure transients and thus

eliminating/minimizing formation impairment. The deviated

underbalanced section of GS 333 is to be drilled with a

Gachsaran field native crude oil and a membrane nitrogen

generation circulating system.

Liquid Phase- The native crude oil was chosen over Diesel

and other drilling fluids because it is the natural reservoir fluid

for this well. This will minimize chances of formation

damage in event of pressure transients and/or from fluid

imbibitions. If the reservoir fluid available on location is too

heavy to achieve UBD conditions with the available nitrogen

rate, the use of diesel will be recommended to initiate UBD

condition with nitrogen. The well will be displaced with the

produced fluid after getting enough oil production.

Gas Phase- Nitrogen was selected as the injection gas because

of its inert nature, economic availability and suitability for this

specific underbalanced drilling project. Nitrogen will be

obtained from the surrounding air and generated onsite, by

NIOCs nitrogen production unit.

Operating Envelope and Optimum Rate- The multiphase

flow behavior in the wellbore during underbalanced drilling isvery complex. The response of the downhole conditions to

changes in various flow parameters must be characterized

prior to the commencement of underbalanced drilling

operations in order to maximize chances of success.

Figure 2 contains a plot of the bottom hole circulating

pressures induced by a variety of nitrogen rates and the

Gachsaran native crude oil injection rates. This plot is

referred to as the operating envelope. Also plotted on the

operating envelope are the various constraints that must be

fulfilled during underbalanced drilling operations. The range

of flow rates that satisfy all of the constraints, defines the

acceptable operating region. A minimum drawdown at the bit

of 200 psi is required to ensure adequate underbalanced

conditions in the well, with a maximum drawdown of 300 psi

to minimize any near wellbore depletion effects.

The target bottom hole circulating pressure at the bit for this

well is 2300 - 2400 psi. The downhole conditions are also

constrained by the ability of the circulating system to

effectively achieve an underbalanced state, provide adequate

hole cleaning, power the BHA and provide well control. The

maximum nitrogen generation capacity of the underbalanced

drilling package, under local conditions, is also factored in.

Typically, the flow rates are also constrained by the maximum

and minimum downhole positive displacement motor

equivalent flow rates [2]. Drilling Fluid: Nitrogen and Native Crude Oil (0.9 SG).

Injection Rates = 600 - 1400 scfm N2 & 240 - 360 gpmCrude Oil.

Optimum Rate = 1400 scfm Nitrogen and 320 gpmCrude Oil

Induced BHCP = 2300 psi (no production); WellheadPressure = 50 psi

Reservoir Pressure = 2622 psi

Min Motor flow = 300 gpm; Max Motor flow = 550 gpm

Min Vertical Velocity = 165 ft/min

H2SHydrogen sulfide (H2S) is a major concern in hydrocarbon

production. Operating problems caused by H2S can include:

severe corrosion and fouling, injection well plugging with iron

sulfide and QHSE problems associated with H2S gases. Trace

amounts of H2S are anticipated. A thorough monitoring

system will be put in place before the underbalanced drilling

phase in preparation for any emergencies. The advice of

Weatherford QHSE department will be fully enlisted. H2S

scavenger is to be supplied by NIOC and to be added to the

Native Crude if needed while drilling reservoir section of this

well underbalanced.

OperationsThe 8.5-inch directional wellbore was drilled with a nitrified

Diesel to a total depth of 2,938 m MD (2,567 m TVD) with

one motor and two bits. The directional bottom hole assembly(BHA) consisted of a 8.5-inch tri-cone bit, a 6-7/8 inch

positive displacement motor (PDM having a 1.5o bent

housing) with a drillstring consisting of 5-inch drill pipe and

heavyweight drill pipe. A MWD tool was incorporated in the

BHA for survey telemetry.

The Top hole of GS-333 directional well was drilled

conventionally and cased with a 9 5/8 casing to 2,630 m MD.

The Shoe track was drilled out to 2630 m with fresh Water,

prior to arrival of UBD equipment and crew. A new BHA with

directional tools were made up and run into the hole.The hole was initially displaced to Diesel and circulated for

two complete circulations. A two-phase flow commenced with

400 gpm of Diesel and 800 scfm of N2. The BHCP before

drilling operations commenced was calculated at 2275 psi. A

total of 307 m of directional wellbore was drilled at 10% to

20% drawdown. Average liquid injection rate was 400 gpm,

and average N2 injection rate was 900 scfm[3].

Production/Losses While DrillingDuring the underbalanced section of this well, no losses were

encountered. Also no oil and gas production were recorded

while drilling the underbalanced drilling section [3].

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Lessons Learned Hydrocarbon based fluids are particularly susceptible tothe increase of density due to natural solids accumulation.

This warranted accurate monitoring of the diesel to make sure

that the re-injected diesel would not exceed 1 % of solids

content. It is important to ensure that minimal drilling fines

are re-circulated back into the wellbore. High finesconcentrations will increase slugging and the likelihood of

BHCP spikes.

It is crucial to minimize the time with the pumps offduring a connection. The rig crew should be on the rig floor

with the tongs ready to break off the Kelly as soon as it is bled

off. Every effort must be made to start the pumps as soon as

possible after a connection has been made. Any time saved

during the pumps-off period will also reduce the amount of

time required to regain circulation prior to drilling ahead and

also reduce pressure transients.

During underbalanced drilling operations, mud weightoften increases as a result of the milling action of the drill bit

on the formation. This was present while drilling on GS-333.

For the 307 m drilled, the density of the diesel changed from

52.5 pcf to 54.5 pcf. Although increasing nitrogen injection

rate compensated this increase in density, it is highly

recommended to reduce the solids in the surface system by

continuous dilution with fresh diesel. Deployment of

centrifuge also may aid to reduce percentage of solid particles

in the active liquid system.

The Data Acquisition System will provide more value tothe UBD Engineering and separation team with the integration

of more data points from the rig and MWD provider. Digital

stroke counters can be added to the rig pumps and taken

directly into the Data Acquisition to provide real-time injectedfluid rates.

Conclusions and Recommendations:1.The NIOC GS-333 well was successfully drilled to the

target total depth.

2.No loss circulations were encountered while drilling UBDsection.

3.During Underbalanced drilling, base liquid weight oftenincreases due to the penetration of new hole, on the

formation. This natural solids accumulation will no doubt

result in increased effective bottomhole circulating

pressures and may make maintenance of underbalanced

conditions difficult. Therefore accurate monitoring ofthe base liquid weight on a continuous basis is essential

for the proper evaluation and monitoring of

underbalanced states.

4.As this was the first directional underbalanced drillingproject, in this Field, a steep learning curve was the case

for everyone involved at various stages of the GS-333

well.

5.Good communication and following the well program isimportant for successful underbalanced drilling

operations.

6.Planned and applied correctly, underbalanced drillingtechnology can address problems of formation damage,

lost circulation and poor penetration rates. The ability to

investigate and characterize the reservoir while drilling is

another important benefit of underbalanced drilling.

7.As gas percentage in the drillstring has a significant effecton conventional MWD survey tool performance.

Alternative survey tools such as EM-MWD or concentric

gas injection needs to be considered for future UBD

wells.

8.The effect of long connection periods could be greatlyreduced by proper operating practices, which include

among others, the use of trained rig crews capable ofmaking connections in a rapid and yet safe fashion.

9.The ability to obtain real time bottom pressure would bean additional benefit to UBD engineering during the

operation. It is highly recommended the use of PWD

measuring tool for the future UBD wells to accurately

measure and analyze bottom hole parameters.

AcknowledgementThe authors wish to thank NIOC for permission to publish this

paper, and to the many whose efforts on the wellsite

contributed to the success of the completion.

References1. Mc Lennan John., et al Underbalanced Drilling Manual

Published by Gas Research Institute Chicago, Illinois, 1997, GRI

Reference No.GRI-97/0236.

2. NIOC, South Drilling Engineering Division, Underbalanced

Drilling Program for well GS 333. December 2004.

3. NIOC, South Drilling Department, Underbalanced End of Well

Report, Well GS 333, Gachsaran Field, 2005.

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Figure 1. Well Profile Diagram

Figure 2. Operational Envelope Native Crude

BHCP vs Oil and Nitrogen Injection Rates

NIOC, GS 333

Gachsaran Field, Iran

1800

2000

2200

2400

2600

2800

3000

400 600 800 1000 1200 1400 1600

N2 Injection rate (scfm)

B

HCP

(psi)

280gpm

60gpm

Max. Motor Flow Rate

80gpm

240gpm

Min. Liquid Velocity

Operating Area

Min. Motor Flow Rate

Static Reservoir Pressure 2,622 psi

100gpm

320gpm360gpm