Stability Chirag

8/11/2019 Stability Chirag

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Chirag Wellbore Stability

Study - Part 1,

Azerbaijan

S/UTG/105/00

Sophie Louise Dowson

UTG Drilling Sunbury

Upstream Technology Group, Sunbury

August 2000 GQS50101


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UNCLASSIFIED

The information contained in this document is the property of BP Exploration. Due

acknowledgement should be made if it is desired to refer to this information in publications or

discussions with third parties.


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UTG Indexing Sheet

TEAM REPORT NO. JOB NO.

U T G D R G U T G / 1 0 5 / 0 0 8 7 1 0 5 2 0

AUTHOR(S) TELEPHONE LOCATION DATE

Sophie Louise Dowson 01932 764541 200 / G27 August 2000

MAIN TITLE

Chirag Wellbore Stability Study - Part 1, Azerbaijan

CLIENT PRINCIPAL RECIPIENT COMMISSIONED BY

Chirag Drilling Team Tom Scoular Tom Scoular

SECURITY CLASSIFICATION UNCLASSIFIED CONFIDENTIAL SECRET

KEYWORDS

Chirag, Wellbore Stability

ACKNOWLEDGEMENT

RECORD PAGE

FOR EXTERNAL CLIENT LISTING DISTRIBUTION

OVERLEAF

ABSTRACT

The main objective of this report was to review existing well data for the Chirag Field to assess further wellbore stability

work requirements for future development drilling. Particular emphasis was given to well A-15 due to spud in August

2000.

Within well A-15, the most troublesome sections identified with regard to instability were the 12.25 pilot and side-track

hole sections. Mud weight recommendations for minimising instability in each hole section are as follows:

26 hole section : static mud weight of 8.6 ppg to 8.7 ppg

17.5 hole section : static mud weight of 12.1 ppg. Dynamic losses experienced in previous wells highlights the need

to maintain a reasonable ECD margin that does not exceed the fracture gradient. Since previous development wells

have experienced chemical instability it is important that drilling mud inhibition is optimised.

12.25 pilot hole and side-track sections : static mud weight of 12.4 ppg. As with the 17.5 hole, occurrences of

chemical instability in previous development wells highlights the importance of optimising drilling mud inhibition.

8.5 hole section : static mud weight of between 11.2 ppg to 11.4 ppg

The most problematic formations within the 12.25 pilot hole section appear to be the Sabunchi and Balakhany. Previous

occurrences of instability may have potentially been attributed to overpressured zones within the Sabunchi and inaddition to intact failure, slippage along pre-existing weakness planes within base Sabunchi / Balakhany. Minimum mud

weight requirements for the 8.5 hole are those required to minimise instability within the shale interbeds. Since the 8.5

hole will be terminated in the Pereriv instability concerns relating to the NKG need not be considered.

Conclusions relating to existing data and analysis needs, for future development drilling, highlight the need for additional

data acquisition above the Sabunchi. Future wellbore stability work identified includes a larger generic ERD study and a

more specific study for the forthcoming A-16 well.

PREPARED BY:

Sophie L. Dowson

.....................................................

APPROVED BY:

Sophie L. Dowson

AUTHORISED FOR ISSUE BY:

Sophie L. Dowson

..........................................................

ISSUE DATE: August 2000


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DISTRIBUTION LIST

NO. OF COPIES NAME LOCATION

1-2 BDM Library Sunbury (122/105)3 UTG Drilling Team Files Sunbury (200/G27)4 Tim Bailey UTG Sunbury (200/G27)5 Sophie Dowson UTG Sunbury (200/G27)

6 - 7 Tom Scoular Chirag Asset8 Nigel Last UTG Sunbury (200/G27)


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CONTENTS

Page No

1. Study Scope 1

2. Principal Conclusions and Recommendations 2

3. Data Review Summary 4

3.1 Review of Chi rag Development Wells 4

3.1.1 26 Hole Section and 20 Casing 4

3.1.2 17.5 Hole Section and 13.375 Casing 5


3.1.4 8.5 Hole Section and 7.0 L iner 9

3.2 Review of Exploration Well GCA-1 10

3.2.1 36 Hole Section and 30 Conductor 10

3.2.2 26 Hole Section and 20 Casing 10



3.2.5 8.5 Hole Section and 7.0 L iner 11

3.3 Previous Report Summary 11

4. A15 Stabil i ty Assessment and Mud Weight Requirements 13

4.1 Planned Tr ajectory and Subsur face Condi tions 13

4.2 Compressive Failure Analysis of I ntact Formation 15

4.3 Fail ur e Along Pre-existing Weakness Planes 20

4.4 Tensile Fail ure and Risk of Losses 25

4.5 Mud Weight Recommendations for A-15 26

5 Data Acquisiti on Requir ements for A15 and Future Well s 30

6. Additional Well bore Stabil ity Work to Support Future ERD Dr il li ng 31

7. References 32


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APPENDIX A: F ield I nformation and Offset Well Review 33

APPENDI X B: Review of Previous Studies for Chir ag 42

B1 Exxon Well bore Stabili ty Report 42B2 BP Review of In-Situ Stresses & Rock Mechanical Properties for Fracturing 47

B3 Notes by Tetsuro Tochi kawa 48

B4 Notes by Dr Nobuo Mori ta 48

B5 Characterisation of Shale Samples fr om Well A-13 49

APPENDIX C: Defini tion of Subsur face Conditions perti nent to A-15 51

C1 I ntact Rock Formation Properties 51

C2 Pre-existing Weakness Planes / Faul ts 56

C3 Prognosed Pore and Fracture Pressures 57

C4 Stress Regime and Direction 59

C5 Principal Stress Magnitudes 60

C5.1 Vertical Stress 60

C5.2 Maximum and M in imum Hori zontal Stresses 63


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S/UTG/105/00 Study Scope

August 2000 Page 1

1. Study Scope

This report has been compiled as the final deliverable for work requested by Tom Scoular of the ChiragAsset, Azerbaijan Business Unit, in March 2000. The main aim of the study was to review existing well

data for the Chirag Field to assess further wellbore stability work requirements for future development

drilling. Particular emphasis was given to well A-15 due to spud in August 2000. Based on the initial

workscope, study objectives were as follows:

Review available data and reports pertinent to wellbore instability

Based on review findings advise whether further wellbore stability analyses required for the Chirag

ERD well programme

Identify additional data required for future analyses and better definition of subsurface conditions

Advise what data, if any, should be obtained from well A-15, due to spud in August 2000

Conduct a stability analyses for well A-15 and recommend mud weight requirements to minimisehole instability


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S/UTG/105/00 Principal Conclusions and Recommendations

August 2000 Page 2

2. Pri ncipal Conclusions and Recommendations

Based on findings from this study conclusions and recommendations may be split into those specific tothe forthcoming A-15 well and data / analysis needs for future development drilling.

Recommendations for minimising instability within the A-15 well are as follows:

Within the 26 hole section a static mud weight of 8.6 ppg is recommended to minimise the risk of

instability. If tight spots are encountered density may be raised to 8.7 ppg.

Within the 17.5 hole section a static mud weight of 12.1 ppg is recommended to minimise the risk

of instability. Dynamic losses experienced in previous wells highlights the need to maintain a

reasonable ECD margin that does not exceed the fracture gradient. Since previous development

wells have experienced chemical instability it is important that drilling mud inhibition is optimised.

Within the 12.25 pilot hole and side-track sections a static mud weight of 12.4 ppg is recommended

to minimise the risk of instability. As with the 17.5 hole, occurrences of chemical instability in

previous development wells highlights the importance of optimising drilling mud inhibition.

The most problematic formations within the 12.25 pilot hole section appear to be the Sabunchi and

Balakhany. Previous occurrences of instability may have potentially been attributed to

overpressured zones within the Sabunchi and in addition to intact failure, slippage along pre-existing

weakness planes within base Sabunchi / Balakhany.

Within the 8.5 hole section a static mud weight of between 11.2 to 11.4 ppg is recommended to

minimise the risk of instability. Uncertainty in mud weight results from uncertainty in stress regimewithin the Pereriv which may be reverse as opposed to extensional. In addition to shear failure of the

intact formation, bedding plane slip may be a further risk of instability within the 8.5 hole although

not considered as severe a risk compared with the Balakhany. Although the majority of the Pereriv

is sandstone and potentially able to be drilled with a nominal overbalance, mud weights

recommended are those required to minimise the risk of instability within shale inter-beds.

A further risk of instability within the 8.5 hole section is a buckling mode of failure in the roof of

the borehole which may occur if the well is drilled within 10oof bedding. Instability may also be

compounded where the well crosses the prognosed thrust fault when slippage along pre-existing

weakness planes may be a further risk. Without knowing the dip and dip direction of the fault at the

point where it crosses the well path, however, analyses cannot be conducted to provide a qualitativeassessment of instability risk. Since the 8.5 hole will be terminated in the Pereriv instability

concerns relating to the NKG need not be considered.

Due to the increased risk of instability within the 12.25 and 8.5 hole sections as evidenced by

occurrences of large caving volumes in previously drilled development wells, it is recommended that

cavings morphology be continuously monitored at the rig site. Such procedures will enable the

correct failure mode to be identified thus allowing the most suitable remedial action to be taken.

Comparing minimum mud weight recommendations with fracture gradient values induced fractures

are not considered to be a risk but will be dependant on the ECD margin. The main risk of losses for

A-15 will be those that may be associated with pre-existing fractures and faults. From results of


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S/UTG/105/00 Principal Conclusions and Recommendations

August 2000 Page 3

work conducted by Tetsuro and notes made by Jake Hossack of BP, risk of losses along the A-15

trajectory are considered to be of medium severity.

Conclusions relating to existing data and analysis needs for future development drilling are as follows:

Within previous exploration, appraisal and development wells data is limited above the Sabunchi

for characterisation of overburden properties

For definition of the overburden gradient and rock properties, density, sonic and gamma ray logs

should be extended to mud line.

Current uncertainty exists in the stress regime and magnitudes of the two horizontal stresses.

Within reservoir formations the vertical stress may no longer be the largest component if the stress

regime is reverse as opposed to extensional

To better constrain stress magnitudes more information on the type and degree of hole failure isrequired. Continual monitoring of cavings at the rig site will help to differentiate between failure

modes. Image log data would then further help to define the degree and extent of failed zones. If

image logs cannot be run four to six arm oriented callipers should be used instead.

For definition of the minimum principal stress magnitude leak off tests should be extended in future

wells to give closure pressure values. Ideally a reopening cycle should also be conducted to define

the tensile strength of the formation.

For stress direction, breakout studies conducted to date take no consideration of wellbore

inclination and azimuth. For this reason, stress direction is currently based on observations of hole

failure within the vertical exploration well GCA-1.

ECD values should be closely monitored in future wells to assess actual downhole values

associated with losses and gains. As wells become longer reach and higher angle there may be a

need to differentiate between losses and gains associated with wellbore breathing and zones drilled

underbalance.

For future development drilling additional stability analyses will be required. Given the complexity

of the structure, however, additional data is needed to better define subsurface conditions. A stress

cube approach is suggested to better define the problem.

Future wellbore stability needs identified for the Chirag Asset include a larger generic ERD study

and a more specific study for the forthcoming A-16 well.


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S/UTG/105/00 Data Review Summary

August 2000 Page 4

3. Data Review Summary

In order to assess the need for further wellbore stability analyses in support of future developmentdrilling options, all pertinent data has been reviewed. A main part of this review included the need to

ascertain subsurface conditions from drilling experience gained to date. Studying drilling reports for all

wells on the Chirag Structure, summary charts have been compiled for each formation. Also collated as

part of the study is information that was contained in previous reports and memos addressing

geomechanical issues.

3.1 Review of Chi rag Development Wells

End of well reports for all fourteen development wells, A-1 to A-14, were used to provide summary

information for all formations drilled. Resulting spreadsheets are presented in Appendix A with asummary of pertinent information provided below for each hole section. In addition to well reports

notes were also made from conversations held with Asset staff about drilling experiences to date.

Information believed to be of relevance is included in the hole section summaries also. All depths

referred to in the text are measured and relative to the rotary table elevation.

3.1.1 26 Hole Section and 20 Casing

The 26 top hole sections of all development wells were drilled through Recent Sediments and for all,

but well A-3, TDd in the Apsheron Formation. Within well A-3, the section was extended further and

TDd in the Akchagyl Formation. In wells A-1, A-2, A-3, A-4, A-5 and A-6, a 17.5 pilot hole was

drilled prior to the section being enlarged with a 26 hole opener. In all other wells the pilot hole drilledwas 12.25 in diameter.

Hole inclinations within this section are near vertical. In the Recent Sediments, inclinations varied from

0o to 12o and in the Apsheron inclination ranges were between 1.25o and 13o. Within the Akchagil

Formation, well A-3 inclination was about 3.5o.

Drilling mud used was always seawater with viscous sweeps. Densities ranged from 8.4 to 8.7 ppg,

although the higher weights were typically spud weights that the hole was displaced with after having

drilled the section.

Overall, hole conditions in this section were good whilst drilling. Only a few problems were reported in

terms of losses and very occasional tight spots. For the majority of cases, good returns were obtained

whilst running casing and cementing. In a few instances, however, losses were reported with the

majority of cases being associated with pumping the tail slurry. Lead cement densities were typically

11.9 ppg, although in well A-1 lead density was a lower value of 11.1 ppg. Tail cement densities were

typically 15.8 ppg.

Regards drilling, losses occurred in well A-3 whilst pumping a 9.5 ppg sweep with the assumed lost

zone in the Recent Sediments around 350m +/- 20m. At 364m, the A-3 hole was washed and reamed to

393m and within the Apsheron Formation the hole was tight at 560m and 460m during a wiper trip.

Within well A-13 while opening up the Pilot hole, the BHA was unable to get past an obstruction in the

Recent Sediments at 390m : returns were lost whilst circulating and working the string. Using a new

assembly, well A-13 was washed and reamed from 390m to 478m. Within well A-14, the hole opener


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conversations with Asset staff it is noted that wells drilled across the Chirag Structure appeared to be

more susceptible to losses (e.g. A-4). Mention was also made that in high angle hole sections some

stuck pipe incidents appear to be associated with sliding towards the base of the 17.5 section. Casing

running operations were generally good with only occasional losses reported. During cementing, losses

were reported for certain wells during both pumping and displacement. Within earlier wells (A-1, A-2)lead cement densities were 12.5 ppg but within all other wells this was increased to 13.5 ppg. Tail

cement densities were the same for all wells, 15.8 ppg.

Regards drilling, a 9.6 ppg brine flow was noted in well A-1 between the 20 and 13 3/8 annulus

which took 14 days to stop. It is believed that the flow was emanating from just above the Surakhany

within the Akchagyl. Within well A-2, pit level fluctuations and an increase in Cl2indicted a further

possible brine flow. For well A-3, drilling records note dynamic losses of about 20 bbls an hour. A

further brine flow was also noted in well A-3 within the 20 and 13 3/8 annulus. It is remarked that

over-pressured salt water formations were not isolated with the most likely cause being loss of

hydrostatic head as the slurry set up together with the existence of a channel behind the casing. Within

well A-4, gumbo problems lead to pack off at 922m during a wiper trip. Losses were also noted in theAkchagyl at 737m. Below 1108m, losses were dynamic with gains reported when static. Tight spots

were also noted in other wells such as A-5 at 800m and A-6, both of which noted high percentages of

fine cuttings. Within well A-7, a wiper trip to TD resulted in the hole packing off with losses noted

also. After having pulled out of the hole and ran back in with a new bottom hole assembly it was

necessary to wash and ream from 788m in the Akchagyl. An inadvertent side-track at 1043m was noted

so the well had to be re-drilled. Within well A-8, wiper trips were conducted every 300m to improve

hole conditions resulting from gumbo related problems. Large cuttings volumes were reported after

trips but cuttings integrity was supposedly good. Within wells A-9, A10, A12, A13 and A14, the use of

a much more inhibitive Quadrill mud system dramatically improved hole conditions. Within well A-9,

overpull was much reduced and hole conditions appeared good with no losses. Within well A-10 the

inhibitive mud system is noted as improving stability and providing good cuttings integrity. A-12 and

A-14 were also drilled supposedly trouble free but A-13 was noted as having occasional gumbo and

losses. Within well A-11, the section was supposedly less challenging so a KCl / PHPA system was

used with addition of Staplex 500. Although better than some of the earlier wells drilled without

addition of Staplex, gumbo problems were still noted with flowline blockage and tight hole on wiper

trips.

Regards casing running, the only occasional losses reported were 15bbls within well A-6.

Regards casing cementing, losses were reported in certain wells. Within well A-1, losses were noted

during circulation prior to cementing, during mixing / pumping and displacement. Within well A-3,

losses were recorded during displacement and within A-5 returns began to diminish towards the end of

pumping. Within well A-7, good returns were reported whilst mixing / pumping but none duringdisplacement. Within A-8, partial to total losses were taken during displacement of the tail slurry.

Within well A-14, partial returns were noted after pumping 398 bbls displacement with lost returns

after 488 bbls. A total of 307 bbls were lost during the cement job.


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3.1.3 12.25 Hole Section and 9.625 Casing

The 12.25 hole sections of all wells were drilled through the remainder of the Surakhany, the Sabunchi

and then either TDd in the Balakhany, the Pereriv or the NKG. 12.25 sections of wells A-1, A-2, A-

7, A-9 and A-14 TDd in the Balakhany and wells A-3, A-8, A-11, A-13 which were TDd in thePereriv all had 8.5 hole sections drilled to well TD within the Pereriv. In certain cases, however, wells

were all drilled in 12.25 to final well TD within either the Pereriv, as for wells A-4 and A-12, or the

NKG, as for wells A-5, A-6 and A-10.

Hole inclinations ranged from 0.5oto 70

owithin the Surakhany, from 12.7

oto 71

owithin the Sabunchi,

from 13oto 76

oin the Balakhany and from 27

oto 85

oin the Pereriv / NKG.

For most of the wells a Saraline Synthetic Based Mud was used. Exceptions were well A-1, A-2, A-3,

A-13 and A-14. Well A-1 original hole and side-track were drilled with a KCl Polymer mud. Wells A-2

and A-3 were both reportedly drilled with a Quadrill system. In the case of wells A-13 and A-14, a

combination fluid was used said to consist of Ultidrill LAO, Novatec LAO and Saraline SBM. For A-

1, drilled with the KCl Polymer, mud weights were initially around 11.9 ppg but had to be raised to

12.8 ppg due to reactive clays. For wells A-2 and A-3, mud weights ranged from 12.7 ppg to 12.9 ppg.

For more recent wells drilled with the a Saraline SBM, mud weight were lower and typically ranged

from 12.0 ppg to 12.6 ppg. Within well A-5 an increase to 12.9 ppg is noted at 2400m depth. Wells A-

13 and A-14 which used a combination fluid were drilled with densities of between 12.1 ppg to 12.4

ppg.

The 12.25 hole was typically the most troublesome section to drill. From conversations with Asset

staff the over-pressured Sabunchi zone is typically associated with washouts and tight hole that

requires back-reaming. The Balakhany is also noted as troublesome with several instances of instability

that is not immediate but appears to have a time dependant element resulting in a large volume of

cavings. Within the initial A-1 well, drilling with too low a mud weight and a mud system which wasnot perhaps inhibitive enough lead to several instability problems. Mud weight had to be increased

accordingly. Within subsequent wells hole conditions appeared to improve with the use of more

inhibitive mud systems and increased mud weights. Hole inclinations within the 12.25 section are

typically greater for more recently drilled wells. Within wells A-2 to A-8 hole drilling conditions were

relatively good with only a few overpulls and tight spots recorded. Within well A-7, the hole did require

several wiper trips to clean up prior to logging but this may possibly be attributed to the fact that the

mud pumps were continually breaking. Within subsequent higher angle wells, however, instability

worsened with several incidences of tight holes and pack offs. Drilling reports note an abundance of

large blocky cavings from base Sabunchi and Balakhany in wells A-9, A-11, A-12, A-13 and A-14.

Within well A-10, hole instability was also noted but appeared to be associated with instability within

the NKG rather than any Formations above. Studying hole inclinations for those wells that reportedhole problems and the abundance of large blocky cavings it is interesting to note that these are typically

all high angle within the Balakhany. Typically inclinations range from 65oto 76o, with the exception of

well A-13 which wad only at 57o through the Balakhany. Given the sudden change in instability for

wells above a certain angle compared with those below, and the evidence of large chunky cavings, it

may be the case that instability is attributed to failure along pre-existing weakness planes than classical

breakout of the intact formation. Regards casing running, losses were reported in certain but typically

ran to TD without any major problems. During cementing, losses were reported for certain wells during

both pumping and displacement. Lead cement densities were typically 14.5 ppg and tail cement

densities typically 15.8 ppg.


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August 2000 Page 8

Within well A-1, the section was drilled to the top of the Pereriv but the pipe became stuck on the trip

out. A high percentage of cuttings were reported between 2355m and 1989m. During the wiper trip,

reamed from 1421m in the Surakhany to 2581m in the Balakhany. During the trip out excessive

overpulls were noted and whilst back-reaming to 2750m, the string became stuck. The hole was side-

tracked. The new hole was drilled to 2077m in the Sabunchi and then mud weight needed to beincreased to 12.8 ppg. Whilst running an LCM spotting assembly into the hole no mud returns were

achieved so density was reduced to 12.6 ppg. Wells A-2 and A-3 were drilled relatively trouble free

with reportedly good hole conditions. Well A-3, however, did have evidence of overpull on tripping and

was tight on wiper trips mainly in the sandstones. Well A-4, the first to be drilled with a synthetic based

mud reportedly had no wellbore stability problems. Calliper traces show and essentially in-gauge hole

with an average diameter of ~13. Well A-5, which was reportedly close to a fault had good hole

conditions throughout its section with only minor overpulls on wiper trips. Within well A-6, there was

never a need to stop and circulate the hole clean. Within A-7, the mud pumps broke down frequently

with several wiper trips and back-reaming required to prepare the hole for logging. Within well A-8, the

section drilled fine although tight spots were noted within the Sabunchi during a wiper trip to the shoe.

While attempting to run tools could not get past 2341m the first time and 2394m the second time, bothwithin the Sabunchi. The third attempt was successful.

Within well A-9, occasional tight spots were noted with the presence of cuttings beds forcing tripping

to stop at 2559m, 2900m and 3279m in the Sabunchi and Surakhany. A high percentage of cuttings

and large cavings were reported. Within well A-10, overpull pulling out and drag running in required

the section to be washed and reamed from 3729m to 3787m. Whilst circulating, abundant fines and

cavings from the NKG were recorded. Pulling out of the hole, overpull was noted to be greater than

normal in the Pereriv and Balakhany. Losses were also reported. One metre of fill in the base of the

hole is believed to have been associated with cuttings beds created by the NKG having been severely

washed out. Within well A-11, the section was fine to 3512m. Performing a wiper trip to 3018m

indicated good hole conditions but when building from 3512m to 3612m in the Balakhany, the drill

string became stuck . Many large blocky cavings were noted with the hole packing off. It is noted that

that from +/-2600m the amount increased with the cavings being identified as most probably being

from the Upper Balakhany. Within well A-12, a steady amount of 3 to 4 cm cavings are noted whilst

drilling / sliding through the Balakhany also. Several tight spots were noted in both the Balakhany and

the Pereriv. Whilst back-reaming through the Upper Balakhany and Sabunchi to prevent getting stuck,

a steady increase in cavings was reported, described as blocky chunks. When tripping back into the

bottom, the hole was tight in the Sabunchi and Balakhany V / VI. It was noted that between 2505m and

3680m, hole condition was good with all problems related to the Upper Balakhany section above

2505m. Logs reportedly show bad hole enlargements of up to 20 in the Sabunchi and Upper

Balakhany. Within well A-13, tight spots are again noted in the Balakhany IX, VII and V, being worst

towards the top. Abundant blocky cavings are also noted but no major downhole losses reported.

Within well A-14, after having drilled / slid to 2850m, a moderate amount of cuttings were reportedbetween 2850m and 2450m whilst circulating bottoms up. These occurrences then became heavy above

2450m in the Sabunchi. Ninety percent of the large blocky cavings came from the base of the Sabunchi.

Losses were also reported whilst drilling between 2850m and 4155m at specific depths of 2810m and

3998m in the Balakhany. Returns were lost at 2622m, 2480m, 2452m, 2219m and 2200m all in the

Sabunchi. Further large blocky cavings from the Balakhany and Sabunchi and further losses were

reported as the well was progressed and back-reaming continued.

Regards casing running, within the side-track of well A-1 it had to be washed from 2544m to 2591m

within the Balakhany. Within well A-4 no losses were supposedly reported whilst running but some

were noted when circulating after the liner hanger was landed. Within well A-5, losses were reported

whilst running and circulating with zero returns through the Sabunchi during running. Losses whilst


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August 2000 Page 9

running were also reported in well A-6. Within wells A-7 and A-8, negligible fluid loss was reported

and runs were made with negligible resistance. Casing within well A-9 was run with no losses. Within

well A-10, losses were recorded whilst running casing and at 3747m it became stuck as the hole packed

off. Finally the casing was circulated and washed down. Within well A-12 and A-14, losses were

reported whilst running.

Regards casing cementing, losses were reported in well A-4, A-5, A-6, A-10, A-11, A-12, A-14. Whilst

circulating prior to cementing losses were recorded in well A-8.

3.1.4 8.5 Hole Section and 7.0 L iner

Within wells A-4, A-5, A-6, A-10 and A-12 wells were TDd in 12.25 hole with no 8.5 section.

Other wells which did have an 8.5 hole section may be split into (i) those that were drilled through

both the remainder of the Balakhany and the Pereriv / NKG and (ii) those that were solely drilled within

the Pereriv Formation. Wells A-1, A-2, A-7, A-9 and A-14 fall into the former category and wells A-3,

A-8, A-11, A-13 into the latter. Well A-7 was the only case where the 8.5 hole was TDd in the NKG.

Within the Balakhany hole inclinations ranged from 10o to 78

oand in the Pereriv / NKG ranged from

10oto 88

o.

A range of mud types were used to drill the 8.5 hole sections. Quadrill was the most popular choice,

used for wells A-2, A-3, A-8 and A-13 with mud weights ranging from 10.3 ppg to 11.1 ppg. Saraline

SBM was used for wells A-9 and A-11 with mud weight ranges of 10.2 ppg to 10.9 ppg. Within well

A-7 an Oil Based Mud was used with a much higher density of 12.1 ppg to 12.4 ppg. Ultidrill with a

density of 10.7 ppg to 10.8 ppg was used to drill A-14. Within well A-1, an 11.6 ppg KCl Polymer

mud was used.

In general, within 8.5 hole sections, the majority of hole instability indicators were within the

Balakhany Formation. Little instability was observed within the Pereriv other than when the hole was

extended into the underlying NKG Formation. In a few instances, the 7 liner passed tight spots but

was always worked past. No major losses were noted whilst running or cementing the liner. A 15.8 ppg

cement slurry was used.

Within well A-1, no problems were noted. Within well A-2, the logging tool temporarily stood up at

3100m in the Pereriv. Within well A-3 it is noted that the use of Quadrill mud prevented washout of

shale layers within the Pereriv. Within well A-7, hole conditions were noted as excellent although

wireline became stuck because hole inclination was too great. The 7 liner had to be washed to 4200m

at which point the hole packed off several times and could not be cleaned effectively. It was notpossible to rotate the liner at any time and not possible to pass 4581m. The liner was subsequently set

at 4575m. Within well A-8 no problems were reported drilling or tripping but logging tools reportedly

became stuck requiring extra trips. Within well A-9, signs of packing off were noted back-reaming the

last stand. Within well A-11 no problems were reported but within well A-13 the calliper log reportedly

shows large washouts at 4200m, the depth at which logs could not pass. A washed out zone between

4095m and 4100m was noted also.

The worst instability within this section was experienced within well A-14. After having drilled the 8.5

hole to a TD of 4970m, the hole packed off and the pipe became stuck at 4387m in the Balakhany

whilst back-reaming to the shoe. After having worked the pipe and started circulating, a steady stream

of sand and small cuttings were noted at the surface. Back-reaming to the shoe again was better than

the previous attempt and whilst tripping back to bottom the problem are was reamed. Tripping back out


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August 2000 Page 10

again several tight spots were noted in the Balakhany but when tripping back into the hole conditions

were much improved. Reportedly the calliper showed several washouts followed by sections of gauge

hole. A decision was made to side-track. Within the side-tracked hole section tight spots in the

Balakhany required back-reaming. The top drive was reportedly stalling with the hole packing off. The

pipe was noted as becoming stuck several times and could not be jarred free. Whilst running the 7liner, several tight spots are noted with it becoming tagged up at 5185m.

3.2 Review of Exploration Well GCA-1

In addition to the fourteen development wells the exploration well GCA-1 was also reviewed as part of

this study. Summary details for each formation are included on the appended spreadsheets with

pertinent information for each hole section detailed below. The well was vertical. All depths referred to

in the text are measured and relative to the rotary table elevation.

3.2.1 36 Hole Section and 30 Conductor

A 12.25 pilot hole was drilled with seawater to 505m to cut through shallow faults where gas could

have been encountered. Unstable conditions and tight pulls were experienced, particularly between

360m and 380m. From the seabed to 310m within Recent Sediments the hole was opened up to 36 and

then a 30 conductor was run.

3.2.2 26 Hole Section and 20 Casing

The remainder of the 12.25 pilot hole was opened up to 26 through the remainder of Recent

Sediments and presumably through the Apsheron Formation and into the Akchagyl (no formation topsavailable for near surface sediments). The 20 surface casing had to be worked through a tight interval

from 360m to 380m. Casing was finally landed and the shoe cemented at 498m.


The 17.5 hole section was drilled through the remainder of the Akchagyl and TDd within the

Surakhany. A PHPA / KCl mud was used with an initial density of 9.5 ppg. At 655m, the well was

observed to be flowing so mud weight was raised to 11.3 ppg and the flow killed. Density was then

raised again with drilling resuming with a mud weight of 11.6 ppg. A further influx was finally

controlled with a density of 12.0 ppg then severe swabbing and tight hole led to a further increase in

mud weight of 12.3 ppg at the casing depth of 1215m. When the 13.375 casing was run there wasevidence of channelling.


The 12.25 hole section was drilled through the remainder of the Surakhany, the Sabunchi and TDd in

the Balakhany. A PHPA / KCl mud was used initially with a density of 12.0 ppg but gradually raised

to 12.4 ppg from 1400m to 1500m to overcome swabbing and tight hole conditions. Mud weight was

further raised to penetrating the Balakhany due to increased gas returns in the mud. 9.625 casing was

run and cemented at 2558m.


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August 2000 Page 11

3.2.5 8.5 Hole Section and 7.0 L iner

The 8.5 hole section was drilled through the remainder of the Balakhany, the Pereriv and TDd in the

NKG. The section was drilled with a KCl Polymer mud with a density of 11.0 ppg. From 2662m to

2715m the hole was cored and then drilling continued from 2715m to 2769m. Mud weight wassubsequently raised to 11.4 ppg due to increase in connection gas, the well not being static and the hole

reportedly tight on connections. A further 102m were then cored from 2769m but when pulling at

2788m severe weather required hanging off the string. With the bit at 2683m for 11.5 hours the string

became stuck and took 15 hours to free. Drilled continued from 2871m to 2958m and when close to TD

mud weight was increased to 11.8 ppg due to instability. The 7" Liner was run without problem.

3.3 Previous Report Summary

Reviewing previous reports and memos made available by the Asset, the main report relating to

wellbore stability is that written by Exxon in 19981. The main objective of the study was to assess

wellbore stability for the forthcoming A-2 well and for several future development well trajectories.

Offset wells were limited to exploration wells GCA-1, GCA-2 and the first development well A-1.

Within the report uncertainty was noted in horizontal stresses and according to report findings stresses

below 2000m appeared more consistent with compressive tectonic forces. Analyses conducted were

soley based on intact formation failure with no consideration of slippage on pre-existing weakness

planes. Mud weights quoted by Exxon to minimise instability are based on different breakout criterion

to those used by BP so direct comparison of results is made more difficult. Comparing Exxons

predictions for well A-2 with actual mud weights used and observations of hole instability the following

are noted:

Within the 17.5 section of well A-2 Exxon report that the well should be stable if drilled with about

8% KCl and mud weights in the range 11.5 - 12.0 ppg. Actual mud used was KCl / PHPA but %salt is not known. Density was increased to 12.0 ppg at 640m with no major wellbore stability

problems reported.

Within the 12.25 section of well A-2 Exxon report that the well should be stable if drilled with an 8

- 12% KCl mud with a densities in the range 12.0 to 13.0 ppg. Actual mud used was a 12.8 ppg

Quadrill system. No major wellbore stability problems were reported.

The above comparison indicates that Exxon predictions for well A-2 were reasonable. That said,

however, mud range limits quoted by Exxon were considered large. If using mud weights towards the

lower end of the range stability may not have been ensured. Limitations that Exxon point to in their

study are (i) uncertainty in stress regime so extensional assumed, (ii) assume no anisotropy inhorizontal stresses so no distinction between different well azimuths and (iii) strength profiles derived

from surface area data from GCA-1 which may not be representative of other locations.

In addition to the Exxon Report, other studies were also reviewed. These include (i) a review of in-situ

stresses and rock mechanical properties for stimulation design by Chris Dyke of BP2, (ii) notes by

Tetsuro Tochikawa3,4 and Dr Nobuo Morita5, and (iii) a Schlumberger Dowell shale characterisation

report6. In the case of the first two, information of relevance relates to data on in-situ stresses and rock

properties. Drilling data (i.e. LOTs) and analyses results imply higher than may be expected horizontal

stresses. For rock properties, results of rock strength and deformation parameters for GCA-1 sandstone

core tested in the laboratory have been presented.


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August 2000 Page 12

Within the Dowell Schlumberger report results of characterisation tests on shale cutttings from well A-

13 were presented and discussed in the context of washouts in the 12.25 hole section. Since using

Synthetic Based Mud, certain sections of 12.25 holes are reportedly enlarged. Characterisation tests

involved determination of moisture content, cation exchange capacity, water activity , X-ray diffraction

analysis and cuttings dispersion tests. Throughout the report hole enlargements are described aswashouts as opposed to breakout with the worst areas associated with the very top and very bottom of

the Sabunchi Formation. Calliper data presented from well A-12 is one arm only so it is unclear what

shape hole enlargements really are. This is important as washouts would certainly imply chemical

instability as opposed to mechanical. After reviewing results of all tests, the report discussion notes that

none of the shale physico- chemical characteristics are directly correlated with the cuttings recovery

data. For this reason, the author concludes that instability is thought to be due to mechanical effects

rather than chemical. In order for this to be investigated further, better knowledge is required on the

shape of hole enlargements since as stated earlier, washouts would be indicative of chemical effects.


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S/UTG/105/00 A-15 Stability Assessment and Mud Weight Requirements

August 2000 Page 13

4. A15 Stabi l i ty Assessment and Mud Weight Requi rements

Having reviewed all offset well data and previous reports / memos, a further scope of this study was toprovide mud weight recommendations for minimising instability in the forthcoming A-15 injector well.

4.1 Planned Trajectory and Subsur face Conditions

The well, to be drilled from the Chirag platform, is due to spud at the beginning of August. The

planned trajectory is presented in Figure 1, below, detailing prognosed formation tops and casing

setting depths.

F igure 1:Planned Trajectory for A-15 with Formation Tops and Casing Points


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August 2000 Page 14

The top hole 26 section will be drilled through Recent Sediments and mid-way into the Apsheron

Formation with the 20 casing shoe set at a measured depth of 476m (475m TVDbrt). The following

17.5 hole will then be drilled through the remainder of the Apsheron, the Akchagyl and into the

Surakhany with the 13.375 casing shoe set at a measured depth of 1394m (1275m TVDbrt). A 12.25

geologic pilot hole will then be drilled through the remainder of the Surakhany, the Sabunchi, theBalakhany and into the Pereriv crossing a thrust fault and TDd at a measured depth of 4700m (3315m

TVDbrt). This hole will then be plugged back and side-tracked. A second 12.25 hole will then be

drilled from a measured depth of 3650m (2669m TVDbrt) through the lower part of the Balakhany and

TDd at top Pereriv. After running 9.625 casing, an 8.5 hole section will be drilled and TDd within

the Pereriv reservoir at a measured depth of 4750m (3373m).

Well inclinations and formations within each hole section are presented in Table 1 below:

Hole Section (TVDbrt in metres) Formations and Hole Inclinations (TVDbrt in metres)

26 (Mud Line to 475) Recent (Mud Line to 407): Inclination 0 to 7.5

Apsheron (407 to 475): Inclination 7.5 to 1017.5 (475 to 1275) Apsheron (475 to 552): Inclination 10 to 12

Ackhagyl (552 to 732): Inclination 12 to 21

Surakhany (732 to 1275): Inclination 21 to 46

12.25 Pilot Hole (1275 to 3316) Surakhany (1275 to 1642): Inclination 46 to 52

Sabunchi (1642 to 1922): Inclination 52

Balakhany (1922 to 2884): Inclination 52

Pereriv (2884 to 3316): Inclination 52

12.25 Side-track Hole (2669 to 2884)

Note: above 2669m, the 12.25 hole will

remain open

Balakhany (2669 to 2884): Inclination 52 to 50

Note: above 2669m, formations open as detailed for pilot

hole

8.5 Side-track Hole (2884 to 3373) Pereriv (2884 to 3373): Inclination 50

Table 1 :Formations and Hole Inclinations for Each Section

In order to provide mud weight recommendations for minimising hole instability, subsurface conditions

along the A-15 well trajectory require definition. This is discussed within Appendix C where

information on rock properties and in-situ stresses considered for A-15 design are detailed. Within

formations above the Balakhany, an extensional regime is assumed with the vertical stress being greater

than the two horizontal components. Within the Balakhany and Pereriv Formations, however,

uncertainty exists in both the stress regime and actual horizontal magnitudes with the possibility of the

vertical component being the least principal stress. For this reason, the following range of stress

scenarios are considered for design below base Sabunchi:

Stress Scenario 1 : Extensional with horizontal stresses from prognosed fracture gradient

Vertical stress = overburden gradient as presented in Figure C6

Minimum horizontal stress = fracture gradient as presented in Figure C4

Maximum horizontal stress = value mid way between vertical stress and minimum stress value

This stress scenario is as that to be used in overburden formations above the Balakhany


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August 2000 Page 16

17.5 Hole Design

Analysis 2 : CASE A with breakout width of 69othrough base Apsheron and Akchagyl Formations

Analysis 3 : CASE B with breakout width of 44o

through top Surakhany

12.25 Pilot Hole Design

Analysis 4 : CASE B with breakout width of 38othrough mid / bottom Surakhany and Sabunchi

Analysis 5 : CASE C (Stress Scenario 1) with breakout width of 38othrough Balakhany and Pereriv



12.25 Side-track Hole Design

Results of Analysis 4 to 7 for formations drilled in the pilot hole that will remain open above side-

tracked section

Analysis 8 : CASE C (Stress Scenario 1) with breakout width of 38othrough base Balakhany



8.5 Side-track Hole Design

Analysis 11 : CASE C (Stress Scenario 1) with breakout width of 40othrough Pereriv



Analyses were conducted using the stability software Stress and Failure in Inclined Boreholes (SFIB).

The Well TRaJectory (WTRJ) module was used to provide minimum mud weight requirements along

the specific A-15 well trajectory. Results of analyses 1, 3, 4 and 11 which dictate minimum mud weight

requirements for each section are presented in Figures 2 to 5. The top and bottom diagrams on the far

left of the output file define profiles for in-situ stresses and rock properties, respectively. Within thecentre, profile and plan views of the well trajectory are shown colour coded with minimum mud weights

required to minimise the risk of compressive failure. Within the far right of the output files, the five

columns of data show depth (MD and TVD brt), breakout to be expected using a specific mud weight,

pore pressure, minimum mud weight required to limit breakouts to within tolerable limit set and the

minimum principal stress made equal to the prognosed fracture gradient for A-15. For the condition

where degree of breakout is shown for a specific mud weight, a default value of 1.49 SG (12.4 ppg) is

set for all analyses.

Results presented only provide summary details and not a complete listing of mud weight requirements

for all data points considered. For this reason, the maximum mud weight in the section may be greater

than that shown in Figures 2 to 5.


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August 2000 Page 17

F igure 2:WTRJ Analysis 1



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August 2000 Page 18



Studying complete listings of results from each analyses, minimum mud weight requirements computed

for each hole section were noted and are follows:


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August 2000 Page 19

26 Hole : 1.05 SG (8.7 ppg) at a depth of 407m TVDbrt at the top of the Apsheron Formation

17.5 Hole : 1.45 SG (12.1 ppg) at a depth of 1237m TVDbrt in the Surakhany Formation

12.25 Pilot Hole and Side-track : 1.47 SG (12.3 ppg) at a depth of 1647m TVDbrt in the SabunchiFormation

8.5 Side-track Hole: 1.33 SG (11.1 ppg) for Stress Scenario 1 to 1.35 SG (11.3 ppg) for Stress

Scenario 3 at a depth of between 2897m to 2952m TVDbrt in the Pereriv Formation

Mud weights calculated are based on the Modified Lade Failure Criterion. This was used as it is

considered a more realistic criterion in softer sediments compared with other more conservative criteria

such as Mohr Coulomb. Comparing Modified Lade results with those that would be obtained using

Mohr Coulomb, discrepancies between the two criterion appear to be worst within the 26and side-

track sections of the well. Table 2 below, compares highest mud weight values computed using

Modified Lade with equivalent densities assuming Mohr Coulomb.

Hole Section Depth

(metres TVDbrt)

Maximum Mud Weight for Section (SG / ppg)

Modified Lade Mohr Coulomb

26 407 1.05 / 8.75 1.12 / 9.33

17.5 1237 1.45 / 12.08 1.47 / 12.25

12.25 Pilot and Side-

track

1647 1.47 / 12.25 1.53 / 12.75

8.5 Side-track -

Stress 1

2897 1.33 / 11.08 1.46 / 12.17

8.5 Side-track -

Stress 3

2952 1.35 / 11.25 1.49 / 12.4

Table 2: Comparison of Modified Lade with Mohr Coulomb

Analyses conducted are linear elastic with no accounting for poro-elastic effects over time where

overbalance may reduce as mud pressure and formation fluid pressure equalise at the wellbore wall

when in communication. The analyses also take no consideration of chemical effects which may worsen

stability if mud chemistry is incompatible with the shale formations being drilled. For this reason, mud

weights computed are based on the use of an oil based mud or similar.

When drilling sandstone sequences, a nominal overbalance is typically required to prevent instability.

The belief is that when drilled the sand grains dilate thereby redistributing induced stresses creating a

plastic zone around the wellbore. Since, however, the stability code SFIB assumes brittle failure,stability in sandstone sequences is difficult to model with mud weights calculated often an over-

prediction of what is actually required. For this reason, mud weights quoted are as those required to

minimise instability of shale sequences. Since all hole sections pass through shale formations mud

weights computed are considered applicable for all sections.


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August 2000 Page 20

4.3 Fail ur e Along Pre-existing Weakness Planes

For analyses 1 to 13, mud weights are based on failure of the intact rock matrix. Within certain

formations, however, instability may be worsened by failure along pre-existing weakness planes. Such

planes may be either bedding features, fractures or faults. Having been provided ranges in bedding dip /dip direction for the Balakhany and Pereriv, additional analyses were conducted to assess what impact

consideration of anisotropy would make on mud weights required to minimise instability risk. The two

main concerns being (i) unfavourable attack angle between wellbore and bedding features, and (ii)

intersection of the well with a thrust fault within the Pereriv.

When considering instability associated with pre-existing weakness planes, failure may be either due to

(i) slippage or (ii) buckling. The former will occur if the well is drilled at an unfavourable azimuth with

respect to both stress orientation and weakness plane dip / dip direction. The second mode of failure

will pose a risk if the well is drilled almost parallel to the feature such that buckling occurs in the roof

of the borehole. This type of failure is typically associated with highly deviated wells drilled through

near horizontal bedding and referred to as bedding parallel failure. An example of bedding parallelfailure observed within a shale sample tested in a laboratory is presented in Figure 6 below.

F igure 6: Bedding parallel failure of a wellbore demonstrated in a laboratory

experiment with a Jurassic shale sample. Montage of SEM photographs7

For the A-15 trajectory, dip and dip direction for bedding in the Balakhany and Pereriv, provided by the

Asset, are as below. Regards the thrust fault to be crossed within the Pereriv, uncertainty currently

exists in its dip and dip direction at the point of intersection.


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August 2000 Page 21

Top Balakhany - dip around horizontal

Top Balakhany IX -dip angle ~ 5oto 045 (approximate to +/- 8o)

Top Balakhany X - dip angle ~ 15oto 045 (approximate to +/- 8o)

Top Pereriv - dip angle ~ 24oto 045 ( +/- 3

o)

Top Pereriv - dip angle ~ 33oto 045 ( +/- 3o)

To assess the likelihood for bedding plane slippage within the shale inter-beds of the Balakhany and

Pereriv, anisotropic analyses were conducted for each of the cases noted above. Analyses of this kind,

however, may only be conducted using the Mohr Coulomb criterion. If A-15 mud weights are to based

on Modified Lade criterion results, anisotropic analyses only serves to provide a qualitative assessment

of bedding plane slippage.

Using the Get FaiLuRe (GFLR) module of SFIB, mud weights required to minimise instability for

tolerable breakout widths were computed for (i) intact failure only and (ii) intact failure incorporating

risk of slippage. If slippage is a risk, the failure zone for a given mud weight will be greater when

considering slippage than intact failure alone. This being the case, mud weights will need to be higherthan that required to prevent failure of the intact formation if the tolerable breakout width is to be

maintained. Using next the Borehole Stress and Failure Orientation (BSFO) module, breakout widths

were computed for mud weights required to minimise intact failure alone and weights required to

minimise both intact failure and slippage. The lower mud weights correspond to those required to

maintain tolerable breakout width based on intact failure only. The higher mud weights are those

required to maintain tolerable breakout width based on both intact rock failure and slippage. Breakout

widths due solely to intact failure are denoted wBO and those corresponding to failure from both intact

shear and slippage are denoted wBO.

Within the Balakhany, the pilot hole section will be drilled at an inclination of 52oand within the side-

track inclination will be slightly lower at 50o. Tolerable breakout within the Balakhany is set at 38

o

based on the highest inclination of 52o. Within the Pereriv a tolerable breakout of 40ois set based on an

inclination of 50oplanned for the 8.5 hole section. Analyses conducted within the Balakhany are based

on the pilot hole trajectory and for the Pereriv are based on the side-track trajectory.


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August 2000 Page 22

Formation

(Depth in m

TVDbrt)

Bedding

Dip / Dip

Dirn.

Min. Mud

Weights with

No AnisotropyMC (ML)

SG / ppg

Min. Mud

Weights with

AnisotropyMC

SG / ppg

Breakout for

No Anisotropy

Mud WeightwBO wBO

Breakout for

Anisotropy

Mud WeightwBO wBO

Top

Balakhany

(1922)

1o/ 045

o 1.491 (1.385) /

12.43 (11.54)

1.493 / 12.44 38 46 32 38

Top

Balakhany IX

(2452)

5o/ 045

o 1.403 (1.270) /

11.69 (10.58)

1.411 / 11.76 38 68 15 38

Top

Balakhany X

(2702)

15o/ 045

o 1.428 (1.295) /

11.90 (10.79)

1.440 / 12.00 38 84 0 38

Top Pereriv

(2884)

24o/ 045o 1.325 (1.175) /

11.04 (9.79)

1.334 / 11.12 40 73 12 40

Base Pereriv

(3290)

33o/ 045

o 1.428 (1.290) /

11.90 (10.75)

1.436 / 11.97 40 56 25 40

Note

MC: Mohr Coulomb Criterion, ML : Modified Lade Criterion

No Anisotropy: Density reqd. for intact formation failure only (tolerable wBO of 38oor 40

o)

With Anisotropy: Density reqd. for intact formation failure & slippage (tolerable wBO of 38oor 40

o)

wBO : breakout width for intact failure alone, wBO : breakout width for intact failure & slippage

Table 3:Results of Anisotropic Analyses to Assess Risk of Bedding Plane Slippage

Within Figures 7 and 8 below, BSFO output files for analyses at Top Balakhany X are presented.

Figure 7 shows the result of the analysis which does not consider slippage effects. Here a mud weight

of 1.43 SG (10.79 ppg) gives a tolerable breakout of 38o. Figure 8 shows that when slippage is

considered the same mud weight will result in a much larger failed borehole circumference.


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August 2000 Page 23

F igure 7:BSFO Output File for Top Balakhany X - Failure without Anisotropy Considered

F igure 8:BSFO Output File for Top Balakhany X - Failure with Anisotropy Considered

To conduct analyses, an assumption had to be made about the strength of the weakness planes. This is

difficult as no data exists to define such parameters. For results presented above, it was assumed that

an equivalent unconfined compressive strength incorporating the weakness planes would be 1/3rdthat of

the intact unconfined compressive strength value. Taking the friction angles along the bedding planes to

be equal to intact friction values, cohesion for each bedding plane was computed from the equation

below:

Cohesion (MPa) = (UCS*(1-sin)) / (2*cos)


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August 2000 Page 25

Considering the risk of bedding parallel failure due to buckling, the only way to minimise risk is to

avoid drilling certain trajectories. In the case of A-15 attack angles between bedding and wellbore

provided by the asset and based on apparent dips are as follows:

37 degrees at top Balakhany32 degrees at top Balakhany IX

22 degrees at top Balakhany X

13 degrees at top Reservoir

4 degrees at base Reservoir

Studies by Okland and Cook7show that wells drilled within 10 degrees of the bedding parallel direction

are at risk of this type of instability. Considering attack angles noted for A-15, bedding parallel failure

is considered a potential threat in the reservoir section. To reduce the risk it is recommended that the

well trajectory is altered to make attack angle between bedding and wellbore more favourable.

4.4 Tensile Fail ur e and Risk of Losses

In addition to minimising the risk of tight hole and stuck pipe by ensuring a high enough mud weight to

prevent compressive failure of the formation, the risk of losses should also be assessed. Two potential

causes of losses are drilling induced fractures and pre-existing fractures and faults. The former will

result from tensile failure of the formation due to drilling with too high a mud weight. In the case of the

latter, partial losses may result if natural fractures / faults are intersected and total losses occur if

fractures / faults are propagated away from the wellbore wall. To be conservative, the fracture gradient

for well A-15 is based on propagation values equivalent to the minimum principal stress. Assuming that

fracture initiation is greater than fracture propagation, fractures will be neither initiated or propagated

if mud weight is maintained below the value of the minimum principal stress. Comparing the highest

static mud weight required to minimise stability in each hole section with the lowest value of minimumstress, typically at the shoe but not always, losses due to induced fractures are not considered to be a

risk but will be dependant on the ECD margin. If ECDs do not exceed the minimum principal stress

new fractures will not be initiated and pre-existing fractures will not be propagated away from the

wellbore. The only exception to this would be in cases where the initiation gradient is less than the

propagation value as is sometimes the case in high angle wells within an extensional regime or for

HPHT wells. For stress scenario 2 within the Balakhany and Pereriv it should be noted that the

minimum stress, still taken to be horizontal, is higher that used in Scenario 1. If such conditions do

exist the corresponding fracture gradient line should be increased by the same amount. In the case of

stress scenario 3, the vertical stress is made equal to the minimum principal value. Taking the fracture

gradient to be equal to the propagation value the upper limit to the drilling window should be made

equal to the vertical principal stress.

Considering that all static mud weights, incorporating ECD margin, should be below the upper bound

to the drilling window the only risk of losses should be along pre-existing faults and fractures. Whether

or not these are partial or total losses will be dependant on (i) the degree of connectivity of the feature

away from the wellbore wall and (ii) the nature of the fracture / fault directly dependant on its stress

history - sealed with the major principal stress perpendicular or open with the minor principal stress

perpendicular. Whilst reviewing previous reports / memos, results of a study conducted by Tetsuro4

were noted. Within this note, occurrence of losses within previous development wells were related to

well azimuth relative to the Chirag structure. All losses reported occurred whilst running the 9 5/8

casing and in terms of depth relate to Top Sabunchi. The database was compiled from development

wells A4 to A12. Corresponding ECD values quoted were derived using the Wellplan program. Results


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August 2000 Page 26

of the losses study implied that their occurrence was sensitive to well azimuth (e.g. A9 experienced no

losses with 13.9 ppg ECD, while A4 experienced major losses with a 13.0 ppg ECD). Tetsuros

explanation for observations noted is related to the azimuth of the well with respect to the maximum

horizontal stress. Believing the maximum direction of compression to be perpendicular to the Chirag

Structure Anticline Trend of NW-.SE, a maximum horizontal stress direction of 60ois assumed. Wellssuch as A4 and A5, drilled parallel to the maximum horizontal stress, are likely to be more at risk of

instability than those drilled perpendicular to this direction. Tetsuro derived the following directionality

- loss relationship:

Most stable wells : up-dip and / cross dip wells with min horizontal stress dirn (330 o)

Medium severity wells : down-dip wells with min horizontal stress direction (150o)

Most unstable : wells with max. horiz. stress (60 / 240)

Dip direction refers to the structural dip of the structure with up dip defined as 330o and down-dip

referred to as 150o. This is not the same as smaller scale formation dips which will also have an effect

on overall stability.

Relating results of Tetsuros work with the A-15 azimuth of between approximately 10oto 40o, the risk

of losses into natural fractures / faults is considered to be medium severity. The well direction falls

between the worst and most favourable drilling direction in with respect to the in-situ stress orientation.

Comparing well azimuth with structural dip, well azimuth lies mid way between up dip and cross dip

directions.

In addition to work by Tetsuro, the risk of losses within A-15 has been discussed in a note by Jake

Hossack8. Jake states that with the maximum regional stress direction running perpendicular to the

strike of the main thrust fault (i.e. ~ 60o), faults that strike perpendicular to the strike should be

considered as high risk in terms of mud losses compared with thrust parallel faults.

4.5 Mud Weight Recommendations for A-15

Based on results presented in preceding sections and findings of the offset well review, the following

static mud weights are recommended for minimising instability in well A-15:

26 Hole : 8.6 ppg with contingency to weight up to 8.7 ppg if tight spots encountered

17.5 Hole: 12.1 ppg

12.25 Pilot Hole and Side-track : 12.4 ppg

8.5 Side-track Hole: 11.2 to 11.4 ppg

Within the 26 hole, offset data indicate that previous development wells were drilled with mud weights

of between 8.4 ppg to 8.7 ppg. Results of stability analyses imply a minimum mud weight requirement

of 8.7 ppg driven by subsurface conditions at the top of the Apsheron Formation. Since analyses may

be slightly conservative within the very top hole soft sediments and considering previous drilling

experience, a minimum mud weight of 8.6 ppg is thought the optimum mud weight for drilling this

section.

Within the 17.5 hole, offset data indicate that previous development wells were typically drilled with

mud weights of between 11.9 ppg to 12.1 ppg with some tight spots noted but also occasional dynamic


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August 2000 Page 27

losses whilst circulating and during wiper trips. Results of stability analyses imply a minimum mud

weight requirement of 12.1 ppg driven by subsurface conditions at 1237m TVDbrt in the Surakhany

Formation. This weight is considered optimum for drilling this section. Since previous development

wells report occurrences of dynamic losses within the 17.5 hole section careful attention must be given

to minimising ECDs. As early development wells experienced several problems related to chemicalinstability it is important that mud type chosen provides optimum inhibition to minimise the occurrence

of washouts.

Within the 12.25 hole, offset data indicate that early development wells were typically drilled with

mud weights as high as 12.9 ppg using a Quadrill mud system. Within later wells drilled with a

synthetic based mud, densities were lower and ranged from 12.0 ppg to 12.6 ppg. The last two wells,

A-13 and A-14, drilled with a combination fluid had density ranges of between 12.1 ppg and 12.4 ppg.

Within offset wells A-2 to A-8 hole conditions were reportedly good with only a few overpulls and tight

spots recorded. Within more recent wells drilled at higher inclinations instability appeared to worsen

with an abundance of blocky cavings noted from the Sabunchi and Balakhany Formations. The report

by Dowell Schlumberger notes hole enlargements within the Sabunchi Formation for wells drilled withsynthetic muds concluding that instability is most likely to be the result of mechanical failure as

opposed to chemical effects.

Results of stability analyses imply that both 12.25 pilot hole and side-track sections of A-15 should be

drilled with a 12.3 ppg mud weight to minimise instability. This value is driven by the over-pressured

Sabunchi Formation based on intact formation failure and assuming a completely inhibitive mud

system. Considering hole problems in more recent wells resulting in large caving volumes, failure along

pre-existing weakness planes may be an additional mode of failure to explain such instability in wells

drilled at high angle through base Sabunchi / Balakhany. Considering ranges in bedding dip within the

Balakhany, analyses incorporating anisotropic effects imply that failure along weakness planes could

be a potential risk for the A-15 trajectory. To minimise the risk of bedding plane slip mud weights may

need to be increased by a further 0.012 SG (0.1 ppg) within zones most at risk. Given that cavings

within previous wells are reportedly from both the Sabunchi and the Balakhany it may be prudent to

assume that this type of formation failure is feasible within both formations. Considering that the

Sabunchi is described as a calcareous claystone with occasional inter-beds of sandstone and the

Balakhany is described as blocky, failure along pre-existing weakness planes does not seem an

unreasonable explanation. Also, within the Sabunchi, thin sand layers may be even more over-pressured

than assumed. For this reason, increasing mud weight by an additional 0.1 ppg is seen to further reduce

the risk of instability within these zones. For the 12.25 hole sections of A-15 a minimum mud weight

of 12.4 ppg is thought the optimum mud weight for drilling this section. Since earlier development wells

experienced occasional problems related to chemical instability it is important that mud type chosen

provides optimum inhibition within this section.

Within the 8.5 hole, offset data indicate that previous development wells were drilled with quite a wide

range of mud weights of between 10.2 ppg to 12.4 ppg. From stability analyses conducted results imply

that a mud weight between 11.1 ppg and 11.3 ppg should be used to minimise instability resulting from

intact formation failure. The range of values quoted result from the variation in stress magnitudes

considered. Given that the Pereriv may be susceptible to bedding plane slippage, mud weights should be

further increased by 0.1 ppg. Optimum mud weight range for drilling this section is 11.2 to 11.4 ppg.

A further risk of instability within this hole section is a buckling mode of failure in the roof of the

borehole when the well is drilled within 10oof bedding. Instability may also be compounded where the

well crosses the prognosed thrust fault. Without knowing the dip and dip direction of the fault at the

point where it crosses the well path, however, analyses cannot be conducted to provide a qualitative

assessment of instability risk. Although the majority of the Pereriv is sandstone and potentially able to


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August 2000 Page 28

be drilled with a nominal overbalance, mud weights recommended are those required to minimise the

risk of instability within shale inter-beds.

Given the risk of slippage along pre-existing weakness planes it is recommended that cavings be

continually monitored at the rig site whilst drilling 12.25 and 8.5 hole sections. This will allow themost likely mode of failure to be identified such that the appropriate remedial action can be taken. If for

example, instability is related to failure along pre-existing weakness planes, raising mud weight once

instability has started will almost inevitably worsen hole conditions as the mud will act to lubricate and

reduce the strength of planes still further. Crack blocking agents in the drilling fluid can retard the

failure by restricting mud invasion. It is important to respond rapidly to sudden changes in cavings rate

whereas a small constant volume of cavings are worth monitoring but may not require immediate

remedial action. Photographs showing examples of typical cavings are presented in Figure 10 and

Table 4 presents key characteristics and suggested remedial actions associated with different caving

types. Differences between cavings delineated along fracture planes and those delineated along bedding

features were presented earlier in section 4.3.

F igure 10:Examples of Typical Cavings (Courtesy of Schlumberger)


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August 2000 Page 29

Caving Type Angular Platy / Tabular Splintery

Failure

Mechanism

Shear failure resulting in

multifaceted fragments

Failure along pre-

existing weakness planes

Tensile failure believed

to result from a poro-

elastic response todrilling too fast through

low permeability shale

Key

Characteristics

Facets are newly

created fracture

surfaces

Facets may be

curviplaner

Facets are non parallel

Failure - two regions

of wellbore separated

by 180 degrees

Majority of caving

surfaces represent pre-

existing planes of

weakness

One or more parallel

surfaces are common

Surfaces tend to be

relatively smooth and

planar Failure initiates on

high side of wellbore

when well is nearly

parallel to a weakness

plane

Elongated platy rock

fragments

Typical lithology - low

permeability shale

Caving surfaces show

plume structure

indicative of tensile

failure

Entire circumferenceof wellbore may be

damaged

Remedial

Action

If mud weight close to

pore pressure: raise

mud weight

If mud weight close to

fracture pressure

maintain mudweight

decrease fluid loss

manage hole

cleaning

Maintain mud weight

Minimise fluid loss

coefficient of drilling

mud

Use crack blocking

additives Avoid back reaming

Manage hole cleaning

Avoid excessive rpm

and drillstring

vibrations

Employ gentle drilling

practices

Raise mud weight

Reduce ROP

Table 4:Key Characteristics and Suggested Remedial Actions associated with Different Caving Type

(Courtesy of Schlumberger)

Comparing minimum mud weight recommendations with fracture gradient values induced fractures are

not considered to be a risk but will be dependant on the ECD margin. The main risk of losses for A-15

will be those that may be associated with pre-existing fractures and faults. From results of work

conducted by Tetsuro and notes made by Jake Hossack of BP, risk of losses along the A-15 trajectory

are considered to be of medium severity.


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S/UTG/105/00 Proposed Work Scope for Main ERD Study

August 2000 Page 31

6. Additional Wellbore Stabil ity Work to Support Future ERD Dr il li ng

Following on from recommendations reported as part of this study, additional wellbore stability workwill be required to support future ERD drilling operations on Chirag. Given current knowledge of

subsurface conditions across the structure, the following tasks are suggested to increase confidence in

future wellbore stability predictions.

Better definition of the pore pressure regime across the structure - pressure cube should be

constructed using predictions from seismic, calibrated with offset well data

Better definition of stress regimes and principal stress magnitudes. Numerical modelling should be

employed to define total stress ranges. Linking this with improved pore pressure predictions an

effective stress cube may be generated

Improve rock strength predictions based on learnings from other Caspian Sea Fields and current

advancements in populating stress cubes with rock property data derived from seismic.

Incorporate faults into stress cube so as to optimise drilling trajectories by avoiding unfavourable

attack angles. Information on losses into faults within previous development wells will help to

differentiate between critical and non critically stressed faults

Better definition of stress direction. Review all available breakout data. Possibility of performing a

multi-well analysis using breakouts within a range of differently oriented well trajectories. Evidence

of compressional failures and / or induced fracturing will also help to better constrain stress

magnitudes.

Optimise drilling practices in parallel with future wellbore stability work. Often hole problems are

not solely the result of mechanical instability. Poor hole cleaning and general operational practices

are often causes relating to pack offs and stuck pipe.

Future wellbore stability needs identified for the Chirag Asset include a larger generic ERD study and a

more specific study for the forthcoming A-16 well.


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S/UTG/105/00 References

August 2000 Page 32

7. References

1. D.E. Nierode; Chirag Wellbore Stability Study, Exxon Production Research Report, January 1998.RA

2. Chris Dyke; Review of In-Situ Stresses and Rock Mechanical Properties for Hydraulic Fracturing /

Frac Packing, Chirag, Azerbaijan, BP Draft Report, October 1997. RA

3. Tetsuro Tochikawa; Sand Control Strategy, Memorandum, April 1999. RA

4. Tetsuro Tochikawa; SBM Losses During 9 5/8 Casing Running and Cementing, File Note. RA

5. Professor Nobuo Morita; Draft Stability Summary Notes - Various, Rock Mechanics and

Production Research Section, Resources and Environmental Engineering, Waseda University. RA

6. Davison and A. Burn; Characterisation of Shale Samples from Well A-13, Chirag Platform,

Azerbaijan, Europe -Cis and Africa Technology Application Centre, July 2000. RA

7. kland and J.M. Cook; Bedding Related Borehole Instability in High Angle Wells, SPE/ ISRM

47285, Volume 1, Eurock Conference Proceedings, 1998. R

8. Jake Hossack of BP, Notes on Faulting within the Overburden and Reservoir. RA

9. Personal Communication: Dowson / Alberty. A

10. Akhmedov; LOT and FIT Study, Azerbaijan Asset Document, 2000. A

11. Horsrund; Estimating Mechanical Properties of Shale from Empirical Correlations, IKU Petroleum

Research. Unsolicited SPE Paper (56017), January 1999. A

12. Marie Scoular; Breakout Study Results. A

Key

RA - Within Main Report and Appendix

R - Within Main Report Only

A - Within Appendix Only


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S/UTG/105/00 APPENDIX A: Field Information and Offset Well Review

August 2000 Page 33

APPENDI X A: F ield I nformation and Offset Well Review

The Gunashli-Chirag-Azeri (GCA) field complex is located approximately 5km southeast of theApsheron Penninsula. The complex lies in water depths ranging from 85m to 300m. Hydrocarbons in

the GCA complex are trapped within an elongated, north-west to south-east trending, south-east

plunging anticlinal feature which extends nearly 50 km in length. The main oil producing intervals in

the field are the Balakhany X and the Pereriv intervals of the Pliocene Productive series. Above the

Balakhany Formation in the Sabunchi Formation, a number of sandstone reservoirs contain gas /

condensate primarily on the crest of the structure. Figure A1 below presents a plan of the GCA

structure with all exploration / appraisal well locations.

.

0 5km

502800

442

0800

512800 522800 532800 542800

44

30800

444

0800

44

50800

502800 512800 522800 532800 542800

442

0800

44

30800

444

0800

44

50800

Developed

Gunashli

PSAboundary

Deepwater

Gunashli

Chirag

Far East

Azeri

GCA-1

GCA-4/4z

GCA-5/5z

GCA-2

Prop.

GCA-6/6z

F igure A1:The GCA Structure Showing Exploration /Appraisal Well Locations

The Chirag Field was discovered in 1984 by the drilling and testing of the exploration / appraisal well

GCA-1. To date fourteen development wells, A-1 to A-14, have since been drilled to access reserves

across the field. Well locations, together with the proposed A-15 trajectory, are presented in Figure A2

below.


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S/UTG/105/00 APPENDIX A: Field Information and Offset Well Review

August 2000 Page 34

F igure A2:Chirag Development and GCA-1 Well Locations

As part of this study, all fourteen development wells and well GCA-1 were reviewed. For each

formation a spreadsheet was compiled to pull together summary information from all wells. Details

recorded include hole size, well inclination, well azimuth, mud type, mud weight, leak off test data and

general notes detailing incidences of stuck pipe and losses associated with drilling and casing

operations. All spreadsheets are presented on the following pages for Recent Sediments, Apsheron,

Akchagyl, Surakhany, Sabunchi, Balakhany and Pereriv / NKG.


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Recent

T

Well Hole Section

Top Depth (MD

brt) Blue :

Casing, Red :

Formation,

Green : KOP

End Depth (MD

brt) Blue : Casing,

Red : Formation,

Green : KOP Incl. Azi.

Mud Type

for Entire

Hole

Section

Mud Weight for

Entire Hole

Section

LOT / FIT for

Formation NOTES FOR ENTIRE HOLE SECTION (Chirag RTE = 36.6m ACSL, Water Depth = 121m)

A1-OH 17.5" Pilot 157.6 416 0 to 1 185 Spud 8.7 Satisfactory drilling26" Opener 20" successfully run & cemented in place with returns (11.5 ppg lead & 15.8 ppg tail)

A1-T1 - - - - - - - -

A2 17.5" Pilot 157.6 409 0 to 5.4

209 to

230 to

90

Sea water

and viscous

pills Sea water

Section drilled to TD of 480m. 20" casing cemented with 11.9 ppg lead & 15.8 ppg tail, full

returns.26" Opener

A3 17.5" Pilot 157.6 415 0 to 3.1

206 to

318

viscous

seawater

and drilled 8.5 to 9.6 ppg

Hole at 605m, total losses while pumping a sweep (9.5 ppg mw). Regained circ. after pulling

BHA into 28" shoe. LCM pills to cure losses. At 364m the bit took weight, washed and reamed

364m to 393m. No returns to TD. Assumed lost zone around 350m +/-20m

Spotted LCM pills tripping from 420m to surface - no returns pumping. Hole opened to 26"

- returns at top of cond. (8.6 - 8.7 ppg). Wiper trip - tight @560m & 460m. Hole displ. to

9.6 ppg - returns lost at ~ 360m. 20" casing to 595m, cmt - no returns

26" Opener

seawater &

viscous

sweeps

A4 17.5" Pilot 157.6 416

0 to 4.8

to 1.92

191 to

232 to

133 to

202

seawater /

PAC 8.5 ppg

Drilled section IN 17.5" TO TD of 482m with good returns reported throughout. Opened up to

26" again with good hole conditions. Prior to pulling out to run casing hole displaced with 8.5

ppg spud mud. WOW and additional trip - good hole conds.

20" casing run to 475.5m with good returns during running and circulating. 11.9 ppg lead

and 15.8 ppg tail - good returns whilst cementing26" Opener

A5 17.5" Pilot 157.6 417

0 to

2.22

174 to

to 187

to 48

seawater /

PAC 8.4 to 8 .7 pp g

Drill to section TD of 485m and open to 26". No losses - drilling / opening. Displace to 8.7 ppg

spud mud prior to final trip out of hole. 20" run to 478m - no losses running / circulating. Cmt

11.9 / 15.8 (lead / tail).

Full to partial returns when pumping cmt slurry, after 473 bbls pumped returns were

completely lost shortly after clear brine 160 ppm observed flowing out of well A3, btwn 28"

& 20". Assumed A3 loss zone had allowed comm. with A5.26" Opener

A6 17.5" Pilot 157.6 417 0 to 5

174 to

103 to

203

seawater /

PAC 8.4 ppg

Well drilled to TD of 479m: good returns reported. Hole opened to 26" - good hole conditions.

Displace to a 8.5 ppg spud mud prior to running casing to TD of 472m: good returns running /

circulating. 11.9 ppg lead & 15.8 ppg tail.

Casing - good returns until 450 bbls lead pumped - lost total returns: started getting trickle

returns of seawater after pumping 900bbls. Returns increased to 5 / 10 % after 1200 bbls

lead pumped. No indication of hydraulic communication btwn. A6 & A326" Opener

A7 12.25" Pilot 157.6 410 0 to 5

188 to

36

seawater

wi th sw eep s 8 .4 pp g

Well drilled to 484m and opened to 26". Hole displaced to 8.7 ppg spud mud. 20" casing run to

274m - worked and washed to 413m, then pulled. Hole cleaned out and deepened to 491m

and again displaced to 8.7 ppg spud mud. 20" ran to 488m - no losses.

11

Stability Chirag

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