Mitigating Exploitation Challenges in Unconventional HPHT Development Field in KGOffshore Basin

Debasish Saha*, Somenath Ghosh, Abinash Mohanta, Bibekananda Mahato, Sikha Mondal, Pratayaya BhadraGujarat State Petroleum Corporation Limited

debasish@gspc.in

Keywords

DDW, HPHT, HPLT, Scaling, degassing

Summary

“Deen Dayal West Field” (DDW) is a HPHT tightgas field comprising of multilayered fluvialsandstone reservoirs of Lower Cretaceous Age.Within the period of trial production from two wellscompleted in these multilayered reservoirs, two mainchallenges surfaced viz. (1) Leakage in productiontubing in well-A & (2) sharp fall in production inwell-B. Trouble shooting through PLT posed anotherchallenge as it required High Pressure - HighTemperature PLT (HPLT) tool with H2S / CO2

resistant mono core cable having sufficient breakingstrength to log at a depth of 5300 m1 within safepulling limit. Present paper details the process inmitigating the challenges in PLT recording andidentification of possible causes which may beaddressed during work over planning. This paperdiscussed elaborately about the problems faced inPLT logging, the way out to handle these adverseconditions and bring out remedial measures for thesick wells.

Introduction

DDW field is located in Krishna Godavari (KG)Basin in Bay of Bengal, Eastern coast of India and isoperated by GSPC. The field is presently indevelopment stage and under trial production. Thewells are drilled to a depth of 5300 m in hostileHPHT conditions where the reservoir temperature isaround 390-400oF and pressure is around 11,500 psi.Moreover, wells are producing hazardous H2S andCO2 gas. The wells have encountered multilayeredtight Sandstone reservoirs of Lower Cretaceous agewith permeability ranging between 0.01 - 1 mD andgross reservoir thickness extending to 550-650 m.

1 All depths mentioned in this paper are Measured Depths (MD)

Wells are generally completed in multilayer withcommingled flow.

Test production from two wells i.e. Well-A & -Bstarted around mid 2014 but within short period of 4-5 months, there was a drop in production in both thewells. Also, problem with completion integrity inWell-A was noticed as there was increased pressurein the annulus. All the indirect studies in Well-Apointed to a possible Tubing Leakage.

Methodology

The job required appropriate planning and executionof PLT logging to (1) identify leakage point in Well-A & (2) assessing layer wise production profiles inboth the wells. Subsequent remedial follow up jobswere carried out based on PLT findings.

Major hurdles in carrying out PLT job in DDW fieldare:(i) Non availability of high temperature PLT tool

(HPLT) which can work at 390oF for sufficientlong hours of recording.

(ii) Requirement of H2S & CO2 compliance monocore cable which should have sufficient safebreaking limit to log at 5300 m.

(iii) Service provider (M/s SASL2) also did not haveprevious experience of recording PLT in such ahostile environment in India. Whatever PLTjobs previously done in India in HPHTconditions were carried out in wells of muchshallower depth and therefore, tool was exposedto HT for lesser time.

2 M/s Schlumberger Asia Services Ltd.

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Mitigating Production Challenges in HPHT Development Field in KG Offshore Basin

Overcoming the problems:

After several rounds of technical discussions, SASLarranged a PLT tool encapsulated in a flask enablingit to work around 390oF. Necessary equipments viz.H2S - CO2 compliant cable, 15K pressure controlequipment were in place to handle adverse wellcondition. Micro planning considering all timings ofevery activities viz. lowering & pulling out, flowstabilization etc. was a key for successfulaccomplishment of the job. "PLT on Paper" exercisewas carried out in coordination with SASLEngineers.

First job was attempted in well-A and the jobsequences are as follows; Temperature & Pressure tools were stacked along

with Dummy in the first run to assess bottom holepressure & temperature before running HPLTtool. Adding these tools with dummy is not anormal practice.

During lowering dummy tool, recordingcontinued and well was flowing through annulusto burner. It resulted in getting an indication oftubing leakage point and also to record bottomhole Temperature & Pressure profiles up to 5300m.

Next run was made with HPLT while the wellwas flowing through annulus.

While lowering the tool, tubing leakage points at1372 m & 1385 m were confirmed.

At 4743 m (i.e. 20 m above the top most perforatedinterval) the tool was not going down and whilepulling up a high tension was observed which wasclose to the safe pulling limit of the cable. Hence, thetool was pulled out of the hole and the job wassuspended at this juncture. Meanwhile during pullingout, the tool failed to work as it exceeded thetemperature due to exposure to HT for about 8 hrs.

Lessons Learnt & Preparation for 2nd Well (Well-B):Following lessons were learnt and provided insight /input for the planning of the job in next well:

HPLT tool can work for a time limit of 8 hr ofexposure in HT condition.

Gas entry noticed in cable after every run. This is acommon phenomenon in high pressure gas wellsand it may cause disruption of connection at thecable head. Degassing3 the cable took 8 to 10 hr inbetween runs.

Based on the previous experience, following stepswere taken as a preparation for next job in well-B:

1. SASL brought 2 new cable drums (with aprovision of one spare drum) so that cabledrums may be swapped instead of waiting longtime for degassing. These are H2S compliantcables and being new, breaking strength will behigher.

2. Pressure greasing of the cables were donethoroughly at their base to minimize gas entry.

3. A new set of HPLT tool was brought for takingup job in well-B.

Following PLT operational sequences were carriedout in Well-B:

Well was taken up for HPLT job and Dummy toolgot held up at 4740 m. Therefore, only top 3 or 4layers could be logged. HPLT tool also got held upat 4740 m.

Recorded 4 hr Shut-in Passes (1 Down & 1 Up)and then well was opened to flow for 2 hr for stableflow condition.

Tool malfunctioned after reaching at held up depthand so it was pulled out, checked at surface and re-run again after degassing the cable. This time itwent up to 4736 m.

Recorded 4 hr shut-in down log & up log and shut-in station recording at different depths in casing.

Flowing Passes, 1 up and 1 down pass at cablespeeds of 1800 ft/hr, 2200 ft/hr and 2700 ft/hr wererecorded in the interval 4670 m to 4736 m (Figure3). Because of bad hole condition, higher recordingspeeds could not be maintained. So, instead ofplanned speeds of 3600 & 4500 ft/hr, logs wererecorded at 2200 ft/hr and 2700 ft/hr.

Flowing station readings were recorded at differentdepths.

3Degassing is the process of releasing of entrapped gas in thearmor of the cable. Cable head is cut and the cable is submergedin water to monitor gas bubble count. Bubbles should be less thanpermissible limit which is < 10-15 Bubbles/min.

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Mitigating Production Challenges in HPHT Development Field in KG Offshore Basin

Finally, during logging operation, Petrophysicistsat site & at base and SASL Engineers activelyparticipated online during entire recording periodfor instant decision related to quality dataacquisition.

Observations

Well-A: Tubing leakage was detected at 1372 m & 1385 m

(Figure 1). Contributing zones were identified qualitatively

from temperature log recorded during dummy run. Out of ten perforated zones, gas entries are noticed

from four zones i.e. Zone-1(4761-4764 m), Zone- 3(4776-4785 m), Zone-4 (4917-4929 m) & Zone-7(4977-4980 m)

Major contribution of gas is from topmostperforation interval i.e. Zone-1

Well-B: Obstruction in casing at 4705–4710 m due to scale

deposition as revealed from caliper logs (Figure 2) Logs could cover only top 3 zones out of 21. Flow condition is very unstable as reflected on

fluctuation in pressure (Figure 3). About 83% of gas contribution is from zones

below Zone-3. Also there is some contributionfrom top zone.

Sharp drop in production is attributed to blockageof perforated intervals and possible formationdamage due to scale deposition (Figure 2 & 4).

Figure 1: Well-A: Tubing leakage reflected on spinner,Temperature & Fluid Density logs

Figure 2: Well-A: Dummy data shows gas entry from 4 zones

Figure 3: Well-B: Logs of Shut-in passes (Up & Down) at 1800ft/hr. Obstruction in casing at 4705–4710 m due to scale depositionas revealed from caliper logs

Follow Up Jobs

Well-A: Based on PLT observations, workover jobwas carried out in the well. The completion stringwas retrieved, well was cleared, all the zones were re-perforated and recompleted in single string. Well wasput back to production.

Well-B: Subsequent to PLT job, upper zones were re-perforated through tubing and the well was put backto production. Initially, there was some increase inproduction, however, production declined againwithin 2-3 months.

GR Resistivity NeutronDensity

CableVelocity

SpinnerVelocity PressureCaliper Temperature

Fluid DensityBubbleCount

11th Biennial International Conference & Exposition

Mitigating Production Challenges in HPHT Development Field in KG Offshore Basin

Figure 4: Well-B: Merged HPLT Log recorded with differentspeed shows variation of Pressure indicating unstable flowcondition. Change of Spinner and Temperature responses indicatefluid entry.

Figure 5: Well-B: Scale came out along with CTU

Conclusions

1. DDW is a HPHT field with reservoir deptharound 4500-5300 m. Trouble shooting ofproduction problem through PLT Logging in thisenvironment of high temperature (390-400oF)with hazardous gas production (H2S & CO2) wasa real challenge faced by GSPC. There was also alimitation of tools in terms of exposure time toHT and limitation of cable in terms of gas entryand low breaking strength.

2. These problems were mitigated throughmeticulous planning, arranging proper tools &equipments and experienced manpower.

3. Though PLT job in Well-A was incomplete, yet itdetected tubing leakage points and gave sufficientclue about production profile. Maximumcontribution was from Zone-1, minorcontributions were from Zone-3, 4 & 7 while rest6 zones were not contributing. Subsequently, thewell was worked over and was put back toproduction. Difficulties faced in this job were alearning experience which helped in further microlevel planning for Well-B.

4. PLT detected scale deposition in Well-B againstthe perforated intervals. Obstruction in casing wasobserved below Zone-4.

5. It was also concluded that fall in production inWell-B is due to deposition of scale / choking ofperforation and possible reservoir damage.

6. In well-B, about 83% of gas contribution fromzones below Zone-3. Subsequent to PLT job,upper 4 zones were re-perforated which resultedin some enhancement of production.

7. HPLT in Well-B also helped GSPC ininvestigating the causes for deposition of scales.

8. The detection of scales has also led to confirmthat there is water production but source of watercould not be identified as PLT tool got held upbelow 3rd zone. Suspected water bearing zoneswere carefully avoided during perforation,however, channeling cannot be ruled out.

Forward plan

Work over and clearing the scale in Well-B seemedto be a very time consuming and expensive task inthis hostile environment. To bring the Well-B back toproduction, it has been planned to make a slimholeside track with barefoot completion to continue gasproduction from these reservoirs taking care of notpenetrating lower water layers. One water zone at theupper level may be isolated with inflatable packer.

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Mitigating Production Challenges in HPHT Development Field in KG Offshore Basin

References

Smolen, James J., 1995, Cased Hole and ProductionLog Evaluation; J. PennWell, Chapters 13, 17-18.

Saha, G.C. & Bose, P.N.S., 1995, ProductionLogging Operations and Interpretations, RTIMD,ONGC.

Saha, Debasish et.al., 2015, Brief Report on HPLTJob in Well-DDW-A, unpublished GSPC report.

Mohanta, Abinash et.al., 2015, Brief Report onHPLT Job in Well-DDW-B, unpublished GSPCreport.

Acknowledgments

Authors are very grateful to GSPC Limited for givingrequired permission to present this paper to SPGIndia conference.

Authors are indebted to Mr. Samir Biswal, Director(Exploration), GSPC Ltd., and Mr. Niladri KumarMitra, Chief Operating Officer, GSPC Ltd., forproviding necessary support and encouragement.Authors are thankful to Mr. Sudhanshu Bakshi,Senior VP (Petrophysics), GSPC Ltd., & Mr. G.C.Saha, VP (Petrophysics), GSPC Ltd. for theirconstant support and guidance during HPLT dataacquisition, planning, execution and interpretation.We also express our heartiest thanks to Mr. PrakashKamble, Asst. Manager (Reservoir), GSPC Ltd., forproviding necessary reservoir data support. We arealso thankful to engineers of M/S SASL for usefuldiscussions.

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