ABSTRACTlarger stress contrast across the wellbore resulting from theoverburden (Sv) and difficulties with drilling in the maximumhorizontal stress (SHmax) direction under the prevailing strikeslip stress conditions in the field1. Several data sets, includingopen hole logs, were integrated through geomechanical analy-ses2 to develop a mechanical earth model (MEM) providingmagnitudes of the three principal in situ stresses, the azimuthof SHmax direction, pore pressure and the rock strength proper-ties along the logged open hole section. Horizontal in situstresses can be calculated using a poroelastic horizontal strainmodel3 and further calibrated by the observed wellbore wallfailure; the result is continuous profiles of stress magnitudesalong the well trajectory, Fig. 1.

Apart from pore pressure, the magnitudes of horizontal

Saudi Aramco has been drilling horizontal and multilateralwells to develop gas fields. Due to a production-induced dropin reservoir pressures, along with the tight nature of the reser-voir rock, development activities have focused more on placingnew wells, completed with multistage hydraulic fracturing, to-ward the minimum horizontal stress (SHmin) direction with thegoal to improve lateral reservoir contact, which enables higherproduction at sustained rates, thereby increasing recoverywhile drilling fewer wells.

Horizontal wells drilled in the SHmin direction are mademore challenging by complex geological conditions and com-pressional in situ stress conditions. The data shows that somewells were drilled without major difficulty while other wellsencountered more problems, leading to stuck pipe events. Adetailed study was conducted to identify the nature of theseproblems and ascertain major controlling factors for this variabledrilling experience. The goal was to make future operationssafer and more efficient through recommendations based on adiagnostic analysis of the observed problems in existing wells.

Analysis of data suggests that excessive borehole breakoutsand a faster rate of penetration (ROP) are the key contributingfactors to the observed drilling challenges. In addition, differ-ential sticking has been found to be a potential risk across highporosity and/or depleted zones. As a result, determining theoptimum mud weight for a given well based on a pre-drill geo-mechanics model was recommended to manage the hole stability.In addition, a safe limit for the ROP, set as a function of holeazimuth, was identified to manage efficient hole cleaning andavoid stuck pipe issues due to pack off. The recommendationsmade based on this analysis enabled successful drilling andtimely completion of several horizontal wells across the field.

INTRODUCTION

Saudi Aramco is pursuing the drilling of horizontal and multi-lateral wells along the minimum horizontal stress (SHmin) direc-tion targeting carbonate gas reservoir development. Theobjective is to maximize reservoir contact by creating trans-verse hydraulic fractures to enhance gas production; however,these horizontal wells are more challenging to drill due to the

Optimum Mud Overbalance and ROP Limits for Managing Wellbore Stability in Horizontal Wells in a Carbonate GasReservoirAuthors: Khaqan Khan and Dr. Hamoud A. Al-Anazi

Fig. 1. In situ stresses and pore pressure profile based on the observed holecondition for a well drilled along the SHmin direction.

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stresses are affected by the rock elastic properties. Varyingrock porosity and mineralogy can cause variable stress con-trast between the Sv and the two horizontal stresses in differentlayers. For a well drilled in the SHmin direction, Sv and SHmax

result in higher stress concentrations (compressive) at the topand bottom of the wellbore wall. Across some zones along thewellbore, when this concentrated stress magnitude is higherthan the value of the effective mud support — the mud weightpore pressure — the wellbore wall can fail and develop break-outs of variable severity, as indicated by caliper data plotted inFig. 1. The resulting rock volume generated due to breakouts,along with drill bit cuttings, needs to be circulated out to avoiddownhole drilling problems such as tight hole, overpull, hightorque and drag, pack off and stuck pipe.

The severity — depth and width — of breakouts decreasesas mud overbalance is increased, Fig. 2. An optimum value ofthis mud overbalance in a given field and reservoir is recom-mended based on a geomechanical analysis. Using optimalmud overbalance will stabilize the wellbore wall and minimizethe breakout’s severity for successful drilling of wells; however,the applied mud overbalance cannot be increased beyond cer-tain limits as this can trigger differential sticking problems inporous and/or depleted zones. Given these limits on optimalmud overbalance, it is expected that breakouts of low tomedium severity will still develop, and to avoid the associateddrilling problems, the resulting rock debris needs to be circu-lated out. Therefore, for safer drilling, apart from optimal mudoverbalance, the rate of penetration (ROP) must be consid-ered. The ROP plays an important role to ensure good clean-ing as a high ROP can generate rock debris — cuttings andcavings — at a faster rate, which poses an extra burden onhole cleaning, particularly in horizontal wells where it is morechallenging4. It follows that it is imperative to study both mudoverbalance and ROP together as part of wellbore stabilitymanagement process.

NATURE OF DRILLING PROBLEMS

Figure 3 shows the drilling experience data from several hori-zontal wells where each data point represents a well with anazimuth — radial lines — varying between 0°, or parallel to

the SHmax direction, and 90°, or parallel to the SHmin direction,in the reservoir. The concentric circles represent the well devia-tion — 0° being a vertical well and the outermost circle being ahorizontal well. The color of the data point symbolizes theseverity of the drilling problems encountered, where red indi-cates that the well could not be drilled according to the plandue to severe and repeated drilling problems, while green indi-cates that the well was drilled according to plan without anysignificant drilling problem. Similarly, the pink and light pinkcolors represent moderate and minor drilling problems, respec-tively — i.e., tight hole, reaming, high torque and drag, etc.These wells were also drilled according to plan, though a fewof them were completed before reaching the planned totaldepth due to reported drilling problems. It can be seen that asthe well azimuth falls close to the SHmin direction, the drillingoperations became more challenging. A pre-drill MEM wasbuilt for all these wells to predict the stable mud weight win-dow. The data suggests that apart from mud weight, therecould be other factors that contribute to the observed drillingproblem, thereby requiring integration of information fromdifferent sources to obtain a comprehensive solution. As well-bore instability can result from a combination of geomechanicsand drilling-related factors, a detailed analysis was required toidentify the nature of these problems and determine majorcontrolling factors for this variable drilling experience.

DATA ANALYSIS

To manage wellbore stability, data from offset wells was ana-lyzed and integrated into a MEM, Fig. 4, and recommenda-tions were made for each planned horizontal well5.

Fig. 2. Effect of mud overbalance on calculated breakout severity at a given depthfor known in situ stresses and rock strength properties. The extent of the whiteregion outside the blue dotted circle (bit size) represents breakout severity. Highermud overbalance stabilizes the wellbore wall and reduces breakout width (�b)and depth (db).

Fig. 3. Drilling experience in horizontal wells drilled in different directions.

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two fields. The increase in mud overbalance for a well azimuthabove 45° azimuth from the SHmax direction suggests that thewellbore wall experiences higher differential stress in thoseconditions, requiring higher overbalance to reduce breakoutseverity. Figure 5b shows that the ROP needs to be droppedgradually — according to the single curve for both fields —and then maintained at 10 ft/hr to 12 ft/hr for well azimuthsbetween 75° and 90°.

The well data presented in Fig. 5 was used to further clas-sify the wellbore stability based on the ROP values and themud overbalance, grouping them into four risk categories,Table 1. Wells falling into risk category 1 are those where boththe ROP and the mud overbalance are within the safe limitsdefined per Fig. 5; these wells were drilled without any majordrilling problems — Wells 1, 2, 10, 12 and 13. Risk category 2includes those wells where either the mud overbalance is belowthe stable limit, resulting in breakout development, Wells 6and 7, or the ROP is above the safe limit, causing a higher rateof cuttings generation, Wells 3 and 5. Such wells are ranked asmedium risk and can experience drilling problems such as tighthole, high torque and drag, and occasional stuck pipe issues.Likewise, if both parameters are exceeded — the mud overbal-ance is low and the ROP is above the safe limit — there is ahigher risk of getting stuck and experiencing a loss of tool andBHA as excess cuttings and cavings generated downhole maybe difficult to circulate out effectively. These wells are classified

The mud weight recommendations were incorporated into adrilling program, and a post-drill analysis was performed toexplain the nature of the drilling problems.

The data analysis indicates that the majority of stuck pipeevents are associated with back reaming and pulling out ofhole. These problems may be attributed to the cuttings andcavings settled at the bottom of the hole. During tripping outof hole or back reaming operations, if upward movement ofthe drillstring is faster than the rate at which the rock debriscan be circulated out, the cuttings/cavings will accumulate be-hind the bottom-hole assembly (BHA) and may cause tighthole or stuck pipe at some depth above the current well depth.Therefore, apart from mud overbalance, the ROP of thesewells was also studied to assess if hole cleaning could be a factorresponsible for the observed problems.

The wells were ranked based on the severity of the drillingproblems observed, and the corresponding mud overbalance,Fig. 5a, and average ROP, Fig. 5b, were plotted as a functionof hole azimuth from the SHmax direction. The color of thedata points represents the severity of drilling problems, as ex-plained earlier. The data shown belongs to wells drilled in twoadjacent fields, Field 1 (F1) and Field 2 (F2), targeting tworeservoir zones vertically separated by a nonproducing thicklayer. The alphanumeric data labels in Fig. 5a indicate the wellnumber (number at left) and the field, either F1 or F2.

Figure 5a indicates that wells with an azimuth up to 45°from the SHmax direction can be drilled with the same overbal-ance of 10 pounds per cubic foot (PCF) to 12 PCF (lb/ft3) inboth fields and with a ROP between 35 ft/hr to 38 ft/hr. Whenthe well azimuth from the SHmax direction increases above 45°— the well gets closer to the SHmin direction — the mud over-balance needs to be increased as per the two curves in Fig. 5afor the two fields, solid curve for F1 and dotted line for F2, tomaintain the wellbore stability. The variable mud overbalancerequirements for the two fields suggest that in situ stress condi-tions and other geomechanical factors may be different for the

Fig. 4. Wellbore stability analysis performed for an offset well using calibrated 1DMEM.

Fig. 5. Variation of drilling experience with well azimuth from SHmax directionunder different drilling parameters in horizontal wells: (a) mud overbalance, and(b) average ROP. The round dot and diamond symbols differentiate data from thetwo reservoirs. The comma-separated numeric value in the data label representsthe well number. F1 and F2 identify the two adjacent fields.

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as high risk and fall under risk category 3, Wells 4, 8 and 9. Wells falling into risk category 2 would require adjustment

in one of the two parameters to achieve stable wellbore condi-tion. For risk category 3 wells, a simultaneous increase of mudoverbalance and reduction in ROP to the safe limit are re-quired to maintain wellbore stability. Extremely high risk wellsare those included in risk category 4, where mud overbalanceis significantly above the stable limit for managing breakouts— stable wellbore — even as ROP is within the safe limit.Such wells, Wells 14 and 15, encountered stuck pipe problemsdue to differential sticking across the permeable zones. The so-lution to this problem is to reduce the mud overbalance andbring it close to the stable mud weight overbalance limit, Fig. 5a.

The lessons learned in how to optimize the drilling practiceswere incorporated together with real-time observations andwellbore stability updating2, 6. This helped successfully over-come the challenges and reduce drilling problems along theSHmin direction, Fig. 6. It can be seen that the number of suc-cessful wells increased from 22% in 2012 to 65% in 2014. In about 25% of the wells in 2014 that experienced severedrilling problems, the problems were mainly differential in na-ture — drilling through more depleted and/or high porosityzones. This problem is being further handled by optimizing themud additive and using appropriate bridging material.

CONCLUSIONS

1. Based on pre-drill geomechanics models and post-drillreviews, safe limits of mud overbalance and ROP havebeen identified for drilling horizontal wells in carbonategas reservoirs.

2. Horizontal wells oriented up to 45° from the SHmax

direction can be drilled safely with the same mudoverbalance — 10 PCF to 12 PCF — in both fields. Forwell azimuths above 45°, horizontal wells drilled in Field 1require a lower mud overbalance to achieve main holestability compared to Field 2.

3. Horizontal wells toward the SHmin direction require a mudweight overbalance of 45 PCF to 50 PCF in Field 2, whilea mud overbalance of 15 PCF to 20 PCF is required forField 1, suggesting spatially variable in situ stresses androck strength properties. For these wells, the ROP shouldbe maintained between 10 ft/hr and 20 ft/hr to ensureproper hole cleaning.

4. The ROP in the study wells was found to vary in the rangeof 20 ft/hr to 50 ft/hr. For wells experiencing the samebreakout severity, those drilled using a 5⅞” bit had anextra burden on achieving hole cleaning efficiency becauseof the reduced annular area — 55% to 75% — comparedto that from an 8⅜” bit. Both ROP and proper mudoverbalance are key to manage hole cleaning efficiency.

5. Most of the drilling problems — tight hole and stuck pipe— are reported while pulling out of hole and/or backreaming to make the connection. It appears that debris/cavings are accumulated behind the BHA, causing restriction.Careful tripping operations — a tripping speed adequate toensure hole cleaning as well as appropriate trippingfrequency — can help reduce these problems.

6. The combination of pre-drill geomechanic studies based onoffset well data, real-time geomechanics support of fieldoperations and post-drill analysis of actual drillingexperience helped to successfully overcome the challengesand reduce drilling problems.

Well # MO ROP Stability Indicators

Risk Category

1 OK OK S 1

2 OK OK S 1

3 OK >> PHC 2

4 < > BO 3

5 OK > PHC 2

6 << OK BO 2

7 < OK BO 2

8 < > BO, PHC 3

9 < > BO, PHC 3

10 OK OK S 1

12 OK OK S 1

13 OK OK S 1

14 >> OK DS 4

15 >> OK DS 4

MO: Mud overbalance.OK: Parameter within the safe limits defi ned by the trend lines in Fig. 5.S: Stable.PHC: Poor hole cleaning (high ROP or excessive breakouts, or both).BO: Breakouts (insuffi cient mud overbalance).DS: Differential sticking (too high mud overbalance).

T Table 1. Classification of drilling difficulty at varied mud overbalance and ROP

Fig. 6. The improvement in drilling performance of wells drilled along the SHmindirection.

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7. The number of horizontal wells drilled toward the SHmin

direction has increased to 65% in 2014, compared to 22%in 2012, after successful implementation of the findings ofthis study.

ACKNOWLEDGMENTS

The authors would like to thank the management of SaudiAramco for their support and permission to publish this article.

This article was presented at the SPE/IADC Middle EastDrilling Technology Conference and Exhibition, Abu Dhabi,UAE, January 26-28, 2016.

REFERENCES

1. Al-Qahtani, M.Y. and Rahim, Z.: “A MathematicalAlgorithm for Modeling Geomechanical Rock Properties ofthe Khuff and Pre-Khuff Reservoirs in Ghawar Field,” SPEpaper 68194, presented at the Middle East Oil Show,Manama, Bahrain, March 17-20, 2001.

2. Ahmed, S., Khan, K., Omini, P.I., Abdul Aziz, A.A.,Ahmed, M., Yadav, A.S., et al.: “An Integrated Drilling andGeomechanics Approach Helps to Successfully Drill Wellsalong the Minimum Horizontal Stress Direction in KhuffReservoirs,” SPE paper 171755, presented at the AbuDhabi International Petroleum Exhibition and Conference,Abu Dhabi, UAE, November 10-13, 2014.

3. Thiercelin, M.J. and Plumb, R.A.: “A Core-basedPrediction of Lithologic Stress Contrasts in East TexasFormations,” SPE Formation Evaluation, Vol. 9, No. 4,December 1994, pp. 251-258.

4. Piroozian, A., Issham, I., Yaacob, Z., Babakhani, P. andIsmail, A.S.: “Impact of Drilling Fluid Viscosity, Velocityand Hole Inclination on Cuttings Transport in Horizontaland Highly Deviated Wells,” Journal of PetroleumExploration and Production Technology, Vol. 2, No. 3,September 2012, pp. 149-156.

5. Ahmed, M., Rahim, Z., Al-Anazi, H.A., Al-Kanaan, A.A.and Mohiuddin, M.: “Development of Low PermeabilityReservoir Utilizing Multistage Fracture Completion in theMinimum Stress Direction,” SPE paper 160848, presentedat the SPE Saudi Arabia Section Annual TechnicalSymposium and Exhibition, al-Khobar, Saudi Arabia, April8-11, 2012.

6. Khan, K., Abdul Aziz, A.A., Ahmed, S. and Ahmed, M.:“Managing Wellbore Instability in Horizontal Wellsthrough Integrated Geomechanics Solutions: A Case Studyfrom a Carbonate Reservoir,” SPE paper 172550,presented at the SPE Middle East Oil & Gas Show andConference, Manama, Bahrain, March 8-11, 2015.

BIOGRAPHIES

Khaqan Khan is a GeomechanicsSubject Specialist with Saudi Aramco’sNorth Ghawar Division of the GasReservoir Management Department.He joined the company in December2012 to assist with various aspects offield development activities focused on

well planning, drilling, completions and stimulation. Prior to joining Saudi Aramco, Khaqan worked with

Schlumberger, starting in 2007, as a Lead GeomechanicsEngineer and Regional Geomechanics Manager in theMiddle East. In 2005, he joined GeoMechanicsInternational Inc. (GMI) as a Geomechanics Specialist,based in Dubai, UAE. After graduate school, Khaqan hadworked as a Geomechanics Engineer with the Center forPetroleum and Minerals at King Fahd University ofPetroleum and Minerals (KFUPM), Dhahran, Saudi Arabia.

During a career spanning more than 15 years, hemanaged and technically led several consulting projects inthe Middle East and elsewhere, focusing on thegeomechanics aspects of reservoir management and fielddevelopment. Khaqan has written and coauthored severalarticles on the subject and has been actively involved inteaching and mentoring of junior staff.

In 1998, he received his M.S. degree in GeotechnicalEngineering from KFUPM.

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Dr. Hamoud A. Al-Anazi is theGeneral Supervisor of the NorthGhawar Gas Reservoir ManagementDivision in the Gas ReservoirManagement Department (GRMD).He oversees all work related to thedevelopment and management of huge

gas fields like Ain-Dar, Shedgum (SDGM) and ‘Uthmaniyah(UTMN). Hamoud also heads the technical committee thatis responsible for all new technology assessments andapprovals for GRMD. He joined Saudi Aramco in 1994 asa Research Scientist in the Research & Development Centerand moved to the Exploration and Petroleum EngineeringCenter – Advanced Research Center (EXPEC ARC) in2006. After completing a one-year assignment with theSouthern Area Reservoir Management Department,Hamoud joined the GRMD and was assigned to supervisethe SDGM/UTMN Unit and more recently the Hawiyah(HWYH) Unit. With his team, he successfully implementedthe strategy of deepening key wells that resulted in the newdiscovery of the Unayzah reservoir in UTMN field and theaddition of Jauf gas reserves in HWYH field. Hamoud wasawarded a patent application published by the U.S. Patentand Trademark Office on September 26, 2013.

Hamoud’s areas of interest include studies of formationdamage, stimulation and fracturing, fluid flow in porousmedia and gas condensate reservoirs. He has publishedmore than 60 technical papers at local/internationalconferences and in refereed journals. Hamoud is an activemember of the Society of Petroleum Engineers (SPE), wherehe serves on several committees for SPE technicalconferences. He is also teaching courses at King FahdUniversity of Petroleum and Minerals (KFUPM), Dhahran,Saudi Arabia, as part of the Part-time Teaching Program.

In 1994, Hamoud received his B.S. degree in ChemicalEngineering from KFUPM, and in 1999 and 2003, hereceived his M.S. and Ph.D. degrees, respectively, inPetroleum Engineering, both from the University of Texasat Austin, Austin, TX.

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