Optimum Mud Overbalance and ROP Limits for Managing Wellbore Stability in Horizontal Wells in a Carbonate Gas Reservoir Authors: Khaqan Khan and Dr. Hamoud A. Al-Anazi ABSTRACT Saudi Aramco has been drilling horizontal and multilateral wells to develop gas fields. Due to a production-induced drop in reservoir pressures, along with the tight nature of the reservoir rock, development activities have focused more on placing new wells, completed with multistage hydraulic fracturing, toward the minimum horizontal stress (S Hmin ) direction with the goal to improve lateral reservoir contact, which enables higher production at sustained rates, thereby increasing recovery while drilling fewer wells. Horizontal wells drilled in the S Hmin direction are made more challenging by complex geological conditions and compressional in situ stress conditions. The data shows that some wells were drilled without major difficulty while other wells encountered more problems, leading to stuck pipe events. A detailed study was conducted to identify the nature of these problems and ascertain major controlling factors for this variable drilling experience. The goal was to make future operations safer and more efficient through recommendations based on a diagnostic analysis of the observed problems in existing wells. Analysis of data suggests that excessive borehole breakouts and a faster rate of penetration (ROP) are the key contributing factors to the observed drilling challenges. In addition, differential sticking has been found to be a potential risk across high porosity and/or depleted zones. As a result, determining the optimum mud weight for a given well based on a pre-drill geomechanics model was recommended to manage the hole stability. In addition, a safe limit for the ROP, set as a function of hole azimuth, was identified to manage efficient hole cleaning and avoid stuck pipe issues due to pack off. The recommendations made based on this analysis enabled successful drilling and timely completion of several horizontal wells across the field. larger stress contrast across the wellbore resulting from the overburden (S v ) and difficulties with drilling in the maximum horizontal stress (S Hmax ) direction under the prevailing strike slip stress conditions in the field 1. Several data sets, including open hole logs, were integrated through geomechanical analyses 2 to develop a mechanical earth model (MEM) providing magnitudes of the three principal in situ stresses, the azimuth of S Hmax direction, pore pressure and the rock strength properties along the logged open hole section. Horizontal in situ stresses can be calculated using a poroelastic horizontal strain model 3 and further calibrated by the observed wellbore wall failure; the result is continuous profiles of stress magnitudes along the well trajectory, Fig. 1. Apart from pore pressure, the magnitudes of horizontal INTRODUCTION Saudi Aramco is pursuing the drilling of horizontal and multilateral wells along the minimum horizontal stress (S Hmin ) direction targeting carbonate gas reservoir development. The objective is to maximize reservoir contact by creating transverse hydraulic fractures to enhance gas production; however, these horizontal wells are more challenging to drill due to the Fig. 1. In situ stresses and pore pressure profile based on the observed hole condition for a well drilled along the S Hmin direction. SPRING 2016 SAUDI ARAMCO JOURNAL OF TECHNOLOGY
stresses are affected by the rock elastic properties. Varying rock porosity and mineralogy can cause variable stress contrast between the S v and the two horizontal stresses in different layers. For a well drilled in the S Hmin direction, S v and S Hmax result in higher stress concentrations (compressive) at the top and bottom of the wellbore wall. Across some zones along the wellbore, when this concentrated stress magnitude is higher than the value of the effective mud support the mud weight pore pressure the wellbore wall can fail and develop breakouts of variable severity, as indicated by caliper data plotted in Fig. 1. The resulting rock volume generated due to breakouts, along with drill bit cuttings, needs to be circulated out to avoid downhole drilling problems such as tight hole, overpull, high torque and drag, pack off and stuck pipe. The severity depth and width of breakouts decreases as mud overbalance is increased, Fig. 2. An optimum value of this mud overbalance in a given field and reservoir is recommended based on a geomechanical analysis. Using optimal mud overbalance will stabilize the wellbore wall and minimize the breakout s severity for successful drilling of wells; however, the applied mud overbalance cannot be increased beyond certain limits as this can trigger differential sticking problems in porous and/or depleted zones. Given these limits on optimal mud overbalance, it is expected that breakouts of low to medium severity will still develop, and to avoid the associated drilling problems, the resulting rock debris needs to be circulated out. Therefore, for safer drilling, apart from optimal mud overbalance, the rate of penetration (ROP) must be considered. The ROP plays an important role to ensure good cleaning as a high ROP can generate rock debris cuttings and cavings at a faster rate, which poses an extra burden on hole cleaning, particularly in horizontal wells where it is more challenging 4. It follows that it is imperative to study both mud overbalance and ROP together as part of wellbore stability management process. Fig. 2. Effect of mud overbalance on calculated breakout severity at a given depth for known in situ stresses and rock strength properties. The extent of the white region outside the blue dotted circle (bit size) represents breakout severity. Higher mud overbalance stabilizes the wellbore wall and reduces breakout width ( b ) and depth (d b ). NATURE OF DRILLING PROBLEMS Figure 3 shows the drilling experience data from several horizontal wells where each data point represents a well with an azimuth radial lines varying between 0, or parallel to Fig. 3. Drilling experience in horizontal wells drilled in different directions. the S Hmax direction, and 90, or parallel to the S Hmin direction, in the reservoir. The concentric circles represent the well deviation 0 being a vertical well and the outermost circle being a horizontal well. The color of the data point symbolizes the severity of the drilling problems encountered, where red indicates that the well could not be drilled according to the plan due to severe and repeated drilling problems, while green indicates that the well was drilled according to plan without any significant drilling problem. Similarly, the pink and light pink colors represent moderate and minor drilling problems, respectively i.e., tight hole, reaming, high torque and drag, etc. These wells were also drilled according to plan, though a few of them were completed before reaching the planned total depth due to reported drilling problems. It can be seen that as the well azimuth falls close to the S Hmin direction, the drilling operations became more challenging. A pre-drill MEM was built for all these wells to predict the stable mud weight window. The data suggests that apart from mud weight, there could be other factors that contribute to the observed drilling problem, thereby requiring integration of information from different sources to obtain a comprehensive solution. As wellbore instability can result from a combination of geomechanics and drilling-related factors, a detailed analysis was required to identify the nature of these problems and determine major controlling factors for this variable drilling experience. DATA ANALYSIS To manage wellbore stability, data from offset wells was analyzed and integrated into a MEM, Fig. 4, and recommendations were made for each planned horizontal well 5. SAUDI ARAMCO JOURNAL OF TECHNOLOGY SPRING 2016
Fig. 4. Wellbore stability analysis performed for an offset well using calibrated 1D MEM. The mud weight recommendations were incorporated into a drilling program, and a post-drill analysis was performed to explain the nature of the drilling problems. The data analysis indicates that the majority of stuck pipe events are associated with back reaming and pulling out of hole. These problems may be attributed to the cuttings and cavings settled at the bottom of the hole. During tripping out of hole or back reaming operations, if upward movement of the drillstring is faster than the rate at which the rock debris can be circulated out, the cuttings/cavings will accumulate behind the bottom-hole assembly (BHA) and may cause tight hole or stuck pipe at some depth above the current well depth. Therefore, apart from mud overbalance, the ROP of these wells was also studied to assess if hole cleaning could be a factor responsible for the observed problems. The wells were ranked based on the severity of the drilling problems observed, and the corresponding mud overbalance, Fig. 5a, and average ROP, Fig. 5b, were plotted as a function of hole azimuth from the SHmax direction. The color of the data points represents the severity of drilling problems, as explained earlier. The data shown belongs to wells drilled in two adjacent fields, Field 1 (F1) and Field 2 (F2), targeting two reservoir zones vertically separated by a nonproducing thick layer. The alphanumeric data labels in Fig. 5a indicate the well number (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 overbalance of 10 pounds per cubic foot (PCF) to 12 PCF (lb/ft3) in both fields and with a ROP between 35 ft/hr to 38 ft/hr. When the well azimuth from the SHmax direction increases above 45 the well gets closer to the SHmin direction the mud overbalance needs to be increased as per the two curves in Fig. 5a for the two fields, solid curve for F1 and dotted line for F2, to maintain the wellbore stability. The variable mud overbalance requirements for the two fields suggest that in situ stress conditions and other geomechanical factors may be different for the SPRING 2016 SAUDI ARAMCO JOURNAL OF TECHNOLOGY Fig. 5. Variation of drilling experience with well azimuth from SHmax direction under different drilling parameters in horizontal wells: (a) mud overbalance, and (b) average ROP. The round dot and diamond symbols differentiate data from the two reservoirs. The comma-separated numeric value in the data label represents the well number. F1 and F2 identify the two adjacent fields. two fields. The increase in mud overbalance for a well azimuth above 45 azimuth from the SHmax direction suggests that the wellbore wall experiences higher differential stress in those conditions, requiring higher overbalance to reduce breakout severity. Figure 5b shows that the ROP needs to be dropped gradually according to the single curve for both fields and then maintained at 10 ft/hr to 12 ft/hr for well azimuths between 75 and 90. The well data presented in Fig. 5 was used to further classify the wellbore stability based on the ROP values and the mud overbalance, grouping them into four risk categories, Table 1. Wells falling into risk category 1 are those where both the ROP and the mud overbalance are within the safe limits defined per Fig. 5; these wells were drilled without any major drilling problems Wells 1, 2, 10, 12 and 13. Risk category 2 includes those wells where either the mud overbalance is below the stable limit, resulting in breakout development, Wells 6 and 7, or the ROP is above the safe limit, causing a higher rate of cuttings generation, Wells 3 and 5. Such wells are ranked as medium risk and can experience drilling problems such as tight hole, high torque and drag, and occasional stuck pipe issues. Likewise, if both parameters are exceeded the mud overbalance is low and the ROP is above the safe limit there is a higher risk of getting stuck and experiencing a loss of tool and BHA as excess cuttings and cavings generated downhole may be difficult to circulate out effectively. These wells are classified
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 defined by the trend lines in Fig. 5. S: Stable. PHC: Poor hole cleaning (high ROP or excessive breakouts, or both). BO: Breakouts (insufficient mud overbalance). DS: Differential sticking (too high mud overbalance). Table T 1. Classification of drilling difficulty at varied mud overbalance and ROP 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 condition. For risk category 3 wells, a simultaneous increase of mud overbalance and reduction in ROP to the safe limit are required to maintain wellbore stability. Extremely high risk wells are those included in risk category 4, where mud overbalance is 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 problems due to differential sticking across the permeable zones. The solution to this problem is to reduce the mud overbalance and bring it close to the stable mud weight overbalance limit, Fig. 5a. The lessons learned in how to optimize the drilling practices were incorporated together with real-time observations and wellbore stability updating 2, 6. This helped successfully overcome the challenges and reduce drilling problems along the S Hmin direction, Fig. 6. It can be seen that the number of successful wells increased from 22% in 2012 to 65% in 2014. In about 25% of the wells in 2014 that experienced severe drilling problems, the problems were mainly differential in nature drilling through more depleted and/or high porosity zones. This problem is being further handled by optimizing the mud additive and using appropriate bridging material. Fig. 6. The improvement in drilling performance of wells drilled along the S Hmin direction. CONCLUSIONS 1. Based on pre-drill geomechanics models and post-drill reviews, safe limits of mud overbalance and ROP have been identified for drilling horizontal wells in carbonate gas reservoirs. 2. Horizontal wells oriented up to 45 from the S Hmax direction can be drilled safely with the same mud overbalance 10 PCF to 12 PCF in both fields. For well azimuths above 45, horizontal wells drilled in Field 1 require a lower mud overbalance to achieve main hole stability compared to Field 2. 3. Horizontal wells toward the S Hmin direction require a mud weight overbalance of 45 PCF to 50 PCF in Field 2, while a mud overbalance of 15 PCF to 20 PCF is required for Field 1, suggesting spatially variable in situ stresses and rock strength properties. For these wells, the ROP should be maintained between 10 ft/hr and 20 ft/hr to ensure proper hole cleaning. 4. The ROP in the study wells was found to vary in the range of 20 ft/hr to 50 ft/hr. For wells experiencing the same breakout severity, those drilled using a 5⅞ bit had an extra burden on achieving hole cleaning efficiency because of the reduced annular area 55% to 75% compared to that from an 8⅜ bit. Both ROP and proper mud overbalance 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 back reaming to make the connection. It appears that debris/ cavings are accumulated behind the BHA, causing restriction. Careful tripping operations a tripping speed adequate to ensure hole cleaning as well as appropriate tripping frequency can help reduce these problems. 6. The combination of pre-drill geomechanic studies based on offset well data, real-time geomechanics support of field operations and post-drill analysis of actual drilling experience helped to successfully overcome the challenges and reduce drilling problems. SAUDI ARAMCO JOURNAL OF TECHNOLOGY SPRING 2016
7. The number of horizontal wells drilled toward the S Hmin direction has increased to 65% in 2014, compared to 22% in 2012, after successful implementation of the findings of this study. ACKNOWLEDGMENTS The authors would like to thank the management of Saudi Aramco for their support and permission to publish this article. This article was presented at the SPE/IADC Middle East Drilling Technology Conference and Exhibition, Abu Dhabi, UAE, January 26-28, 2016. REFERENCES 1. Al-Qahtani, M.Y. and Rahim, Z.: A Mathematical Algorithm for Modeling Geomechanical Rock Properties of the Khuff and Pre-Khuff Reservoirs in Ghawar Field, SPE paper 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 and Geomechanics Approach Helps to Successfully Drill Wells along the Minimum Horizontal Stress Direction in Khuff Reservoirs, SPE paper 171755, presented at the Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, November 10-13, 2014. 3. Thiercelin, M.J. and Plumb, R.A.: A Core-based Prediction of Lithologic Stress Contrasts in East Texas Formations, SPE Formation Evaluation, Vol. 9, No. 4, December 1994, pp. 251-258. 4. Piroozian, A., Issham, I., Yaacob, Z., Babakhani, P. and Ismail, A.S.: Impact of Drilling Fluid Viscosity, Velocity and Hole Inclination on Cuttings Transport in Horizontal and Highly Deviated Wells, Journal of Petroleum Exploration 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 Permeability Reservoir Utilizing Multistage Fracture Completion in the Minimum Stress Direction, SPE paper 160848, presented at the SPE Saudi Arabia Section Annual Technical Symposium and Exhibition, al-khobar, Saudi Arabia, April 8-11, 2012. 6. Khan, K., Abdul Aziz, A.A., Ahmed, S. and Ahmed, M.: Managing Wellbore Instability in Horizontal Wells through Integrated Geomechanics Solutions: A Case Study from a Carbonate Reservoir, SPE paper 172550, presented at the SPE Middle East Oil & Gas Show and Conference, Manama, Bahrain, March 8-11, 2015. BIOGRAPHIES Khaqan Khan is a Geomechanics Subject Specialist with Saudi Aramco s North Ghawar Division of the Gas Reservoir Management Department. He joined the company in December 2012 to assist with various aspects of field 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 Geomechanics Engineer and Regional Geomechanics Manager in the Middle East. In 2005, he joined GeoMechanics International Inc. (GMI) as a Geomechanics Specialist, based in Dubai, UAE. After graduate school, Khaqan had worked as a Geomechanics Engineer with the Center for Petroleum and Minerals at King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia. During a career spanning more than 15 years, he managed and technically led several consulting projects in the Middle East and elsewhere, focusing on the geomechanics aspects of reservoir management and field development. Khaqan has written and coauthored several articles on the subject and has been actively involved in teaching and mentoring of junior staff. In 1998, he received his M.S. degree in Geotechnical Engineering from KFUPM. SPRING 2016 SAUDI ARAMCO JOURNAL OF TECHNOLOGY
Dr. Hamoud A. Al-Anazi is the General Supervisor of the North Ghawar Gas Reservoir Management Division in the Gas Reservoir Management Department (GRMD). He oversees all work related to the development and management of huge gas fields like Ain-Dar, Shedgum (SDGM) and Uthmaniyah (UTMN). Hamoud also heads the technical committee that is responsible for all new technology assessments and approvals for GRMD. He joined Saudi Aramco in 1994 as a Research Scientist in the Research & Development Center and moved to the Exploration and Petroleum Engineering Center Advanced Research Center (EXPEC ARC) in 2006. After completing a one-year assignment with the Southern Area Reservoir Management Department, Hamoud joined the GRMD and was assigned to supervise the SDGM/UTMN Unit and more recently the Hawiyah (HWYH) Unit. With his team, he successfully implemented the strategy of deepening key wells that resulted in the new discovery of the Unayzah reservoir in UTMN field and the addition of Jauf gas reserves in HWYH field. Hamoud was awarded a patent application published by the U.S. Patent and Trademark Office on September 26, 2013. Hamoud s areas of interest include studies of formation damage, stimulation and fracturing, fluid flow in porous media and gas condensate reservoirs. He has published more than 60 technical papers at local/international conferences and in refereed journals. Hamoud is an active member of the Society of Petroleum Engineers (SPE), where he serves on several committees for SPE technical conferences. He is also teaching courses at King Fahd University 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 Chemical Engineering from KFUPM, and in 1999 and 2003, he received his M.S. and Ph.D. degrees, respectively, in Petroleum Engineering, both from the University of Texas at Austin, Austin, TX. SAUDI ARAMCO JOURNAL OF TECHNOLOGY SPRING 2016