Abstract

The gas accumulation field under review is located in a highly stressed geological setting with tight sandstone reservoir overlain by wellbore instability prone formations. Drilling vertical wells over time became challenging with depleted sandstone reservoir requiring lower mud weight than required to minimize breakouts in overlying shale formations (Barakat, Mabrouk, Al Bashair and Miqrat). There was increase in drilling BHA held ups, logging tool stuck and NPT in recent wells. Hydraulic fracturing jobs were performed to obtain enhanced flow through the tight reservoir which itself increases cost for the wells. With increase in production requirement, horizontal wells were introduced to increase reservoir length which posed higher risk of wellbore instability in build-up section through shale formations and risk of differential sticking in long depleted open hole reservoir section.

A geomechanics model was constructed to represent the state of stress and mechanical properties of the overburden and reservoir to conduct wellbore stability analysis and simulate shear failures in offset wells for the mud weights used. Laboratory measured mechanical properties, closure pressure and breakdown pressure from hydraulic fracturing jobs, leak-off tests and drilling records collected from the offset wells were used to calibrate rock strength and stress profile. Wellbore stability analysis showed tendency of breakouts in shale formations with the mud weight used in vertical wells. Vertical pilot holes are usually drilled with 12.5kPa/m-12.7kPa/m. With increase in wellbore inclination, the mud weight requirement will increase by 0.8kPa/m-1.4kPa/m depending on the build-up inclination. Review of pilot hole sonic data showed anisotropy in the shale formations. Drilling in minimum horizontal stress direction as per plan would require higher mud weights (by 0.4-0.6kPa/m) than drilling parallel to maximum horizontal stress direction.

The geomechanical study helped in taking a number of critical decisions in the well design including the overburden shales and sandstone reservoir to be drilled in different open hole sections. This will enable weak shales to be drilled with higher mud weight in the range of 12.9kPa/m-13.9kPa/m to mitigate risk of stuck pipe in build-up section. Further, 7inch liner should be set to isolate build up section prior drilling the depleted reservoir with 10.6kPa/m-10.8kPa/m to minimize reservoir damage and differential sticking. Hole cleaning in critical build up section was addressed through proper mud additives to reduce torque and drag due to additional cavings as a result of breakout occurrence.

Findings from geomechanics study helped to minimize wellbore instability through key decisions on mud weight, kick off depth and BHA type during drilling operations. Both the build-up and reservoir sections were drilled successfully with minimum drilling related NPT. The mud weights were increased in steps based on field observations with the Geomechanical study results as guidance. This helped to maximize rate of penetration while reducing overbalance pressure across the overburden and reservoir sections.

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