Engineered High Performance Water Based Mud Application and Integrated Geo-Mechanics Help Drill Through Challenging Shale Formation with Record ROP in the Fold Belt Kirthar Concession – A Case Study from Pakistan

Abstract

The Kirthar Concession is located in a Fold and Thrust Belt region with an active strike-slip component, causing borehole instability due to in-situ stress induced complexities such as weak bedding planes, rubble zones, and micro fractures through shales. The primary shale failure mechanism is pore pressure transmission (PPT) through weak bedding planes, worsening over time, which make Laki and Ranikot shales difficult to drill. Severe Blocky, tabular, and splintery cavings, pack-offs, and excessive reaming events are common problems – leading to days lost conditioning hole prior to running casing, wellbore collapse and stuck pipe, and expensive remedial solutions. The 1D Geo-mechanic model in place has limitations due to data inadequacy because of lack of Extended LOTs, unavailability of density log to surface, lack of cores in shale, uncertainty in depletion (collapse pressure) and stress anisotropy, and due to poor resolution of seismic data owing to terrain – rendering a 3D model challenging to achieve. Since the reservoir is tight and produced by hydraulic fracturing, wells are drilled in the preferred Fracturing direction, not conducive to wellbore stability mitigation. This paper describes the integrated application of an engineered high performance water based mud system with an updated geo-mechanical model, providing performance similar to an Oil Based Mud system in terms of clay inhibition and hydration prevention, ROP enhancement, and wellbore stability. For minimizing PPT; insoluble aluminium complexes, and nano-sized deformable sealing polymer (d50 = 100-200 nm) along with nano-technology based bridging and sealing additives were used – providing effective particle size distribution and sealing across shale micro fractures and weak bedding planes, all while maintaining a high over balance pressure for borehole stability. An optimum salinity level was utilized for the drilling fluid to prevent elevation of pore pressure at near well bore area, preventing shale collapse due to chemo-poroealastic mechanism. A pre-drill geo-mechanical model was developed for predicting a safe mud window, addressing borehole stability challenges. The model was updated in quasi real time with mud weight increased where necessary, assisting in timely interventions to prevent wellbore collapse. The Laki Shale was drilled without any major issues, and the Ranikot formation was drilled with a record Rate of Penetration (ROP) of 10 m/hr - lessening condition trips significantly and wells delivered safely. The paper discusses the major uncertainties addressing the geo-mechanical model, primarily in overburden pressure, calibration of pore pressure, minimum horizontal stress magnitude, and rock strength – and proposes possible way forward to mitigate these.