First Experience Drilling Overpressured Shale and Depleted Limestone in a Single Hole Section in Kuwait Using Highly Engineered Drilling Fluid – A Case History

Abstract Drilling sidetracks has become a viable means to access unexplored reservoir in a complex field in northern Kuwait. Historically, for re-entry projects on existing wells, the casing is run and cemented to isolate the pressurized shale formation prior to drill through the highly depleted limestone reservoir. Due to the well design constraints, the reservoir is commonly drilled slim hole 3 7/8" that would require drilling multiple and costly sidetracks to improve production rates, thus decreasing investment returns. To improve well economics on a new three (3) well project, the Operator decided to drill the sidetrack through the dual pore pressure gradient formations as a single 6" hole section. All three (3) wells experienced wellbore instability in the upper (pressurized) formation due low mud weight run with the intention of limiting the risk of formation losses. This instability led to pack off and stuck pipe events due to an accumulation of cavings in the annulus. Based on lessons learned and results achieved on previous applications on another project, the drilling team decided to investigate options to improve drilling practices and optimize fluid design. The revised strategy was to drill one single stage 6" sidetrack through the upper high pore pressure formation, then drill the depleted reservoir with use of Managed Pressure Drilling (MPD) technique and an engineered Non-Aqueous Drilling Fluid (NADF). The fluid was designed, and pilot tested with the objective of strengthening and stabilizing the stressed formation while minimizing the risk of losses risk in the depleted reservoir. The addition of multi-functional sealing polymers, ultra-low invasion fluid loss additives and bridging materials to the previous fluid formulation stabilized the upper interval and allowed the mud weight to be safely lowered from 17.0 ppg to 9.5 ppg to drill the depleted limestone formation. Utilization of a tested and validated NADF lubricant contributed to enhanced fluid lubricity while building the trajectory and drilling the horizontal section to target Total Depth (TD). Despite some minor stuck pipe events and fluid losses that were promptly cured with a customized loss remediation plan, the sidetrack was successfully drilled and completed ahead of schedule and in fewer days compared to the previous wells.