Reducing Drilling Risks in ERD Wells Through Geomechanical Solutions

Abstract

Abstract In the quest to achieve higher production, extended reach drilling (ERD) wells are planned to be drilled in the Bassein cluster, offshore India, threatened by high risk of wellbore stability after severe hole problems drilling low-angle exploration and appraisal wells. The key to successful drilling is to predict optimal mud weight windows for safe and stable ERD wells with a casing plan to drill the critical buildup section from 30° to 75° through overlying shales. The in-situ stress profile is built and further calibrated with closure pressures obtained during extended leakoff tests and stress testing with the modular formation dynamics tester. Rock mechanical properties are estimated to history match actual failures with predicted failures using the geomechanical model and mud weight used while drilling. Reported cavings and stuck tool incidents in shale formations are at depth intervals where the estimated breakout mud limit exceeds the mud weight used during drilling of offset wells. There is variation of rock elastic properties like Young's modulus and Poisson's ratio with direction in shale. Intermediate lower-strength coal and shale layers are seen in the Panna sandstone against overgauged hole condition observed in offset wells. Stable mud weights to avoid breakouts are in range of 11.6 to 12.5 ppg depending on well azimuth and deviation across different shale layers. Variations in minimum stable mud weight are in the range of 0.3 to 0.5 ppg based on well azimuth with reference to minimum horizontal stress direction from same platform. Even with depletion of 1.2 ppg in hydrostatic limestone, the minimum mud weight required to safely drill the horizontal section is 8.7 to 8.9 ppg due to lower- strength layers. Well deviation should remain same in the 12.25-in. section to avoid hole cleaning issues in shale. The breakout-mud loss window varies between 1.3 and 1.8 ppg at high well deviation in the 12.25-in. section. The 20-in. casing shoe plan was modified by setting deeper by 100 to 150 m to obtain a higher mud weight window. Lower core recovery at the top of the target limestone reservoir coincides with overgauged hole conditions seen across all offset wells. The local normal fault regime identified against regional strike-slip geological setting. The modifications resulted in significant reduction of nonproductive time with no lost-in-hole incidents and time savings of 1 week on each ERD well compared to offset wells. The good hole condition made it possible to acquire formation pressure measurements across the field to update the reservoir model and understand the structural connectivity of different target layers.