Managing Geomechanics Principles for Proper Handling of Wellbore-Stability Problems in Shale and Depletion Effects on Formations While Drilling

Abstract

Abstract This paper describes two types of hole problems and wellbore stability solutions for drilling operations in North Sea fields: (1) severe shale cavings and wellbore collapse; (2) drilling through cap rock and reservoir formations impacted by large pressure depletion and reservoir compaction effects. Field cases of wellbore collapses and failures in shale formations during high-angle drilling are presented along with a systematic program of comprehensive geomechanics investigation and evaluation necessary to prevent such events. The program includes analyzing drilling and log data analysis for rock strength and earth stress model construction, laboratory triaxial stress-strain tests on well core samples considering bedding plane inclination effects, 3D analytical modeling and 3D elastic-plastic numerical modeling analysis for determining optimum mud-weight (MW) windows for secure and stable well drilling. Continuous well production can also present problems for in-field drilling as the severe pressure depletion and reservoir compaction will cause significant reduction in stresses and formation fracture gradient. These effects may occur not only in the reservoir, but may also extend significantly upward into the cap rock formations depending on the reservoir/formation stiffness contrast, reservoir size, thickness and depth, etc. Therefore, the optimum MW must be examined and recalculated based on the insitu stresses that can be altered by this effect. In summary, this paper will include the following: Analysis of actual drilling cases of severe shale caving and wellbore collapses/failures;Laboratory triaxial stress-strain tests on well cores considering bedding plane effects;Effective drilling guidelines based on modeling analysis of troublesome shale formations;Coping with reduced stresses and formation fracture gradient due to reservoir depletion and compaction effects;Use of wellbore strengthening material for drilling (and possible reduction in number of casing strings);Case histories of severe wellbore instability and lessons learned.