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Abstract Zubair Formation is the deepest producing Cretaceous reservoir in North Kuwait. This 1,400 ft thick formation was deposited in deltaic to paralic depositional environment with complex sand/shale sequence, structural geometry, mineralogical composition and lateral extent. Drilling wells of any profile has witnessed high non-productive time due to severe wellbore instability issues in the form of stuck pipes, tight holes, hole pack-offs and jarring/fishing operations. So, a number of vertical wells were drilled to drain the reservoir– an economic challenge. Current strategic goal in this matured reservoir is to exploit multiple but thin pays by maximizing reservoir contact with high angle multi-lateral wells. An integrated 3D Geomechanics study was carried out in two phases. In the first phase, responsible failure mechanisms for wellbore instability were identified: stress induced breakouts, washouts and cavings, failure and fluid invasion associated with shale bedding planes at high deviation and osmotic pressure transmission between Zubair shales and drilling fluid system. Water sensitivity of clays and presence of micro-fractures were also studied on cores of this trouble making formation. In the second phase, calibrated well based 1D Geomechanical models; 3D structural model with high definition faults, facies models indicating lithological changes and drilling experience of latest high angle wells were integrated into a 3D Geomechanical model. The 3D model was tested with data from several offset wells and it was capable of explaining the wellbore failure of these wells. This 3D geomechanical model also helped in predicting mud weight window for any proposed high angle well trajectories. Mitigation measures from the study included drilling with Oil Based Mud or High performance water based mud systems with model derived mud weights, micronized sealing polymer to seal-off the laminations and micro-fractures, marble grade Calcium carbonate or resilient graphite to plug wider fractures and high salinity of mud to avoid time-sensitive osmotic flow. After implementing these recommendations, six horizontal wells have been drilled successfully. The study has given further confidence to implement an aggressive field development plan for optimal depletion. The paper discusses complex reservoir architecture, drilling complications and how the integrated study helped to achieve a breakthrough in development planning. |
