The well design has been changed over last 55 years of development in Zubair Formation. It is the deepest producing Cretaceous reservoir in North Kuwait. This 1,400 ft thick formation was deposited in deltaic to Paralic depositional environments with complex sand/shale sequence, structural geometry, mineralogical composition and lateral extent. Drilling wells of any profile has been more difficult than the shallower reservoirs overlying it. The wells have 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. During initial phase lasting over 4 decades, vertical wells were drilled to drain the oil column which was thicker in most part of the Field. With water encroachments from bottom and edge, thinner pay Sands in multiple but thin pays are needed to be exploited by maximizing reservoir contact with high angle multi-lateral wells for effective production.
Drilling complications are inherent in Zubair since beginning even with vertical and deviated wells. Current transition to horizontal and high angle wells was possible with integrated studies. In the first phase of mitigating stability, responsible failure mechanisms for wellbore 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 model was tested with data from several offset wells and it was capable of explaining the wellbore failure of these wells. It was used 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.
The integrated study was implemented and six horizontal wells and a highly deviated well have been drilled successfully. The well designs and trajectories have been modified to drill along different azimuths of stress field with turns and up dip/down dip movements. Structurally complex and faulted blocks could be crossed effortlessly. The study has given further confidence to implement an aggressive field development plan for optimal depletion of undrained areas. Current strategy is to have vertical and deviated wells also for thicker reservoirs as they have the advantage of well interventions.
The paper discusses complex reservoir architecture, drilling complications and how the integrated study helped to achieve a breakthrough in development planning.
