Detailed description of core (19,500 ft) from 57 wells in the Cretaceous (Late Albian) Mauddud Formation, Sabiriyah and Raudhatain Fields, Northern Kuwait, formed the basis for an updated depositional model. Numerous reservoir units occur within carbonate-dominated, mixed carbonate-clastic, and siliciclastic-dominated intervals. Core description and interpretation software (WellCAD/CoreCAD) was used to record observations and interpretations in digital format which was integrated into a master database for geocellular modeling, static, and dynamic reservoir modeling. The predictive depositional model demonstrates lithofacies distribution, primary depositional and secondary diagenetic controls on porosity occurrence, distribution of high permeability 'thief' zones common in both fields, and distribution of flow baffles and barriers within the vertical sequence. The Lower Mauddud comprises carbonate-dominated shallowing-upward cycles deposited in a carbonate ramp environment and localized clastic wedge deposits and associated tidal channels. The Upper Mauddud evolved to carbonate platform geometry with bioclastic shoals, rudist banks and patch reefs, and restricted back-reef and lagoonal highstand deposits. Late highstand deposition culminated in widespread subaerial exposure near the top of Upper Mauddud. Most porosity development and distribution is controlled by depositional fabric and texture, bioturbation, and dissolution during subaerial exposure at cycle tops. Widespread exposure surfaces and related brecciation and dissolution associated with late highstand deposition are major contributors to thief zone development. Several marine flooding events influence widespread and correlative vertical flow baffles. 'Bank and tidal channel' topography is dominant in the upper Lower Mauddud, where shoals and patch reefs developed between tidal channels. Tidal channels were filled in part with poor to non-reservoir clastics, creating baffle zones. Bioclastic shoals comprise Upper Mauddud reservoir units. Middle-late highstand reservoirs are composed of bank crest rudist buildups, patch-reefs and shoals. Late highstand deposits record increasingly restricted depositional setting, increase in frequency and duration of subaerial exposure, and development of karst brecciation with associated dissolution.

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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.