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.

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Maximizing oil production by implementing appropriate latest technology is of utmost importance to meet the KOC's strategic goals. The Burgan Formation in the Sabiriyah field is of Cretaceous age and consists of three reservoirs viz, Lower, Middle and Upper Burgan. The Upper Burgan is broadly divided into seven lithological units. Among them, Upper Burgan 2 (UB2) and Upper Burgan 4 (UB4) litho units demonstrates comparatively good reservoir quality with sufficient lateral extension. The Upper Burgan reservoir in Sabiriyah field is very heterogeneous and complex in nature. The reservoir was developed through vertical wells all these years. However, to enhance production contribution from this complex reservoir, a paradigm shift in development strategy was adopted. Accordingly, a first ever non-conventional well (NCW) was planned, successfully drilled and completed with ICD. The detailed geological studies and information from dynamic data were integrated in identifying the target. The twenty feet UB2 layer in less explored eastern part of the field was finalized as the target for NCW and the proposed location was further evaluated using full field modelling. The UB2 layer in that location has three sand units viz. upper, middle and lower. Each sand unit was separated by a prominent shale layer. The NCW was planned in such a manner to target all the sand units of UB2 optimally and stratigraphically step down /drill through the shale units in between the sand units. The NCW was drilled with the horizontal trajectory in the UB2 layer as per the plan. A total of 1800 feet of lateral section was encountered with an average porosity of 20 per cent while the permeability reached up to 1 Darcy in certain intervals. The test results of this well are highly encouraging, paving the way for future NCWs to enhance production from the Upper Burgan Reservoir.

The Cretaceous (Albian) Upper Burgan Formation has been on production for circa 60 years in the Raudhatain and Sabiriyah fields in North Kuwait and significant resources remain. Key to an optimized development of this important reservoir is the introduction of Slant Well technology incorporating the use of sliding sleeve completions. The Upper Burgan Formation in the Raudhatain Field consists of four major sand dominated layers that comprise the main producing zones. These likely represent high stand deposits that formed during a progradation of the delta system across the region. Separating the sand layers are non-reservoir mudrocks that were deposited in marginal marine settings and act as baffles or barriers to fluid flow. In order to maximize production from the Upper Burgan, KOC has instigated the use of high angle slant well technology which allows greater reservoir contact at the well bore for each sand layer. As well, all four layers are in contact with the well bore thereby maximizing the oil drainage more efficiently. Vertical wells achieve the same result, but the difference being far greater reservoir contact with the four major layers has been successfully accomplished with the Slant well, and this has afforded higher production rates. The use of the Slant well however is selective and horizontal well application is still in use in both fields where good sand continuity in a specific layer has been interpreted. Currently the non-conventional wells make up approximately 25 percent of the producers in the Raudhatain Upper Burgan, the rest being vertical wells. Drilling the Slant well is a challenge primarily due to the long intervals of shale that are encountered between the sand layers. Shale sloughing is the major problem. The Upper Burgan is a mature reservoir with a weak edge water drive. Pressure depletion is a concern that is mitigated with the use of an active sea water and effluent water flood program that is now in place. The first Slant well to be drilled in the Upper Burgan was RA-000A. The well, located in the southern part of the Raudhatain Field was drilled to a depth of 9858 feet with an inclination of 80 degrees through the productive intervals. The well was drilled to the top of the Upper Burgan and a 7 inch casing was landed at this depth and cemented. The productive interval was drilled with a 6 1/8 inch hole size having a length of 850 feet. The well was then completed with a sliding sleeve ICD (Inflow Control Device). The ICD completion in the Slant well allows more uniform production from different regions of the well bore with less draw down. This will minimize the water coning effect. The sliding sleeve can be used to shut off water producing intervals during the life of this producer. The successes and challenges of this well are explored in this paper.