The Lower Cretaceous Zubair formation in Kuwait comprises oil-bearing sands intercalated with shale sequences. Historically, drilling into this formation presented major wellbore instability issues that include hole pack-off, stuck pipe and logging tools, high over-pull, tight zones while tripping, and severe hole washouts. These well problems have a significant impact on well costs and timeline.Wells drilled in every orientation have experienced instability problems in this formation. Vertical wells have encountered major washouts and difficulties with wireline logging operations. Deviated wells have been even more unstable and often required sidetracks -greatly increasing well costs. This paper analyzes these wellbore instability problems, including the failure mechanisms, and presents the actions taken to resolve them. A solution to these instability issues is presented, which was derived by building geomechanical models using well data, drilling problem analysis, core inspection, and core -based rock mechanical test results.We used chemoporoelastic and anisotropic geomechanical models to simulate the behavior of the Zubair formation while drilling in vertical and deviated holes to understand the wellbore instability experiences.Based on the analysis, changes in mud weight, reformulation of the mud system, and modified drilling practices were incorporated in the well plan of new high-angle wells. Success was achieved in drilling the wells and running the casing in this formation with deviations as high as 80°. The study helped to achieve a large reduction in indirect NPT due to wellbore instability. This experience is also a key learning and input for designing future complex trajectories. It is expected that a major measurable impact in the form of smoother operations and optimized well cost will occur during the drilling campaign based on recommendations from this analysis.

The Zubair Formation in Raudhatain Field is a prolific producer of oil for last 5 decades. Currently, it produces the maximum amount of dry oil in North Kuwait. The Formation was deposited in paralic to offshore environments giving rise to reservoirs of variable quality and heterogeneity. An integrated study on structural and stratigraphic interactions led to discovery of bypassed oil in an extended area. The discovered area was developed with four producers which resulted in sustaining production from Zubair. The concept could be useful in discovering more thin pay sands for future development. Identification of nature of the faults, pattern and compartmentalization are the key contributors to the process of locating bypassed oil areas. Faults were mapped from wide ranging seismic signatures varying from refection discontinuity to attribute variations supplemented by information from geology, drilling, well logs and reservoir performance. Fault calibrations from well cuts were useful to define the throws and distinct faults were identified to have varying throws with depth. Strike slip faults were mapped from more seismic attributes. The fault seal analyses were carried out from performances of multiple reservoirs which are in mature stages including the ones lying 4000 ft above the target zones, variation in fluid contacts with time and space, juxtaposition, structural back stripping and fill-spill history of the traps. An east-west trending fault Zone with several secondary faults was identified from multiple criteria. It is the most dominant fault trend and separate the southern part of Raudhatain field from main field and has significant control over fluid distribution during later stages of field life. Lack of communication across the fault was observed from pressure response in overlying Mauddud reservoir. Distinctly low reservoir pressure in the main field during early stages of Mauddud injection got inverted during later stages of water injection provided the direct proof of fault sealing. Water encroachment mapping in Lower Burgan was supplemented to the study showed island of dry oil in Layered sands while the area in the main field showed water. Three wells drilled close the fault zone indicated drilling complications from brecciated rocks directly associated with faulting. Finding thick oil columns in attic zone gave credence to the concept of fault sealing in this area. Another well lying in the graben area shows a thick immovable oil zone in Burgan massive sand which is not seen in anywhere else in the field. This immovable strip is the fault zone which enabled significant up-structure migration of oil. Sand discontinuity due to shorter channels and rapid variation in water depths in adjacent areas contributed to the existence of by-passed oil. The oil bearing layers are stratigraphically controlled with lower net/gross. More arenaceous intervals show communication across the faults. One of the eight identified fault compartments was drilled with an appraisal well and it showed 79 ft of bypassed oil in Zubair. Heavy oil/Tar is normally observed close to faults: NMR logs and pressure response indicated presence of light oil. The reserves added by the finding would last for 30 years with existing producers in the area. The effort helped in extending the platue period of the reservoir. Being closer to oil-water-contact and with significant volume of residual oil in flushed zones, the area is the target for future exploitation with the appropriate facilities and completion technology.

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.

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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