TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe Early Aptian carbonates of the Mauddud Formation form giant hydrocarbon reservoirs in North Kuwait. Reservoir description and distribution of rock properties in 3D space are challenging due to inherent reservoir heterogeneity. A robust depositional model driven by sequence stratigraphy, petrophysics tuned to dynamic data and innovative static modeling techniques were used to characterize this complex reservoir.The Mauddud carbonate sedimentation took place in a low angle ramp setting. The basal part of Mauddud consists of shales and low energy carbonates deposited in a transgressive systems tract. The main reservoir was deposited during the subsequent highstand systems tract. High-energy inner ramp grainstones preserve the best primary porosity and permeability. Reservoir quality deteriorates in mid ramp to inner ramp wackestones and mudstones.Diagenetic carbonate concretions destroy porosity and permeability. It is more pronounced in mud-rich packstone / wackestone fabrics. Early hydrocarbon emplacement has terminated concretion growth in crestal areas of the field whereas concretion formation and subsequent reservoir degradation continued in the water leg through late diagenetic stages. Rudistic floatstones, radially fractured concretions and small-scale fractures in low-porosity brittle rocks are the main thief zones in the reservoir.Through the integration of core, openhole logs, production logs, and pressure transient analysis, a deterministic permeability model has been developed that characterizes the reservoir. Logs have been reprocessed to identify zones of secondary porosity (enhanced permeability) and fractureprone zones. Porosity-Permeability transforms for matrix properties and fracture-prone intervals were developed. This methodology results in log-derived permeability profiles that match production log profiles and well test Kh estimates.A fine Geological model with 85 layers and 2.5 million cells has been built to capture the primary depositional units. The horizons bounding the flow units are major flooding surfaces. The lithofacies associations have been modeled as composite objects restricted to facies belts. As porosity was observed to be decreasing towards the flank, trend modeling has been used to model the effective porosity. Another geological model with 166 layers was built to capture the small-scale heterogeneity caused by vuggy zones and fractures. The vugs and fractures have been modeled as objects restricted within an area demarcated by poorer seismic coherence. The Matrix permeability was enhanced by vuggy permeability and fracture permeability.The paper describes the challenges in reservoir description and static modeling of this complex reservoir in detail.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe Upper Zubair Formation is a giant reservoir consisting of 350 ft of excellent quality sandstone. This main producing reservoir has a 100 thick tar mat historically encountered near oil water contact. Recently, a well completed in the crestal part of structure above tar mat zone and in good quality sand did not contribute to oil production and presence of tar was suspected. A re-look at identification, origin and distribution of the tar mat has become critical for formulating future development strategy of the reservoir.

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.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe Upper Zubair Formation is a giant reservoir consisting of 350 ft of excellent quality sandstone. This main producing reservoir has a 100 thick tar mat historically encountered near oil water contact. Recently, a well completed in the crestal part of structure above tar mat zone and in good quality sand did not contribute to oil production and presence of tar was suspected. A re-look at identification, origin and distribution of the tar mat has become critical for formulating future development strategy of the reservoir.

The first horizontal well (RA-156) was successfully drilled and completed in the Lower Burgan sandstone reservoir of Raudhatain Field, North Kuwait. The objective of the well was to produce water-free (dry) oil at high rate from a difficult-to-reach portion of the reservoir. Dry production is especially desirable due to constrained wet crude processing capacity. The well successfully delivered the expected rate, making this well the highest-rate producer in the field. Uncertainty in structure and stratigraphy are inherent in the area due to weak/noisy seismic reflections and rapid change in reservoir facies in marine-influenced channel deposits. Innovative correlation schemes and detailed reservoir description were used to reduce uncertainties. The trajectory was designed along the direction of maximum principal stress to overcome borehole instability problems. The horizontal-section was drilled with minimum overbalance to avoid potential reservoir damage. Well-site data including logging-while-drilling (LWD), cuttings, and drilling data were used to identify faults along the well and to optimize the selection of kick-off point (KOP) and end-of-build (EOB). Subtle markers were identified while drilling, and the trajectory was geo-steered within the channel sand while maintaining maximum standoff from a rising oil-water contact (OWC). Absence of shale along 3000-ft of horizontal/high angle section was instrumental in faster drilling. Low mud weight was used to minimize potential formation damage caused by prolonged formation exposure to high-density mud. The completion of the well was designed to deliver longterm high-rate dry oil production. The horizontal section of the well was cased and cemented to ensure zonal isolation. A perforation strategy was developed and implemented to encourage even flow from the reservoir along the productive section to avoid local water coning, facilitate future well intervention and increase productivity index (PI) by reducing potential skin. External casing Packers were used to further ensure isolation of across a fault in the horizontal section. 5–1/2" tubing with appropriate jewelry was selected to accommodate the expected high rate and to ensure well integrity. The details of the multidisciplinary approach and the lessons learnt during planning and implementation of this horizontal well are summarized in this paper. Introduction Horizontal well technology has made rapid strides since the pioneering efforts of drilling in offshore Italy in the late 1970's. In addition to improving well productivity1, horizontal wells have enjoyed success in combating coning/cresting problems, in accessing bypassed oil2, and in converting resources to reserves. In Kuwait, horizontal wells have not performed up to expectation due to various reservoir issues3. Three horizontal wells in Mauddud reservoir of North Kuwait have productivity somewhat higher than a vertical well. RA-156 is the first of its kind to be drilled in the Lower Burgan clastic reservoir. The Lower Burgan is the main producing reservoir in Raudhatain Field, North Kuwait. Of late, the reservoir is witnessing a complex pattern of water movement in different layers leading to increasing water cuts. The existing water handling facility has reached its limit due to de-salter constraints, and there is a need to produce additional dry oil to maintain oil output at the target level. The reservoir has been exploited with vertical wells, which experience coning heights up to 40 ft. A horizontal well was envisaged to circumvent the problem.