Significant wellbore instability problems are being experienced during the drilling of horizontal wells in a shaly sand member of the Khafji reservoir in Zuluf field. This paper presents the results from a case study that integrates detailed rock mechanics and swelling tests with information from petrophysical logs and core properties acquired to evaluate, define and predict the instability mechanism in this portion of the Khafji reservoir.

The study has tackled the effect of drilling fluid on shale strength and swelling. Additionally the effects of water activity, osmosis and hydraulic diffusion on shale stability were investigated. The mechanical properties and stress field in the khafji shale was determined. The results provided recommendations to minimize instability problems encountered during drilling. All drilling fluids that have water phase including the oil-based drilling fluid were found to cause instability problems. However all-oil drilling fluid was found to maintain shale strength. Drilling mud salinity to encourage resverse osmosis was determined based on measurement of shale-pore-water salinity. The critical mud weight window was calculated considering the Chemoporoelastic properties of the Khafji shale.

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