Wellbore Instability of Shale Formation; Zuluf Field, Saudi Arabia

Abstract

Abstract 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 reverse 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. Introduction Drilling through shale formation usually causes over 90% of wellbore stability problems. These problems can be a simple washout to complete collapse of the hole. The problems of shale stability are related to the mechanical properties (strength and deformation under stress), the drilling fluids properties (weight, chemical makeup and brine concentration), the in-situ stress field, time dependant temperature, and time spent in open hole. Drilling extended-reach wells with long open hole intervals has been increasing. Oil based mud (OBM) have been the industry choice for difficult drilling. Their application has been typically justified on the basis of borehole stability, fluid loss, filter cake quality, lubricity, and temperature stability. Water-based muds (WBM) are attractive replacements from a direct cost point-of-view. Past efforts to develop improved WBM for shale drilling have been hampered by a limited understanding of the drilling fluid/shale interaction phenomenon. This limited understanding has resulted in drilling fluids designed with non-optimum properties required to prevent the onset of borehole instability. Modeling has been used to determine the mud weight window to minimize wellbore instability. In this study we used PBORE-3D developed through the Rock Mechanics Consortium of the Oklahoma University.