Effect of Fluid Exposure on Mechanical Properties of Organic-Rich Shale and Field Applications

Understanding the effect of fluid exposure on organic-rich shale mechanical properties is crucial in design of working fluids used in shale reservoir drilling and hydraulic fracturing. A series of triaxial compression tests and friction coefficient measurements were conducted on dry and fluid exposure specimen of three typical organic-rich shales in China. Fluid-type and time-dependent variation of mechanical properties were significantly observed between dry and fluid exposure specimen. These variations are attributed to the specific fabric feature of organic-rich shale and different fluid–shale interaction mechanisms, which is predominantly hydration for water-based fluid and friction reduction for oil-based fluid. Fluid exposure effect will increase the risk of wellbore instability through increasing pore pressure and decreasing shale strength. It also influences the propagation and orientation of hydraulic fractures, strengthens proppant embedment, and induces fines migration and colloids agglomeration/precipitation in hydraulic fracturing. Improve sealing performance, apply clay stabilizer and surfactant, manage pH value, and increase drilling rate are mainly effective methods to prevent or mitigate adverse effect of fluid exposure on shale oil/gas well drilling and production performance. These fluid effect control strategies have been successfully applied in the shale reservoir drilling in Sichuan Basin and Ordos Basin.