The pore pressure change and swelling pressure development are two main concerning factors in shale wellbore instability issue. This study investigated water-base drilling fluid interaction with shale formation and the drilling fluid physicochemical effects on shale formation pressure and shale swelling through mathematical modeling, numerical simulation analysis and experimental tests. The study mainly focuses on the drilling fluid pressure, drilling fluid chemical, drilling fluid salinity and shale property effects on shale/drilling fluid interaction. It integrates the water movement into shale with shale pore pressure change, shale water content change and shale swelling pressure development. The experimental tests on shale pore pressure changing with different fluid chemicals have been conducted.

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