Theoretical analysis and numerical simulation of drilling fluid interaction with shale

A numerical simulation model has been developed to determine the interaction between water-base drilling fluids and shale. The physico-chemical effects on shale pore pressure, water content and swelling pressure have been investigated. Some experimental tests concerning the problem have been also conducted.; The developed simulation model integrates fluid movement into or out of the shale with shale pore pressure, the change in water content and the induced hydration stress. The influences of hydraulic, chemical and electrical potentials along with the drilling fluid and shale properties on the interaction phenomenon have been studied. The time required to equilibrate the fluid driving force has been investigated.; The results of the theoretical analysis and some experimental tests suggested that shale behaves as a non-ideal semi-membrane. Thus, when drilling fluid is brought in contact with shale, an osmotic pressure is generated thus causing a change in shale pore pressure. it was also found that when the drilling fluid chemical potential is larger than that of shale pore fluid, pore pressure increases. Otherwise, pore pressure decreases. It was found that the magnitude of pore pressure change is mainly dependent on the chemical potential difference between drilling fluid and shale pore fluid, shale membrane property, and confining pressure.; The study indicated that drilling fluid salinity and confining conditions are two major factors for hydration stress development in shale, while the effect of hydraulic potential of drilling fluid is not significant for intact shale under moderate overbalance drilling (overbalance 500psi). It was also noted that the water content change is primarily dependent on the salinity difference between drilling fluid and pore fluid.; Experimental tests were conducted to verify osmotic pressure transmission process and water content change in shale due to wellbore fluid change. Verification of mathematical model with test data has shown good agreement.; In addition, a model to estimate the membrane coefficient was developed. An analysis demonstrated that membrane coefficient is mainly related to the type of salt, salt concentration, confining pressure and shale cation exchange capacity. The effects of drilling fluid properties and shale properties, such as streaming potential, CEC and pore size on diffusivity and permeability were studied.