Seepage mode in lamina-developed shale oil reservoirs under strong heterogeneous and strong fluid-solid coupling——Taking Jiyang Depression in China as an example

Abstract The laminae of different lithologies is a typical sedimentation of shale oil reservoirs, and the resulting strong heterogeneous and strong fluid-solid coupling seriously affect shale oil productivity. Therefore, the strong permeability heterogeneous and mechanical heterogeneous of lamina-developed shale were first qualified by the permeability experiments and quasi triaxial mechanical experiments on shale cores of different directions from Jiyang Depression, which revealed strong brittleness in horizontal cores and elasticity in vertical cores. Then, the strong fluid-solid coupling in lamina-developed shale was investigated by permeability stress sensitivity test and CT scanning, which presents the elasticity deformation of matrix in vertical direction, resulting in dynamic opening and closing of laminae and strong fluid-solid coupling in horizontal direction during the fracturing shut-in and production (FSP). Based on online nuclear magnetic resonance (NMR), fluid migration between pores of different scales and laminae during the FSP was discovered. To detailedly describe the fluid seepage mode in lamina-developed shale oil reservoirs during FSP, an oil-water two-phase flow model coupling shale matrix, laminae and hydraulic fractures was simulated to describe the variation in saturation, pressure, relative permeability, and streamlines of water in lamina-developed shale cores, based on the permeability heterogeneous and stress sensitivity. The results show that During the hydraulic fracturing, the horizontal brittleness and vertical elasticity created a seepage channel of shale matrix, horizontal seams and vertical hydraulic fractures. During the shut-in, the expanded seepage area after the laminae are opened and the extremely high pressure gradient from the seam to the matrix cause the fracturing fluid to invade the matrix and displace some shale oil, gradually balancing the pressure among the three seepage media. During the production, the extremely high pressure gradient shifts from the matrix to seams and hydraulic fractures, and oil and water are simultaneously extracted. However, due to stress sensitivity and relative permeability variation, some fracturing fluids remain in the shale formation.