Seepage mode in lamina-developed shale oil reservoirs under strong heterogeneous and strong fluid-solid coupling in Jiyang depression of China.

The laminae of varying lithologies are characteristic of shale oil reservoirs, with their pronounced heterogeneity and fluid-solid coupling significantly impacting oil productivity. To this end, this study initially quantified the permeability and mechanical heterogeneity in lamina-developed shale through permeability tests and quasi triaxial mechanical experiments on shale cores from different orientations in the Jiyang Depression. These tests revealed marked brittleness in horizontally oriented cores and elasticity in vertically oriented cores. Subsequently, the strong fluid-solid coupling in lamina-developed shale formations was hereby investigated by permeability stress sensitivity experiments and CT scanning, which presented the 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 in pores of different scales and laminae during the FSP was discovered. To detail 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 further simulated, based on the permeability heterogeneous and stress sensitivity, to delve into the variation in saturation, pressure, relative permeability, and streamlines of water in lamina-developed shale cores. The results showed that during the hydraulic fracturing, the horizontal brittleness, and vertical elasticity created a seepage channel composed of shale matrix, horizontal seams, and vertical hydraulic fractures. During the shut-in period, the expanded seepage area resulting from the opening of the laminae, combined with the extremely high pressure gradient from the seam to the matrix, facilitated the invasion of fracturing fluid into the matrix, displacing shale oil and gradually balancing the pressure among the three seepage media. During production, the extremely high pressure gradient shifted from the matrix to seams and hydraulic fractures, and oil and water were simultaneously extracted. However, due to stress sensitivity and variations in relative permeability, fracturing fluid remained trapped in the shale formation.