Experimental Investigation of Fracture Effects on Pore Fluid Mobilization and Seepage Behavior in Continental Shale Oil Reservoirs

Continental shale oil reservoirs are characterized by nanoscale pore systems, ultra-low matrix permeability, and poor fluid mobility, making fractures the dominant factor controlling effective development. In this study, imbibition-NMR joint measurement tests, depletion-NMR joint measurement tests, single-phase fluid threshold pressure gradient tests, and two-phase relative permeability tests were conducted, and typical clay-bearing shale cores were selected to systematically investigate pore fluid mobility and seepage behavior under reservoir conditions, with particular emphasis on the differences between matrix and fractured systems. The results indicate that fluid flow in matrix cores is strongly restricted by nanoscale pores, exhibiting distinct nonlinear seepage and threshold pressure gradient effects, which significantly limit effective recovery. Under these conditions, both imbibition and depletion processes show relatively low recovery degrees and limited pore mobilization. In contrast, the introduction of fractures markedly enhances reservoir connectivity and improves seepage conditions, reduces flow resistance, and promotes mass transfer between matrix and fractures. This facilitates the continuous migration of crude oil from nanoscale pores into fracture channels, thereby remarkably enhancing fluid mobilization. Furthermore, fractured systems exhibit a broader range of mobilizable pore sizes and improved two-phase flow capacity. The innovation of this paper lies in demonstrating through multiple experiments that the presence of fractures effectively enhances the flow capacity of the matrix fluid, as fractures can substantially reduce the flow distance of the fluid within the shale matrix. Overall, fractures serve not only as primary flow pathways but also as the key controlling factor for achieving large-scale fluid mobilization and effective recovery in clay-bearing shale oil reservoirs. These findings provide important insights for optimizing fracturing strategies and enhancing shale oil development.