关键词: 页岩油, 启动压力梯度, 渗流机制, 时间效应, 边界层

Abstract: In micro-nano pore throats of shale oil reservoirs, strong physicochemical interactions between crude oil molecules and pore walls induce a significant boundary layer effect, imposing additional resistance that must be overcome for fluid initiation. Conventional experimental techniques, including the pressure differential-flow, bubble, and equilibrium methods, fail to precisely capture the instantaneous onset of fluid motion. This technical limitation constrains the fundamental understanding of shale oil percolation laws, necessitating the development of advanced testing methodologies. Leveraging the time-dependent fluid initiation behaviors, this study proposes a novel characterization method for identifying the initiation moment via pressure-time curve analysis. By monitoring the dynamic changes in core pressure gradients under constant injection, the initiation process is categorized into an elastic energy storage stage and a concurrent elastic-flow stage. The threshold pressure gradient is determined by identifying the divergence point between reservoir core and sealed steel core curves. Compared with conventional approaches, this method offers clear physical interpretability and operational simplicity while eliminating the multi-flow rate testing complexities. Experimental results indicate that the oil-phase threshold pressure gradients for the calcareous and dolomitic shale and the Member 7 of Yanchang Formation shale are 1.346 MPa/m and 0.114 MPa/m, respectively. This disparity is primarily governed by the dominant pore-throat radius, lipophilicity, and movable fluid saturation.

Key words: shale oil, threshold pressure gradient, flow mechanism, time effect, boundary layer