Effect of Flow Velocity on Multi-Scale Response of Natural Core Pore Environment in Heavy Oil Reservoir

Pore-environment adjustment occurs during continuous flow in fine-grained sandstone, yet NMR, CT and SEM responses are not one-to-one. Montmorillonite-bearing sandstone was studied: 12 cores matched by porosity and gas permeability from 60 samples were divided into four flow levels (0.1, 0.5, 1.0 and 1.5 mL/min), with three parallel cores in each group. White oil (4.5 mPa·s) was flowed at 40 °C and 5 MPa. NMR and CT were measured at 2, 4, 6 and 8 h after saturation, and SEM was performed before and after flow. Group means show short relaxation rising and long relaxation falling continuously, with changes at the same time point generally increasing with flow rate. At 8 h, the relative change rate of short relaxation increased from 6.31% to 70.86%, long-relaxation attenua-tion from 6.26% to 62.91%, the CT small-pore fraction increased by 0.89–17.46 percentage points, and the large-pore fraction decreased by 1.81–22.32 percentage points. In high-er-flow groups, relaxation peaked at 4 h and CT pore proportion at 6 h. Even after nor-malizing for cumulative fluid volume, higher-flow groups responded more strongly. These trends support particle release, redistribution and local retention, and the parallel design strengthens the flow-rate–pore-response correlation.