Pore Size Scale Oil Mobilization and Fractal Evolution of Residual Oil Occurrence Spaces in Deep-Water Sandstone: Influence of Waterflooding Mode

Waterflooding intensity strongly affects oil–water migration in deep-water sandstone reservoirs with bottom edge water, yet oil mobilization across different oil-occupied pore scales and the evolution of residual oil occurrence spaces under different waterflooding modes remain unclear. Constant high-rate and rate-increasing waterflooding experiments were conducted on deep-water sandstone cores. Online nuclear magnetic resonance (NMR) monitoring and fractal analysis were used to distinguish oil-occupied pore scale intervals from oil-phase T2 signals and quantify the evolution of oil saturation and fractal dimensions under different waterflooding intensities. In 0–100 pore volumes (PV), rate-increasing waterflooding yielded slightly lower recovery than constant high-rate waterflooding. During 100–1000 PV, however, its recovery increment reached 8.68–12.39 percentage points versus 2.91–6.01 percentage points under constant high-rate waterflooding, increasing final recovery by 5.25–6.17 percentage points. Oil mobilization was dominated by macropore and mesopore intervals, with limited micropore mobilization. In 100–1000 PV, rate-increasing waterflooding caused larger oil saturation reductions in macropore and mesopore intervals (4.43–10.60 and 1.75–3.99 percentage points) than constant high-rate waterflooding (3.05–4.42 and 0.01–1.30 percentage points). Fractal dimensions increased during waterflooding, with the largest changes occurring in the micropore interval. Fractal dimension increments in mesopore and macropore intervals were 0.09–0.12 and 0.04–0.06 under rate-increasing waterflooding versus 0.04–0.07 and 0.03–0.05 under constant high-rate waterflooding. The results provide core scale insight into the relationship between injection history, late-stage oil depletion, and the T2 distribution of remaining oil in deep-water sandstone.