Study on the distribution patterns of water-drive residual oil in Weizhou W oilfield based on micro-CT scanning

This study targets the high and ultra-high water-cut Weizhou W Oilfield in Beibu Gulf, where almost all prior residual oil studies relied on indirect numerical simulation or post-displacement static characterization, lacking continuous in-situ pore-scale dynamic observation across graded displacement rates. We couple time-lapse micro-CT scanning with customized in-situ core displacement apparatus, to trace the full dynamic evolution process of residual oil at a 4.87 μm pixel resolution under four controlled, sequential water injection rates. We systematically calibrate and define four distinct residual oil forms with clear, quantitative equivalent radius thresholds specifically suited for Beibu Gulf offshore sandstone reservoirs. They are connected clusters (> 45 μm), continuous network structures (25–50 μm), film-like/multi-pore aggregates (15–25 μm), and dispersed isolated droplets (< 15 μm). Different from the widely held prior assumption that higher displacement rate always brings higher recovery benefit, we identify a non-monotonic evolution rule and a unique optimal 0.1–0.5 mL/min displacement window that delivers the maximum effective recovery of connected cluster and network residual oil, while effectively avoiding the sharp, counterproductive surge of hard-to-produce isolated residual oil proportion (from 6.13% to 48.02% once the rate exceeds this window to 1 mL/min) caused by excessive water scouring. These findings fill the critical pore-scale dynamic characterization gap for local high water-cut offshore sandstone reservoirs, and provide direct actionable experimental support for operators to avoid the commonly encountered “high injection, low efficiency” paradox during actual field waterflood adjustment, eliminating the risk of permanently trapping massive residual oil in untouchable dead-end pores that cannot be easily recovered in subsequent tertiary oil recovery operations.