Microscopic Pore-Throat Mobilization Characteristics and Conversion Timing Strategies for CO2 Injection After Waterflooding in Reservoirs with Different Properties: A Case Study of Block X, Huabei Oilfield

Due to varying development histories of reservoirs with different physical properties in Block X, Huabei Oilfield, the remaining oil distribution and microscopic mobilization limits after waterflooding remain unclear, and the subsequent CO2 transition timing urgently requires clarification. In this study, high-temperature and high-pressure core displacement experiments combined with online nuclear magnetic resonance (NMR) were conducted to simulate CO2 injection after waterflooding to water cuts of 50%, 80%, and 100% and compared with continuous gas injection (CGI) to reveal the effect of different switching timings on the oil recovery and pore-throat mobilization for the three reservoir classes tested in this study. Results indicate that waterflooding primarily mobilizes pores larger than 0.1 μm, while crude oil in small pores (<0.1 μm) is difficult to effectively displace. After switching to CO2 flooding, the mobilization efficiency across all pore-throat classes increases significantly, with Class IV reservoirs exhibiting the largest enhancement in micropore recovery—from 5.8% after waterflooding to 34.5% after CO2 flooding, representing an increment of approximately 28.70 percentage points—demonstrating that CO2 can significantly expand the effective mobilization range relative to waterflooding. The underlying microscopic mechanism is that the degree of waterflooding alters the oil–water distribution within the core: at low-water-cut stages, remaining oil is continuously distributed, allowing injected CO2 to contact crude oil through sufficient diffusion and dissolution, which is interpreted as providing a mobility-control effect that helps suppress gas channeling; at high-water-cut stages, remaining oil is segmented and trapped in micropores, limiting CO2–crude oil contact, while long-term waterflooding establishes preferential water pathways that promote localized gas channeling and restrict mass transfer. These findings provide experimental evidence for the class-specific design of post-waterflooding CO2 injection strategies.