NMR T 2-Based Mechanistic Study of Residual Oil Distribution during Pulsed Waterflooding in Low- to Medium-Permeability Cores

Abstract Conventional waterflooding in low- to medium-permeability (LMP) sandstone reservoirs is prone to preferential flow, early water channeling, and insufficient sweep efficiency. To clarify the effects of pulsed waterflooding on oil mobilization and residual oil distribution in different pore-size domains, continuous and pulsed waterflooding experiments were conducted in single-core and dual-core parallel systems, and low-field nuclear magnetic resonance (LF-NMR) T2 measurements were used to characterize fluid changes in different pore size ranges before and after flooding. The results show that the recovery factor of single-core continuous waterflooding exhibited a nonmonotonic relationship with the injection rate, reaching a maximum of 32.53% at 0.20 mL/min. An excessively low injection rate provided insufficient displacement pressure, whereas an excessively high injection rate strengthened preferential flow, both of which were unfavorable for oil mobilization in small pores and poorly connected pore throats. Single-core pulsed waterflooding showed clear schedule dependence, with the highest final recovery of 39.64% achieved under the 15/15 min schedule. In dual-core parallel pulsed waterflooding, the recovery of the high-permeability core was consistently higher than that of the low-permeability core, reaching 35.83% and 19.48%, respectively, under a 30/30 min schedule. The NMR T2 results indicate that continuous waterflooding mainly mobilized oil in large-pore domains, whereas pulsed waterflooding promoted displacement toward small pores and poorly connected pore throats through viscous displacement during injection, pressure re-equilibration during shut-in, and capillary-driven fluid exchange. Overall, pulsed waterflooding can improve pore-size-resolved oil mobilization in LMP cores, but its effectiveness is jointly controlled by the injection/shut-in schedule, pore throat connectivity, flow resistance, and flow partitioning in parallel systems.