Experimental study on the mechanism of unstable water injection for oil displacement in low-permeability sandstone reservoirs

Abstract

To address the technical challenges of strong interlayer and intralayer heterogeneity and poor water flooding performance in low permeability sandstone reservoirs, systematic physical simulation experiments and numerical simulation studies of unstable water injection were conducted. A parallel core device was used to simulate interlayer heterogeneous reservoirs, and the displacement effects of four water injection methods were comparatively analyzed: continuous water injection, equal-duration intermittent injection, step injection, and short-injection-long-shut-in. A dual-layer composite core was employed to investigate the fluid crossflow patterns under intralayer heterogeneous conditions. A three-dimensional numerical simulation model was constructed to quantitatively optimize key parameters such as water injection cycle and injection-production ratio, and to elucidate the oil recovery enhancement mechanism of unstable water injection. Experimental results showed that the short-injection-long-shut-in method achieved the best oil displacement effect for low permeability layers, with a recovery factor increase of 3.5%, and an increase of 1.1% for high permeability layers. Unstable water injection in the dual-layer core stimulated the interlayer fluid crossflow process, with a recovery factor increase of 2.8%. Numerical simulation optimization determined that the optimal water injection cycle for interlayer heterogeneous reservoirs was 4 months with an injection-production ratio of 1.0, while for intralayer heterogeneous reservoirs, the optimal cycle was 6 months with an injection-production ratio of 0.9. Mechanism analysis revealed that unstable water injection activates the capillary imbibition effect through pressure oscillation, drives fluid exchange between high and low permeability layers, expands the swept volume of injected water, and effectively improves the mobilization of low permeability layers, providing theoretical support and technical basis for the scientific and efficient development of low permeability reservoirs.