Laboratory physical simulation study on water flooding EOR mechanisms and patterns in FM fault- fractured-vuggy reservoirs

The FM fault-fractured-vuggy volatile oil reservoir exhibits significant heterogeneity and rapid production decline. Furthermore, there are notable differences in the effects of depletion development, and the impact of volatile oil degassing on water injection parameters remains unclear. This paper established artificial cores that replicate both single-fractured-vuggy and multi-fractured-vuggy full-diameter systems, reflecting a typical reserves ratio of pores, fractures, and caves in the FM oilfield. Concurrently, water injection experiments were conducted for both single-well and multi-well units after natural energy depletion had occurred. The results indicated favorable development when single-well units opened the lower part of the fracture-vuggy zone, allowing for depletion below bubble pressure at an early stage. Due to dissolved gas flooding at the top of reservoirs, crude oil depletion recovery reached over 67%. The filling of fractures and caves led to delayed degassing: single-phase seepage before degassing was influenced by reservoir physical properties. In contrast, multiphase seepage after degassing was affected by fluid viscosity. The single-well units employed large-volume water injection huff-n-puff techniques, resulting in a pronounced gravity oil displacement effect. This method improved cumulative crude oil recovery by more than 5%. Additionally, it was recommended that multi-well units adopt a deep injection and shallow production development strategy, with continuous water flooding enhancing oil recovery by over 40%. However, during the asynchronous injection-production stage, the recovery improvement was limited to only 1%.