Experimental study on CO2 flooding within a fractured low-permeability reservoir: Impact of high injection rate

The rapid decay of reservoir energy in the late development of fractured low-permeability reservoir leads to poor development benefit. Injecting CO2 into such reservoir at high pressure and rate can rapidly replenish reservoir energy and fully utilize the advantages of CO2 flooding, which is an efficient approach for CCUS-EOR. Previous laboratory studies have mainly been conducted under conventional low injection rate, with little discussion on the impact of high injection rate (above 20 times the conventional low injection rate) on CO2 flooding, especially regarding the limited reports on the effect of high injection rate on CO2 flooding within a fractured lowpermeability reservoir. In this paper, microscopic visualization and fractured-core experiments of CO2 flooding at different injection rates were conducted. The variations in microscopic displacement front and sweep characteristics under different injection rates were clarified. The influence of CO2 injection rate on the macroscopic and pore scale oil production characteristics in fractured low-permeability cores under different injection modes were analyzed. The results indicate that injection rate significantly influences the microscopic displacement characteristics. The final sweep coefficient at 10 mu L/min was 25.07 %, and the pre-breakthrough sweep coefficient accounted for approximately 90 %. While for 200 mu L/min, the final sweep coefficient was 34.83 %, and the post-breakthrough sweep coefficient accounted for above 50 %. During CO2 flooding in fractured lowpermeability core, the oil production before CO2 channeling dominates the final oil recovery. Compared to 0.1 mL center dot min- 1, oil recovery increases by above 20 % at a CO2 injection rate of 4 mL center dot min- 1 at 6 PV. The highpressure gradient formed by high injection rate enhances CO2 diffusion, promoting the oil mobilization in near-fracture and deep matrix. And it results to a decrease in the minimum pore scale of oil mobilization. Soaking enhances the interaction between CO2 and oil as well as the fluid exchange between fracture and matrix. It further reduces the minimum pore scale of oil mobilization, and the enhancing effect of soaking gradually strengthens with increasing injection rate. The results can provide theoretical reference for application of CO2 flooding within a fractured low-permeability reservoir.