Experimental and numerical study on influencing factors of replacement capacity and slickwater flowback efficiency using pre-CO2 fracturing in tight oil reservoirs

CO2 as pre-fracturing fluid technology, with dual advantages of CO2 fracturing and water fracturing, has broad application prospects in the development of unconventional oil and gas, and it is also one of the important ways of carbon utilization and carbon storage. In this paper, an accurate experiment was designed first time to explore the replacement capacity of CO2 under different discharge pressure gradients using cores with different sizes. The fracturing fluid flowback law and enhance oil recovery ability were also studied by the self-designed experiment. Before the test, the core was split axially from the center and the core fracture was filled with mixed 40-70 mesh quartz sand and AB glue to simulate the actual formation fracture. Then a dual-permeability compositional model was established to explore the field scale fracturing fluid flowback. The performances of slickwater fracturing and pre-CO2 fracturing were compared. The effects of flowback pressure gradient, CO2 injection volume, and soaking time on the flowback efficiency and oil recovery were also analyzed. The results show that the efficiency of CO2 replacement of oil increases with the increase of flowback pressure difference and is 15% higher than that of slickwater. CO2 pre-fracturing can significantly improve the flowback efficiency of slickwater and enhance oil recovery. As the pressure gradient increases, the flowback efficiency of slickwater increases, which shows that the flowback pressure gradient needs to be higher than 10 MPa to achieve economic output. With the proportion of CO2 injection increased from 20% to 58%, the CO2 flowback efficiency increased from 37.5% to 44.1%, and the oil recovery factor increased from 11.2% to 14.7%. With the increase of soaking time, slickwater imbibed into matrix pore throat according to the capillary force, meanwhile, CO2 diffused into deep formation. The degree of slickwater and CO2 backflow decreased accordingly. Considering the pressure change, stimulation effect and flowback efficiency during the soaking period, the optimal soaking time is 6 h in exper-iment and 15 d in field production. This study is expected to provide theoretical guidance for CO2 pre-fracturing in tight oil reservoirs.