Experimental Study on Enhanced Pre-CO2 Assisted Flowback of Fracturing Fluid in Tight Reservoirs

Pre-CO2 fracturing is a prominent technique for enhancing production in tight oil and gas reservoirs by enlarging stimulated reservoir volume and improving single-well productivity. This paper aims to investigate the influence of different injection pressures of pre-CO2 on the flowback of fracturing fluids. Flowback experiments, with the pre-CO2 being the displacing fluids, were conducted to compare and analyze the flowback rate under different injection pressures. Using nuclear magnetic resonance testing analysis, the flowback characteristics of fracturing fluids in different scales of pore spaces were clarified. The experimental results show that pre-CO2 has a significant effect on the flowback of fracturing fluids. As the injected pressure exceeds the supercritical state, the flowback rate of fracturing fluids decreases with the increasing pre-CO2 pressure. The flowback rate of fracturing fluids is the largest (85.53%) when the injection pressure is 10 MPa. During pre-CO2 assisted flowback, the fracturing fluid in large pores is mainly displaced, while more fracturing fluid remains in small pores and cannot be mobilized. This study reveals the migration mechanism of fracturing fluid in different pore spaces in the presence the pre-CO2 operation and provides a theoretical basis for designing a reasonable flowback scheme and improving the tight oil and gas recovery.