Evaluation of matrix energization effect of pre-fracturing method in tight reservoirs- A comparative study of CO2, N2, and water

Abstract

The exacerbation of the greenhouse effect has created an urgent global demand for large-scale utilization of CO2, making it a significant concern for industries worldwide. During the fracturing process, CO2 possesses unique properties that enable the formation of intricate fracture networks, presenting promising applications. However, the potential of CO2 as a medium for enhancing recovery through fracturing remains largely unexplored. This paper highlights the priority of using CO2 as an energized fluid by comparing the pressure and oil recovery performance with N2 and water. Initially, CO2-oil swelling tests were conducted to assess the impact of CO2 injection on the physical properties of crude oil. Subsequently, the matrix energization performance of the three fracturing fluids was analyzed from two perspectives: pressure augmentation and pressure maintenance. The Online Nuclear Magnetic Resonance (NMR) method is proposed to further evaluate the oil replacement performance of fracturing fluids. The experiment results suggest that nitrogen is more effective in raising reservoir pressure, hydraulic fracturing exhibits superior formation energy maintenance, and CO2 demonstrates favorable performance in both aspects. Meanwhile, CO2 remains capable of recovering the largest amount of oil, with a recovery of up to 10.19%, and the oil recovery for nitrogen and water is 9.39% and 6.37%, respectively. Finally, a field-scale numerical model was developed to evaluate the matrix energization performance and oil recovery efficiency. In contrast to water and nitrogen, CO2 exhibits a heightened capacity for oil recovery. However, this advantage is proved to be constrained when the injection volume of CO2 is limited. CO2 can fully leverage its benefits when a substantial injection volume is employed, and reasonable optimization of CO2 injection amount can maximize the benefits.