Research progress and prospects of CO2 fracturing for developing unconventional energy sources

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As the world faces the challenges of meeting energy demand, reducing reliance on conventional oil and gas resources, and improving global climate conditions, developing unconventional energy sources as alternatives to conventional oil and gas resources and lowering carbon emissions have become critical issues. CO2 fracturing, a novel fracturing technology, offers the benefits of water conservation, environmental protection and enhanced recovery of unconventional energy sources. This paper reviews three types of CO2 fracturing: Supercritical CO2 (Sc-CO2) fracturing, CO2 foam fracturing and quasi-dry CO2 fracturing: from both basic research and applied research perspectives. This paper also analyzes the basic principle, fracturing mechanism, advantages and disadvantages of Sc-CO2 fracturing, discusses the basic principle, fracturing fluid, experiment and molecular dynamics simulation research of CO2 foam fracturing, summarizes the quasi-dry CO2 fracturing, and compares three fracturing methods. The basic research results indicate that Sc-CO2 fracturing can induce shear failure, activate natural fractures, reduce fracture net pressure, create complex multi-level fracture networks, penetrate deeper into rock fractures, and thus improve productivity. By adding foaming agents and foam stabilizers/thickeners, the stability and salt tolerance/temperature resistance of CO2 foam fracturing fluids can be enhanced, which ultimately increases the recovery unconventional oil and gas reservoirs. Quasi-dry CO2 fracturing doesn't necessitate specialized sealed sand mixing equipment, making it easy to prepare. Moreover, it achieves integrated drag reduction and efficient sand transport. The field application results further verify the feasibility and application potential of CO2 fracturing systems. However, the theoretical research and technical process of exploiting unconventional energy sources using Sc-CO2 fracturing, CO2 foam fracturing and quasi-dry CO2 fracturing systems are still immature. This paper discusses the challenges and problems that may arise in the future CO2 foam fracturing, and provides some suggestions, which offer theoretical guidance and technical support for the green development of unconventional oil and gas reservoirs.