Progress and Prospects of Supercritical CO2 Application in the Exploitation of Shale Gas Reservoirs

The breakthrough of clean and efficient shale gas stimulation technology and reduction of global carbon emissions is in a critical period. The injection of supercritical carbon dioxide (SCCO2) into shale gas reservoirs, fracturing the reservoir and displacing preadsorbed CH4, enhancing gas recovery, and completing CO2 geological storage, is regarded as an optimum scheme due to its engineering advantages and high adsorption capacity and preference in shale. This investigation systemically summarizes the theoretical understanding and research progress on SCCO2 fracturing technology and the displacement of CH4 by CO2 in shale gas development practice and theoretical simulation in the past decade and proposes the current challenges and prospects. The fracture space distribution characteristics of hydraulic fracturing and SCCO2 fracturing and the stimulation effectiveness of the shale gas reservoir are compared. The low viscosity, high diffusion coefficient, and zero surface tension of SCCO2 bring up its distinctive advantages for fracturing engineering. And the recovery of CH4 is much higher than that from hydraulic fracturing. The adsorption capacity and preference for CO2 are greater than those for CH4, caused by its molecular structure, properties, thermodynamics, and kinetics. Shale gas reservoirs are widely distributed and enormous resources. Several shale gas reservoirs in several basins are deemed to store CO2 by several to tens of gigatons. However, the wide commercial application of CO2 has been limited due to (i) the high cost of CO2 capture and transportation, (ii) low sand carrying capacity, nonuniform distribution, and easy settlement of proppant, (iii) the complex coupling mechanism of CO2-CH4-H2O-shale, and (iv) the environmental threat after CO2 storage. Therefore, it is urgent to develop a thickener and cosolvent with economical, clean, safe, and environmentally friendly characteristics that is highly compatible with SCCO2. High-precision reduction models of the structural characteristics and pore-fracture distribution characteristics of kerogen and mineral components are necessary to simulate competitive adsorption of CO2 and CH4. Additionally, a perfect monitoring system for CO2 leakage risk should be established near CO2 storage sites to avoid impact and harm to the atmosphere, groundwater, and surface ecosystems.