Review of CO2 Fracturing and Carbon Storage in Shale Reservoirs

Unconventional oil and gas resources, such as shale gas and tight oil, are increasingly important in the energy structure; however, these reservoirs have poor physical properties and must be transformed for commercial development. Hydraulic fracturing is widely used in unconventional reservoir transformation. However, because of issues, such as water resource consumption, environmental protection, and poor stimulated reservoir volume (SRV), this technology faces increasing limitations. Potential advantages render CO2 fracturing to be an ideal alternative to hydraulic fracturing. This review discusses the current achievements in hydraulic and CO2 fracturing, evaluates the current research status and hotspots from a technical perspective, and introduces CO2 pre-enhanced fracturing. CO2 fracturing possesses a high backflow rate, environmental friendless, increased formation energy, and adsorption and plugging removal for increased production and can sequester carbon. However, CO2 possesses low viscosity, high friction, and high filtration loss, which render it to be unfavorable for the entry of proppants and for oil and gas migration, in addition to viscosity fingering (low sweep efficiency). Combining the advantages of hydraulic and CO2 fracturing, engineers have proposed CO2 pre-enhanced fracturing and applied it in oilfields, achieving good results, which is a good development trend. Presently, basic research on CO2 pre-enhanced fracturing and the geological storage of CO2 are research focuses and hotspots. Basic research on CO2 pre-enhanced fracturing mainly involves CO2 injection-induced stress field and reservoir property changes, the subsequent impact on fracture propagation, and the optimization of complex fracture network (CFN); CO2 sequestration involves high-confidence quantitative risk analysis for specific reservoir projects.