Review of CO<sub>2</sub> Fracturing and Carbon Storage in Shale Reservoirs
Lei Han, Xian Shi, Hongjian Ni, Weidong Zhang, Qi Gao
Abstract
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 CO 2 fracturing to be an ideal alternative to hydraulic fracturing. This review discusses the current achievements in hydraulic and CO 2 fracturing, evaluates the current research status and hotspots from a technical perspective, and introduces CO 2 pre-enhanced fracturing. CO 2 fracturing possesses a high backflow rate, environmental friendless, increased formation energy, and adsorption and plugging removal for increased production and can sequester carbon. However, CO 2 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 CO 2 fracturing, engineers have proposed CO 2 pre-enhanced fracturing and applied it in oilfields, achieving good results, which is a good development trend. Presently, basic research on CO 2 pre-enhanced fracturing and the geological storage of CO 2 are research focuses and hotspots. Basic research on CO 2 pre-enhanced fracturing mainly involves CO 2 injection-induced stress field and reservoir property changes, the subsequent impact on fracture propagation, and the optimization of complex fracture network (CFN); CO 2 sequestration involves high-confidence quantitative risk analysis for specific reservoir projects.