Review of fundamental studies of CO₂ fracturing: Fracture propagation, propping and permeating

CO₂ fracturing is one potential technique to relieve environmental issues related to the massive hydraulic fracturing of hydrocarbon reservoirs. We summarize fundamental studies on overall procedures of CO₂ fracturing and analyze research progress related to fracturing, the propping of the induced fractures and permeating CO₂ into, then recovering hydrocarbons from, the formation. The key controlling characteristics in CO₂ fracturing at each stage are defined, together with a definition of their relative dominance. Fractures generated by CO₂ fracturing are typically viewed as of superior complexity but increased tortuosity. Proppant transport during CO₂ stimulation is evaluated through consideration of particle settling, remobilization and flowing behaviours. New views of permeability evolution in propped fractures as a function of CO₂ saturation are presented. Correlations among each procedure are revealed to identify common issues and key technical details illuminated through multidisciplinary efforts. The field case studies of CO₂ fracturing are collected for the analysis of hydraulic parameters and then compared against water-based fracturing. The mismatch between pumping rate and CO₂ viscosity is highlighted, suggesting that the role of wellbore friction is an important topic requiring resolution. Suggestions for the optimization of CO₂ thickening, the usage of fine proppants and injected form of CO₂ are discussed and illustrated. Other open questions remain with respect to the nature of CO₂-rock interactions and their resultant impact on permeability evolution and fracture generation – key issues are identified for future investigations to promote the popularization of CO₂ fracturing for the concurrent and complementary recovery of native hydrocarbons and sequestration of carbon emissions.