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

Abstract

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.