A 3D coupled numerical simulation of energised fracturing with CO2: Impact of CO2 phase on fracturing process

Engineered fractures play a critical role in enhancing energy extraction efficiency. In this study, energised fracturing with CO₂, as an alternative approach to conventional water-based hydraulic fracturing, is investigated via numerical simulations. We validated the CO₂ finite element-based fracturing model against analytical as well as CO₂-fracturing laboratory experiments, then utilised the model to investigate the effects of pressure-temperature dependent properties of CO₂ on energised fracturing process. To account for the temperatures expected in a real field, four cases with injection temperatures of CO₂ varying between 250 K and 350K, under both isothermal and adiabatic conditions have been considered. In the adiabatic conditions, the temperature variation during compression of CO₂ is captured using the Joule-Thompson coefficient, assuming no thermal exchange between the CO₂ and the surrounding medium. The results highlight the significant influence of CO₂ phase on the fracturing process, during the pressurisation stage, as well as post-breakdown, the speed of fracture growth after the breakdown and subsequent depressurisation and associated cooling of CO₂. In the designed cases, the phase-change from gas to liquid or supercritical occurs during the pressurisation and prior to breakdown, while the phase remains unchanged post breakdown and during fracture propagation. Liquid CO₂ presents a fast-pressurising process while gaseous CO₂ undergoes a lengthy compression stage. Supercritical CO₂ is the best performing as the pressurisation is not too lengthy, while the instantaneous post breakdown fracturing is significant. Results show that higher temperature of supercritical CO₂ is causing larger instantaneous fracture propagation as it has lower viscosity for the given in situ stresses (>10 MPa).