Supercritical carbon dioxide fracturing of granite from conventional to superhot geothermal conditions

Our previous studies on hydraulic fracturing in granite at 200°C to 450°C under triaxial stress condition revealed that infiltration of low-viscosity water at near-/super-critical temperatures stimulates preexisting microfractures and creates dense fracture networks which is favorable for extraction of geothermal energy. Presently, fracturing experiments have been carried out at similar conditions to examine feasibility of low-viscosity supercritical carbon dioxide (SC-CO2) adoption for application at various geothermal conditions. In this study, three fracturing experiments were conducted on cylindrical granite samples at 200°C and 450°C, with a range of differential stress, where liquid CO₂, which was preheated to the experimental temperature before entering the borehole, was injected at 1 ml/min. At 200°C, 85 MPa axial stress and 40 MPa confining stress were applied. Meanwhile at 450°C, 90 MPa axial stress with 40 MPa or 25 MPa confining stress was applied. As a result, 50 MPa breakdown pressure was observed at 200°C. At 450°C, 47 MPa and 17 MPa of breakdown pressure were observed for the experiment at 40 MPa and 25 MPa confining stress respectively. As the theory predicted that in the case of non-penetrating fluid, breakdown pressure will be approximately twice the magnitude of confining stress, these low breakdown pressures indicated fluid penetration. Furthermore, borehole pressure profiles suggested that pore pressures were close to borehole pressure. In addition, X-ray CT on the samples revealed that complex fracture patterns were developed. It has been discovered that stress states at breakdown events were close to Griffith failure criterion. The low-viscosity SC-CO₂ has allowed stimulation of preexisting microfractures so that the rock failed in accordance to Griffith theory, in which fractures generated in various locations. Hence, favorable complex fracture patterns were generated. The results demonstrated the feasibility of SC-CO₂ adoption to replace water in fracturing application at a wide range of geothermal conditions, due to its capabilities to return low breakdown pressure and to induce complex fracture network as well as to sequester CO₂ at certain amount.