A study on the efficiency of supercritical CO2 fracturing in hot dry rocks using coupled finite-discrete element method

Despite widespread use, reservoir stimulation by water hydraulic fracturing has drawbacks, including difficulties in areas with water shortages. To address challenges associated with hydraulic fracturing, supercritical carbon dioxide (ScCO2) fracturing has been proposed as a suitable alternative to address water-use issues. The main characteristic of ScCO2 is its low viscosity at supercritical conditions, which makes it particularly useful as a working fluid for stimulation and fracture generation. This study utilizes a 2D fully coupled thermo-hydro-mechanical (THM) model, developed using the combined finite-discrete element method (FDEM), as the tool for fluid injection in hot dry rock (HDR). Two different models are considered: the continuum model and fracture models with discrete fracture networks (DFN). First, the employed method is verified against some available theoretical and experimental data to validate its capabilities in this regard. Once verified, models are analyzed and three influencing and controlling factors are examined: in-situ stresses, fluid viscosity, and fluid flow rate. The results show that the performance of ScCO2 injection is highly dependent on the magnitude and difference of ground stresses. It is revealed that ScCO2 performs better in anisotropic stress fields, whereas its efficiency in isotropic stress fields is lower. It is shown that changes in ScCO2 viscosity have negligible effects on fracture development patterns and breakdown pressure. In addition, fluid flow rate plays a crucial role in both breakdown pressure and the ultimate fracture pattern. The higher the flow rate, the more fluid-driven fractures are generated, the higher the breakdown pressure, and the shorter the time to reach it. Results indicate that a higher fluid flow rate is not recommended for ScCO2 injection, as this change is less efficient at improving fluid flow. In fractured reservoirs, 12 interaction types between ScCO2 and pre-existing fractures are identified; some are newly identified, such as branching at fracture tips and rebranching, making this fluid more efficient than other working fluids. Also, comparisons are made between ScCO 2 and water injection. The results indicate that ScCO2 generates many more fractures, longer fracture lengths, lower breakdown pressures, and longer post-breakdown pressure times. The longer post-breakdown pressure means more stable fluid flow in the fractures, an extended fluid-immersed zone, and thus a larger stimulated area.