Micromechanical analysis of hydraulic fracturing in the toughness-dominated regime: implications to supercritical carbon dioxide fracturing

Supercritical carbon dioxide (SC-CO2) is considered as an ideal non-aqueous fracturing fluid due to its superior properties of liquid-like density, gas-like viscosity, high compressibility, and diffusivity. This study aims to investigate the micromechanical behavior of SC-CO2 fracturing in both intact and fractured rock samples by using a coupled fluid-solid discrete element method (DEM) model. A new numerical algorithm for hydraulic fracturing in the toughness-dominated regime is developed by assuming that the pressure in the whole fracture is uniform. This new numerical algorithm could achieve a much higher computational efficiency compared with the conventional hydromechanical scheme in DEM. Hydraulic fracturing cases using high-viscosity fracturing fluid are also performed for comparison. The results indicate that the fracture propagation induced by SC-CO2 tends to be less smooth and continuous, more asymmetric, and tortuous compared to that induced by viscous fluid. Besides, the low-viscosity fluid like SC-CO2 can lead to a lower breakdown pressure, and the fluid leak-off into the rock matrix can result in a lower breakdown pressure and higher fracture propagation pressure. The simulations also illustrate that SC-CO2 fracturing tends to create a more complex and productive fracture network if the pre-existing natural fractures are involved. As a result, we can conclude that SC-CO2 could be an alternative fracturing fluid to induce a more effective fracture network for hydrocarbon production.