Crack Propagation during the Process of Carbon Dioxide Fracturing Based on Molecular Dynamics Simulations

Carbon dioxide (CO2) fracturing is a powerful strategy for enhancement of oil recovery by creating more fractures in the formation so that it can efficiently improve the flow conditions of oils and gases underground. In the present study, we explore the whole process of crack propagation by injecting CO2 molecules into the formation based on the molecular dynamics simulation. For the I-type crack of a rock sample, the details on the crack propagation are demonstrated, accompanied by the damage process near the crack tip. Next, the fracture toughness of the I-type crack is calculated according to the critical intensity factor and the energy balance equation, and the two results are in good agreement. Moreover, the CO2 fracturing process is probed when the rock includes natural fractures, and all of the cracks are finally merged after propagation. The effects of CO2 fracturing and hydraulic fracturing for breaking rocks are compared, and it is found that the first method is more conducive to the formation of multiple fractures and complicated crack surfaces. The reason is attributed to the expansion effect of the CO2 molecules, and the diffusion coefficient and the radial distribution function are calculated. These findings are beneficial to depict the details of rock fracturing and provide some inspirations on inventing effective techniques of oil and gas development.