Effect of fluids on the critical energy release rate of typical components in shale and andesite by molecular simulations

The critical energy release rate (Gc) is a key parameter in numerical simulations of hydraulic fracturing, which may be affected by a fluid. Molecular dynamics (MD) simulations of minerals' tensile failure can be performed to gain insights into the mechanisms relevant to the critical energy release rate at the microscale. The methodology of calculating the critical energy release rate for solid-fluid systems is challenging. In this study, we conduct extensive MD simulations for solid-vacuum and solid-fluid systems. Typical components in shale and andesite, including quartz, muscovite, and kerogen, are selected in our investigation. The effect of H2O and CO2 on the critical energy release rate is analyzed. Fracture propagation and fluid invasion in fractures are also monitored. The results show that quartz and muscovite are brittle in H2O and CO2 and kerogen has very pronounced ductile behavior. H2O can reduce the critical energy release rate of quartz and muscovite slightly, but may increase that of kerogen. The effect of CO2 on quartz and muscovite is mild, while it reduces Gc of kerogen significantly. The implication is the creation of a much higher surface area in kerogen by CO2 than by H2O, which is in line with large-scale observations.