Shale softening induced by CO2 injection in the absence and presence of water: An experimental investigation based on nanoindentation

Injecting CO2 into shale gas formations to improve recovery and sequestrate CO2 is regarded as a promising technology. Understanding the softening behavior of shale induced by CO2 injection is crucial for the engineering decision of shale gas development and CO2 geo-sequestration. In this study, the softening behavior of shale induced by CO2 injection was investigated based on nanoindentation. Emphasis was placed on analyzing the differences in shale softening behavior in the absence and presence of water. CO2 injection caused alteration of shale mineral composition and mechanical properties. The contents of quartz and muscovite increased after treatment, while albite, anorthite, and kaolinite decreased. The nanoindentation results were analyzed using the Gaussian mixture model and upscaled using Mori-Tanaka model. Three mechanical phases (i.e. soft phase, intermediate phase, and stiff phase) were identified through Gaussian mixture model. Under the condition of CO2 treatment, both the hard and soft phases softened, while the intermediate phase did not. Under the condition of CO2+H2O treatment, all three phases - hard, intermediate, and soft - experienced significant softening. The macroscale elastic modulus of shale sample was estimated through Mori-Tanaka method. Compared with untreated sample, the decrease in macroscale elastic modulus of CO2 treated sample is 10.92 %, and that of CO2+H2O treated sample is 45.76 %. The results of this study show that shale softening induced by CO2 injection is more severe in the presence of water than that in the absence of water.