Influence mechanism of brine-gas two-phase flow on sealing property of anisotropic caprock for hydrogen and carbon energy underground storage

As the simplest saturated hydrocarbon, methane is an important hydrogen and carbon energy. The stability and sealing of caprock are keys to ensure the safe storage of energy in the process of hydrogen and carbon energy underground storage. A fully coupled two-phase flow model is established to analyze the migration mechanism of methane in caprock. This model considers the characteristics of dual porosity medium composed of caprock fractures and matrix, including the seepage of methane and brine. In the model, the replacement process of methane and brine exists in the fracture network, and the dynamic adsorption desorption process of methane exists in the matrix. Under the action of tectonic stress, obvious differences are observed in the fracture and permeability in different directions. All these characteristics are considered in the fully coupled two-phase flow model. The restraining and strengthening effects of caprock in the process of methane leakage are analyzed. The results show that the gas induces the adsorption expansion of the caprock matrix and reduces the fracture opening and permeability of the caprock. With the gradual invasion of gas into the caprock, the sealing characteristics of caprock show the evolution of self-inhibition first and then self-enhancement. With the increase in dip angle, gas tends to invade vertically rather than horizontally. Realizing gas–brine replacement is easier with the existence of large fractures. This study provides an effective numerical analysis method, which can be used to evaluate the sealing efficiency in the caprock in underground methane storage.