Numerical modeling of self-limiting and self-enhancing caprock alteration induced by CO 2 storage in a depleted gas reservoir

Abstract

This paper presents numerical simulations of reactive transport which may be induced in the caprock of an on-shore depleted gas reservoir by the geological sequestration of carbon dioxide. The objective is to verify that CO 2 geological disposal activities currently being planned for the study area are safe and do not induce any undesired environmental impact. In our model, fluid flow, mass transport and mineral alteration are induced in the caprock by penetration of high CO 2 concentrations from the underlying reservoir, where it was assumed that large amounts of CO 2 have already been injected at depth. The main focus is on the potential effect of precipitation and dissolution processes on the sealing efficiency of caprock formations. Concerns that some leakage may occur in the investigated system arise because the seal is made up of potentially highly-reactive rocks, consisting of carbonate-rich shales (calcite + dolomite averaging up to more than 30% of solid volume fraction). Batch simulations and multi-dimensional 1D and 2D modeling have been used to investigate multicomponent geochemical processes. Numerical simulations account for multiphase advection, aqueous diffusion, fracture–matrix interactions (advective and diffusive exchange of species between fractures and matrix rock), gas phase participation in multiphase fluid flow and geochemical reactions, and kinetics of fluid–rock interactions. The sensitivity of CO 2 concentrations to geochemical processes and parameters is investigated by conceptualizing different mass transport mechanisms (i.e. diffusion and mixed advection + diffusion). The most relevant mineralogical transformations occurring in the caprock are described, and the feedback of these geochemical processes on physical properties such as porosity is examined to evaluate how the sealing capacity of the caprock could evolve in time. The simulations demonstrate that the occurrence of some gas leakage from the reservoir may have a strong influence on the geochemical evolution of the caprock. In fact, when a free CO 2-dominated phase migrates into the caprock through pre-existing fractures, or through zones with high initial porosity acting as preferential flow paths for reservoir fluids, low pH values are predicted, accompanied by significant calcite dissolution and porosity enhancement. In contrast, when fluid–rock interactions occur under fully liquid-saturated conditions and a diffusion-controlled regime, pH will be buffered at higher values, and some calcite precipitation is predicted which leads to further sealing of the storage reservoir.