Identifying Hydrogeochemical Conditions for Fault Self-Sealing in Geological CO 2 Storage

Abstract

Injection of anthropogenic CO (Formula presented.) into a subsurface reservoir will significantly impact the geochemistry, porosity, and permeability of the reservoir. If a fault or fracture penetrates the reservoir, CO (Formula presented.) -enriched brine may migrate into that fault, eventually sealing it via precipitation or opening it up via dissolution. The goal of this study was to identify and quantify such conditions of fault self-sealing or self-enhancing. We found the Damköhler number ((Formula presented.)) provides a meaningful framework for characterizing the propensity of (fault) systems to seal or open up. We tailored a spatiotemporally varying (Formula presented.) framework and applied it to simplified fault models with eight conditions derived from four geologic compositions and two reservoir conditions. The four geologic compositions were chosen such that three of them were representative of distinct geologic end-members (sandstone, mudstone, and dolomitic limestone) and one was a mixed composition based on an average of three end-member compositions. The two sets of (Formula presented.) - (Formula presented.) conditions chosen included one for CO (Formula presented.) in a gaseous phase (“shallow conditions”) and the other for supercritical phase CO (Formula presented.) (“deep conditions”). Simulation results suggest that fault sealing via carbonate precipitation was a possibility for shallow conditions within limestone and mixed composition settings. The concentration of cations in the water was found to be an important control on the carbonate precipitation. A key conclusion suggested by the results of this study is that carbonate precipitation in the near-surface (top 50–100 m) depths of a fault is the most likely mechanism of “self-sealing” for most geological settings.