Influence of Rock Mineralogy on Reactive Fracture Evolution in Carbonate-Rich Caprocks.

Abstract

Fractures present environmental risks for subsurface engineering activities, such as geologic storage of greenhouse gases, because of the possibility of unwanted upward fluid migration. The risks of fluid leakage may be exacerbated if fractures are subjected to physical and chemical perturbations that alter their geometry. This study investigated this by constructing a 2D fracture model to numerically simulate fluid flow, acid-driven reactions, and mechanical deformation. Three rock mineralogies were simulated: a limestone with 100% calcite, a limestone with 68% calcite, and a banded shale with 34% calcite. One might expect transmissivity to increase fastest for rocks with more calcite due to its high solubility and fast reaction rate. Yet, results show that initially transmissivity increases fastest for rocks with less calcite because of their ability to deliver unbuffered-acid downstream faster. Moreover, less reactive minerals become persistent asperities that sustain mechanical support within the fracture. However, later in the simulations, the spatial pattern of less reactive mineral, not abundance, controls transmissivity evolution. Results show that a banded mineral pattern creates persistent bottlenecks, prevents channelization, and stabilizes transmissivity. For sites for geologic storage of CO2 that have carbonate caprocks, banded mineral variation may limit reactive evolution of fracture transmissivity and increase storage reliability.