WEAKENING EFFECTS OF LOW-FRICTION MICROSTRUCTURAL FILMS IN CO2 ALTERED RESERVOIR ROCKS AND CAPROCKS

The occurrence of natural and induced seismicity is related to ambient stress fields, the propensity for slip instability, and strength of rocks. Furthermore, these factors are known to be related to the corresponding mineralogical characteristics and textures of the rocks. The dissolution of CO₂ into the brines of saline aquifers and caprocks may result in significant changes in the geochemical environment and in the strength, stability and permeability of the reservoir and caprock, induced by the alteration of mineralogical composition and texture. This has implications for modes of deformation during injection, induced seismicity and the security of storage. We examine the impact of mineralogical and microstructural alteration due to CO₂ injection in an analog reservoir and caprock (Crystal Geyser, Utah). Fe-coating (hematite) of cementation between tectosilicate grains are dissolved and altered into low-friction films (goethite) surrounding the grains (Major et al. 2014). We perform 2D Distinct Element Modeling (DEM) to simulate fault gouge materials consisting of phyllosilicates, tectosilicates, and surrounding low-friction films under slip events. The mechanical response of grain-grain contact is represented by a linear-elastic contact model with rotational resistance and a slip-weakening friction law. Results of end-member (unaltered and altered gouge materials) show that the presence of such low-friction films can greatly reduce shear strength of the bulk gouge while promoting velocity-strengthening behavior. These results suggest that the presence of trace quantities of key minerals, such as low-friction films, may serve as a key mechanism of weakening and creep behavior in CO₂ altered rocks and faults - with implications for the potential for induced seismicity and the evolution of permeability during inflation of the reservoir.