The influence of grain coatings on the shear strength, slip-stability, and permeability of CO₂-altered reservoir rocks and caprocks

Carbon sequestration involves injecting large amounts of CO2 into the subsurface. Dissolved CO2 in saline aquifers can chemically alter the mineralogy of reservoir rocks and caprocks. This alteration may greatly influence mechanical properties such as shear strength, slip stability (a-b parameter in rate state friction), and permeability of affected zones. Previous laboratory studies on CO2-altered sandstone retrieved from an analog reservoir and caprock (Crystal Geyser, Utah) show significant reduction in fracture toughness after CO2 alteration (Major et al. 2014). Microstructural analysis of these samples suggests that the possible weakening mechanism is dissolution of the original grain coating, hematite (before alteration), and reprecipitation of goethite (after alteration). Distinct element modeling shows that alteration can reduce the shear strength of analog samples by ~40% (Wang et al. 2017). However, the associated change in slip stability, and permeability during fault reactivation is poorly understood. In this study, we perform confirmatory double-direct-shear laboratory experiments on end member compositions, i.e. hematite (before alteration) and goethite (after alteration) coated quartz gouge under recreated in situ conditions – and for control samples without grain coating. Slip stability parameters are captured by systematically varying shear velocity. Frictional healing and creep is captured by slide-hold-slide procedures with various hold durations. Shear-parallel permeability is measured at presumed steady-state conditions. Hematite coated gouge yields higher peak shear strength than quartz without coating but shows similar residual shear strength. The hematite coated quartz gouge can potentially undergo unstable slip – suggested by negative (a-b) values. Conversely, Goethite coated quartz gouge shows a noticeable reduction in shear strength with velocity strengthening behavior – suggested by positive (a-b) values. Permeability of both hematite-coated quartz and goethite-coated quartz undergo a trajectory of initial increase followed by a decline. Hematite-coated quartz shows higher initial permeability increase followed by sharp decline in permeability, while goethite-coated quartz shows moderate initial increase and follow-up decrease in permeability.