Experimental investigation of the hydro-mechanical response of soft rocks in the CO2 injection process - Impacts of fracture roughness and brittleness index on the evolution of permeability behavior

Understanding the hydro-mechanical response of a reservoir and its surrounding rocks to the CO2 pressure build-up during CO2 injection process is a key challenge in the safe implementation of CO2 capture and storage (CCS) technologies. These responses are expected to depend on the type of formation properties, such as hardness of the reservoir rocks. This study investigates permeability behavior of various soft sedimentary rocks that are commonly in candidate reservoirs, considering geological CO2 storage in Japan during shear-fracturing processes. Both the pre- and post-failure regimes were examined using direct shear-fracturing tests with simultaneous measurements of permeability. Our results demonstrate that the permeability of the tested specimens varied widely during shear-fracturing, depending on their failure mechanisms (i.e., brittle or ductile). The magnitude of the change in permeability during shear-fracturing was consistent with the geometry of the fracture patterns formed in the rock matrix after the experiment, indicating that, during slipping, the permeability behavior may be related to the fractured plane roughness. We found that the brittleness index was linearly correlated with the maximum change in permeability in most samples; this correlation provides insight into the hydro-mechanical responses of soft sedimentary rocks for assessments of the CO2 seal integrity in caprocks and compartments of reservoir rocks and should be considered when conducting numerical simulations of CO2 sequestration sites.