Importance of Water‐Clay Interactions for Fault Slip in Clay‐Rich Rocks.

Clay‐rich rocks are integral to subduction zone dynamics and of practical importance, for example, as barriers in nuclear waste and CO2 repositories. While the effects of swelling strain on the self‐sealing capabilities of these rocks are relatively well‐established, the implications of polar fluids interacting with charged clay particles on the frictional behavior, and the role of swelling stress in initiating slip in critically stressed faults, remain ambiguous. To address these uncertainties, we conducted triaxial friction experiments using saw‐cut samples, with the upper half composed of Opalinus claystone (OPA) and the lower half of Berea sandstone (BER). The frictional strength of the non‐wetted OPA‐BER interface was estimated based on experiments at confining pressures of 4–25 MPa and constant axial loading rate (0.1 mm/min). Fluid injection friction experiments were performed using decane (non‐polar fluid) or deionized water (polar fluid) at 10 and 25 MPa confining pressures and constant piston displacement control. Macroscopic mechanical data were complemented by distributed strain sensing on the sample surface. Compared to decane, the frictional strength of the OPA‐BER interface tended to decrease when injecting water, which is attributed to phyllosilicate lubrication and the transition of the OPA from a solid rock to an incohesive mud near the saw‐cut surface. When injecting water, slip was initiated during initial hydration of the OPA‐BER interface, although the apparent stress state was below the yield stress. To explain this behavior, we propose that the swelling stress is a crucial factor that should be integrated into the effective stress model. Plain Language Summary: Clay‐rich rocks are important in subduction zones and for practical applications like nuclear waste containment and CO2 storage. A unique property of clay is the ability to swell by incorporation of water. If swelling deformation is constrained, swelling stress can develop. While the enhanced sealing ability of fractures due to swelling is well‐known, the effect of water‐clay interactions on friction and the contribution of swelling stress to fault reactivation remain unclear. We studied these effects with laboratory experiments where Opalinus claystone (OPA) was sheared against Berea sandstone. First, using experiments without fluids, we established that our set‐up is representative for claystones. In further experiments, we injected a non‐polar fluid that does not interact with clay. We compared these with experiments where we injected water, which is polar and therefore interacts with the charged clay. The frictional strength tended to decrease when injecting water, likely due to water acting as a lubricant and transforming OPA into a slurry. Surprisingly, movement along the interface started although the applied stress was below the threshold. We explain this by the contribution of swelling stress to the overall stress state. These findings improve our understanding of clay‐rich rock behavior in nature and practical applications. Key Points: The frictional strength of clay‐rich rocks tends to decrease in the presence of a polar fluid compared to a non‐polar fluid or no fluidFrictional slip in clay‐rich rocks may be initiated although the apparent effective stress state is below the yield stressH2O‐clay interactions may contribute to reactivation of faults by altering the mechanical properties and the build‐up of swelling stresses [ABSTRACT FROM AUTHOR]