Importance of Water-Clay Interactions for Fault Slip in Clay-Rich Rocks by Rast et al.
Researchers at the RMPLab are using rock deformation experiments to better understand the frictional strength of clay-rich rocks in the presence of a polar fluid.
"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."