Collapse of Reacted Fracture Surface Decreases Permeability and Frictional Strength. Issue 12 (28th December 2019) - Spokas, K.
Abstract: Geochemical and geomechanical perturbations of the subsurface caused by the injection of fluids present risks of leakage and seismicity. This study investigated how acidic fluid flow affects hydraulic and frictional properties of fractures using experiments with 3.8‐cm‐long specimens of Eagle Ford shale, a laminated shale with carbonate‐rich strata. In low‐pressure flow cells, one set of samples was exposed to acidic brine and another set was exposed to neutral brine. X‐ray computed tomography and energy‐dispersive X‐ray spectroscopy revealed that samples exposed to acidic brine were calcite‐depleted and had developed a porous altered layer, while the other set showed no evidence of alteration. After reaction, samples were compressed and sheared in a triaxial cell that supplied normal stress and differential pore pressure at prescribed sliding velocities, independently measuring friction and permeability. During the initial compression, the porous altered layer collapsed into fine particles that filled the fracture. This effectively impeded flow and sealed the fracture, resulting in fracture permeability to decrease 1 to 2 orders of magnitude relative to the unaltered fractures. This is a favorable outcome in subsurface applications where the goal is to reduce leakage risks. However, during shear the reacted fracture had lower frictional strength because the fine‐grained particles in the collapsed layer prevented the formation of interlocking microasperities. Therefore, coupled geochemical and geomechanical processes that could favorably seal fractures could also increase the likelihood of induced seismicity. These findings have important implications for geological carbon sequestration, pressurized fluid energy storage, geothermal energy, and other subsurface technologies. Plain Language Summary: Underneath the surface of the earth lie many layers of rock formations. Some of these formations contain open pores where fluids can be stored. Natural examples include groundwater and oil formations. Many engineering applications utilize this space underground as storage. One example is the injection of greenhouse gasses for climate change mitigation. To prevent the fluids from leaking to the surface, storage rock formations are often located where overlying rock formation can contain fluids. Several risks include the inducement of small earthquakes and the leakage of fluids toward the surface. These risks are most prominent along rock fractures, where rock material is most susceptible to earthquakes and fluid can easily flow. In addition, the injection of fluids into the subsurface can produce acids that exacerbate these risks. In this study, we conducted experiments and found that these acids weaken fracture surfaces. Once exposed to mechanical stress, these surfaces collapse and seal the fracture. However, this collapse also decreases the frictional strength of the fracture. For relevant energy activities, these finding are helpful for minimizing the risks of injecting fluids underground. In context of climate change mitigation, these findings will help appropriate site selection to store greenhouse gas emissions safely. Key Points: Flow of acidic brine through fractures in mineralogically heterogeneous rocks selectively dissolves calcite and creates a porous layer Calcite‐depleted porous layer collapses under normal stresses, filling the fracture with fine particles, decreasing permeability Layer of fine particles lowers fracture frictional strength because it prevents the formation of interlocking microasperities