Impact of Shear Fracture Structure on Chemical Reactions and Fracture Permeability

Fracture structure controls fluid movement and therefore must exert control over the impact of chemical reactions on fracture permeability and mechanical properties. Although mineral precipitation in fractures is ubiquitous in nature, it has been difficult to replicate in laboratory settings. As a result, we have little experimental guidance to understand fundamental controls on the location, rates, and timing of precipitation. In this study, we combine experimental observations and a theoretical framework to interpret the distribution of mineralization observed at in situ conditions. We used triaxial direct-shear coreflood methods to fracture shale and anhydrite while injecting barium-laden water. Dissolution of calcium carbonate and calcium sulfate released anions that resulted in rapid precipitation of barium carbonate and barium sulfate as observed by timelapse x-ray radiography. Subsequent x-ray tomography and scanning electron microscopy revealed precipitation concentrated in low-aperture regions of the shear fracture system that appeared enhanced by fracture gouge and debris. The uneven distribution suggested that relatively small amounts of precipitate could have significant impacts on fracture permeability. The observed distribution of precipitates can be explained by Frash et al.'s (2019) theory that shear fracture systems develop characteristic en échelon structures that consist of offset shear and tensile segments. The shear segments have tight apertures and concentrate fracture gouge, while the tensile segments have open apertures and are relatively free of fragments. Precipitation is enhanced in tight shear segments due to a combination of high rock surface area and low reactive fluid volume. Conversely, precipitation is less favorable in the low-surface area, large-aperture tensile segments. These geometric effects are magnified by shear fracture gouge that is highly reactive due to a combination of high-surface area particles, the production of fresh mineral surfaces, and the production of strained/destabilized material. A mineralized en échelon geometry would allow fracture sealing to occur in the presence of persistent open tensile segments. This suggests an enhancement of permeability anisotropy with flow parallel to shear increasingly cut off by precipitation.