Rapid Mineral Precipitation During Shear Fracturing of Carbonate-Rich Shales

Flow through fractures in low-permeability reservoirs targeted for emerging energy technologies can be controlled by geochemically mediated deformations that remain poorly understood. Here, a series of three triaxial direct shear experiments was conducted to evaluate how fractures generated at representative subsurface conditions respond to penetration of reactive fluids with a focus on the role of mineral precipitation resulting from fluid-rock interactions. Carbonate-rich shale cores (66-95% calcite) were directly sheared with BaCl₂-rich fluids under 3.5 MPa confining stress. Experiments were conducted within an x-ray computed tomography (xCT) scanner to capture reactions in 4D. X-ray radiographs were collected continuously while xCT scans were taken at critical points in each test and segmented to quantify changes in precipitate and fracture volumes. Reaction products were identified in thin sections with a scanning electron microscope.

All tests evidenced non-uniform precipitation of barium carbonates (BaCO₃) along fracture surfaces generated in shearing. The extent of precipitation increased with increasing calcite content and led to an 80% permeability reduction in the most calcite-rich core. X-ray radiographs indicated precipitation occurred rapidly within 10-20 minutes of shear fracturing. Precipitates generally formed in fractures with narrow apertures and extensive fragmentation, which provided greater reactive surface area, but were also highly localized due to complex feedback among mineralogical, geochemical, and structural heterogeneities. The results highlight idiosyncrasies of precipitation processes whereby nucleation and growth may be highly localized throughout a geologic system despite uniform thermodynamic favorability. In a broader context, this study demonstrates the role of fluid-rock interactions in precipitation reactions that can work to counteract mechanical fracture closure by propping or sealing apertures. Such reactions are more challenging to predict than precipitation resulting from mixing of incompatible fluids but can influence or control fluid transport, particularly in shales where even small amounts of precipitates can dramatically reduce fracture permeability if they localize in narrow pore throats or along critical flow paths.