Modification of fracture geometry by calcite precipitation

Geochemical interactions during the injection/withdrawal of fluids into the subsurface can modify fracture apertures through dissolution and/or precipitation of minerals. Modification of fracture apertures by reactive flows is strongly affected by non-reactive, non-wetting fluids that limit the fracture surface area and void volume that is accessible to the reactive fluids. An experimental investigation was performed to examine the controls on mineral precipitation within a fracture when mineral precipitation occurs with and without the generation of gas. Several differences were observed in the results from the experiments using the two chemical approaches for generating precipitates. The pore-filling precipitates (particles formed in solution) were more uniform in size and were thicker than the surface adhering precipitates (particles formed on fracture surfaces). The acoustic response for fractures with pore-filling precipitates reached a steady-state after 2 hours while fractures with surface adhering precipitates took longer to equilibrate because of the presence of CO₂ that was generated during the chemical reaction at liquid-surface interfaces. In surface adhering precipitation case, the generation of CO₂ limited accessibility of calcium carbonate pre-nucleation polymers to the entire void volume of the fractures. On the other hand, fractures with pore-filling precipitates exhibited a more uniform acoustic response across the fracture plane than fractures with surface adhering precipitates. Understanding the effect of mineral precipitation on acoustic wave attenuation provides a path forward for long-term monitoring of seal integrity.