Roles of chemical and mechanical processes on the evolution of fracture permeability in carbonates

We explore how fracture permeability in confined carbonates evolves due to flow of reactive fluids. Core plugs of the Capitan Massive Limestone are saw-cut to form a consistent axial fracture that is subsequently roughened to simulate a natural fracture with controlled topography. Either distilled water or 0.176 M NH₄Cl solutions are circulated while initial fracture roughness, influent fluid pH, and confining stresses are controlled. Throughout the experiment we measure the fluid flow rate and chemical composition of the effluent fluid. The cubic law is used to infer the retreat or advance rate of the fracture walls from the evolution of permeability. By taking measurements in regimes of both increasing and decreasing permeability we quantitatively constrain the transition between fracture-gaping and fracture-closing modes of behavior. We parameterize this transition using the ratio of mechanically- to chemically-controlled dissolved mass fluxes and compare it to the effective hydraulic aperture velocity. The transition value of unity for the mass flux ratio logically coincides with a predicted net effective hydraulic aperture velocity that asymptotes to zero. These results offer a first opportunity of constraining the transition between stress-dominated and dissolution-dominated mechanisms of permeability evolution for stress-sensitive fractures.