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A fully coupled hydrological–mechanical–chemical (HMC) model is developed and applied to explain enigmatic spontaneous changes in permeability that develop within a fracture in limestone under simulated in situ conditions (Water Resour Res 2004;40:W03502). The water flow-through test was concurrently monitored for water and dissolved mineral mass efflux and periodically imaged by X-ray CT to provide redundant constraints on fracture evolution by dissolution and precipitation. The resulting data, exhibiting a spontaneous switch between permeability-decreasing, and permeability-increasing, are evaluated using the HMC model. This model represents these two distinct behaviors: apertures first reduce, under presumed net dissolution, and then preferentially open as net dissolution is even more positively apparent. In the model, dissolution mechanisms within the fracture can be switched from mixed-mode (a combination of pressure dissolution and free-face dissolution) to single mode (pressure dissolution or free-face dissolution). This allows both permeability reduction as mineral mass is net removed from contacting asperities, and permeability enhancement as free-face dissolution localizes along a solution channel, to be accommodated and quantified. The model successfully replicates experimental measurements in limestone. |
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