Modifications of carbonate fracture hydrodynamic properties by CO ₂-acidified brine flow

Acidic reactive flow in fractures is relevant in subsurface activities, such as CO₂ geological storage and hydraulic fracturing. Understanding reaction-induced changes in fracture hydrodynamic properties is essential for predicting subsurface flows, such as leakage, injectability, and fluid production. In this study, X-ray computed tomography scans of a fractured carbonate caprock were used to create three-dimensional (3D) reconstructions of the fracture before and after reaction with CO₂-acidified brine (Ellis, B.; Peters, C.; Fitts, J.; Bromhal, G.; McIntyre, D.; Warzinski, R.; Rosenbaum, E. Deterioration of a fractured carbonate caprock exposed to CO ₂-acidified brine flow. Greenhouse Gases: Sci. Technol. 2011, 1, 248-260). As expected, mechanical apertures were found to increase substantially, doubling and even tripling in some places. However, the surface geometry evolved in complex ways, including "comb-tooth" structures created from preferential dissolution of calcite in transverse sedimentary bands and the creation of degraded zones, i.e., porous calcite-depleted areas on reacted fracture surfaces. These geometric alterations resulted in increased fracture roughness, as measured by surface Z₂ parameters and fractal dimensions D_f. Computational fluid dynamics (CFD) simulations were conducted to quantify the changes in hydraulic aperture, fracture transmissivity, and permeability. The results show that the effective hydraulic apertures are smaller than the mechanical apertures and the changes in hydraulic apertures are nonlinear. Overestimation of the flow rate by a factor of 2 or more would be introduced if fracture hydrodynamic properties were based on mechanical apertures or if hydraulic aperture is assumed to change proportionally with mechanical aperture. The differences can be attributed, in part, to the increase in roughness after reaction and is likely affected by contiguous transverse sedimentary features. Hydraulic apertures estimated by the one-dimensional (1D) statistical model and two-dimensional (2D) local cubic law (LCL) model are consistently larger than those calculated from the CFD simulations. In addition, a novel ternary segmentation method was devised to handle the degraded zones, allowing for a bounding analysis of the effects on hydraulic properties. We found that the degraded zones account for less than 15% of the fracture volume but cover 70-80% of the fracture surface. When the degraded zones are treated as part of the fracture, the fracture transmissivities are 2-4 times larger because the fracture surfaces after reaction are not as rough as they would be if one considers the degraded zone as part of the rock. Therefore, while degraded zones created during geochemical reactions may not significantly increase mechanical aperture, this type of feature cannot be ignored and should be treated with prudence when predicting fracture hydrodynamic properties.