科研之友
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摘要
Acidic reactive flow in fractures is relevant in subsurface activities, such as CO2 geological storage and hydraulic fracturing. Understanding reaction-induced changes in fracture hydrodynmic 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 CO2-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 in reacted fracture surfaces. These geometric alterations resulted in increased fracture roughness, as measured by surface Z(2) parameters and fractal dimensions D-f. Computational fluid dynamics (CFD) simulations were conducted to quantify the changes in hydraulic apertaure, fracture transmissivity, and permeability. The result shows that the effective hydraulic aperatures are smaller than the mechanical aperatures and the changes in hydraulic aperatures 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 aperatures or if hydraulic aperture is assumed to change proportionally with mechanical aperture The difference 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 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 he ignored and should be treated with prudence when predicting fracture hydrodynamic properties.
- 出版日期2013-8
