Modifications of Carbonate Fracture Hydrodynamic Properties by CO₂-Acidified Brine Flow

Abstract

Acidic reactive flow in fractures is relevant in subsurface activities, such as CO 2 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 CO 2 -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 Z 2 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.

Quantifying fracture geometry with X-ray tomography: Technique of Iterative Local Thresholding (TILT) for 3D image segmentation

“…The iso-surfaces of the fractures were generated from the binary 3D image using an open-source 3D mesh generator iso2mesh with minimum smoothing to preserve the roughness [74], and the sum of the areas of the surface meshes provides an estimate of the fracture surface area. Surface roughness was quantified by the surface Z 2 parameter, which has been used and documented in details in previous studies [21,75]. The surface Z 2 parameter is the root mean square of the elemental surface slope, and has a larger value for a rougher surface for which the local surface slope varies substantially.…”

“…Over the past two decades, X-ray computed tomography (xCT) has become a valuable tool for 3D visualization and characterization of geological specimens [1][2][3][4][5][6][7][8][9], including fracture geometries [10][11][12][13][14][15][16][17][18][19][20][21], pore networks [22][23][24][25][26][27][28][29][30][31], crystal sizes [32,33], and mineral phases [3,[34][35][36][37][38][39]. xCT imaging is indispensable for non-destructive observation of geometry of fractures, which is important because fractures provide preferential flow conduits and often dominate mass transfer in geological materials.…”

“…xCT imaging has been used to provide valuable insights in core-flow experiments designed to investigate fractures in the context of geohydrological, geochemical, and geomechanical processes of the deep subsurface, such as CO 2 geological storage [20,30,40] and oil and gas operations [13-15, 17, 41-46]. Quantitative characterizations of fracture geometries have advanced our understanding of fracture hydrodynamics [13,17,21,[47][48][49], reactivity [11,24,25,35,36,47,[50][51][52][53], and mechanics [16,19,37].…”

“…For fractured media, the algorithms commonly used for segmentation of porous media are generally applicable [11,14,19,21,52,64]. For example, Karpyn et al [14] adopted a thresholding-based approach to differentiate water and oil phase from rock matrix, and Deng et al [21] devised a ternary thresholding method to identify the fracture, reaction-altered rock matrix, and unaltered rock matrix, while Gouze et al [11] and Noiriel et al [52] applied the region growing method.…”

“…For example, Karpyn et al [14] adopted a thresholding-based approach to differentiate water and oil phase from rock matrix, and Deng et al [21] devised a ternary thresholding method to identify the fracture, reaction-altered rock matrix, and unaltered rock matrix, while Gouze et al [11] and Noiriel et al [52] applied the region growing method. However, there are challenges to direct application of those methods to fracture segmentation.…”