ABSTRACT:

I describe an experimental system that allows application of a uniform confining stress to transparent analog, variable-aperture fractures during reactive fluid flow experiments. The fractures are fabricated by mating a rough, nonreactive surface (glass) with a smooth reactive surface (KH2PO4). This system permits unobstructed, continuous measurement of transmitted light through the fracture during experiments, providing high-resolution (80 x 80 ?m) measurements of localized changes in fracture apertures. A preliminary dissolution experiment in a fracture subjected to a steady confining pressure exhibited an initial increase in fracture transmissivity as the reactive surface dissolved. However, gradual erosion of contacts caused stresses to increase resulting in sudden decreases in fracture transmissivity. This process repeated itself leading to periodic transmissivity oscillations over the duration of the experiments.

1. INTRODUCTION

Mineral dissolution and precipitation in the subsurface can lead to significant changes in porosity and permeability. In fractured systems, individual fractures often provide the dominant pathways for fluid flow, such that relatively small changes in fracture aperture can significantly influence transport properties. Anthropogenic processes, such as CO2 sequestration or nuclear waste isolation, that cause localized perturbations from chemical equilibrium, may amplify these rock-water reactions. For example, subsurface CO2 sequestration in deep saline aquifers can lead to mineral alterations after the injected CO2 dissolves into resident fluids. The addition of a confining stress during chemical alteration of fracture surfaces forces the surfaces into constant contact and dissolution of contacting asperities can lead to fracture closure; countering these effects, dissolution in the open regions of the fracture causes local increases in fracture aperture. These two mechanisms compete to control the overall impact on fracture transmissivity of coupled mineral dissolution and mechanical deformation. This often results in dissolution-induced decreases in fracture transmissivity [1-3]. However, because data are often limited to measurements of bulk permeability and influent/effluent fluid chemistry, quantifying the relative influence of stress-induced permeability reductions and dissolutioninduced permeability increases is not possible. Recent studies of coupled geochemical/geomechanical alteration have included efforts to quantify changes in fracture aperture. Durham et al. [4] used a surface profilometer to measure the pre- and post-experiment fracture-surface topography and numerically reconstructed the initial and final aperture fields. Results suggested faster dissolution around contacts might have caused the eventual collapse of contacting asperities. However, the results were limited to measurements of the initial and final fracture surfaces and occasional measurements of fluid chemistry during the experiment. X-ray computed tomography (CT) imaging provides a promising approach for directly measuring fracture apertures during experiments. Polak et al. [5] observed the growth of a fracture-scale dissolution channel during an experiment coinciding with a switch from decreasing to increasing transmissivity. Early measured transmissivity reductions were attributed to pressure enhanced dissolution of contacting asperities [6]. Clearly the spatial distribution of mineral alteration within a fracture has a significant influence on the impact on transport properties. However, to date, resolution limitations of X-ray CT have impeded detailed mapping of aperture alterations, except in very small (0.6 x 1.2 cm) cores [7] making

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