Abstract

Compressional and shear waves propagated across single fractures are sensitive to the size and distributions of voids and contact area in a fracture. Fracture geometry is often altered by geochemical reactions among pore fluids and rock. In this study, acoustic monitoring was performed while two chemical solutions were flowed into a fracture to induce chemical precipitation. Mineral precipitation along the fracture plane was not uniform and significant gas bubble evolution occurred during the chemical invasion. Compressional wave amplitudes were significantly affected by the size and distribution of gas bubbles. Once the chemical invasion was halted, bubble formation was minimized but the redistribution of bubbles still occurred. Transmitted wave amplitudes increased in regions with significant amounts of mineral precipitation. The detection of mineral precipitation within a fracture is possible but requires an understanding of the effect of each component that arises from reactive flow on acoustic signals.

1. INTRODUCTION

Fractures in the Earth’s subsurface are subjected to natural and induced physical processes that alter a fracture. For example, a change in stress on a fracture deforms the void geometry and affects the amount of contact area between the two fracture surfaces. Geochemical reactions between pore fluids and the host rock also alter fracture void geometry through dissolution and mineral precipitation. The ability to detect and monitor alterations to fractures using geophysical methods requires a link between a measured geophysical response and a property (or properties) of the fracture.

Compressional and shear waves propagated across single fractures are sensitive to the size and distributions of voids and contact area in a fracture [1, 2]. The complexity of this fracture topology is captured by fracture specific stiffness which is an effective parameter that captures the deformed state of a fracture topology under stress [1, 3]. Fracture specific stiffness increases with an increase in contact area and a decrease in fracture aperture [4-7]. Normal and shear fracture specific stiffness are used in many theoretical and numerical approaches to wave propagation in fractured media to represent the complexity of fracture topology (for example see: [8-14]). From the theory for single fractures, an increase in fracture stiffness results in an increase in transmitted wave amplitude and a decrease in group time delay. Few studies have examined the effect of chemical alteration of fractures on fracture stiffness and/or elastic wave transmission [15]. In this study, we examined the effect of mineral precipitation in a fracture on compressional wave transmission to determine the effect of geochemically altering fracture void geometry.

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