Abstract

The interdependent effects of mechanical stresses and fluid-rock reactions govern the evolution of fracture permeability. Yet, the ability to predict whether the coupled chemical and mechanical processes will enhance or diminish the permeability of heterogeneous fractures remains elusive. In this paper, we present a modeling framework based on a previously developed model that calculates the combined effect of reactive-flow and geomechanical deformation on a heterogeneous fracture. We use synthetic aperture fields with varied micro-scale roughness and correlation length to investigate the effect of geometrical heterogeneities on the evolution of fracture apertures due to reactive flow. An Increase in fracture fluctuations amplitude enhances the development of channels in the fracture and can shift the dissolution regime from advection-dominated to reaction-dominated. Whereas an increase in correlation length reduces the number of channels while enhancing their widths. Anticipating such differences in permeability evolution is important in engineering application involving fluid-rock interactions, such as acid stimulation, for effective and efficient reservoir permeability enhancement.

1. INTRODUCTION

Fractures often control transport properties in rock formations. Quantifying fracture permeability is crucial for the long-term isolation in geologic formations of (i) injected greenhouse gases (CO2) for climate change mitigation [1, 2], (ii) waste fluids for underground disposal [3], and (iii) nuclear waste for secure storage [4, 5]. The successful isolation of these fluids within rock formations requires robust predictions of fracture permeabilities, particularly in caprock formations relied upon to provide structural seals [6, 7]. Also, quantifying fracture permeability is important for the production of fluids through enhanced hydrocarbons extraction [6] and geothermal systems [8, 9] for energy purposes.

The combined effects of fluid-rock reactions and mechanical stresses govern the evolution of permeability and porosity of reservoir and seal systems subjected to reactive fluids (e.g. [9-12]). Three processes govern the evolution of fracture porosity and permeability during reactive flow: (i) fluid flow through the fracture aperture for a given pressure drop or flow rate, (ii) concomitant evolution of the aperture and fluid composition due to chemical mass transfer across the fluid-rock interface, and (iii) aperture evolution due to mechanical stresses applied on the fracture surfaces. While normal compressive stresses deform the fracture and decrease its permeability, reactive fluid-flow can both increase and decrease fracture permeability by means of mineral dissolution and precipitation, respectively. Moreover, the combined effect of mechanical deformation and reactive flow on permeability can be dramatically different from the effect of mechanical deformation and reactive flow separately [13-15]. Tuning the flow rate or the reaction rate can result in the onset of reaction instabilities at certain length scales [16, 17]. Here we show how the onset of reaction instabilities is influenced by fracture heterogeneities characterized by the correlation length and fluctuation amplitude of the aperture. In the following sections, we present our model, use the model to confirm previously reported results on the effect of reactive flow and mechanical deformation on fracture aperture evolution, and investigate the influence of geometrical heterogeneities of the fracture aperture on the evolution of fracture permeability.

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