Experimental observations of deformation caused by mineral dissolution in variable-aperture fractures

摘要

Problems such as CO2 sequestration, petroleum production and nuclear waste isolation involve the potential for rock-water reactions. Mineral alteration resulting from reactive fluid flow can lead to significant changes to fracture transport properties. At depth, these processes are further influenced by stresses in the host rock. To quantitatively explore these coupled processes, we built a new experimental apparatus designed to directly measure changes in fracture aperture in analog fractures subjected to the combined influence of a reactive fluid and an applied normal stress. Light transmission techniques provided direct measurements of the changing fracture aperture at high spatial resolution during two experiments in identical fractures with an initial mean fracture aperture of 95 μm. The two experiments were carried out at values of the dimensionless Damkohler number (Da = reaction rate/advection rate) that differed by a factor of 2. The high-Da experiment resulted in the formation of a large-scale dissolution channel in the middle of the fracture and regions with little dissolution and slow closure of the fracture surfaces. By contrast, the low-Da experiment exhibited relatively uniform dissolution across the width of the fracture, with locally enhanced dissolution in small aperture regions. This resulted in increased stresses in contacting asperities and eventual damage of the asperities accompanied by large (up to 50 μm), instantaneous displacements of the surfaces and corresponding reductions in fracture aperture. The results demonstrate the importance of the spatial variability of dissolution rates, which are controlled by both local reaction kinetics and hydrodynamics, in fractures deforming because of combined dissolution and mechanical stress.