Modeling the Evolution of the Fracture Permeability in Granite due to Free-face Dissolution and Pressure Solution

Abstract:

This paper focuses on the evolution of the fracture permeability due to water-granite long-term interactions when deionized water flows through the fracture surface. Laboratory-scale batch experiments have been conducted by Yasuhara et al (2011), wherein artificial fractures are subject to a mechanical confining pressure, variable differential hydraulic pressures and different applied temperatures. The aqueous geochemical system involved in the chemical weathering of granite is investigated in the first place which is a mixture of several kinetic reactions corresponding to mineral dissolution and a series of equilibrium reactions corresponding to potential derivatives in the aqueous solution. As fracture surfaces are in contact under confining stress, mineral dissolution rates may be different at hydrostatically stressed open pore and at asperity contacts under non-hydrostatic stress. Especially at asperity contacts, intergranular pressure solution may accelerate mineral dissolution rates whose driving force is represented as the chemical potential difference between a stressed contact and a hydrostatically stressed open pore (Taron and Elsworth (2010)). To better understand dominant mechanisms in the system, a reactive transport model including both the free-face reactions and the pressure solution is developed in the open-source simulator OpenGeoSys. Fracture aperture is updated as a result of the mass removal from the open-pore walls and the contacting asperities. The study presents impacts of mineral composition and their spatial distribution on the permeability evolution.

References

Yasuhara, H., Kinoshita, N., Ohfuji, H., Lee, D.S., Nakashima, S., and Kishida, K. (2011), Temporal alteration of fracture permeability in granite under hydrothermal conditions and its interpretation by coupled chemo-mechanical model. Applied Geochemistry 26: 2074–2088.

Taron, J., and Elsworth, D. (2010). Constraints on compaction rate and equilibrium in the pressure solution creep of quartz aggregates and fractures: Controls of aqueous concentration. Journal of Geophysical Research: Solid Earth (1978–2012), 115(B7).