A coupled chemo-mechano conceptual model is presented to follow the evolution of fracture permeability observed in flow-through experiments in a single rock fracture in granite. The experiments are conducted under constant confining pressures of 5 and 10 MPa with differential water pressures ranging 0.01-0.5 MPa, and temperatures of 20 and 90 °C. Permeability measured shows a monotonic decrease with time, via apparent steady state after relatively short periods at 20 °C. A presented model addresses the two dissolution processes at contacting asperities and free walls within fractures, and also describes the multi-mineral dissolution behavior, showing a capability that the evolution of fracture aperture (or related permeability) may be followed with time under an arbitrary temperature and pressure conditions. Predictions utilizing the model proposed in this study show a relatively good agreement with the experimental measurements, although an abrupt reduction observed is incapable of being replicated, that is due to an unaccounted effect in the current model.
An understanding of the flow and transport characteristics of porous/fractured rocks is of significant importance to the effective recovery of energy resources from the subsurface, and for the safe and long-term isolation of energy-related by-products (e.g., high level radioactive wastes and CO2). Notably, under relatively high stress and temperature conditions, mechanically-and chemically-mediated processes may alter the pore structures in rocks, resulting in an irreversible evolution of the flow and transport behavior. Enhancement may result from mechanical dilation as an adjunct to shear deformation with overprinted effects of chemical dissolution within the pore spaces. Whereas degradation may result from reversible or irreversible compaction attributed to mechanical effects, dissolution at the composed grain contacts, and clogging of pore spaces by mineral precipitation.
Pressure solution [1], [2], which is composed of the serial processes of the interfacial dissolution at contacts, interfacial diffusion, and free-face precipitation, is believed to be one of the dominant mechanisms on the chemically-driven compaction. This complex process influences flow and transport characteristics within fractures, and should be well-examined.
In this work, especially focusing on rock fracture, we have conducted flow-through experiments in a granite single fracture under stressed and temperature conditions. The applied stresses (5 and 10 MPa) and temperatures (20 (or 25) and 90 °C) are moderate, but may exert significant influences on the flow and transport behavior. Moreover, a conceptual model, accounting for pressure and free-face dissolutions, is presented to describe the evolution in fracture topology mediated by stress- and temperature-dependent dissolutions, and then validity and usability of the model are examined by comparing the predictions with the measurements.
A model presenting in this work can describe mechanically- and chemically-mediated processes of pressure solution and free-face dissolution. Pressure solution incorporates the three serial processes; dissolution at the contacts, diffusion along the interfacial water film, and precipitation on pore walls. First, dissolution at the contacts provides a source of mass into the pore via the interface.
