Thermo-hydro-mechano-chemical couplings that define the evolution of permeability in rock fractures.

摘要

Coupled thermal-hydraulic-mechanical-chemical (THMC) processes exert significant influences on the evolution of the mechanical and transport properties of rocks.; In this work, quantitative models are developed to explain significant and anomalous observed changes in the permeability of fractures circulated by hydrothermal fluids. Permeabilities are shown to reduce under net dissolution, and to anomalously switch between permeability reduction and enhancement with no change in ambient conditions. This anomalous behavior is attributed to the significant role of pressure solution in driving the redistribution of mineral mass within fractures. A lumped-parameter model is developed to represent this behavior. This model linked processes of dissolution at the stressed interfaces of grain-to-grain contacts, diffusive transport of dissolved matter from the interface to the pore space, and finally precipitation at the less-stressed surface of the grains. This behavior has been applied to both hydraulically closed and open systems, and: (i) to describe controls on the compaction of quartz aggregates, as analogs for compacting sedimentary basins, and to follow the evolution of porosity and related permeability (Chapter 1), (ii) to explain the significant observation that under certain circumstances net dissolution from contacting rock fractures results in a net decrease in porosity and permeability (Chapter 2), and (iii) to examine the frictional restrengthening attributed to the growth and welding of grain contact areas in the simulated fault gouge (Chapter 3).; The lumped parameter model, incorporating the role of pressure (dis)solution, diffusion, and precipitation, has replicated the observed monotonic closure of a natural fracture in novaculite under constant effective stresses and at moderately elevated temperatures. However, companion experiments of permeability evolution in limestone (Chapter 4) and another novaculite (Chapter 5) sample do not show similar monotonic decreases in permeability; the spontaneous switching in permeability change from aperture reducing to aperture increasing without significant changes in environmental conditions, and are not explicable by the same simple model.; The inability of lumped models to consistently follow the evolution in fracture permeability where dissolution dominates suggests the significant control exerted by fracture topography and void structure. A distributed parameter model is developed and applied to represent the evolution in fracture aperture mediated by the significant processes of pressure solution and free-face dissolution (Chapter 6). (Abstract shortened by UMI.)