Short-Timescale Chemo-Mechanical Effects and their Influence on the Transport Properties of Fractured Rock

Abstract

Abstract-Anomalous changes in permeability are reported in fractures circulated by fluids undersaturated with respect to the mineral host. Under net dissolution and net removal of mineral mass, fractures may alternately gape or seal, depending on the prevailing mechanical and chemical conditions. The influence on transport properties is observed to be large, rapid, and irreversible: Permeabilities may change by two orders of magnitude in a month, and the direction of permeability change may switch spontaneously, for no apparent change in environmental forcing. These behaviors are apparent in continuous circulation experiments conducted on fractures in novaculite and limestone, intermittently imaged by X-ray CT. In novaculite, permeability reduces by two orders of magnitude as silica is net removed from the sample. Surprisingly, these changes can occur at modest temperatures ($80°C) and stresses ($3.5 MPa), where compaction progresses as temperatures are incremented. Isothermal ($20°C) circulation tests in limestone show similar compaction driven by pressure solution. Where circulation remains undersaturated in Ca, the change in permeability spontaneously switches from net reduction to net increase as a wormhole forms. The surprising magnitude and rapidity of these changes are investigated in the context of the competition between stress-and chemistry-mediated effects.

Chemically and mechanically mediated influences on the transport and mechanical characteristics of rock fractures

“…Direct observation of pressure solution was evaluated by favorable comparison of change in fracture aperture with anticipated mass removal rate measured in the experiment. This behavior is also consistent with observations of the sectioned fracture, post-test, where SEM micrographs show dissolution sites and the blunting and welding of asperity contacts in a natural fracture that had been separated prior to the test [22]. Careful characterization of the fracture topography and observations of equilibrium changes in the hydraulic apertures were available for the experiments [12] and were fit to the model of Eq.…”

Experimental Poroviscoelasticity of Common Sedimentary Rocks

“…Although, the extrapolation to natural conditions yields enormous timescales (tens to hundreds of years) in order to observe time‐dependent deformation of minerals (Karato, ). However, some experiments on low‐porosity rock (Elsworth & Yasuhara, ) have shown that reactions involving reactive fluids (as in CO 2 storage) are fast (hours to weeks), and under relatively modest loads (~10 6 –10 7 Pa) can lead to pressure solution and effectively viscous deformation.…”

Permeability evolution in carbonate fractures: Competing roles of confining stress and fluid pH

“…Therefore, one of the most likely mechanisms is pressure solution as observed elsewhere Elsworth and Yasuhara, 2006;Yasuhara et al, 2006b].…”

“…Additionally, in our experiments, no particulate matter is observed in the effluent fluid, which suggests that a concept of particle‐detachment might be inappropriate as an explanation for aperture reduction. Therefore, one of the most likely mechanisms is pressure solution as observed elsewhere [ Polak et al ., ; Elsworth and Yasuhara , ; Yasuhara et al ., ].…”

“…As a result, we employ a lumped‐parameter model to interpret the dissolution‐dominant evolution of fracture permeability in the following. In our model, permeability evolution is controlled by the competition between two mechanisms: pressure solution at contacting asperities and dissolution at the free‐face surfaces of fractures [ Elsworth and Yasuhara , ; Yasuhara et al ., ]. The details of this lumped parameter model are as follows.…”