Short-Timescale Chemo-Mechanical Effects and their Influence on the Transport Properties of Fractured Rock
Pure and Applied Geophysics, 163 (10):
2051--2070 (2006)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 prevailingmechanical 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 conductedon 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 (~80C) and stresses (~3.5 MPa), where compactionprogresses as temperatures are incremented. Isothermal (~20C) circulation tests in limestone show similar compaction drivenby 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 surprisingmagnitude and rapidity of these changes are investigated in the context of the competition between stress- and chemistry-mediatedeffects.