A. S. Grader

Permeability reduction of a natural fracture under net dissolution by hydrothermal fluids

[1] Flow-through tests are completed on a natural fracture in novaculite at temperatures of 20� C, 80� C, 120� C, and 150� C. Measurements of fluid and dissolved mass fluxes, and concurrent X-ray CT imaging, are used to constrain the progress of mineral dissolution and its effect on transport properties. Under constant effective stress, fracture permeability decreases monotonically with an increase in temperature. Increases in temperature cause closure of the fracture, although each increment in temperature causes a successively smaller effect. The initial differential fluid pressure-drop across the fracture increases by two orders of magnitude through the 900 h duration of the test, consistent with a reduction of an equivalent hydraulic aperture by a factor of five. Both the magnitude and rate of aperture reduction is consistent with the dissolution of stressed asperities in contact, as confirmed by the hydraulic and mass efflux data. These observations are confirmed by CT imaging, resolved to 35 microns, and define the potentially substantial influence that benign changes in environmental conditions of stress, temperature, and chemistry may exert on transport properties. INDEX TERMS: 5104 Physical Properties of Rocks: Fracture and flow; 5114 Physical Properties of Rocks: Permeability and porosity; 5194 Physical Properties of Rocks: Instruments and techniques; 5134 Physical Properties of Rocks: Thermal properties; 8135 Tectonophysics: Hydrothermal systems (8424). Citation: Polak, A., D. Elsworth, H. Yasuhara, A. S. Grader, and P. M. Halleck, Permeability reduction of a natural fracture under net dissolution by hydrothermal fluids, Geophys. Res. Lett., 30(20), 2020, doi:10.1029/2003GL017575, 2003.

The Evolution of Permeability in Natural Fractures - The Competing Roles of Pressure Solution and Free-Face Dissolution

Abstract Fracture permeabilities are shown surprisingly sensitive to mineral dissolution at modest temperatures (c. 20°–80°C) and flow rates. Net dissolution may either increase or decrease permeability, depending on the prevailing ambient THMC conditions. These behaviours have important ramifications for constitutive laws for flow and transport. Flow-through tests are completed on a natural fracture in novaculite at temperatures of 20°C, 80°C, 120°C, and 150°C, and on an artificial fracture in limestone at 20°C. Measurements of fluid and dissolved mass fluxes, concurrent X-ray CT and imaging, and post-test sectioning and SEM are used to constrain the progress of mineral dissolution and its effect on transport properties. For the novaculite, under constant effective stress, fracture permeability decreased monotonically with an increase in temperature, with fracture permeability reducing by two-orders-of-magnitude over the 900 h test. For the limestone, an initial decrease in permeability over the first 935h of the test, switched to a net increase in permeability as distilled water was subsequently circulated for the final 500h of the test.