Amir Polak

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.

A mechanistic model for compaction of granular aggregates moderated by pressure solution

[1] A model is presented for the compaction of granular aggregates that accommodates the serial processes of grain-contact dissolution, grain-boundary diffusion, and precipitation at the pore wall. The progress of compaction and the evolution of the mass concentration of the pore fluids may be followed with time, for arbitrary mean stress, fluid pressure, and temperature conditions, for hydraulically open or closed systems, and accommodating arbitrary switching in dominant processes, from dissolution, to diffusion, to precipitation. Hindcast comparisons for compaction of quartz sands [Elias and Hajash, 1992] show excellent agreement for rates of change of porosity, the asymptotic long-term porosity, and for the development of silica concentrations in the pore fluid with time. Predictions may be extended to hydraulically open systems where flushing by meteoric fluids affects the compaction response. For basins at depths to a few kilometers, at effective stresses of 35 MPa, and temperatures in the range 75� –300� C, rates of porosity reduction and ultimate magnitudes of porosity reduction increase with increased temperature. Ultimate porosities asymptote to the order of 15% (300� C) to 25% (75� C) at the completion of dissolution-mediated compaction and durations are accelerated from a few centuries to a fraction of a year as the temperature is increased. Where the system is hydraulically open, flushing elevates the final porosity, has little effect on evolving strain in these precipitation-controlled systems, and depresses pore fluid concentrations. These effects are greatest at lower temperatures. INDEX TERMS: 5120 Physical Properties of Rocks: Plasticity, diffusion, and creep; 5139 Physical Properties of Rocks: Transport properties; 8045 Structural Geology: Role of fluids; 8160 Tectonophysics: Evolution of the Earth: Rheology—general;

Tracer diffusion from a horizontal fracture into the surrounding matrix: measurement by computed tomography

The vertical diffusion of NaI solution from a horizontal fracture into and within the surrounding matrix was tracked and quantified over time using an artificially fractured chalk core (30x5 cm) and a second-generation X-ray computed tomography (CT) scanner. The different tracer-penetration distances imaged in the matrix above and below the horizontal fracture are indicative of a greater tracer mass penetrating into the lower matrix. The enhanced transport in the matrix below the fracture was related to the Rayleigh-Darcy instability induced by the density differences between the heavier tracer solution in the fracture (1.038) and the distilled water that had initially resided in the matrix. Our observations suggest that below the fracture, the tracer is propagated by an advection-diffusion process that is characterized by both higher rates and higher concentrations relative to its propagation by diffusion above the fracture. The experimental results suggest that the prediction of contaminant migration in a rock intersected by both vertical and horizontal (e.g. along bedding planes) fractures is difficult because of density effects that result in different solute-penetration rates.

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.

Compaction and Diagenesis of Sandstones – the Role of Pressure Solution

Abstract A model is presented for the compaction of granular aggregates that accommodates the serial processes of grain-contact dissolution, grain-boundary diffusion, and precipitation at the pore wall. Importantly, this treatment follows the progress of grain interpenetration as contact areas grow, mass transport lengths increase, and rate-limiting processes may switch with the progress of compaction. A simple repeating closed system incorporates two stressed grains in contact and enables the progress of compaction, and the evolution of the mass concentration of the pore fluids to be followed with time, for arbitrary mean stress, fluid pressure, and temperature conditions. Hindcast comparisons with experimental results for the compaction of quartz sand in a closed system ( Elias and Hajash, 1992 ) show excellent agreement for rates of change of porosity, the asymptotic long-term porosity, and for the development of silica concentrations in the pore fluid with time.