Hideaki Yasuhara

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;

Applicability of Enzymatic Calcium Carbonate Precipitation as a Soil-Strengthening Technique

AbstractA grouting technique for enzymatic calcite precipitation is evaluated. Urea and calcium salt, at various concentrations, are mixed with a concentration-fixed enzyme to obtain the optimal precipitation of CaCO3. The optimally combined solution is injected into sand samples in small PVC cylinders. Then, the improvement in small-scale samples is observed. The combination, approved for small-scale tests, is further used for larger-scale tests. The porosity distribution within the soil is evaluated by sampling the treated sand at different locations. A precipitation ratio up to 80% can be obtained using a small amount of the enzyme. The results show that the in situ enzymatic CaCO3 precipitation technique may be feasible for use in larger-scale applications. A multiphysics simulator that considers the calcite precipitation reaction during the transport of the solution is adopted to predict the evolution of the porosity. The predicted porosities are compared with the measured porosities. The results sho...

Long‐term observation of permeability in sedimentary rocks under high‐temperature and stress conditions and its interpretation mediated by microstructural investigations

In this study, a series of long-term, intermittent permeability experiments utilizing Berea sandstone and Horonobe mudstone samples, with and without a single artificial fracture, is conducted for more than 1000 days to examine the evolution of rock permeability under relatively high-temperature and confining pressure conditions. Effluent element concentrations are also measured throughout the experiments. Before and after flow-through experiments, rock samples are prepared for X-ray diffraction, X-ray fluorescence, and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy to examine the mineralogical changes between pre and postexperimental samples, and also for microfocus X-ray CT to evaluate the alteration of the microstructure. Although there are exceptions, the observed, qualitative evolution of permeability is found to be generally consistent in both the intact and the fractured rock samples—the permeability in the intact rock samples increases with time after experiencing no significant changes in permeability for the first several hundred days, while that in the fractured rock samples decreases with time. An evaluation of the Damkohler number and of the net dissolution, using the measured element concentrations, reveals that the increase in permeability can most likely be attributed to the relative dominance of the mineral dissolution in the pore spaces, while the decrease can most likely be attributed to the mineral dissolution/crushing at the propping asperities within the fracture. Taking supplemental observations by microfocus X-ray CT and using the intact sandstone samples, a slight increase in relatively large pore spaces is seen. This supports the increase in permeability observed in the flow-through experiments.

Effect of Magnesium as Substitute Material in Enzyme-Mediated Calcite Precipitation for Soil-Improvement Technique

The optimization of enzyme-mediated calcite precipitation was evaluated as a soil-improvement technique. In our previous works, purified urease was utilized to bio-catalyze the hydrolysis of urea, which causes the supplied Ca2+ to precipitate with CO32− as calcium carbonate. In the present work, magnesium chloride was newly added to the injecting solutions to delay the reaction rate and to enhance the amount of carbonate precipitation. Soil specimens were prepared in PVC cylinders and treated with concentration-controlled solutions composed of urea, urease, calcium, and magnesium chloride. The mechanical properties of the treated soil specimens were examined through unconfined compressive strength (UCS) tests. A precipitation ratio of the carbonate up to 90% of the maximum theoretical precipitation was achieved by adding a small amount of magnesium chloride. Adding magnesium chloride as a delaying agent was indeed found to reduce the reaction rate of the precipitation, which may increase the volume of the treated soil if used in real fields because of the slower precipitation rate and the resulting higher injectivity. A mineralogical analysis revealed that magnesium chloride decreases the crystal size of the precipitated materials and that another carbonate of aragonite is newly formed. Mechanical test results indicated that carbonate precipitates within the soils and brings about a significant improvement in strength. A maximum UCS of 0.6 MPa was obtained from the treated samples.

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.

An evaluation of the effects of fracture diagenesis on hydraulic fracturing treatment

The rates and magnitudes of fracture permeability on commercially available proppants were determined from flow through experiments at different stresses and temperatures for hydraulically open and closed systems. A pressure solution model describing multi-mineral dissolution behavior is extended to accommodate the specific nature of the compaction of the synthesized proppant with different chemical compositions. Mechanisms include multi-mineral dissolution, transport, and re-precipitation at the contacting asperities and the free walls within the fractures, resulting in a loss of porosity in proppant packs. The mechanistic model used recovered thermodynamic and kinetic data for mineralogical composition of available proppants within rigid-walled fluid pressure only furnace and ambient stress quadcell reactors. Under reservoir temperature of 191°C and stresses of 65.5 MPa, these ensemble data suggest that proppant packs may compact by up to 10% over the period of a few years.

Stress- and Chemistry-Mediated Permeability Enhancement/Degradation in Stimulated Critically-Stressed Fractures

This work has investigated the interactions between stress and chemistry in controlling the evolution of permeability in stimulated fractured reservoirs through an integrated program of experimentation and modeling. Flow-through experiments on natural and artificial fractures in Coso diorite have examined the evolution of permeability under paths of mean and deviatoric stresses, including the role of dissolution and precipitation. Models accommodating these behaviors have examined the importance of incorporating the complex couplings between stress and chemistry in examining the evolution of permeability in EGS reservoirs. This document reports the findings of experiment [1,2] and analysis [3,4], in four sequential chapters.

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.