Asperities in a fracture subjected shear offsets prop the fracture open, and are key to providing permeability necessary for efficient fluid circulation in enhanced geothermal systems (EGS). Changes in these asperities and the resulting permeability over time are controlled by many factors including mineral dissolution and precipitation in the fractures and by mechanical deformation of the asperities and the rock. These factors are functions of the temperature, pressure, fluid chemistry, mineralogy, and stress state of the system. We conducted laboratory experiments to quantify fracture permeability changes under conditions that could be expected in EGS reservoirs (effective fracture normal stress up to several tens of MPa, temperature up to 300°C). Our objective has been to gain a better understanding of how different rock types, mineral compositions, textures, and fracture surface topography affect the life span of fractures, enabling selection of reservoir rock to optimize long-term fracturing effectiveness. In the current experiment, we used rock samples obtained from a borehole at Brady Hot Springs: a meta-mudstone-volcanic sample from beneath 4800 ft, a meta-dacite sample from beneath 3900 ft, and an altered tuff from beneath 2300 ft. For our tests we developed and built a specialized laboratory apparatus which applies normal stress on a disk-shaped rock sample containing an induced offset extensile fracture perpendicular to the core axis. In this apparatus, we flow water radially through the aperture and measure the aperture change, fracture permeability, and effluent chemistry. Using this system, we performed extended duration tests (several weeks to several months) to assess differences in the time-dependent changes in permeability. Although the rock samples were from different depths and therefore were naturally subjected to different in-situ effective confining stress and temperature, the experiments were conducted using 20.7 MPa effective stress and at 250°C, with distilled water as the pore fluid. All three samples of different rock types exhibited decreasing fracture aperture, with change slowing with increasing time. These decreases, however, were not sufficient to result in significant changes in the permeability of the fracture for the samples from 4800ft and 3900ft. In contrast, the altered tuff sample from 2300 ft exhibited large reductions in both fracture aperture and permeability in less than 2 weeks from the start of the experiment. To understand the long-duration fracture compaction experiment, we characterized rock mineralogy and mechanical properties (from P and S-wave velocities), and core-scale mineral density variations resulting from dissolution and precipitation using X-ray CT scans. Before and after the tests we measured the surface profile of both fracture surfaces, to identify preferential mineral precipitation and dissolution sites that can affect the flow paths, as well as mechanical damage. All samples exhibited clear chemical reactions on the fracture surfaces as a result of the experiments. Particularly, the meta-dacite (2.5 month test) and the altered tuff (2 week test) samples exhibited large reductions in the rock density around the fluid inlet, indicating significant selective dissolution of the minerals. The chemical changes on the fracture surfaces were localized for the two samples exhibiting little permeability changes. In contrast, the altered-tuff sample showed large reductions in permeability, and exhibited uniformly distributed surfaces alternation. Additional detailed post-experiment sample characterization and modeling of the tests has begun to allow a more generalized interpretation of the results.

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