Abstract
Superhot geothermal environments in granitic crusts of approximately 400–500 °C are a frontier of geothermal energy. In the development of such environments, there is a concern of a reduction of permeability of fractured granite due to the formation of fine particles of amorphous silica induced by the phase change from subcritical water to supercritical water or superheated steam. However, the formation of silica particles and a resultant reduction in permeability have not been demonstrated to date. Therefore, experiments were conducted on the formation of amorphous silica particles with various combinations of temperature (430–500 °C) and pressure (20–30 MPa), in which the phase change of Si-containing water from liquid to either supercritical fluid or vapor was induced. Amorphous silica nanoparticles occurred under all conditions with smaller particles for higher temperature. The permeability of fractured granite was also observed to decrease significantly within several hours during injection of the particles into rock at 450 °C and 30 MPa under a confining stress of 40 MPa, with slower permeability reduction at a smaller number of particles or in the presence of larger aperture fractures. The present study suggests that the nanoparticles are likely to form and destroy the permeability in superhot geothermal environments, against which countermeasures should be investigated.
Highlights
Superhot geothermal environments in granitic crusts of approximately 400–500 °C are a frontier of geothermal energy
Accessing geothermal environments at temperatures exceeding the critical temperature of water at approximately 2–4 km depth is expected to increase productivity and sustainability in geothermal energy utilization because such superhot or supercritical geothermal environments, demonstrated by drilling in Italy1–3, Iceland[4,5,6], the United S tates7,8, Mexico[9] and Japan[10], could provide supercritical water or superheated steam with high specific enthalpies of greater than or equal to ca. 2 MJ kg−111–16
It is valuable to find enhanced geothermal system (EGS) technologies that are appropriate for superhot geothermal e nvironments[28] such as hydraulic fracturing to produce or reproduce permeable fracture networks through which a heat transmission fluid circulates between injection and production wells, and means to keep the permeability of the fractured rock adequately high for profitable and sustainable energy production because permeability changes impact energy production in any type of geothermal system[29,30,31,32,33]
Summary
Superhot geothermal environments in granitic crusts of approximately 400–500 °C are a frontier of geothermal energy. In the development of such environments, there is a concern of a reduction of permeability of fractured granite due to the formation of fine particles of amorphous silica induced by the phase change from subcritical water to supercritical water or superheated steam. Significant aspects of the environments are the elevated efficiency of mineral plastic processes[18,19], the retrograde quartz solubility[7,20,21] and enhanced rates in the fracture healing and sealing by water–rock reactions[22,23,24] These could cause the loss of the permeable fracture networks that transmit and store geothermal fluids. Study suggested that permeability may decrease as a result of the formation of fine amorphous silica particles when the phase change of water induces a highly supersaturated state of amorphous s ilica[36]. It was suggested that such silica particles can have a significant impact on the hydrology and mechanical behavior of hydrothermal systems in volcanic areas
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