Formation of Superaitical Geothermal Resevoirs by Means of Supercritical Water-Induced Cracking

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The research project determined the feasibility of utilizing the interactions between supercritical water and rock-forming minerals in the creation of an artificial subsurface geothermal reservoir in a deep-seated rock mass, in which temperature and pressure conditions exceed the critical point for water. The main results obtained in this study are summarized below.1.Application of confining pressures in high temperature testing was shown tot be effective in order to mitigate the thermal stress induced micro-cracking due to thermal expansion mismatch of the rock-forming minerals by conducting ultrasonic wave velocity measurements.2.Permeability measurements were made for several types of granitic rocks using thick-walled cylindrical specimens. The rock specimens included cores taken from the deep-seated high temperature rock mass at an actual geothermal field site. It has been shown that the permeability measured jumped significantly when the condition of water exceeded the critical po … More int for water regardless of the rock types used in this study. The significant increase in permeability was shown to be due to grain-scale microcracking induced from the interaction between the supercritical water and rock-forming minerals. The extent of the permeability increase was less for the rock specimens taken from the deep-seated rock masses than that for the rock types obtained from outcrops.3.Tri-axial compression tests were conducted in order to examine the effect of stress on the above-mentioned supercritical water induced cracking phenomenon. It has been demonstrated that the microcrack density at the vicinity of the final fracture plane was signifcantly larger than that at the far-field when the temperature was greater than the critical temperature. The increase in the microcrack density points out that the supercritical water induced cracking was accelerated by the effect of stress. The accelerated cracking is newty named as supercritical water enhanced stress corrosion cracking. The experimental observation may suggest the potential use of the supercritical water enhanced stress corrosion cracking the expansion of hydraulically-induced geothermal reservoirs.4.We undertook long-term thermal extraction numerical modeling analysis, for a supercritical rock mass, which incorporated fluid flow characteristics (i. e. circulating water) and thermal transfer in the surrounding rock mass. We considered the effects of temperacure and pressure in the supercritical fluid regime, as well as fluid density, viscosity etc. Based on the numerical results, it has been shown that there is an optimum injection rate to extract the heat energy from the supercritical reservoir combined with subcritical temperacure rock mass. Less