Fracture Generation in Artificial Geothermal Reservoirs Under Supercritical Water Conditions

Abstract

This paper discusses the generation of micro-fractures in a granite under a super critical water environment. In order to create an artificial pathway of water in a geothermal reservoir with limited permeability, hydraulic stimulation technology is commonly employed. In this study, simulated hydraulic stimulation tests were performed using thick-walled cylindrical specimens of 45 mm outer diameter, under temperatures up to 600 ìC and c o d i g pressures up to 100 MPa. The experimental results showed that there was no macroscopic fracturing in the high temperature regime where the predominant fluid flow occurred. The permeability of the granite was also measured using the same cylindrical specimen configuration as was used in the simulated hydraulic stimulation tests. The permeability test results showed that the permeability of the granite was enhanced drastically when the temperature exceeded the critical point of water, whilst no significant increase in the permeability was observed under the subcritical water condition. Optical microscopy of the microstructural change revealed that the enhanced permeability was due to the formation of micro-fractures under the supercritical water environment. This laboratory-scale test result suggests that it may be possible to generate a micro-fracture network by injecting water into a high temperature rock mass whose conditions exceed the critical point of water and to extract the heat energy through the generated fracture network from the supercritical rock mass.