Fundamental investigations on mechanical and hydraulic characteristics of the beyond-brittle geothermal reservoir

A new concept of Enhanced Geothermal System (EGS), in which geothermal fluids are produced from a fractured reservoir created artificially within an originally semi-brittle or ductile basement, has been proposed. This new geothermal system potentially has a number of advantages including: (a) simpler design and control of the reservoir, (b) nearly full recovery of injected water, (c) sustainable production, (d) lower cost when developed in relatively shallower zones in compression tectonic settings, (e) large potential quantities of energy extraction from widely distributed semi-brittle or ductile zones, (f) the establishment of a universal design/development methodology, and (g) suppression of felt earthquakes from/around the reservoirs. To assess the potential of the new geothermal system, the "Japan Beyond-Brittle Project (JBBP)" has also been recently initiated, and the authors have conducted fundamental investigations on mechanical and hydraulic characteristics of the new type of reservoir, in which the rock first experiences hydraulically and/or thermally induced brittle failure, and then subjected to the temperature and pressure conditions where the rock exhibits semi-brittle or ductile stress-strain behavior at the original condition. Gypsum aggregate specimens have been used in the present study, because brittle, semi-brittle and ductile stress-stain behaviors of the specimen can be controlled only with confining stress level at the room temperature. At confining stresses up to 40 MPa, tri-axial compression and fluid flow experiments have been conducted on the specimens without and with fracture. It has been indicated that stress-strain behavior is independent of existence of fracture. Moreover, permeability of the specimen with fracture is much higher than that without fracture at conditions of brittle, semi-brittle, and ductile stress-strain behaviors. On the other hand, permeability of the specimen with fracture is not so different from that without fracture at conditions of a transitional behavior between semi-brittle and ductile stress-strain behaviors.