We present a large-scale testing system for determining the precise physical mechanisms at work when ambient temperature water is injected into a high temperature geothermal reservoir. The poly-axial testing chamber can apply up to 14 MPa loads on 3 independent axes using flat-jacks as the loading mechanism. The chamber can be flooded by up to 300? C steam, so that the rock can be brought to in situ equilibrium. Heat can also be applied by four 2 kW heaters around the block perimeter, with the additional application of modeling hot dry rock systems. Steam at an appropriate quality is used as the pore fluid so that field electro-resistive behavior and in situ effective stresses can be accurately modeled. To account for the actual fractured nature of the in situ rock mass, size effects of fracture toughness, and to undo averaging of electro-resistive properties, the specimens will be 260 mm cubic assemblages made up of outcrop rock representative of reservoir rock. Fracturing of the sample will be monitored by a dense array of resistivity sensors, which will be used to create a real-time tomograph. The paper describes the full system and gives examples of the suitability of using electro-resistive tomography for forward and inverse imaging.

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