Heat production from lab-scale enhanced geothermal systems in granite and gabbro

Lab-scale cold water circulation test results and analysis are presented as a proxy for field-scale heat mining in enhanced/engineered geothermal systems. The cold water circulation tests were conducted in hydraulically stimulated igneous rock blocks heated to a uniform temperature. The test blocks had a five-spot system with one injection well at the center and four producers drilled about 9 cm away from it. Two kinds of igneous rocks (granite and gabbro) with different permeability were tested. Due to the heterogeneity of the rock and the notch at the injection well, different fracture geometries with different number of producers connected by the hydraulic fracture were obtained. The circulation tests illustrate for the first time a number of interesting phenomena in a laboratory setting. The great potential of geothermal energy has been demonstrated by nearly 50W of power produced by the cold water flowing through a hydraulic fracture in a mini-EGS reservoir, yielding a good heat extraction percentage of 6.24%similar to 7.85% after 2000 ml of cold water injection. Also, the cooling effect on the injection pressure and thus the fracture aperture has been clearly observed in the experiments i.e., with continued cold water injection the pressure drops, indicating fracture aperture increase due to thermal stress. In addition, reservoir growth during circulation has been observed based on the AE record and production data. Production well competition has also been observed when multiple producers were connected to the injection well by the induced hydraulic fracture, emphasizing the necessity of proper field management to maximize heat production. The test result indicates that a more complex flow path enhances heat production, and early thermal breakthrough and undesirable fracture propagation can be avoided by close monitoring of AE, pressure, and production responses.