Hydro-mechanical parameters of Cornubian and Odenwald reservoir granitoids with focus on fracture stiffness testing

For the resource development, geothermal systems need to be improved to increase the profitability of the investment. One aspect to support this aim is the reservoir productivity, a key parameter, which depends on the hydraulic and mechanical properties of the reservoir formation. In order to develop possible improvement strategies for the profitability enhancement of geothermal reservoirs and/or nuclear waste repositories, hydraulic and mechanical properties of artificial generated fractures were investigated. The importance of the fracture geometry yields the fracture network of the geothermal system. As a result, the influence of stress exerted on single fractures was exploited. Hereby, the fracture aperture represents a key parameter for several other parameters such as the fracture permeability and fracture stiffness. Therefore, experiments of progressive and constant cyclic loading were performed to analyze the fluid flow inside of fractured rock samples from geothermal reservoirs. The specimens analyzed in this research project are the Odenwald Granodiorite which was extracted from the Bergstrasse in Heppenheim, Germany, and the Cornwall Granite from the St. Austell pluton in Cornwall, England. The progressive cyclic loading test (PCL) was performed with confining pressure maxima of 15 MPa, 30 MPa, 45 MPa, and 60 MPa. Within the constant cyclic loading test (CCL), the maximum pressure was raised up to 60 MPa to ensure reproducibility. Axial and lateral strain deformation were measured with LVDT extensometers to calculate the fracture and matrix deformation. Fracture stiffness, -permeability, and -closure were evaluated from the collected dataset. Moreover, the fracture geometry was taken into account by 3D surface scans to display fracture aperture distribution and to model the change in surface structure and its impact on the fracture behavior. The fracture stiffness visualized for both granitoids is similar in terms of values and behavior, despite their different origin and, respectively, their petrographical composition. Moreover, the PCL displayed a linear trend of the fracture stiffness in the 1st cycle and before exceeding the previous stress maximum. This feature transformed into a non-linear trend when exceeding the previous stress level. The transition seems to be related to a stress-memory effect and the behavior of the ‘Kaiser effect’ for acoustic emissions. Both features were additionally detected in the fracture permeability results. The outcome of a similar research by Kluge et al. (underreview) with the Flechtingen Sandstone shows the same characteristics in a different domain for the fracture stiffness, -permeability, and –closure values. Last but not least, the fracture permeability reduction turned out very similar in the PCL and the CCL test, a result that contrasts the outcome of Kluge et al. (under review). Experimental and theoretical results on single fractured rock specimens are discussed and display the importance of fracture stiffness on geothermal systems.