Changes In Hydraulic Properties of The Geothermal Fracture Induced By Cyclic Loading-Unloading Processes

One of the greatest challenges of the twenty-first century is mitigating climate change and greenhouse effects in conjunction with uptrend energy demand. Hot dry rock (HDR) geothermal energy, one of the most promising low-carbon resources to alleviate climate change, attracts significant attention from scientists and decision-makers. Sustainable and profitable energy production from the HDR requires a comprehensive understanding of the coupled effects of elastic and plastic deformation on the hydraulic evolution (e.g., fracture aperture, permeability and conductivity) of geothermal fracture. In this work, four flow-through tests were conducted on granite samples with a rough single fracture at 25-180 ℃. Each test was performed on three cycles of loading-unloading processes within a confining pressure of 5-30 MPa. Experimental results indicated that the hydraulic properties are negatively correlated with confining pressure in a logarithmic manner. The coupled effects of elastic and plastic deformation induced by stress loading are the main factor affecting fracture hydraulic properties. Plastic deformation is associated with the mineral grains crush that occurs in fracture contacting asperities, which is a permanent and irreversible process. Compared to the results at 25 ℃, a larger reduction in permeability is observed at 180 °C, as revealed by a maximum reduction in hydraulic aperture up to more than 40% at this temperature. This means that larger plastic deformations in the fractures are associated with higher temperatures. Therefore, the addition of proppant is very important for sustainable geothermal development for fractured geothermal reservoirs under high-stress conditions. The free-face dissolution was well confirmed by ion concentration detection from the effluent solution, which is beneficial for the low permeability geothermal reservoir due to its positive effect on fracture hydraulic properties, especially for the high-temperature geothermal fractures. As the number of loading-unloading cycles increases, the hysteresis effect induced by plastic deformation becomes less and less according to the cyclic test. Compared to the first loading-unloading stage, the maximum decline of permeability decreases from 73% to 6% and 1% on the second and third stages, respectively. However, pressure dissolution under the long-term stress loading should be further discussed, because it may disrupt the self-propping balance of geothermal fracture asperities.