Coupled effects of elastic and plastic deformation on hydraulic properties of the geothermal fracture induced by cyclic loading-unloading processes

Sustainable and profitable energy production from Enhanced Geothermal System (EGS) requires a comprehensive understanding of the coupled effects of elastic and plastic deformation on the hydraulic evolution of geothermal fractures. Four flow-through tests were conducted on granite samples with a rough single fracture at 25-180 °C. 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 occurring in fracture contacting asperities, which is a permanent and irreversible process. Compared to the results at 25 °C, a larger reduction in permeability is observed at 180 °C, as revealed by a maximum reduction in hydraulic aperture up to >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 ion concentration detection of the effluent solution confirms the existence of free-face dissolution in high-temperature scenarios. Generally, free-face dissolution is beneficial to low-permeability geothermal reservoirs due to its positive effects on fracture hydraulic properties. As the number of loading-unloading cycles increases, the hysteresis effect induced by plastic deformation becomes less and less. Compared to the first loading-unloading stage, the maximum decline of permeability decreases from 73% to 6% and 1% during the second and third stages, respectively. However, pressure dissolution under long-term stress loading should be further investigated, because it may disrupt the self-propping balance of geothermal fracture asperities.