Multiscale Analysis of Fracture Deformation in Enhanced Geothermal Systems due to Thermal Cooling Effect

Abstract:

In the enhanced geothermal systems, fractures propped open by asperity pairs provide primary conduits for heat exchange between injected cold water and hot rock. The fracture deformation in response to the thermal cooling could exert significant impact on the hydraulic flow and heat recovery efficiency. It is of particular importance to investigate the fracture deformation by considering microscale asperity failure. In this study, the deformation of an asperity pair in response to thermal cooling is numerically simulated by a damage mechanics based model, which is also validated by a benchmark. The result shows that the effect of thermal cooling on asperity deformation is negligible at mild temperature difference of water and rock. However, at high temperature difference, most likely near the injection well, fracture asperities would undergo a catastrophic damage resulting in the loss of bearing capacity. In addition, the relation of asperity deformation and temperature difference is obtained. To upscale asperity deformation into fracture deformation, a synthetic fracture with asperities is generated by a stochastic method. The multifractal distribution of fracture aperture is employed in a Monte Carlo distribution. The generated fracture remains the same distribution of fracture aperture in a scaling sense. The fracture deformation in response to different temperature difference and loading conditions is analyzed. It shows that thermal cooling is likely to cause abrupt fracture deformation due to failure of some propped asperities. It could change the topology of flow channels in the fractures and leads to different flow channeling.