Laboratory Investigation on Cryogenic Fracturing of Hot Dry Rock Under Triaxial-Confining Stresses

摘要

Hot dry rock (HDR) has high potential of thermal energy, which can be exploited by hydraulic fracturing to extract heat and generate electricity. Because of great depth (2~6 km) and consequent high temperature (150~650 °C) and high pressure, the stimulation of HDR is difficult. Purpose of this work is to study the feasibility of cryogenic fracturing on HDR. We conducted a series of laboratory experiments on rock samples under triaxial-confining stresses. To generate sharp thermal shock, high-temperature granitic rock specimens (100~700 °C) were immersed into liquid nitrogen (LN) (-196°C) for several hours before the stimulation tests. Then, two schemes were considered: gas fracturing with and without LN treatment. Subsequently, fracture patterns were identified and discussed. Finally, two correlations for both LN rapid-cooling and naturally-cooling stimulations were developed to predict the breakdown pressure based on the experimental data. The results show that compared with untreated specimen, the breakdown pressure for LN treated specimen can be reduced by 9-51%. Particularly, LN has superior fracturing efficiency on rock specimens above 200°C. The major reason could be attributed to the high thermal gradient and specific frost force induced by LN fluids, which can cause strong local tensile stress expand the micro-pore structures inside the high-temperature rock samples. Furthermore, with the increasing temperature difference between rocks and fracturing fluid, especially with the assistance of LN rapid-cooling effect, more complex fracture networks or connected pathways can be generated. The key findings of this work are beneficial for providing an efficient approach to explore HDR resources in geothermal development.