Laboratory investigation on cryogenic fracturing of hot dry rock under triaxial-confining stresses

Hot dry rock (HDR) has substantial potential as a thermal energy source. It can be exploited by hydraulic fracturing to extract heat and generate electricity. Because of its great depth (2-6 km) and consequent high temperature (150-650 degrees C) and high pressure, the stimulation of HDR is difficult. The purpose of this study is to study the feasibility of cryogenic fracturing on HDR. We conducted a series of laboratory experiments on rock samples under triaxial-confining stresses. Two schemes were utilized: gas fracturing with and without liquid nitrogen (LN) (-196 degrees C) treatment. For the LN treatment, high-temperature granitic specimens were immersed in LN for several hours before the fracturing tests. Three critical factors were considered, namely, rock specimen temperature (100-600 degrees C), axial stress (5-20 MPa), and lateral stress (5-20 MPa). Subsequently, the fracture patterns were identified and correlations for both LN rapid-cryogenic and natural-cryogenic stimulations were developed to predict the breakdown pressure based on the experimental data. Finally, the field applications for LN cryogenic stimulation in HDR reservoir were discussed and a thermal-hydraulic coupled transient numerical model was established to provide suggestions of LN injection schedule for an effective cryogenic stimulation. The results show that the LN cryogenic stimulation could reduce the breakdown pressure level by 9-51% more than that of the untreated specimens. Particularly, LN provides superior fracturing efficiency on rock specimens above 200 degrees C. This could be attributed to a sharp thermal gradient induced by LN, which can cause the strong local tensile stress to expand the micro-pore structures inside the high-temperature rock samples. Furthermore, with the assistance of LN, larger fracture apertures and better-connected fracture networks inside the rocks can be generated. Additionally, LN treatment, or rapid cryogenic fracturing, plays an increasingly important role when high-temperature rocks are subjected to higher anisotropic stresses. The key findings of this study could provide an efficient approach to explore HDR resources for geothermal development.