Hydraulic fracturing of granite under real-time high temperature and true triaxial stress

Abstract

Hydraulic fracturing in the exploitation of hot dry rock (HDR) resources could significantly enhance the permeability and heat production of the reservoir. However, the fracturing mechanism of HDR at high temperatures is still not fully understood. In this study, hydraulic fracturing experiments at room temperature and 200 °C were performed respectively on granite under different true triaxial stress to analyze their different fracturing mechanisms. Optical microscope and nuclear magnetic resonance were applied to identify pore and crack characteristics of fractured samples from micro- to macro-scale. The test results show that hydraulic fracturing at 200 °C can significantly reduce the breakdown pressure and fracture initiation pressure under the same stress condition compared to hydraulic fracturing at room temperature. The wellbore pressurization stage at 200 °C deviates distinctly from linearity. The cloud fracture with multi-scale crack, rather than a dominant fracture at room temperature, was formed at 200 °C even under a horizontal stress difference of 20 MPa. Moreover, the nuclear magnetic resonance result shows an increase in fracturing volume caused by the increment of micro-scale crack in the fractured sample at 200 °C. The main reason for the above transition is that the pore pressure diffusion at 200 °C generates more micro-scale cracks.

Keywords

high temperature / hydraulic fracturing / true triaxial stress / granite / deep rock mechanics / geothermal

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