Using true-triaxial systems to study the failure behavior of decimeter-sized granite under reservoir-relevant conditions of hydraulic fracturing and temperature

Abstract:

Enhanced geothermal systems (EGS) have been receiving an increasing attention for its considerable potential for renewable energy source. The hydraulic stimulation has been wieldy used for reservoir enhancement through either hydro-fracturing or hydro-shearing for an efficient heat extraction. However, most laboratory experiments have been performed at room temperatures using conventional triaxial systems with the intermediate principal stress equaling minimum principal stresses. The field failure and fracturing behavior in EGS reservoirs deserve an investigation for field-relevant thermal and stress conditions, in order to create an extensive fracture network with high heat exchange efficiency. In this work, hydraulic fracturing tests were conducted on 300 mm-sized cubic granite samples with different mineralogy of high and low quartz contents both at room and field temperature conditions of up to 400°C. The fracturing failure curve was captured through the true-triaxial system equipped with an acoustic emission (AE) system, compatible with high-temperature conditions up to 250 °C, to monitor the cracking pressure field and newly formed fractures. Meanwhile, the fracture distribution before and after hydraulic fracturing was observed through μm-CT (computed tomography) and XRF (X-ray fluorescence) for fluorescent tracers on cut samples at cylindrical sizes as large as 200 mm in height and 150 mm in diameter. The results yield a better understanding of 1) multiple-well simulation from borehole pressure and crack connection observation; 2) mechanism and behavior of crack propagation under different intermedia principle stresses; 3) influence of temperature and mineralogy on hydraulic fracturing; and 4) the comparison between the failure for samples with pre-existing fissures and not.