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Hydraulic fracturing has been widely employed to enhance the permeability of tight geological formations including deep geothermal reservoirs. However, due to the complex in-situ stresses, high-temperature conditions and heterogeneity of the formations, hydraulic fracturing under deep geothermal conditions is poorly understood to date. The aim of the current study is, therefore, to investigate the effect of reservoir depth, temperature, and sample heterogeneity during hydraulic fracturing and the influences of rock micro-structure on fracture propagation. A series of hydraulic fracturing experiments was conducted on two Australian granite types under a wide range of confining pressures from 0 to 60 MPa and temperatures from room temperature to 300 °C simulating different geothermal environments. The corresponding micro-structural effects on the rock matrix were investigated employing high-resolution CT imaging using the IMBL facility of the Australian Synchrotron. According to the results, the breakdown pressure of reservoir rock linearly increases with reservoir depth (confining pressure). However, with increasing temperature breakdown pressure linearly decreases. This corresponds to the linear reduction of tensile strength measured by high-temperature Brazilian tensile tests. In addition, CT images showed that the injection of cold water into hot rock can result in a porous zone with porosity ranging from 2 to 3% close to the wellbore due to thermally-induced inter- and intra-crystalline cracks. In this condition, fluid leak-off is high and the measured fracture aperture of the main hydraulic fracture is relatively small. Further, fracture propagation paths and apertures are mainly controlled by the stress state and the heterogeneity of the rock matrix. It was found that fractures tend to propagate along preferential paths, mainly along grain boundaries and in large quartz and biotite minerals (grain size > 0.3 mm) and minerals with pre-existing micro-cracks.

Thermochemical conversion of waste tire blended with forestry waste (waste blends) into syngas can help support energy security, provide solutions for the management of rapidly growing waste tires, climate change and environmental degradation. In this paper, co-pyrolysis of waste tire and pine bark mixtures was examined under N2 atmosphere at 900 °C using a lab-scale fixed-bed reactor test facility. The focus was on the evolutionary behavior of different components in the syngas (flow rates of CO, H2, CmHn hydrocarbons), total syngas yield, overall energy efficiency and solid residues characteristics. Insight into the effect of different blend fractions of in several discrete ratios of waste tire (W) and pine bark (P), termed as (W:P) = 0:100, 25:75, 50:50, 75:25, and 100:0 was also obtained. Synergistic effects on the syngas and solid yield under the defined blend ratios of waste tire and pine bark were reported. Results showed that peak flowrate of H2, CO, total syngas increased with increase in pine bark in the mixture, but the peak flow rate of CmHn decreased. H2, CmHn, total syngas, solid residues yield and overall energy efficiency showed synergistic interaction to varied extent. With increase of pine bark in blends, mole fraction of CmHn decreased while CO increased. BET surface area characterization of the solid residue indicated there was an interaction on the surface of waste tire and pine bark during co-pyrolysis supported by SEM of the char residue. These results help provide some practical insights on clean energy recovery from mixtures of waste tire and forestry wastes using thermochemical conversion.