An experimental study of creation of optimal fracture network for heat extraction from engineered geothermal reservoirs

Enhanced geothermal system has been successfully able to increase the permeability of the reservoir and hydro-fracturing is the conventional method of creating artificial fracture system. But more precise technique is required to reduce the cost of the geothermal energy extraction and this will require proper understanding of the thermo-mechanical behaviour of the reservoir rocks i.e. Granites. Current research work has focused on the thermo-mechanical properties of granites under various temperature and strain rate condition. Granites of two different areas i.e. Bundelkhand granite, India and Harcourt granite, Australia have been investigated for the research work. 4 different strain rates i.e. 0.05 mm/min, 0.5 mm/min, 5.0 mm/min and 50 mm/min at 3 different temperatures i.e. room temperature (25 °C), 200 °C and 400 °C were considered for experimental analysis. It was observed that at room temperature, with increasing strain rate the uniaxial compressive strength of the rock increases and the same trend is also observed at higher temperatures conditions. Whereas at low strain rate the compressive strength decreases with increase in temperature, while the trend is irregular at higher strain rates. Bundelkhand granites have thermal conductivity values between 3.1 to 3.6 W/m.K whereas that of Harcourt granite is 2.4 W/m.K. Sieve analysis of the fragmented particles due to UCS test suggested that there is no effect of strain rate on particle size distribution whereas higher temperatures produces more finer particles. It was observed that more than 75% of the fragmented particles are having diameter of greater than 11.3 mm. Effective size i.e. D10 was analysed and found that at room temperature (25 °C) 10% of fragmented particles are finer than 3.0 mm whereas at 200 °C and 400 °C 10% of finer particles are finer than 1.9 mm and 1.5 mm respectively. Numerical simulation results of the numerical simulation are quite analogous to the lab experimental data. Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of the Indian Institute of Technology Bombay, India and Monash University, Australia.