Experiments on the thermally enhanced permeability of tight rocks: A potential thermal stimulation method for Enhanced Geothermal Systems

Creating a good fracture network in hot dry rocks is the key to the success of an Enhanced Geothermal Systems project. Instead of creating large-scale fractures, the proposed thermal stimulation method has clear advantages by generating small-scale cracks, increasing crack density, and forming fracture networks in tight rocks. Granite samples from a tight formation in Lijin County in China were thermally treated at various temperatures to evaluate the permeability enhancement and the feasibility of thermal stimulation. Scanning electron microscope (SEM) method was also used to investigate the changes in interior structure in granite rocks. The experiments show that the threshold temperature of Lijin granites is around 300 degrees C, the permeability can improve approximately 2 orders and the porosity can increase more than 5 times when heating over 600 degrees C. Both crack length and width increase about 10 folds under the experiment conditions, and the original mineral disappearance and new mineral generation were observed. Thermal cracking under high temperatures plays a key role in permeability and porosity enhancement. Numerical simulation results show that the heated zone is very limited if only heating in a borehole. To expand the affected zone and improve energy recovery, a new thermal stimulation method combining microwave excitation with proppants to heat inside hydraulic fractures is proposed and discussed.