Crack propagation patterns and factors controlling complex crack network formation in coal bodies during tri-axial supercritical carbon dioxide fracturing

Supercritical carbon dioxide (SC-CO2) fracturing, as an environmentally-friendly waterless fracturing mining technology, has attracted more and more attention during the safe mining of coal resources. The propagation pattern of fracturing cracks, including the type, number, length, and direction of crack propagation, is the key basis for the design of coal seam fracturing. The direct observation of the crack propagation pattern in fracturing coal body at the engineering site is quite problematic, so laboratory tests are used to study the coal body fracturing process. Based on the tri-axial SC-CO2 fracturing test system, the crack propagation pattern in the SC-CO(2)fractured coal body (SC-CO2-FCB) was studied under thermo-hydro-mechanical coupling conditions. SC-CO2-FCB sample tests were conducted at different crustal stresses, fracturing fluid injection rates, and fracturing fluid temperatures. The injection pressure evolution was analyzed; the type, number, length, and propagation directions of the sample surface cracks were determined, and the crack distribution in SC-CO2-FCB was obtained under the effects of temperature field-seepage field-stress field coupling. From three aspects of fracturing media, fractured substances, and their interactions, the formation mechanism of complex cracks in SC-CO2-FCB was revealed, which provided theoretical guidance for applying and promoting SC-CO2 fracturing in coal mines.