Thermally-induced cracking behaviors of coal reservoirs subjected to cryogenic liquid nitrogen shock

The benefits of using cryogenic liquid nitrogen shock to enhance coal permeability have been confirmed from experimental perspectives. In this paper, we develop a fully coupled thermo-elastic model in combination with the strain-based isotropic damage theory to uncover the cooling-dominated cracking behaviors through three typical cases, i.e. coal reservoirs containing a wellbore, a primary fracture, and a natural fracture network, respectively. The progressive cracking processes, from thermal fracture initiation, propagation or cessation, deflection, bifurcation to multi-fracture interactions, can be well captured by the numerical model. It is observed that two hierarchical levels of thermal fractures are formed, in which the number of shorter thermal fractures consistently exceeds that of the longer ones. The effects of coal properties related to thermal stress levels and thermal diffusivity on the fracture morphology are quantified by the fracture fractal dimension and the statistical fracture number. The induced fracture morphology is most sensitive to changes in the elastic modulus and thermal expansion coefficient, both of which dominate the complexity of the fracture networks. Coal reservoir candidates with preferred thermal-mechanical properties are also recommended for improving the stimulation effect. Further findings are that there exists a critical injection temperature and a critical in-situ stress difference, above which no thermal fractures would be formed. Preexisting natural fractures with higher density and preferred orientations are also essential for the formation of complex fracture networks. The obtained results can provide some theoretical support for cryogenic fracturing design in coal reservoirs.