Evolution of mechanical damage and porosity stress sensitivity under stepwise loading of liquid nitrogen freeze-thaw differential coal rank

As an innovative water-free fracturing technology, liquid nitrogen fracturing is capable of effectively improving coal-seam permeability. Nevertheless, fractures induced by this technology are readily closed under in-situ stress, which restricts the permeability-enhancing efficiency. Changes in pore structure and stress sensitivity of coal subjected to vertical in-situ stress after liquid nitrogen fracturing therefore need to be investigated, so as to optimize process parameters of liquid nitrogen fracturing and formulate reasonable schemes for gas-drainage spacing and proppant arrangement. Typical low-rank, medium-rank and high-rank coals are selected as research subjects. Combined with liquid nitrogen freeze-thaw cycles, mechanical tests and nuclear magnetic resonance (NMR) technology, variations in mechanical properties of three coal ranks under different liquid nitrogen freeze-thaw times, as well as the pore-structure evolution of freeze-thaw-treated coal samples under stepwise axial-stress loading, are explored. Relationships of porosity stress sensitivity with coal rank, stress and freeze-thaw times are further established. The results demonstrate that the compressive strength, elastic modulus and wave velocity of the three types of coal samples are reduced after liquid nitrogen freeze-thaw cyclic treatment, and significant negative correlations are identified between these mechanical parameters and cycle times. Within a certain stress range, pores and pore throats are shrunk under axial stress. Such pore-shrinkage behavior is aggravated for freeze-thaw-treated coal samples owing to the degradation of mechanical properties, and the maximum pore-closure degree reaches 0.679%. Damage variables determined from porosity, acoustic velocity and elastic modulus of freeze-thaw-treated coal are positively correlated with freeze-thaw times. The two-parameter damage variable defined by porosity and wave velocity exhibits higher accuracy, and the damage degree follows the order of lignite > bituminous coal > anthracite. Both the stress sensitivity coefficient and porosity compressibility decrease linearly under low-stress conditions. When the porosity compressibility is greater than zero, these two parameters are elevated with the increase of freeze-thaw times. Among the three types of coal ranks tested, the stress sensitivity is ranked as lignite > bituminous coal > anthracite. It is revealed that liquid nitrogen freeze-thaw treatment markedly exacerbates the tendency of pore closure and damage failure of coal under stress. Theoretical references are provided for the quantitative analysis of fracture closure behavior of liquid nitrogen fractured coal under in-situ stress and the optimization of gas-drainage parameters.