Nuclear Magnetic Resonance (NMR) Study on Micro Porosity Damage of Coal Body by Gas Desorption

To describe the damage evolution law of microscopic pores at the microscopic scale, gas adsorption and desorption tests based on the low-field nuclear magnetic resonance technique (LF-NMR) were conducted under different gas pressures and desorption rates. The experimental results show that the P₁ peak of adsorbed gas is significantly larger than the primary adsorption, the peak apex of adsorption is higher, the peak width (transverse relaxation time T₂ range) is more extensive, and the enhanced inter-pore connectivity is more favorable to gas infiltration and transport. With the increase of gas desorption rate, the damage development of coal samples is higher, which improves the inter-pore connectivity, thus the re-absorption T_2GM decreases. The damage factor D is defined from the NMR T₂ spectrum concentration index, and the damage factor D increases with the increase of gas pressure and gas desorption rate. Considering the role of desorption gas on crack propagation, a model is established to theoretically explain the effect of gas desorption on coal damage evolution. This study is significant for coal and gas outbursts prevention and coalbed methane utilization.