LNMR experimental study on the influence of gas pressure on methane adsorption law of middle-rank coal

In order to reveal the influence of gas pressure on the gas adsorption law of middle-rank coals, four groups of typical middle-rank coals in China were used as the research objects, using saturated, centrifugal, drying equipment and Low-field Nuclear Magnetic Resonance (LNMR) system to carry out coal samples. The pore structure characterization test and the isotherm adsorption test under different gas pressure were carried out. First, the coal sample's pore size distribution, pore roar distribution, porosity and permeability and other parameters were analyzed. Then, the gas adsorption isotherm process was quantitatively studied based on the established relaxation time (T-2) spectrum area conversion model. Finally, the relationship between gas pressure and gas adsorption in coal reservoir was discussed. The experimental results show that the pore structure of middle-rank coal is mainly composed of small pores, and a small number of large pores are distributed. Middlerank coal has low permeability and good gas storage capacity. Under the same gas pressure, with the increase of adsorption time, the gas adsorption amount gradually increased, and the gas adsorption increment gradually decreased. After 7 h, the adsorption equilibrium is reached. Under different gas pressure, the gas pressure has a proportional relationship with the initial gas adsorption speed, which conforms to the Langmuir adsorption theory with the amount of adsorbed gas, and has a quadratic function relationship with the amount of nonadsorbed gas. When the gas pressure is higher than 2.4 MPa, the time required to reach adsorption equilibrium is shortened to 5 h. Gas pressure has no obvious effect on the micropore structure of middle-rank coal, but it will promote the expansion of middle-large pore structure, and its expansion effect is enhanced with the increase of gas pressure.