Influence of gas components on the determination of gas pressure in coal seams under in-situ pressure-preserved coring

Determining the pressure and content of coal seam gas based on pressure-preserved coring is an effective method for quickly and accurately calculating gas parameters. Coal seam gas is a multi-component gas mixture dominated by methane. Given that coals show different adsorption capacities for different gases and that the components in the mixed gas exhibit different partial pressures, the proportions of gas components can influence the calculation results of the content and pressure of coal seam gas. To accurately determine the in-situ gas pressure in coal seams based on the pressure-preserved coring, this study investigated the influence of gas components on the determination of gas pressure by combining numerical simulation with theoretical calculation. Based on the analysis of the evolutionary process of coal-core gas pressure in the coring device, this study introduced an adsorption model for multi-component gas into the gas migration theory. Then, we developed an equation for the evolution of dual-medium gas pressure considering the effects of multicomponent gas and applied this equation to numerical simulation. The numerical simulation results are as follows: (1) After coal cores entered the core barrel, the gas pressure in fissures decreased rapidly and then increases gradually, the gas pressure in the matrix decreased slowly, and the free-gas pressure in the core barrel rose gradually from the initial 0.1 MPa. The three pressures reached equilibrium after a few hours, with the gas pressure after equilibrium being much less than the initial pressure of coal cores. (2) The CO₂ component had the greatest impact on the gas pressure after equilibrium, which, however, was the lowest for gas consisting only of pure CH₄. Furthermore, this study deduced an equation for calculating in-situ gas pressure in coal seams based on the pressure-preserved coring that considers the effects of multi-component gas. For the gas pressure in coal seams obtained through the reverse calculation based on the set gas equilibrium pressure, it agreed well with the data from numerical simulation, with a Pearson correlation coefficient of 99.89%. For the gas pressure in coal seams determined through the reverse calculation based on the equilibrium pressure in the coring device, it decreased with increased CO₂ or CH₄ component and decreased N₂ component and reached the minimum for coal seam gas consisting only of pure CH₄. Therefore, when determining the in situ gas pressure in coal seams based on pressure-preserved coring assuming that coal seam gas consists only of pure CH₄, the determined in-situ gas pressure in coal seams may be underestimated, especially in the case of a high proportion of the CO₂ component. Therefore, to accurately determine the gas pressure in coal seams, it is necessary to consider the influence of the components of coal seam gas.