A preliminary study of pressure-preserved coring calculation principle and method for in-situ pressure in deep coal seams

The accurate acquisition of in-situ gas pressure in coal seams is a key link in the chain of the prevention and control of gas disasters. Presently, it is still necessary to explore accurate, low-cost, and time-saving methods for determining gas pressure. Based on the in-situ pressure-preserved coring for deep coal seams, this study proposed a calculation method for in-situ gas pressure in deep coal seams and developed the calculation process. Using the constructed numerical model of the gas pressure evolution in the pressure-preserved space, this study verified the reliability of the proposed method by comparison with the theoretical and numerical simulation results. Accordingly, this study further explored the evolutionary pattern of the gas pressure in the pressure-preserved space under different in-situ gas pressures in coal seams. The results indicate that (1) the indicated value of gas pressure in the pressure-preserved space is not the in-situ gas pressure in coal seams. When the indicated value was 0.3 MPa, the in-situ gas pressure of coal seams was about 0.38 MPa; (2) a higher gas pressure after equilibrium in the pressure-preserved space corresponds to a higher proportion of in-situ free gas and a higher proportion of the mass of free gas after equilibrium; (3) with the conversion of gas in the pressure-preserved space from the in-situ state to a new equilibrium state, the proportion of the mass of free gas increases gradually, while that of the mass of adsorbed gas decreases correspondingly; (4) after coring, the gas pressure in the pressure-preserved space evolves from rapid increase to slow increase and then to equilibrium since the pressure of the adsorbed gas in the matrix decreases slowly while the pressure of free gas decreases rapidly; (5) a higher permeability of coal cores corresponds to a shorter time required for the gas pressure in the pressure-preserved space to reach equilibrium. However, the final gas pressure in the pressure-preserved space after equilibrium is independent of the permeability of coal cores.