Evolutionary pattern and a calculation method of in situ gas pressure based on pressure-retaining coal cores

Objective and MethodsThe in situ gas pressure of coal seams represents a crucial parameter for the safe production of coal mines and the assessment of coalbed methane (CBM) resources. To overcome current limitations in determining the in situ gas pressure, such as prolonged measurement cycles and the influence of multiple parameters on the measurement results, this study conducted isothermal adsorption and volumetric expansion desorption experiments on pressure-retaining coal cores with varying moisture contents using a self-constructed experimental system for gas pressure evolution in pressure-retaining coal cores. As a result, the evolutionary pattern of gas pressure in pressure-retaining coal cores was determined. Accordingly, following the theoretical calculation principle of in situ gas pressure in coal seams, formed by the in situ pressure- and gas-retaining coring process for deep coal seams, this study proposed a correction model for gas pressure calculation that considered moisture content and verified its reliability. Results and ConclusionsThe results indicate that: (1) Influenced by high gas pressure, the gas pressure within the pressure-preserved space more easily reaches a new equilibrium. During the isothermal adsorption process, the gas pressure exhibits a staged evolution characteristic, where the gas pressure in the pressure-preserved coal core first rapidly decreases to a certain value, then increases before gradually stabilizing. (2) Due to the pore characteristics of the coal sample being dominated by mesopores and having a large micropore volume, the higher the moisture content of coal leads to greater resistance to gas adsorption, resulting the longer the equilibrium time and the lower the adsorption pressure drop. (3) The gas pressure of the coal samples was calculated based on the expansion desorption experiments of pressure-preserved coal cores with different moisture contents. The average relative errors of the gas pressure calculation results under various moisture contents were 2.22%, 1.29%, 0.39%, and 0.80%, respectively. The corrected gas pressure calculation method demonstrated high accuracy. The proposed approach offers promising potential as a reliable, efficient, and accurate means for determining of in-situ coalbed gas pressure.