The evolution law of gas pressure in pressure-preserved coal cores and its calculation method

Objective and Methods Coalbed in-situ gas pressure is a crucial parameter for coal mine safety production and coalbed methane resource assessment. Addressing the current limitations of long measurement cycles and the influence of multiple parameters on the measurement results of in-situ coalbed gas pressure, an experimental system for the evolution of gas pressure in pressure-preserved coal cores was independently constructed. Experiments on isothermal adsorption and expansion desorption of pressure-preserved coal cores under different moisture contents were conducted. Based on revealing the evolution law of gas pressure in pressure-preserved coal samples, a theoretical calculation principle for "in-situ coalbed gas pressure" formed by the "in-situ pressure-preserved gas-preserved coring process for deep coal seams" was proposed. A correction model for gas pressure calculation considering moisture content was developed and its reliability was verified. Results and Conclusions The 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 pressurepreserved 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, in the isothermal adsorption experiments of pressure-preserved coal cores with different moisture contents, the higher the moisture content, 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 across all moisture contents. Ensuring reliable calculation accuracy, the use of equilibrium gas pressure within the pressure-preserved space to calculate in-situ coalbed gas pressure is expected to provide a reliable and convenient calculation method for the determination of in-situ coalbed gas pressure.