Simulated test of gas loss calculation during the freezing coring process inunderground coal seam

The freezing coring technology can not only perform the fixed-point sampling in coal seams, but also effectively reduce gas leakage during the sampling process. It has broad prospects for application in the precise measurement of gas content in coal seams. To accurately evaluate the gas loss of coal sample during the freezing coring process, the simulated tests of gas desorption during coring were carried out under the different external heats and the equilibrium pressures (1−4 MPa) by using a simulation platform for gas adsorption/desorption on gas bearing coal. The gas desorption characteristics in low-temperature conditions were also studied. Based on the graphical method, the three diffusion models were adopted to analyze the gas desorption curves under low-temperature conditions, and the fitting performance of the gas loss estimation models were evaluated. The results show that the tube frictional heat generated during the conventional coring process greatly increases the gas loss; and the gas desorption amount gradually increases with the temperature rise in the core tube wall. When the friction heat of the tube wall is 60, 70, 80, and 90 ℃, the desorption amounts within 30 min are 6.587, 7.082, 7.460, and 7.981 cm3/g, respectively; and the increments in desorption amounts compared to that of 30 ℃ are 13.71%, 22.25%, 28.78%, and 37.77%, respectively. During the freezing coring, there is a back-flow phenomenon in the desorption, which is caused by the coal sample cooling leading to the pressure inside the sample tank lower than atmospheric pressure. Through the correction tests of the pure back-flow of coal without gas, the gas loss during the freezing coring gradually decreases as the external heat of the tube wall. When the external heat of the tube wall is 60, 70, 80 and 90 ℃, the desorption amounts within 30 min are 3.578, 3.842, 4.215, and 4.76 cm3/g, respectively; and the desorption inhibition rates reach 40%−46%, compared with the conventional coring. At low temperatures, the desorption amount gradually increases with the rise of adsorption pressure, but the growth rate gradually decreases. The diffusion coefficient during the freezing coring is much reduced compared to the conventional coring, and it shows a linear decrease with cooling. The fitting accuracy of the gas desorption curve at low temperatures via the logistic growth model is significantly better than that of the \begin{document}$ \sqrt{t} $\end{document} model and the exponential model, with a loss estimation error of less than 0.5%. Therefore, the logistic growth model can meet the needs of gas loss estimation during the freezing coring in coal seams.