Low-Temperature Response Characteristics of Coalbed Methane Desorption under High and Low Destruction Degrees.

Freeze-coring technology can effectively reduce the amount of gas loss during the sampling process and improve the accuracy of gas content measurements in underground coal seams. In this study, high- and low-damage coals were selected as test objects to investigate whether the freeze-coring technique is universally applicable to inhibit gas desorption in high- and low-damage coals. In this paper, the pore structure of the test coal samples was first tested using an ASAP2020 specific surface area analyzer, and then a nonfreezing and freezing simulation test was carried out on high- and low-damage coals using a self-developed freezing coring response test platform. The results showed that the gas desorption curves of both high- and low-damage coal samples followed the pattern of rapid increase in the early stage, slow increase in the middle stage, and stability in the late stage under both conditions; freezing conditions significantly reduced the gas desorption during the sampling process, and the difference in gas desorption between high- and low-damage coals was reduced; the gas desorption inhibition rate of high-damage coals was higher at an external heating temperature of 60 °C under freezing conditions; at an external heating at an external heat temperature of 90 °C, the gas desorption inhibition rate of low-damaged coal was higher in the early stage, and the gas desorption inhibition rate of high-damaged coal was higher in the later stage; freeze coring had a significant inhibition effect on the gas desorption of both high- and low-damaged coal types, which verified that the inhibition effect of freeze coring on the gas desorption of high- and low-damaged coal samples was universal. It provides a basis for the future application of freeze-coring technology in coal mines.

Study on the Equivalent Average Temperature Variation of the Coal Core during the Freeze Coring Process.

In the freeze coring process, the core tube is subjected to cutting heat, frictional heat with the coal wall, and refrigerant action, which causes the temperature of the coal core to be different at different positions and at different times. The equivalent average temperature is proposed to represent the change law of the whole temperature of the coal core and to provide the temperature boundary condition for calculating gas loss. Relying on the self-developed simulation platform for the freezing response characteristics of gas-containing coal, a temperature change simulation test of the freezing core under different external heat conditions was carried out, and the freezing core heat transfer model was constructed with the help of COMSOL to analyze the coal core radial temperature changes during the freeze coring process. Because the drilling sampling time of the freeze coring process is short and there is a thermal isolation device between the drill bit and the core tube, the influence of cutting heat is ignored when the model is established, and only the coal core diameter is studied. The results show that the law of equivalent average temperature of the coal core with time is consistent with the experimental law, which is divided into three stages: rapid decline, slow decline, and relative stability. The temperature drop amplitude and rate of the equivalent average temperature of the coal core decrease with increasing external heat temperature. For example, when the external temperature is 60, 70, 80, and 90 °C, the limit temperatures of the equivalent average temperature of the coal core are -36.301, -30.358, -23.956, and -18.899 °C, respectively.