Methane content in coal seam is an essential parameter for the assessment of coalbed gas reserves and is a threat to underground coal mining activities. Compared with the adsorption-isotherm-based indirect method, the direct method by sampling methane-bearing coal seams is apparently more accurate for predicting coalbed methane content. However, the traditional sampling method by using an opened sample tube or collecting drill cuttings with air drilling operation would lead to serious loss of coalbed methane in the sampling process. The pressurized sampling method by employing mechanical-valve-based pressure corer is expected to reduce the loss of coalbed methane, whereas it usually results in failure due to the wear of the mechanical valve. Sampling of methane-bearing coal seams by freezing was proposed in this study, and the coalbed gas desorption characteristics under freezing temperature were studied to verify the feasibility of this method. Results show that low temperature does not only improve the adsorption velocity of the coalbed gas, but also extend the adsorption process and increase the total adsorbed gas. The total adsorbed methane gas increased linearly with decreasing temperature, which was considered to be attributed to the decreased Gibbs free energy and molecular average free path of the coalbed gas molecular caused by low temperature. In contrast, the desorption velocity and total desorbed gas are significantly deceased under lower temperatures. The process of desorption can be divided into three phases. Desorption velocity decreases linearly at the first phase, and then, it shows a slow decreases at the second phase. Finally, the velocity of desorption levels off to a constant value at the third phase. The desorbed coalbed gas shows a parabolic relation to temperature at each phase, and it increases with increasing temperature at the first phase, and then, it poses a declining trend with increasing temperature at the rest phases. The experimental results show that decreasing the system temperature can restrain desorption of coalbed methane effectively, and it is proven to be a feasible way of sampling methane-bearing coal seams.

中文翻译：

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冻结法采样含甲烷煤层：冻结温度下煤层气的解吸与抑制特性

煤层中的甲烷含量是评估煤层气储量的重要参数，并且对地下煤炭开采活动构成威胁。与基于吸附等温线的间接方法相比，通过采样含甲烷煤层的直接方法显然可以更准确地预测煤层气含量。但是，传统的采样方法是使用打开的采样管或通过空气钻孔操作收集钻屑，这会导致采样过程中煤层气的严重损失。通过使用基于机械阀的压力取芯器的加压采样方法有望减少煤层气的损失，而通常由于机械阀的磨损而导致故障。这项研究提出通过冷冻对含甲烷煤层进行采样，研究了冻结温度下煤层气的解吸特性，验证了该方法的可行性。结果表明，低温不仅提高了煤层气的吸附速度，而且延长了吸附过程，增加了总吸附气量。甲烷的总吸附量随温度的降低呈线性增加，这被认为是由于低温导致煤层气分子的吉布斯自由能和分子平均自由程的降低。相反，在较低温度下，解吸速度和总解吸气体显着降低。解吸过程可分为三个阶段。解吸速度在第一阶段线性下降，然后在第二阶段缓慢下降。最后，解吸速度在第三阶段稳定在恒定值。解吸的煤层气在每个阶段都与温度呈抛物线关系，并且在第一阶段随着温度的升高而增加，然后在其余阶段随着温度的升高而呈下降趋势。实验结果表明，降低系统温度可以有效抑制煤层气的解吸，被证明是一种可行的含甲烷煤层采样方法。