Due to the inherent deposition environment and extraction challenges in a coal seam, quantification of its gas transport behavior serves as the prerequisite to predict and evaluate accurately dynamic coalbed methane (CBM) recovery. In this work, a dual-porosity/single-permeability (DPSP) model is proposed for evaluating the gaseous methane flow performance by considering both the free gas density gradient and pressure gradient. More specifically, methane extraction dynamics was monitored via on-site boreholes with different spacings, and the two key gas flow parameters (i.e., diffusion coefficient and permeability coefficient) were determined and validated by minimizing the deviations between the field measurements and the simulated production profiles. Sensitivity analysis was conducted to examine the effect of both gas flow rate in a borehole and gas mass transfer flux from coal matrix to fractures on the production performance. The gas flow rate was found to be very sensitive to the permeability coefficient at the early extraction stage and then dominated mainly by the diffusion coefficient during the later extraction stage. The gas mass transfer flux from the matrix subsystem to the fracture subsystem exhibited distinct peak features due to the hysteresis of gas desorption in coal matrix. To extract methane in a coal seam, gas pressure in the matrix subsystem decreases much less than that in the fracture subsystem. Compared with the traditional single-porosity/single-permeability model, the newly proposed DPSP model allowed us to correct the overestimated CBM production and identify the underlying gas flow mechanisms. By introducing a critical extraction radius (CER) during a CBM extraction process, a theoretical equation associated with the CER, original methane content in a coal seam and methane extraction time was eventually formulated so that the CER and borehole spacing in some coal seam areas with different methane contents can be determined in a convenient and accurate manner.

由于煤层固有的沉积环境和开采挑战，对其瓦斯输运行为的量化是准确预测和评估动态煤层气（CBM）采收率的先决条件。在这项工作中，提出了一种双孔隙度/单渗透率（DPSP）模型，通过考虑自由气体密度梯度和压力梯度来评估气态甲烷的流动性能。更具体地说，通过不同间距的现场钻孔监测甲烷提取动态，并通过最小化现场测量和模拟生产剖面之间的偏差来确定和验证两个关键的气流参数（即扩散系数和渗透系数） 。进行了敏感性分析，以检查钻孔中的气体流量和从煤基质到裂缝的气体传质通量对生产性能的影响。研究发现，在萃取早期，气体流量对渗透系数非常敏感，而在萃取后期，气体流量主要受扩散系数支配。由于煤基质中气体解吸的滞后性，从基质子系统到裂缝子系统的气体传质通量表现出明显的峰值特征。为了提取煤层中的甲烷，基质子系统中的瓦斯压力下降远小于裂缝子系统中的瓦斯压力。与传统的单孔隙度/单渗透率模型相比，新提出的DPSP模型使我们能够纠正高估的煤层气产量并确定潜在的气体流动机制。通过在煤层气抽采过程中引入临界抽采半径（CER），最终建立了与CER、煤层原始瓦斯含量和瓦斯抽采时间相关的理论方程，使得部分煤层区域的CER和钻孔间距与可以方便、准确地测定不同甲烷含量。