Peng Zhigao, Liu Shenggui, Deng Zongwei, Feng Haoxiong, Xiao Ming
College of Civil Engineering, Hunan City University, Yiyang 413000, China.
Hunan Engineering Research Center of Structural Safety and Disaster Prevention for Urban Underground Infrastructure, Hunan City University, Yiyang 413000, China.
ACS Omega. 2023 Jun 6;8(24):21677-21688. doi: 10.1021/acsomega.3c01152. eCollection 2023 Jun 20.
Under specific conditions, moisture in natural coal seams can be adsorbed in the pores of the coal matrix, reducing the amount of methane adsorption sites and the effective area of the transport channels. This makes the prediction and evaluation of permeability in CBM exploitation more challenging. In this paper, we developed an apparent permeability model of coalbed methane coupling viscous flow, Knudsen diffusion, and surface diffusion which considers the effects of adsorbed gas and moisture in the pores of the coal matrix on the permeability evolution. The predicted data of the present model are compared with those of other models, and the results show good agreement, verifying the accuracy of the model. The model was employed to study the apparent permeability evolution characteristics of coalbed methane under different pressure and pore size distribution conditions. The main findings are as follows: (1) moisture content increases with saturation, with a slower increase for smaller porosities and an accelerated non-linear increase for porosities greater than 0.1. (2) Gas adsorption in pores decreases permeability, further weakened by moisture adsorption under high pressure but negligible at pressures below 1 MPa. (3) Higher water saturation weakens gas transport capacity, especially with pore sizes smaller than 10 nm. (4) The non-Darcy effect weakens with higher initial porosity, and neglecting moisture adsorption may significantly deviate from actual values in modeling methane transport in coal seams. The present permeability model can capture the transport characteristics of CBM in moist coal seams more realistically and is more applicable for predicting and evaluating the gas transport performance under dynamic variations of pressure, pore size, and moisture. The results in this paper can explain the transport behavior of gas in moist, tight, porous media and also provide a foundation for coalbed methane permeability evaluation.
在特定条件下，天然煤层中的水分会吸附在煤基质的孔隙中，减少甲烷吸附位点的数量和传输通道的有效面积。这使得煤层气开采中渗透率的预测和评估更具挑战性。本文建立了一个考虑煤基质孔隙中吸附气和水分对渗透率演化影响的煤层气耦合粘性流、克努森扩散和表面扩散的表观渗透率模型。将本模型的预测数据与其他模型的数据进行比较，结果吻合良好，验证了模型的准确性。该模型用于研究不同压力和孔径分布条件下煤层气的表观渗透率演化特征。主要研究结果如下：（1）含水量随饱和度增加，孔隙率较小时增加较慢，孔隙率大于0.1时呈加速非线性增加。（2）孔隙中的气体吸附降低渗透率，在高压下水分吸附会进一步削弱渗透率，但在1MPa以下的压力下可忽略不计。（3）较高的含水饱和度会削弱气体传输能力，尤其是对于孔径小于10nm的情况。（4）非达西效应随初始孔隙率的增加而减弱，在模拟煤层甲烷传输时忽略水分吸附可能会导致与实际值有显著偏差。本文提出的渗透率模型能够更真实地捕捉煤层气在潮湿煤层中的传输特性，更适用于预测和评估压力、孔径和水分动态变化下的气体传输性能。本文的结果可以解释气体在潮湿、致密多孔介质中的传输行为，并为煤层气渗透率评估提供基础。
