Li Zhiqiang, Li Pengfei, Wang Lin, Liu Yanwei, Li Lin
MOE Engineering Center of Mine Disaster Prevention and Rescue, Henan Polytechnic University, Jiaozuo, Henan454000, China.
Collaborative Innovation Center of Coal Work Safety and Clean High Efficiency Utilization, Henan Polytechnic University, Jiaozuo454000, China.
ACS Omega. 2022 Dec 23;8(1):1606-1617. doi: 10.1021/acsomega.2c07144. eCollection 2023 Jan 10.
The permeability of coal exhibits multiscale characteristics in space and time, which is caused by the presence of micro and nanopores in coal. Water, free gas, and adsorbed gas are common engineering fluids in coal seams during gas extraction. Thus, it is of significance to study the multiscale characteristics and mechanisms of seepage-diffusion of different fluids in coal for gas extraction engineering. Experiments of seepage-diffusion for He, CH, and water are carried out using ϕ50 × 100 mm cylindrical coal. It is found that the apparent diffusion coefficient for He, CH, and water are not a constant but a variable that decays dynamically with time. The phenomenon is independent of fluid properties and determined only by the multiscale characteristics of pores in coal, and then a novel model of multiscale dynamic apparent diffusion that can accurately describe the full-time process of flow in various fluids is proposed. The mechanism of dynamic seepage-diffusion for different fluids in coal is elucidated based on a new proposed geometrical model of multiscale pores. At the early stage of flow, fluids first flow in or out of the largest pores outside coal, and at the later stage of flow, fluids flow in the micro and nanopores inside the coal matrix. The pore sizes through which the fluid flows decrease with time, which leads to a dynamic attenuation of the apparent diffusion-permeability with time. The initial apparent permeability for He/CH shows a "U"-shaped pattern of decreasing and then increasing with the decrease of gas pressure. When the gas pressure is lower than the turning point, the slip effect dominates, making the initial apparent permeability decrease with the increase of gas pressure. When the gas pressure is higher than the turning point, the effect of the pressure difference of the gas dominates, and increases with the increase of gas pressure. When water flows in multiscale pores, it occupies the space of large pores, making the homogeneity of pores increase, and the decay coefficient of apparent permeability becomes smaller. The multiscale permeability can explain the reason for the rapid attenuation of coalbed methane production in the late stage. It is of great significance for coalbed methane productivity prediction.
煤的渗透率在空间和时间上呈现出多尺度特征，这是由煤中微孔和纳米孔的存在所导致的。水、游离气和吸附气是煤层气开采过程中常见的工程流体。因此，研究不同流体在煤中的渗流扩散多尺度特征及机理对煤层气开采工程具有重要意义。采用直径50×100mm的圆柱形煤样进行了He、CH和水的渗流扩散实验。研究发现，He、CH和水的表观扩散系数不是常数，而是随时间动态衰减的变量。该现象与流体性质无关，仅由煤中孔隙的多尺度特征决定，进而提出了一种能准确描述各种流体全时流动过程的多尺度动态表观扩散新模型。基于新提出的多尺度孔隙几何模型，阐明了不同流体在煤中的动态渗流扩散机理。在流动初期，流体首先流入或流出煤体外的最大孔隙，在流动后期，流体流入煤基质内部的微孔和纳米孔。流体流动通过的孔径随时间减小，导致表观扩散渗透率随时间动态衰减。He/CH的初始表观渗透率随气体压力降低呈先减小后增大的“U”形变化。当气体压力低于转折点时，滑脱效应占主导，使初始表观渗透率随气体压力升高而降低。当气体压力高于转折点时，气体压力差的作用占主导，初始表观渗透率随气体压力升高而增大。当水在多尺度孔隙中流动时，它占据大孔隙空间，使孔隙均匀性增加，表观渗透率衰减系数变小。多尺度渗透率可以解释煤层气开采后期产量快速衰减的原因。这对煤层气产能预测具有重要意义。
