Permeability is a critical parameter in coalbed methane (CBM) recovery and received increasing attention in recent years. The slip effect and effective stress exert competing influences on permeability, with coal exhibiting varying sensitivities to effective stress depending on their pore structures. The presence of water further complicates these interactions, affecting both the slip effect and permeability. This study investigates the pore structure and permeability characteristics of four coal cores at varying water contents using low-field nuclear magnetic resonance (NMR) and pulse pressure decay (PPD) methods. An enhanced apparent permeability model was developed by incorporating water content, effective stress, and the slip effect. The dynamic variations of compressibility coefficient, slip coefficient, and intrinsic permeability for Cores C-F were theoretically examined based on the refined model, and the critical pore pressures at which the slip effect becomes significant were identified. The results indicate that cores with larger average pore sizes exhibit more pronounced changes in fracture compressibility coefficients as water content increases. Additionally, the slip coefficient decreases with increasing pore pressure and is notably lower at reduced water contents. Intrinsic permeability increases more significantly with pore pressure at higher water content, with cores having larger average pore diameters showing greater sensitivity to these changes. The critical pore pressure, where the slip effect becomes significant, increases with water content and is higher in cores with smaller average pore sizes. Finally, various coefficients are proposed to quantitatively assess changes in fracture compressibility, slip coefficients, intrinsic permeability, and critical pore pressures under varying water content conditions, enabling more accurate analysis of permeability behavior.

中文翻译：

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试验研究及模型改进对煤渗透率的影响：有效应力、滑移效应和含水率的影响


渗透率是煤层气 （CBM） 回收的关键参数，近年来受到越来越多的关注。滑移效应和有效应力对渗透率产生竞争性影响，煤对有效应力的敏感性取决于其孔隙结构。水的存在使这些相互作用进一步复杂化，影响滑移效应和渗透性。本研究使用低场核磁共振 （NMR） 和脉冲压力衰减 （PPD） 方法研究了不同含水量下 4 个煤芯的孔隙结构和渗透率特性。通过结合含水量、有效应力和滑移效应，开发了增强的表观渗透率模型。基于改进模型，从理论上研究了岩心 C-F 的压缩系数、滑移系数和内禀磁导率的动态变化，并确定了滑移效应变得显着的临界孔隙压力。结果表明，随着含水量的增加，平均孔径较大的岩心在裂缝压缩系数上表现出更明显的变化。此外，滑移系数随着孔隙压力的增加而降低，在含水量降低时明显降低。在较高的含水量下，内禀渗透率随着孔隙压力的增加而增加得更显着，具有较大平均孔径的岩心对这些变化表现出更大的敏感性。滑移效应变得显着的临界孔隙压力随含水量的增加而增加，并且在平均孔径较小的岩心中较高。 最后，提出了各种系数来定量评估不同含水率条件下裂缝压缩率、滑移系数、内禀渗透率和临界孔隙压力的变化，从而能够更准确地分析渗透率行为。