煤炭是我国的主体能源，是能源安全的稳定器和压舱石。煤体中存在大量的不规则孔隙、裂隙，这些孔裂隙是瓦斯储存和运输的主要空间。孔裂隙的几何形态、尺寸、分布特征和拓扑结构直接影响煤体的孔隙率、渗透率、饱和度等物理特性。煤体内部非均质性强，孔裂隙位置、大小随机分布，导致其中的流体流动规律十分复杂。明晰煤体孔裂隙结构特征及其对瓦斯渗流的影响，对于预防瓦斯突出灾害，保障煤矿安全开采至关重要。本文基于 CT 扫描技术对煤体孔裂隙结构进行表征，结合数值模拟和相似物理模拟实验，揭示煤体中瓦斯渗流规律。具体内容如下：
    通过 CT 扫描成像技术对预制型煤进行扫描，利用 AVIZO 软件提取煤体内部的孔隙、裂隙、煤基质分布，将原始切片进行滤波降噪、阈值分割等操作，获得清晰的煤体图像，运用 Volume Rendering 进行煤体三维可视化建模。选取表征单元体（REV）建立孔隙网络模型（PNM），对 REV进行孔隙、喉道等分布特征分析，为研究瓦斯渗流奠定模型基础。
    基于固有渗透率和表观渗透率公式，并结合 Knudsen 数，建立了经过滑移修正的气体表观渗透率模型，模型中考虑了孔裂隙尺寸，分形维数，迂曲度维数等的影响。通过FLUENT 软件对煤体中的瓦斯渗流进行数值模拟，结果表明瓦斯渗流速度随压力梯度升高而增大，瓦斯在孔裂隙收缩或弯折处压力变化剧烈。瓦斯平均速度与压力梯度呈非线性关系，即发生非达西渗流。
    基于孔隙-裂隙双重介质模型，建立了受载煤体渗透率模型，并与实验结果进行对比，验证了模型合理性。通过 COMSOL 软件进行数值模拟，获得了煤体裂隙演化规律，查明了煤体裂隙演化与瓦斯渗流之间的联系。发现煤体内裂隙演化分四个阶段：裂隙渐生阶段、裂隙速生阶段、裂隙贯通阶段、应力残余强度阶段。随着煤体轴压增大，体积应变随瓦斯压力升高呈升高趋势，弹性阶段煤体渗透率呈降低趋势，当达到应力峰值时，煤体渗透率达到最小。随着瓦斯压力增大，煤体渗透率增大趋势更加显著并达到峰值。
    对相似模拟实验中被保护煤层煤体进行 CT扫描，提取煤体裂隙并构建 PNM模型，发现被保护层采动裂隙在采空区中部区域发育最为充分，裂隙体积发育具有一定对称性。根据煤体 CT 裂隙图像计算裂隙分形维数，发现随着工作面推进，分形维数先减小，随着被保护层煤体产生横向裂隙，分形维数逐渐增大，最终分形维数趋于稳定。煤体内扩展裂隙呈现压密-缓慢扩张-加速扩张-稳定扩张-缓慢闭合-压密的变化规律，具有一定的周期性。采用 COMSOL 和 ABAQUS 有限元软件开展保护层开采与瓦斯渗流气固耦合数值模拟，得到上覆岩层应力场、位移场和渗流场分布特征。发现随着工作面推进 50m-80m，上覆岩层垂直应力拱的跨度、拱高不断加大，呈现扩散趋势演化，卸压区域呈现上窄下宽近似梯形形状分布。推进至120m-160m，渗透率变化集中体现在保护层顶板发生塑性变形产生的裂隙网络区域内，与卸压区域分布一致。待保护层开采结束后，保护层与被保护层间覆岩内部裂隙发育逐渐贯通，煤体充分卸压，促进了瓦斯流动。

Coal is the main energy source in China and acts as the stabilizer and ballast of energy security. There are a large number of irregular pores and fractures within the coal body, which serve as the primary spaces for gas storage and transportation. The geometry, size, distribution characteristics, and topology of these pores and cracks directly influence the porosity,permeability, saturation, and other physical properties of the coal. The coal body exhibits strong non-homogeneity, with a random distribution of pore and fracture locations and sizes,complicating the law of fluid flow within it. Clarifying the structural characteristics of coal body pores and cracks and their impact on gas seepage is essential for preventing gas protrusion disasters and ensuring the safety of coal mining. This paper characterizes the pore and fracture structure of the coal body based on CT scanning technology, and uses numerical simulation and similar physical simulation experiments to reveal the gas seepage law in the coal body. The specific content includes:

The prefabricated coal is scanned by CT scanning imaging technology, and AVIZO software is used to extract the distribution of pores, fractures, and coal matrix inside the coal body. The original slices undergo filtering and noise reduction, threshold segmentation, and other operations to obtain a clear image of the coal body. Volume Rendering is used to carry out the three-dimensional visualization modeling of the coal body. The pore network model (PNM) is established by selecting the characterization unit body (REV), and the distribution characteristics of pores and throats are analyzed for the REV, which lays the model foundation for the study of gas seepage.

  Based on the intrinsic permeability and apparent permeability equations and combined with the Knudsen number, a slip-corrected apparent permeability model of gas is established, in which the effects of pore-slit size, fractal dimension, tortuosity dimension, and so on are taken into account. Numerical simulation of gas seepage in the coal body is carried out by FLUENT software, and the results show that the gas seepage velocity increases with the increase of pressure gradient, and the gas pressure changes drastically at the contraction or bending of pore cracks. The average gas velocity is nonlinearly related to the pressure gradient, i.e., non-Darcy seepage occurs.

  Based on the pore-fracture dual medium model, the permeability model of the loaded coal body was established and compared with the experimental results to verify the rationality of the model. Numerical simulation was carried out by COMSOL software to obtain the coal body fracture evolution law and identify the connection between coal body fracture evolution and gas seepage. It was found that the fracture evolution in the coal body was divided into four stages: fracture gradual generation stage, fracture rapid generation stage, fracture penetration stage, and stress residual strength stage. As the axial pressure of the coal body increases, the volumetric strain tends to increase with the gas pressure, the permeability of the coal body in the elastic stage tends to decrease, and the permeability of the coal body reaches the minimum when the stress peak is reached. With the increase of gas pressure, the trend of coal permeability increase is more significant and reaches the peak.

  CT scanning of the coal body of the protected seam in the similar simulation experiment extracts the coal body fractures and constructs the PNM model. It is found that the mining fractures of the protected seam are most fully developed in the central area of the mining hollow zone, and the fracture volume development exhibits a certain symmetry. The fractal dimension of the fractures is calculated based on the CT fracture images of the coal body, showing that the fractal dimension decreases initially with the advancement of the working face and then gradually increases with the generation of transverse fractures in the coal body of the protected seam, eventually stabilizing. The expanding fractures in the coal body display a change rule of compacted-slowly expanding-accelerating expansion-stabilized expansion-slowly closing-compacted, exhibiting a certain periodicity. COMSOL and ABAQUS finite element software are used to carry out numerical simulation of protective layer mining and gas seepage gas-solid coupling, obtaining the distribution characteristics of the stress field, displacement field, and seepage field in the overlying rock layer. It is found that as the working face advances 50m-80m, the span and height of the vertical stress arch of the overlying rock layer increase continuously, showing a diffusion trend, and the pressure relief area exhibits a trapezoidal shape distribution with a narrow top and a wide bottom. Advancing to 120m-160m, the permeability change is concentrated in the fracture network area generated by plastic deformation of the top plate of the protective layer, consistent with the distribution of the decompression area. After the mining of the protected layer is finished, the cracks in the overburden between the protected layer and the protected layer gradually open, and the coal body is fully unpressurized, promoting the flow of gas.

[1] 程远平, 俞启香. 中国煤矿区域性瓦斯治理技术的发展 [J]. 采矿与安全工程学报, 2007, 24(8): 5.

[2] 薛东杰. 不同开采条件下采动煤岩体瓦斯增透机理研究[D].北京：中国矿业大学, 2013.

[3] 袁亮. 低透高瓦斯煤层群安全开采关键技术研究 [J]. 岩石力学与工程学报, 2008, 27(7): 10.

[4] 李晓红, 卢义玉, 赵瑜, 等. 高压脉冲水射流提高松软煤层透气性的研究 [J]. 煤炭学报, 2008, 33(12): 1386-1390.

[5] 林柏泉, 孟凡伟, 张海宾. 基于区域瓦斯治理的钻割抽一体化技术研究及应用[C]. 中国煤矿瓦斯治理国际研讨会. 中国煤炭工业协会, 2010.

[6] 林海飞, 李树刚, 成连华, 等. 覆岩采动裂隙带动态演化模型的实验分析 [J]. 采矿与安全工程学报, 2011, 28(2): 6.

[7] 刘厅. 深部裂隙煤体瓦斯抽采过程中的多场耦合机制及其工程响应[D].徐州：中国矿业大学, 2019.

[8] 张芝文, 何光福, 夏帆. 多孔炭材料的制备及其应用研究进展 [J]. 2020, 21(02):34-42.

[9] 王安民, 魏迎春, 李勇, 等. 准南地区煤储层孔隙结构特征及其煤岩学控制因素[C]. 中国地质学会. 中国地球物理学会, 2016.

[10] 段超超, 傅雪海, 刘正. 沁水盆地阜生煤矿煤储层物性特征 [J]. 煤矿安全, 2018, 49(10): 183-186+190.

[11] 李娜娜, 刘会虎, 桑树勋. 基于压汞-低温液氮联孔与核磁共振分析的煤中孔径分布对比研究 [J]. 煤矿安全, 2024, 55(02): 1-9.

[12] 姚艳斌, 刘大锰, 蔡益栋,等. 基于NMR和X-CT的煤的孔裂隙精细定量表征 [J]. 中国科学：地球科学, 2010, 40(11):1598-1607.

[13] 石钰, 马玉华, 李树刚,等. 基于CT扫描技术的煤样三维重构及气体表观渗透率研究 [J]. 矿业安全与环保, 2024, 51(01): 27-35.

[14] 车禹恒. 基于X-ray μCT扫描的煤孔隙瓦斯微观渗流各向异性特征研究 [J]. 矿业安全与环保, 2021, 48(04): 12-17.

[15] HERIAWAN M N, KOIKE K. Coal quality related to microfractures identified by CT image analysis [J]. International Journal of Coal Geology, 2015, 140: 97-110.

[16] 张文政, 邱磊. 基于CT三维重构的煤孔隙结构表征及分析 [J]. 煤炭技术, 2018, 37(12): 327-329.

[17] ZHAO Y, LIU S, ZHAO G F, et al. Failure mechanisms in coal: Dependence on strain rate and microstructure [J]. Journal of Geophysical Research Solid Earth, 2015, 119(09): 6924-6935.

[18] 曹广祝, 仵彦卿, 丁卫华. 低渗透压力条件下砂岩渗透性质的CT试验 [J]. 煤田地质与勘探, 2005, 33(04): 59-62.

[19] 王广荣, 薛东杰, 郜海莲, 等. 煤岩全应力-应变过程中渗透特性的研究 [J]. 煤炭学报, 2012, 37(01): 107-112.

[20] HAN F, BUSCH A, KROOSS B M, et al. Experimental Study on Fluid Transport Processes in the Cleat and Matrix Systems of Coal [J]. Energy & Fuels, 2010, 24(10): 117-125.

[21] ZHANG R, AI T, LI H, et al. 3D reconstruction method and connectivity rules of fracture networks generated under different mining layouts [J]. International Journal of Mining Science and Technology, 2013, (06):863-871.

[22] 尚锁贵, 高科超, 高强勇, 等. 裂缝发育程度对低孔隙度岩石渗流特性的影响 [J]. 科学技术与工程, 2023, 23(23): 9809-9819.

[23] 王亚, 葛丽珍, 路研,等. 基于核磁共振驱替实验的低渗透砂岩流体可动性及剩余油赋存特征研究 [J]. 油气地质与采收率, 2023, (06): 22-31.

[24] 苗杰. 低渗煤岩大孔隙结构三维重构及渗流模拟[D].焦作：河南理工大学, 2017.

[25] 巩剑南. 煤基多孔介质三维重构与渗流模拟研究[D].徐州：中国矿业大学, 2020.

[26] MOSTAGHIMI P, BLUNT M J, BIJELJIC B. Computations of Absolute Permeability on Micro-CT Images [J] Springer-Verlag, 2013, 45(01):103-125.

[27] MIAO J, LV R, LIN] X. Quantitative 3D spatial characterization and flow simulation of coal macropores based on μCT technology [J]. Fuel, 2017, (15):199-207.

[28] XIANGYU C, GANG W. Numerical simulation of gas flow based on three-dimensional reconstruction using computed tomography [J]. 2016, (03):74-82.

[29] YIN S H, WANG L M, PAN C Y, et al. Fluid flowing characteristics in ore granular with fine interlayers existed [J]. Zhongguo Youse Jinshu Xuebao/chinese Journal of Nonferrous Metals 2017, 27(3): 574-581.

[30] BO W J, HUI H Y, YANG J U, et al. LBM Simulation on Gas Seepage of Porous Rock Based on 3D Reconstructed Model [J]. Coal Engineering, 2014.

[31] LIU D, ZHOU G, MIAO Y, et al. Numerical Simulation Investigation for Stress Deformation and Water Injection Seepage of Coal Microstructure under Uniaxial Compression [J]. Journal of energy engineering, 2021, (5): 147.

[32] 申建. 论深部煤层气成藏效应 [J].煤炭学报, 2011, 36(09): 1599-1600.

[33] 杨天鸿,徐涛,刘建新,等.应力-损伤-渗流耦合模型及在深部煤层瓦斯卸压实践中的应用 [J].岩石力学与工程学报, 2005, 24(16):6.

[34] 赵洪宝, 尹光志, 李小双. 突出煤渗透特性与应力耦合试验研究 [J]. 岩石力学与工程学报, 2009, 28(S2):3357-3362.

[35] 魏建平, 吴松刚, 王登科, 等. 温度和轴向变形耦合作用下受载含瓦斯煤渗流规律研究 [J]. 采矿与安全工程学报, 2015, 32(1): 168-174.

[36] 张春会, 于永江, 岳宏亮, 等. 考虑Klinbenberg效应的煤中应力-渗流耦合数学模型 [J]. 岩土力学, 2010, 31(10): 3217-3222.

[37] 李治豪.煤岩受载下孔裂隙渗流特性研究[D].内蒙古自治区：内蒙古科技大学,2021.

[38] VIDAL-GILBERT, S., PEREIRA, et al. Desorption-induced shear failure of coal bed seams during gas depletion [J]. International Journal of Coal Geology, 2015.

[39] SOMERTON W H, SYLEMEZOLU I M, DUDLEY R C. Effect of stress on permeability of coal [J]. International Journal of Rock Mechanics & Mining Sciences & Geomechanics Abstracts, 1975, 12(5-6): 129-145.

[40] PALMER I, MANSOORI J. How Permeability Depends on Stress and Pore Pressure in Coalbeds: A New Model [J]. Spe Reservoir Evaluation & Engineering, 1996, 1(06): 539-544.

[41] SHI J Q, DURUCAN S. Drawdown Induced Changes in Permeability of Coalbeds: A New Interpretation of the Reservoir Response to Primary Recovery [J]. Transport in Porous Media, 2004, 56(1): 1-16.

[42] CUI X, BUSTIN R M, CHIKATAMARLA L. Adsorption-induced coal swelling and stress: Implications for methane production and acid gas sequestration into coal seams [J]. Journal of Geophysical Research Solid Earth, 2007, 112(10):12-28.

[43] 周世宁. 煤层瓦斯赋存与流动理论 [M]. 煤层瓦斯赋存与流动理论, 1999.

[44] 冯增朝, 郭红强, 李桂波, 等. 煤中吸附气体的渗流规律研究 [J]. 岩石力学与工程学报, 2014, 33(02): 3601-3605.

[45] 周军平, 鲜学福, 姜永东, 等. 基于热力学方法的煤岩吸附变形模型 [J]. 煤炭学报, 2011, 36(3): 468-472.

[46] WANG D, LV R, WEI J, et al. An experimental study of seepage properties of gas﹕aturated coal under different loading conditions [J]. Energy Science and Engineering, 2019.

[47] 孔祥国. 动载荷下含瓦斯煤动力学行为及瓦斯放散特征研究[D].徐州：中国矿业大学.

[48] 邓志刚, 齐庆新, 李宏艳, 等. 采动煤体渗透率示踪监测及演化规律 [J]. 煤炭学报, 2008, (03):273-276.

[49] 程伟. 储层微观结构的随机模型及其渗流规律的数值模拟[D].武汉：武汉工业学院, 2012.

[50] 周世宁, 孙辑正. 煤层瓦斯流动理论及其应用 [J]. 煤炭学报, 1965, (1): 26-39.

[51] 林柏泉, 周世宁. 含瓦斯煤体变形规律的实验研究 [J]. 中国矿业学院学报, 1986, (03):12-19.

[52] 郭勇义, 周世宁. 煤层瓦斯一维流场流动规律的完全解 [J]. 中国矿业学院学报, 1984, (02): 22-31.

[53] 孙培德. 瓦斯动力学模型的研究 [J]. 煤田地质与勘探, 1993, 21(1): 7-13.

[54] LIU J, CHEN Z, ELSWORTH D, et al. Linking gas-sorption induced changes in coal permeability to directional strains through a modulus reduction ratio [J]. Elsevier, 2010, 83(05):21-30.

[55] 田森.开采下保护层抽采瓦斯覆岩裂隙演化增透规律研究[D].辽宁：辽宁工程技术大学, 2016.

[56] 傅雪海, 秦勇, 姜波, 等. 山西沁水盆地中-南部煤储层渗透率物理模拟与数值模拟 [J]. 地质科学, 2003, (02):221-229.

[57] 张伟, 赵博, 郭晓阳, 等. 重复采动影响下含瓦斯岩层瓦斯运移规律 [J]. 矿业安全与环保, 2024, 51(01): 61-69.

[58] 张拥军, 于广明, 路世豹, 等. 近距离上保护层开采瓦斯运移规律数值分析[C]. 中国岩石力学与工程学会工程安全与防护分会. 第2届全国工程安全与防护学术会议论文集(下册). 青岛理工大学土木工程学院, 2010.

[59] SHI J Q, DURUCAN S. A Model for Changes in Coalbed Permeability During Primary and Enhanced Methane Recovery [J]. Spe Reservoir Evaluation & Engineering, 2005, 8(4): 291-299.

[60] SEIDLE J P, JEANSONNE M W, ERICKSON D J. Application of Matchstick Geometry To Stress Dependent Permeability in Coals[C]. proceedings of the SPE Rocky Mountain Regional Meeting, 1992.

[61] SHAO J F,ZHOU H, CHAU K T. Coupling between anisotropic damage and permeability variation in brittle rocks [J]. International Journal for Numerical & Analytical Methods in Geomechanics, 2005.

[62] SHAO J F, LU Y F, LYDZBA D. Damage Modeling of Saturated Rocks in Drained and Undrained Conditions [J]. Journal of Engineering Mechanics, 2004, 130(6): 733-740.

[63] 崔兆帮. 川南地区龙马溪组孔隙特征与页岩气赋存 [D].徐州：中国矿业大学, 2017.

[64] 陈海栋, 程远平, 蒲毅彬, 等. 卸载过程中型煤损伤特性研究 [J]. 煤炭科学技术, 2013, 41(03): 84-87.

[65] 张成林. 矿用超声波风速传感器测量精度补偿研究 [D], 徐州：中国矿业大学, 2023.

[66] KLINKENBERG L J. The permeability of porous media to liquids and gases [J]. American Petroleum Institute Drilling and Production-Practice, 1941.

[67] 姜瑞忠, 原建伟, 崔永正, 等. 基于TPHM的页岩气藏多级压裂水平井产能分析 [J]. 天然气地球科学, 2019, 30(01): 95-101.

[68] JAVADPOUR F, FISHER D, UNSWORTH M. Nanoscale Gas Flow in Shale Gas Sediments [J]. Journal of Canadian Petroleum Technology, 2007, 46(10): 55-61.

[69] FREEMAN C, MORIDIS G J, BLASINGAME T A. A Numerical Study of Microscale Flow Behavior in Tight Gas and Shale Gas Reservoir Systems [J]. Transport in Porous Media, 2011, 90(01):253.

[70] YU B M,CHENG P .A fractal permeability model for bi-dispersed porous media[J].International Journal of Heat and Mass Transfer, 45(14):2983-2993.

[71] YUAN M J , YU B M , et al. Geometrical models for tortuosity of streamlines in three-dimensional porous media[J].The Canadian Journal of Chemical Engineering, 2006, (3):84.

[72] DENN M. Process fluid mechanics [M]. Process fluid mechanics, 1980.

[73] 张文娟, 王媛, 倪小东. Forchheimer型非达西渗流参数特征分析 [J]. 水电能源科学, 2014, (1): 52-54.

[74] 赵明, 郁伯铭. 数字岩心孔隙结构的分形表征及渗透率预测 [J]. 重庆大学学报, 2011, 34(04): 88-94.

[75] 陈勉, 陈至达. 多重孔隙介质的有效应力定律 [J]. 应用数学和力学, 1999, 20(11): 1121-1121.

[76] 李波波, 高政, 杨康, 等. 温度与孔隙压力耦合作用下煤岩吸附–渗透率模型研究 [J]. 岩石力学与工程学报, 2020, 39(04): 668-681.

[77] 李存谊. 电渗联合真空预压现场试验研究和数值分析[D].杭州：浙江大学, 2017.

[78] 应宏伟, 章丽莎, 谢康和, 等. 坑外地下水位波动引起的基坑水土压力响应 [J]. 浙江大学学报:工学版, 2014,9, (03):492-497.

[79] 乔彤.渗透各向异性土体中水下隧道渗流量及衬砌外水压力研究[D].杭州：浙江大学,2022.

[80] 薛熠, 高峰, 高亚楠, 等. 采动影响下损伤煤岩体峰后渗透率演化模型研究 [J]. 中国矿业大学学报, 2017, 46(03): 521-527.

[81] ZHU W C, WEI C H. Numerical simulation on mining-induced water inrushes related to geologic structures using a damage-based hydromechanical model [J]. Environmental Earth Sciences, 2011, 62(1): 43-54.

[82] CONNELL L D, LU M, PAN Z. An analytical coal permeability model for tri-axial strain and stress conditions [J]. International Journal of Coal Geology, 2010, 84(2): 103-114.

[83] 申建军, 刘伟韬. 煤层开采底板突水通道形成的断裂力学机制探讨 [J]. 煤炭工程, 2016, 48(S2): 128-130.

[84] 王汉鹏, 李清川, 袁亮, 等. 煤与瓦斯突出模拟试验型煤相似材料研发与特性分析 [J]. 采矿与安全工程学报, 2018, 35(06): 1227-1283.

[85] 王晶, 王晓蕾. 下保护层开采时被保护层裂隙发育与渗透特征 [J]. 采矿与岩层控制工程学报, 2021, 3(03): 62-70.