前    言

深地能源高效、安全和绿色开采对保障我国十四五国家能源战略安全具有重要意义，而煤层气（煤矿瓦斯）是赋存在煤层及煤系地层的烃类气体，是优质清洁能源。一方面煤层气在增加清洁能源供应、减少温室气体排放中扮演正面角色；另一方面煤矿瓦斯在深地煤矿安全生产中扮演负面角色。不用于美国等能源强国在页岩气（油）革命中将能源输入国地位扭转为能源输出国，我国在煤层气和页岩气赋存地质条件和基于国情的煤与瓦斯共采策略中难以采取完全类似美国的战略。事实上，我国煤层气赋存深度约2000m以浅，与美国页岩气埋藏深度一致，开展煤层气（煤矿瓦斯）革命或许更具优势。煤炭固体资源和煤层气气体资源共存禀赋但状态不同，目前绝对意义上共采是不存在的，井工采煤同时辅助抽采降低瓦斯浓度、再集中回收是实现煤与瓦斯共采的主流形式；而理想的非共采形式为暂不考虑采煤而是以地上钻探辅助以水力压裂抽采的方式来开采煤层气资源，目前这两种开采方式在国内是共存的，各有利弊。本书主要针对深地煤与瓦斯共采中的问题开展理论和实验研究，期望能为我国低渗透煤岩中煤层气开采增透提供理论和技术支撑。

长期以来，瓦斯作为灾害源给煤矿安全生产带来严重损失，尤其是近些年，我国煤矿生产安全形势好转，但瓦斯造成的人员死亡事故仍然不断。中长期我国对能源发展的需求是巨大的，煤炭作为基础能源必须得到保障，而瓦斯富含甲烷，作为煤层气资源在我国存量巨大，又是可靠的前景资源。尤其是美国、澳大利亚等国家开展了页岩气与煤层气革命，创造了巨大的经济效益。我国煤层赋存条件具有高地应力、高瓦斯含量但低渗透等突出特点，尤其是低渗透严重阻碍了瓦斯的有效畅通，给煤层气开采带来严重技术困难，给瓦斯聚集产生灾害提供了条件。目前瓦斯增透技术发展及运用是煤层气开采中的热点与难点，尤其是针对我国特殊煤层赋存条件的增透技术还未得到充分发展，其主要原因是针对煤层增透的力学机理缺乏系统性地认识，未能从本质上为抽采技术的创新发展提供理论支撑，因此煤与瓦斯共采增透理论体系地建立是我国煤层气资源开采创新技术的理论源泉。笔者在充分总结国内外关于采动应力及瓦斯渗流特性研究现状基础上，结合理论研究、室内试验及数值仿真等多种手段，研究了三种不同开采条件下采动煤岩体瓦斯增透机理及裂纹扩展规律，具体研究内容如下：

1、研究工作面前方煤岩体瓦斯渗透率分布对合理抽采瓦斯具有技术指导意义。通过简化孔隙、裂隙等效力学模型，建立标准圆柱煤岩体试件的等效力学模型，推导含单一裂隙纯煤试样的等效轴向、径向与体积应变，并推广应用到含多种裂隙的多种介质组合结构，通过试验验证理论力学模型具有较好的可靠性。以淮南张集矿11-2煤层工作面回采为原型，通过数值计算得到三种不同开采的支承压力峰值集中系数，并推导出支承压力与水平应力分布表达式，其能综合考虑开采条件、影响范围与采动卸压产生的体积膨胀变形。建立体积应变与渗透率之间的多项式关系式，并给出采动条件下不同开采方式下的体积应变分布曲线与渗透率分布曲线，根据其各阶段的特征差异划分为不同的阶段，为工作面合理抽采瓦斯提供了理论依据。

2、保护层开采作为一种典型的煤与瓦斯安全开采形式在煤矿生产中具有重要的意义。通过由半无限开采积分模型求解得到岩体内部位移场表达式并与相似模拟被保护层沉降曲线对比，研究发现理论模型可以较好地反映煤层实际变形。建立了“两带”裂隙分布模型及煤层简化力学模型，通过正交设计的全应力应变渗透试验发现，瓦斯渗透主要分为三个过程，发现瓦斯渗透急剧变化在体积应变达到0.015处，对比理论体积应变分布曲线，得出体积应变沿沉降范围总体上呈对称分布，在中心区域存在一个体积应变大于0.015的范围，可见其正处于渗透率急剧增加阶段，其卸压增透效果最好。研究结果为被保护层瓦斯卸压增透计算提供了参考。

3、基于潘一矿煤田地质背景，开展相似模拟试验，通过Matlab软件实时捕获标记点位置并通过像素点演算其坐标，计算得到采场体积应变分布，其可有效地反映采场膨胀-压缩变形分布规律。同时进一步开展全应力应变渗透试验，认为瓦斯渗透主要分为三个过程，并建立体积应变与渗透率的耦合关系方程，最后绘制出渗透率的采场分布。研究发现随着工作面向前推进，无论是体积膨胀与渗透率演化分布都是一致的，被保护层渗透要滞后于保护层，并且随着垮落区的形成与再压密，其渗透率也逐渐减小，形成类似蝌蚪状分布，且为动态过程。总体上分析，被保护层变形明显滞后于工作面采空区，并且渗透率也小于采空区。

4、室内真实有效模拟煤矿采动力学行为与瓦斯流动规律对防治煤与瓦斯突出灾害认识机理有着重要的指导意义。试验基于河南平煤股份八矿已14-14120工作面（深度约690m）加工煤样，利用含瓦斯煤热流固耦合三轴伺服渗流装置开展煤气耦合渗透实验。根据三种不同开采条件下工作面前方支承压力与水平应力分布规律设计加卸载方案，研究结果为工作面合理抽采瓦斯，防止煤与瓦斯突出、瓦斯超限等安全开采提供必要的理论支持。通过上述研究发现，无论是相似模拟试验，还是室内三轴试验，裂纹的空间生成、表面粗糙程度及联通都是影响瓦斯流通的主导因素。而裂纹的完备定量描述是目前的难点，对建立裂纹特征与瓦斯渗透关系带来了困难。

5、应用逾渗理论建立了以单元裂隙块体为格点单元的采动裂隙逾渗网格点阵模型，基于HK算法编制了逾渗集团标定及有关逾渗参量的计算程序，建立了采动裂隙图像采集处理、逾渗集团标定、逾渗特性分析的较为可靠的研究方法。计算了不同采宽时的逾渗参量：采动裂隙集团大小分布、总采动裂隙集团数、集团平均大小、逾渗分维、逾渗概率，并分析各参量相互之间及与采宽、裂隙率、压力等的关系。

6、基于裂隙聚团演化过程表现出的临界特征，利用逾渗理论，建立上覆岩层的逾渗模型，并分别分析沿走向和沿倾向条件下采动裂隙的逾渗特性。根据相似模拟试验结果，分析采动裂隙演化逾渗特性及周期来压之间的关系。所建逾渗模型中逾渗概率、裂隙率、逾渗团大小、竖向破断裂隙概率、离层裂隙概率可以较为完备地定量描述裂隙特征，为建立裂隙与渗透率等相关参量的定量关系提供了合适的数学载体。

由于不同开采条件下采动煤岩体瓦斯增透机理研究涉及多学科理论和方法，有许多理论和实际问题仍有待于深入探讨和研究。本书可以作为高等院校有关专业的教学参考和有关研究人员的参考。本书在纠正以往文献的讹误的同时，自身也会产生新的谬误。因此，书中难免有不足之处，敬请读者批评指正。

本书所收录的本课题组成果，在研究过程中曾经得到国家自然科学基金和国家973项目的多次资助，没有这些资助，难以完成这些成果。

中国矿业大学(北京)

2020年09月于北京

Preface

   The high-efficiency, safe and green mining of deep energy and resources is of great significance to guaranteeing the energy strategic security of China in the 14th Five-Year Plan. The coalbed methane is a hydrocarbon gas that exists in coal seams and is high-quality clean energy. On the one hand, CBM plays a positive role in increasing the supply of clean energy and reducing greenhouse gas emissions. On the other hand, the CBM plays a negative role in the safe production of coal in deep. Not to be used by the United States and other energy-powered countries in the shale gas (oil) revolution to turn the status of an energy importing country into an energy-exporting country, China has difficulty in the geological conditions for the occurrence of coalbed methane and shale gas and the strategy of co-excavating coal-gas. The depth of coal-bed methane in China is generally 2,000m or less, which is the same as the shale gas in the United States. So, it may be more advantageous to carry out the coal-bed methane revolution in China. Coal as the solid resource and coal-bed methane as gas resources coexist but have different states. At present, the so-called co-mining does not exist in an absolute sense. Indeed, coal mining combined the extraction of gas, which could effectively reduce gas concentration before coal mining is the mainstream way to achieve co-mining of coal-gas. Moreover, the ideal non-co-mining form is directly using ground drilling with hydraulic fracturing technology to mine CBM. These two extracting ways coexist in China, and each has its advantages and disadvantages. This book mainly focuses on theoretical and experimental research on the problems of co-excavation of coal-gas in deep. It is expected to provide theoretical and technical support for the development of coalbed methane from low-permeability coals in China. This book refers to the basic theory of enhanced CBM and some engineering cases in China. The main research context is as follows:

   The mining-induced evolution of coal permeability in front of the working face is the basis to arrange the reasonable craft for coal-gas co-extraction. By simplifying the mechanical model of the cracks, a hybrid model of fracture network and coal matrix is established. The equivalent model containing one single crack effectively shows a good accuracy to predict the axial, hoop and volumetric strain of the coal cylinder under various confining pressure. Then, the model is promoted to a variety of mediums containing the composite structures. The field verification is carried out at the 11-2 coal face in Huainan Zhangji Mine. Three stress concentration coefficients of abutment pressures are calculated through the numerical calculation. Besides, the theoretical expressions of abutment pressure and horizontal stress are derived, which could comprehensively consider the mining layouts, the distribution range and the dilatancy of mining-induced coal. Further, a polynomial correlation between coal permeability and volumetric strain is established, which effectively describes the evolution of volumetric strain and permeability distribution under various mining layouts.

Study on permeability distribution of coalface has guiding significance in reasonable exploitation of gas. An equivalent mechanical model is established by simplifying the characteristics of pores and cracks. From the establishment of the equivalent mechanical model for a standard cylindrical coal-rock specimen, the equivalent formulas for axial, radial and volumetric strains are derived for coal-rock samples containing single fracture and then are extended to the application in a variety of media combinations containing multiple cracks. The theoretical mechanical model has a good reliability with experimental verification. Based on the prototype of 11-2 coal seam mining face of Zhangji Mine in Huainan, the peak concentration factors of abutment pressure under three typical mining layouts are obtained by numerical calculation. An expression for the distribution of abutment pressure and horizontal stress is derived considering different mining layouts, different stages and volume expansion induced by mining. By establishing a polynomial relationship between volumetric strain and permeability, the distribution of volumetric strain and permeability under different mining layouts is obtained and divided into different stages according to their various characteristics. It may provide a theoretical basis for the coal mine methane.

Another mining layout termed the protective coal seam mining has great significance in coal production as a typical form of co-extraction of coal-gas in deep coal. The deformation field of overburden strata is solved employing the semi-infinite integral model, and the actual deformation of coal seam can be better explained by this theoretical model by the contrast of subsidence curves of protected coal seam between theoretical result and physical model test. A two-zone model of fracture distribution is established. It is found that there are three main processes of permeability evolution from an experimental observation by the orthogonal design of the coupled stress-permeability tests. The distribution of volumetric strain of protected coal seam shows a total symmetry and there is a proper match of a surge of permeability corresponding to the serious dilatancy.

Field observation is further made considering the geological conditions of Pan-Yi mine. In the physical model, the classical test named the similar simulation was carried out to investigate the evolution of volumetric strain distribution by monitoring the movable mark. The field deformation of dilatancy and compression could be directly and effectively presented. Further, the coupling correlation between volumetric strain and permeability is proposed to investigate the field evolution of the gas flow. The large-scale evolution of gas flow in the overburden strata by the physical modelling could be effectively visualized based on the proposed correlation between dilatancy and permeability. It is found that with the working face advancing, the distribution of both volumetric strain and permeability also has analogously dynamic distribution process. However, the evolution of gas flow in the protected coal seam lags far behind the protective coal seam.

In the lab through the triaxial compression test, the coupling mechanism of the gas flow in the confined coal cylinders has important guiding significance to understanding the prevention of coal-gas outburst. The coal samples were selected from the working face labelled 14-14120 at the depth of 690m in Henan Pingdingshan No.8 Mine and the experiment has been carried out. According to the comparative distribution of abutment pressure and horizontal stress, the real loading and unloading path is designed corresponding to the increment of abutment pressure and decrement of horizontal stress. It is found that whether it is a similar simulation test or an in-lab triaxial test, the generation of spatial fractures, surface roughness and connection of fracture network are all influencing factors that affect gas flow. The comprehensively quantitative description of fractures is still difficult, which will bring a huge hurdle to establish the relationship between the fracture network and gas flow in low-permeability coal.

By the percolation theory, we constructed the percolation model taking unit damage block as lattice point element, and compiled a program to label percolation clusters and calculate the percolation parameters, and thus established a reliable integrated research method for realizing the acquisition, processing of fracture image, percolation cluster labelling, and percolation behaviour analysis. It is calculated that percolation parameters including cluster size distribution of fracture, total clusters of the mining-induced fracture network, the average size of clusters, fractal dimension, percolation probability, and analyzed the mutual relations among percolation parameters and the relation between each percolation parameter with mining width, fracture probability and pressure.

Finally, based on the critical connection of the fracture network, the percolation model of the fracture network evolution in the overburden strata has been effectively established to describe the evolution of mining-induced fracture network along the strike and dip. According to the physical modelling of mining behaviour, the relationship between the percolation characteristics and periodic abutment pressure is obtained. In the percolation model, the fracture evolution could be statistically calculated perfectly and quantitatively by the proposed parameters including percolation probability, fracture rate, size of percolation group, and the probability of fractures through or along coal seam. The percolation model could provide an appropriately mathematical carrier for the relationship of fractures and other physical or mechanical parameters such as permeability.

As the research on the mechanism of permeability enhancement in low-permeability coal under different mining layouts involves multidisciplinary theories and methods, many theoretical and practical issues that need to be discussed and studied in depth. This book can be used as a reference for teaching-related majors in colleges and universities and related researchers. While this book corrects the errors in the previous literature, it also produces new fallacies. Therefore, it is inevitable that there are deficiencies in the book. Readers are welcome to criticize and correct.

The results of this research group included in this book have been funded many times by the National Natural Science Foundation of China and the National 973 Project during the research process. Without these funds, it is difficult to complete these results.

Xue Dongjie

China University of Mining and Technology (Beijing)

September 2020