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中煤阶煤结构演化的Raman光谱表征

邵燕, 陈小珍, 李晔熙, 左家琦, 崔曦, 蒋恒宇, 李美芬

邵 燕,陈小珍,李晔熙,等. 中煤阶煤结构演化的Raman光谱表征[J]. 煤炭科学技术,2023,51(8):295−303. DOI: 10.13199/j.cnki.cst.2022-0718
引用本文: 邵 燕,陈小珍,李晔熙,等. 中煤阶煤结构演化的Raman光谱表征[J]. 煤炭科学技术,2023,51(8):295−303. DOI: 10.13199/j.cnki.cst.2022-0718
SHAO Yan,CHEN Xiaozhen,LI Yexi,et al. Raman spectroscopy characterization of structural evolution in middle-rank coals[J]. Coal Science and Technology,2023,51(8):295−303. DOI: 10.13199/j.cnki.cst.2022-0718
Citation: SHAO Yan,CHEN Xiaozhen,LI Yexi,et al. Raman spectroscopy characterization of structural evolution in middle-rank coals[J]. Coal Science and Technology,2023,51(8):295−303. DOI: 10.13199/j.cnki.cst.2022-0718

中煤阶煤结构演化的Raman光谱表征

基金项目: 

国家自然科学基金资助项目(U1910204,41772165,41572144)

详细信息
    作者简介:

    邵燕: (1996—),女,山西平定人,硕士研究生。E-mail:ssyan1996@163.com

    通讯作者:

    李美芬: (1982—),女,山西应县人,副教授,博士。E-mail:limeifen1982@126.com

  • 中图分类号: TD315

Raman spectroscopy characterization of structural evolution in middle-rank coals

Funds: 

National Natural Science Foundation of China(U1910204,41772165,41572144)

  • 摘要:

    发生在中煤阶阶段的第二次煤化作用跃变导致煤的许多物理化学性质出现了转折性变化,而聚集态结构变化可能是导致这种转折性变化的根本原因。为详尽研究中煤阶煤结构演化特征及其与第二次煤化作用跃变的关系,选取了6个跨越第二次煤化作用跃变的中煤阶煤样(Ro,max=1.10%~1.63%),应用拉曼光谱法(Raman)对其进行结构表征,并利用分峰拟合软件分别对其一级模和二级模光谱曲线进行分峰拟合,在此基础上计算了相关结构参数。结果表明:Raman结构参数随Ro,max的演化不是线性的,反映煤结构演化的复杂性,据Raman结构参数的演化特征,可以将Ro,max=1.10%~1.63%阶段的煤化作用分为3个阶段,转折点分别位于Ro,max=1.30%和Ro,max=1.50%附近,正好与前人发现的第2次和第3次煤化作用跃变发生的位置相当,说明Raman结构参数可以反映煤化作用跃变的发生,同时也表明Raman光谱是一种研究煤结构的有效手段。第1阶段为Ro,max=1.10%~1.30%,以长链脂肪族结构裂解生成液相物质为主,同时断裂后的较短链的脂肪烃及芳环上的脂肪族取代基会形成脂肪环结构,煤结构的支链化程度增加,阻碍了芳香体系之间的定向排列,芳香体系排列有序度达到最差,表现为WG最小(G峰位置),FG/D最大(F为两峰半高宽比),AD/AG最小(A为峰面积),AS/A1增加,A(2G)R/A2大幅减小;第2阶段为Ro,max=1.30%~1.50%,上一阶段形成的脂肪环发生芳香化作用,导致芳香C—H结构含量增加,无定形碳含量达到最少,芳香化程度及芳香结构有序度均增加,表现为A(GR+VL+VR)/ADA(GR+VL+VR)/AGFG/D大幅减小,AD/AG增加,WGd(G-D)快速增加;第3阶段为Ro,max=1.50%~1.63%,一方面,第二阶段形成的芳香环之间发生缩聚反应,导致A(2G)R/A2减小,另一方面,芳香环体系间的各种桥键继续断裂,导致一些小尺寸芳香结构的生成,表现为A(2G)R/A2减小,WG小幅减小,A(GR+VL+VR)/ADA(GR+VL+VR)/AG增加。这些结果是深入理解煤化作用跃变机制及煤化作用机制的基础。

    Abstract:

    The second coalification jump which occurred during the middle-rank led to abrupt changes of many physical and chemical properties of coal, and the change of the aggregate structure may be the fundamental reason. In order to investigate the structural evolution characteristics of middle-rank coal and its relation with the second coalification jump in detail, the structure characteristics of six middle-rank coals (Ro,max=1.10%−1.63%) that across the second coalification jump were studied by Raman spectroscopy, and the structural parameters were calculated by fitting the first-order and second-order Raman spectrum using the fitting software. The results indicated that the evolution of Raman structural parameters with Ro,max is not linear, reflecting the complexity of the structural evolution of coal. According to the evolution characteristics of Raman structural parameters, the coalification during the stage of Ro,max=1.10%−1.63% can be divided into three stages. The turning points are located near Ro,max=1.30% and Ro,max=1.50%, respectively, which are exactly equivalent to the positions of the second and the third coalification jump discovered in previous research. It indicated that the Raman structural parameters can reflect the occurrence of the coalification jump, moreover, Raman spectroscopy is an effective method to study the coal structure. The first stage is Ro,max=1.10%−1.30%, the long-chain aliphatic structures cracked and the remained shorter-chain aliphatic hydrocarbons and aliphatic substituted structures on the aromatic rings will form new alicyclic structures, which caused the branched degree increases and hindered the alignment of aromatic systems in coal. The order degree of aromatic system is thus reached the least near Ro,max=1.30%, with the smallest WG, the largest FG/D, the smallest AD/AG, the increase of AS/A1, and the significant decrease of A(2G)R/A2. In the second stage of Ro,max=1.30%−1.50%, the aromatization of the alicyclic structures formed in the previous stage resulted in an increase in the content of aromatic C—H structure and the least of amorphous carbon structure. Besides, the degree of aromatization and aromatic structural both increased, which showed that A(GR+VL+VR)/AD, A(GR+VL+VR)/AG and FG/D decreased significantly, AD/AG increased, WG and d(G-D) increased quickly. The last stage is Ro,max=1.50%−1.63%, the condensation reaction occurred between the aromatic rings formed in the second stage, leading to the reduction of A(2G)R/A2. Meanwhile, the various bridging bonds between aromatic ring systems continued to break, resulting in the formation of some small-scale aromatic structures, as evidenced by a decrease in A(2G)R/A2, a small decrease in WG, and an increase in A(GR+VL+VR)/AD and A(GR+VL+VR)/AG. These results are the basis for deeply understanding the mechanism of coalification jump and coalification.

  • 随着浅部煤炭储量的日益减少,我国中部地区煤炭资源开发逐渐转向地层深部,倾斜厚煤层开采比例越来越高,且大部分煤层由于透气性低实施预抽煤层瓦斯虽能达到开采要求,但仍需进行卸压瓦斯抽采[1-3]。综采面覆岩瓦斯缓渗区(采空区上覆岩层发生垮落堆积后,瓦斯气体渗流、扩散更困难的局部区域)的准确判定有助于提高瓦斯抽采系统布置的精准性,对大幅提高卸压瓦斯抽采效果,降低瓦斯超限频率至关重要。

    目前国内外众多学者针对煤层开采后裂隙场空间形态及覆岩变化特征开展了大量研究。GHABRAIE[4]、DAVID[5]等研究了覆岩移动带的变形失稳规律。钱鸣高等[6-7]提出了“O”形圈模型,并建立了描绘整体岩层移动的“关键层”理论。袁亮等[8-10]提出了高位环形裂隙体模型。杨科等[11]提出了覆岩采动裂隙演化类似于“∩”形高帽状。李树刚等[12-15]提出了采动裂隙“椭抛带”理论,并建立了多因素影响下采动裂隙“椭抛带”压实区演化综合效应模型。刘洪永等[16]通过数值仿真研究了不同推进速度下优势瓦斯通道时空分布特征,并建立了基于采动裂隙“椭抛带”理论的优势瓦斯通道时空形态理论模型。以上学者的研究主要是针对近水平煤层,而倾斜煤层上覆岩层的裂隙演化规律相较于近水平煤层具有一定差异。为此,伍永平等[17-18]建立了非对称约束条件下大倾角煤层开采“关键层”覆岩结构特征模型。解盘石等[19]研究了大倾角近距离煤层群长壁采场顶板破断垮落特征,并对采场顶板易发生弯曲破断的位置进行了确定。ZHOU等[20]揭示了不同煤层倾角条件下覆岩位移场的变化规律,并利用对称性指数对覆岩位移场的非对称演化进行了深入研究。

    近年来,随着分形−岩石力学理论的不断发展,越来越多的学者借助分形理论开展了覆岩裂隙网络定量研究。谢和平等[21-23]应用分形维数对放顶煤巷道裂隙的分布和复杂程度进行了表征,并得出了采动岩体裂隙分布的自相似规律。周宏伟等[24]结合分形理论研究了开采宽度、采场矿压及岩层沉降等因素影响的覆岩裂隙分形维数动态演化规律。WANG等[25]研究了上行开采条件下覆岩裂隙网络的分形特征,发现随着开采宽度的增加,裂隙网络的分形维数呈现出快速上升、缓慢增加、稳定变化三阶段分布特征。杨滨滨等[26]研究了近距离煤层重复采动条件下覆岩裂隙时空演化特征,得出沿工作面推进方向上、下行开采覆岩裂隙分形维数分别呈“马鞍型”和“梯型”变化。赵毅鑫等[27]研究了浅埋超大采高工作面覆岩裂隙演化及能量耗散规律,得出覆岩裂隙分形维数在关键层破断前后呈上升−稳定−下降趋势。ZHAO等[28-30]研究了不同采高、煤层倾角、回采率下瓦斯运移优势通道的分布特征,并基于分形理论揭示了采动裂隙在工作面不同分区内的分形维数演化规律。

    上述研究成果为开展覆岩瓦斯缓渗区裂隙演化规律研究提供了理论依据,能够有针对性地描述覆岩瓦斯缓渗区的变化特征,但目前围绕覆岩瓦斯缓渗区进行分域研究较少。因此,笔者以山西某高瓦斯矿井为原型,通过物理相似模拟试验及理论分析研究,得到了覆岩瓦斯缓渗区分域方法及分形演化特征。研究结果为后期在倾斜厚煤层仰斜综采面确定卸压瓦斯抽采钻孔(巷道)终孔(巷)时抽采系统的精确布置提供了一定的理论基础,对采空区卸压瓦斯精准高效抽采具有一定的指导意义。

    以山西某高瓦斯矿井综采工作面为原型,该工作面主采15煤。工作面走向长度2 081 m,倾向长度180 m,平均埋深410 m,煤层平均厚度4.5 m,平均倾角30°,沿顶板一次采全高。煤层中含夹矸1~3层,厚0.38~1.10 m,煤层直接顶为泥岩,基本顶为中砂岩。

    试验采用平面模型,模型尺寸为1 100 mm×150 mm×800 mm(长×宽×高)。根据相似模拟试验原则,物理模型需满足相似三定理,如力学、几何等物理量与试验原型相似,依据工作面实际参数对物理模型相似常数进行计算[29],具体见表1

    表  1  物理模型相似常数
    Table  1.  Physical model similarity constants
    参数时间几何强度容重应力泊松比
    相似常数101001501.51501.0
    下载: 导出CSV 
    | 显示表格

    根据试验原型上覆岩层分布情况对物理模拟试验相似材料配比进行计算(表2)。在进行物理模型搭建时,先调节试验台至30°,然后将应力传感器安装于试验台底部,最后依据配比表称取沙子、淀粉及石膏,加水混合搅拌后逐层进行铺设,并在相邻岩层间均匀覆盖云母片作为层间分层材料,直至铺设完成。在物理模型搭建完成后,先调节试验台至水平,然后待模型自然晾干3~4周后进行位移测点及顶部配重布置,如图12所示。

    表  2  物理模型相似材料配比
    Table  2.  Physical model similar material ratio
    序号岩层厚度/cm质量/kg
    沙子淀粉石膏煤粉
    20粉砂岩7.04.130.110.4500.43
    19砂质泥岩4.04.190.200.3000.45
    1812煤0.54.230.090.362.110.68
    17砂质泥岩5.04.190.200.3000.45
    16细砂岩5.04.130.220.3300.43
    15铝质泥岩4.04.180.100.4000.45
    14细砂岩2.04.130.220.3300.43
    13砂质泥岩7.04.190.200.3000.45
    12石灰岩3.04.100.290.2900.47
    1113煤0.54.230.090.362.110.68
    10砂质泥岩5.04.190.200.3000.45
    9细砂岩6.04.130.220.3300.43
    8粉砂岩4.04.130.110.4500.43
    7石灰岩7.04.100.290.2900.47
    614煤1.04.230.090.362.110.68
    5砂质泥岩5.04.190.200.3000.45
    4粉砂岩3.04.130.110.4500.43
    3中砂岩7.04.130.140.3500.43
    2泥岩3.04.190.150.4000.45
    115煤4.54.230.090.362.110.68
    下载: 导出CSV 
    | 显示表格
    图  1  二维物理试验模型
    Figure  1.  Two-dimensional physical experiment model
    图  2  应力传感器及测点布置
    Figure  2.  Stress sensor and measurement point layout

    在模拟工作面推进时,为减小模型回采过程中边界效应对试验结果造成影响,先在模型两侧各留10 cm煤柱,接着开切眼宽8 cm,之后依次2、3 cm循环仰斜开采,确保与现场实际来压步距相似[31]。每次开采结束,待覆岩发育稳定分别测量记录离层量、破断裂隙密度、煤层底板应力及岩层垮落形态。

    覆岩离层量分布规律如图3所示,在工作面回采过程中,开切眼侧和工作面侧垮落覆岩受铰接梁限制,岩层间离层裂隙充分发育,为瓦斯气体扩散提供横向通道,伴随着上覆岩层垮落,岩层间离层裂隙重新达到稳定状态,岩层间裂隙离层量的突变为判断覆岩瓦斯缓渗区边界提供一定的支撑[28]。由图3可知,在采空区两侧的裂隙区内,覆岩具有较大的离层量,而在采空区中部的缓渗区内,由于垮落覆岩受压实程度强,上下相邻岩层间的离层裂隙更易闭合消失,从而离层量较小。在缓渗区中部的全缓渗区内覆岩离层量均小于1.5 m,此区域内瓦斯气体的扩散将明显更为困难。

    图  3  覆岩离层量分布
    Figure  3.  Distribution of overburden separation amount

    覆岩破断裂隙密度分布规律如图4所示,随着工作面推进,受集中应力作用采场上部岩层周期性垮落,在各岩层内形成竖向破断裂隙,为瓦斯气体运移提供竖向通道,覆岩破断裂隙密度的突变为判断覆岩瓦斯缓渗区边界提供一定的支撑[28]。由图4可知,在下滑效应的作用下,垮落岩块沿煤层倾向不断向开切眼侧进行滑落,造成工作面侧的垮落空间增大而开切眼侧垮落空间减少,同时受开切眼侧垮落岩块的限制,相邻岩块之间会形成反倾斜砌体结构,导致开切眼侧的破断裂隙密度明显大于工作面侧。此外,在缓渗区中部的全缓渗区内破断裂隙密度达到最小值2.5条/m,此区域内瓦斯气体的运移将明显受阻。

    图  4  覆岩破断裂隙密度分布
    Figure  4.  Distribution of overburden fracture density

    煤层底板应力分布规律如图5所示,利用提前布置在试验台底部的应力传感器,对工作面推进过程的煤层底板应力进行采集。由图5可知,采空区底板的应力相较于回采前明显变小且在裂隙区和缓渗区交界处,底板应力会有明显变化。这是由于在采空区两侧的裂隙区内,垮落覆岩之间易形成具有支撑作用的铰接结构,而在采空区中部的缓渗区内,下部已垮落覆岩可直接受到上部覆岩的压实作用,从而导致如图5中局部放大区域所示,中部缓渗区的应力集中系数比裂隙区的应力集中系数大,且在缓渗区中部的全缓渗区内应力集中系数达到最大值0.28。

    图  5  煤层底板应力分布
    Figure  5.  Stress distribution of coal seam floor

    通过对上述仰斜开采覆岩瓦斯缓渗区边界特征参数分布规律的研究,得到缓渗区宽度分布如图6所示。由图6可知,离层量对应工作面侧裂隙−缓渗过渡区的宽度最大为12.6 m,破断裂隙密度对应开切眼侧裂隙−缓渗过渡区的宽度最大为14.9 m,煤层底板应力对应全缓渗区的宽度最大为35.1 m。此外,离层量对应开切眼侧裂隙−缓渗过渡区的宽度小于工作面侧裂隙−缓渗过渡区的宽度,而破断裂隙密度和煤层底板应力对应开切眼侧裂隙−缓渗过渡区的宽度均大于工作面侧裂隙−缓渗过渡区的宽度。

    图  6  覆岩瓦斯缓渗区宽度分布
    Figure  6.  Width distribution of overburden gas slow permeability zone

    由于采动覆岩的裂隙分布具有很好的分形特征[32],因此可用分形维数定量描述裂隙演化程度。模型开采过程中共发生包含初次来压在内的7次来压,每次来压后用高清相机记录覆岩垮落照片,然后再对其进行切片划分,最后将所划分切片逐次批量导入自研的“计盒−关联”分形维数一体化综合分析软件进行二值化处理和分形分析,所得分形维数越高,表明切片区域的裂隙发育情况越复杂。覆岩瓦斯缓渗区垮落角扩展规律如图7所示,由图7可知,覆岩瓦斯缓渗区两侧垮落角随工作面的持续推进均表现出逐渐减小的变化规律。具体表现为从缓渗区初次形成至缓渗区充分发育期间,缓渗区开切眼侧的垮落角从68.3°减小到44.7°,减小幅度34.6%;缓渗区工作面侧的垮落角从76.2°减小到53.5°,减小幅度29.8%。此外,受煤层倾角影响,每次来压工作面侧的垮落角均大于开切眼侧。

    图  7  覆岩瓦斯缓渗区垮落角扩展规律
    Figure  7.  Caving angle expansion law of overburden gas slow permeability zone

    覆岩瓦斯缓渗区宽度扩展规律如图8所示,工作面开采后,上覆岩层发生破断及垮落,在两侧裂隙区内产生大量的离层、破断裂隙,而采区中部缓渗区由于受压实程度强,裂隙不断闭合,造成在两者交界处横向分形维数发生明显变化。由图8可知,覆岩瓦斯缓渗区宽度随工作面的持续推进表现出逐渐增大的变化规律。具体表现为从缓渗区初次形成至缓渗区充分发育期间,缓渗区的宽度从16.3 m增大到52.1 m,缓渗区沿横向不断发生扩展。

    图  8  覆岩瓦斯缓渗区宽度扩展规律
    Figure  8.  The width expansion law of overburden gas slow permeability zone

    覆岩瓦斯缓渗区高度扩展规律如图9所示,受采动影响,覆岩发生垮落在其顶部形成铰接结构,由于铰接结构下部覆岩受压实作用强,可得缓渗区上边界处于每次来压形成铰接结构的下部。由于铰接结构区域分形维数大,在缓渗区顶部与铰接结构交界处纵向分形维数发生明显增高。由图9可知,覆岩瓦斯缓渗区高度随工作面的持续推进表现出逐渐增大的变化规律。具体表现为从缓渗区初次形成至缓渗区充分发育期间,缓渗区的高度从19.2 m增大到38.4 m,缓渗区沿纵向不断发生扩展。

    图  9  覆岩瓦斯缓渗区高度扩展规律
    Figure  9.  Height expansion law of overburden gas slow permeability zone

    煤层在开采过程中,上覆岩层会发生不同程度的复杂移动和变形,同时伴随着离层裂隙与破断裂隙的产生。通过对覆岩瓦斯缓渗区动态扩展规律的研究,构建如图10所示覆岩瓦斯缓渗区分域准则。

    图  10  覆岩瓦斯缓渗区分域准则
    Figure  10.  Division criteria of overburden gas slow permeability zone

    为更好地对覆岩瓦斯缓渗区空间位置进行界定,按照构建的覆岩瓦斯缓渗区分域准则对缓渗区的上边界和外边界进行理论计算[33-34]

    1)缓渗区上边界模型。在采空区纵向方向,将缓渗区上边界定义为距煤层底板的距离,记为Su

    $$ {S_{\rm{u}}} = \sum\limits_{i = 1}^i {{h_i}} $$ (1)

    式中:hi为第i层岩层的厚度,m。

    采场上覆第i层悬空岩层可用固支梁结构进行代替,且岩层上部均匀施加有载荷$ {q_i} $,则上覆第i层岩层的悬空距$ {l_{si}} $与回采工作面推进距离L之间关系可用式(2)表示,其发生初次破断时的极限破断距$ {l_{si{\text{max}}}} $可用式(3)表示,具体如下:

    $$ {l_{si}}{\text{ = }}L - \left(\sum\limits_{i = 1}^{i - 1} {{h_i}\cot {\beta _{{\text{q1}}}}} + \sum\limits_{i = 1}^{i - 1} {{h_i}\cot } {\beta _{{\text{q2}}}}\right) $$ (2)
    $$ {l_{si{\text{max}}}} = {h_i}\sqrt {2{R_T}/{q_i}} $$ (3)

    式中:$ {\beta _{{\text{q1}}}} $、$ {\beta _{{\text{q2}}}} $分别为工作面上下两巷处覆岩破断角,(°);RT为第i层岩层的极限抗拉强度,MPa;qi为岩层承受载荷,kN,根据关键层理论[35],${q_i} = $$ \dfrac{{{E_i}{h_i}^3({\gamma _i}{h_i} + \cdots {\text{ + }}{\gamma _n}{h_n})}}{{{E_i}{h_i}^3 + {E_{i + 1}}{h_{i + 1}}^3 + \cdots + {E_n}{h_n}^3}}$;$ {\gamma _i} $为岩层容重,kN/m3Ei为岩层的弹性模量,GPa。

    假设岩层“砌体梁”结构向上传递至上覆第i层硬岩层时支撑其弹性基础满足Winkler地基模型,则上覆第i层硬岩层达到极限破断距时的最大弯曲下沉量$ {y_i} $可用式(4)表示[35],其下方的空间自由高度$ {\varDelta _i} $可用式(5)表示,具体如下:

    $$ {y_i} = \frac{{{q_i}}}{{{E_i}{I_i}}}\left[ {\frac{{12\alpha - 1}}{{24}}{l_h}4 + \left( {\frac{{\sqrt 2 }}{{\omega {l_h}}} + \frac{1}{2} - \alpha } \right)\frac{{{l_h}^2}}{{{\omega ^2}}}} \right] $$ (4)
    $$ {\varDelta _i} = M - \left(\sum\limits_{i = 1}^{i - 1} {{h_i}({k_{si}} - 1)}\right) $$ (5)

    式中:$ {I_i} $为岩层惯性矩,m4;$ {l_h} $为岩层极限破断距之半,m;$ \omega = {(K/{E_i}{I_i})^{1/4}} $,$\alpha = \left( {\sqrt 2 {\text{ }}{\omega ^2}{l_h}^2 + 6\omega {l_h} + 6\sqrt 2 {\text{ }}} \right)/ [6\omega {l_h}\times (2 + \sqrt 2 \omega {l_h})]$,K为弹性地基系数,$ K = {\left( {{E_0}/{d_0}} \right)^{1/2}} $,$ {E_0} $为地基弹性模量,GPa,$ {d_0} $为垫层厚度,m;$ {k_{si}} $为岩层残余碎胀系数;M为煤层采高,m。

    当采场上覆第i层岩层发生破断,则需符合以下条件:

    $$ \left\{ {\begin{array}{l} {{l_{si}} > {L_{si\max }}} \\ {{y_i} < {\varDelta _i}} \end{array}} \right. $$ (6)

    将式(2)—式(5)代入式(6),即:

    $$ {\left\{ {\begin{array}{l} {L - \left(\displaystyle\sum\limits_{i = 1}^{i - 1} {{h_i}\cot\; {\beta _{{\text{q1}}}}} + \displaystyle\sum\limits_{i = 1}^{i - 1} {{h_i}\cot\; } {\beta _{{\text{q2}}}}\right) > {h_i}\sqrt {\dfrac{{2{R_T}}}{{{q_i}}}} } \\ {\dfrac{{{q_i}}}{{{E_i}{I_i}}} \left[ {\dfrac{{12\alpha - 1}}{{24}}{l_h}4 + \left( {\dfrac{{\sqrt 2 }}{{\omega {l_h}}} + \dfrac{1}{2} - \alpha } \right)\dfrac{{{l_h}^2}}{{{\omega ^2}}}} \right] < M - \left(\displaystyle\sum\limits_{i = 1}^{i - 1} {{h_i}({k_{si}} - 1}\right) } \end{array}} \right. }$$ (7)

    因此,当工作面采高和岩层力学参数确定后,可利用式(7)对岩层是否满足破断进行判据,从而对缓渗区的上边界进行确定。

    2)缓渗区外边界模型。在采空区横向方向,缓渗区外边界定义为距临近侧煤柱的水平距离,记为So。由于区域呈环形,因此,需对走向、倾向两个方向的外边界进行界定,其中走向、倾向方向的外边界分别记为SosSod

    $$ \left\{ {\begin{array}{*{20}{l}} {{S_{{\rm{os}}}} = {F_{{\rm{os}}}} + {F_{\rm{w}}}} \\ {{S_{{\rm{od}}}} = {F_{{\rm{od}}}} + {F_{\rm{w}}}} \end{array}} \right. $$ (8)

    式中:$ {F_{{\rm{os}}}} $、$ {F_{{\rm{od}}}} $为走向、倾向方向裂隙区外边界距临近侧煤柱的水平距离,m;$ {F_{\rm{w}}} $为裂隙区的宽度,m。

    由于缓渗区的外边界即裂隙区的内边界,因此首先对裂隙区走向、倾向方向的外边界进行确定。在走向方向上,走向覆岩破断角$ {\beta _s} $可用式(9)计算;在倾向方向上,根据覆岩移动变形特征,倾向覆岩破断角$ {\beta _d} $需按照一定系数进行角度修正[36],具体如下:

    $$ {\beta _s} = {\text{arc}}\cot \frac{{\displaystyle\sum\limits_{j = 1}^m {{h_j}{\text{cot}}\;{\beta _j}} = \displaystyle\sum\limits_{j = 1}^n {{h_k}\cot\; {\beta _k}} }}{{\displaystyle\sum\limits_{j = 1}^m {{h_j} = \displaystyle\sum\limits_{j = 1}^n {{h_k}} } }} $$ (9)
    $$ {\beta _d} = {\text{arc}}\cot \frac{{\displaystyle\sum\limits_{j = 1}^m {{h_j}{\text{cot}}\;{\beta _j}} = \displaystyle\sum\limits_{j = 1}^n {{h_k}\cot \;{\beta _k}} }}{{\displaystyle\sum\limits_{j = 1}^m {{h_j} = \displaystyle\sum\limits_{j = 1}^n {{h_k}} } }} + kA $$ (10)

    式中:mn分别为第一关键层下方直接顶板岩层数量和煤层至缓渗区上边界范围内的关键层数量;$ {h_j} $、$ {\beta _j} $和$ {h_k} $、$ {\beta _k} $为分别为第j层岩层的厚度与破断角和第k层关键层及其载荷层的总厚度与组合破断角;k为与覆岩性质有关的修正系数,取值0.3~0.8;A为煤层倾角,上、下山侧分别取正、负值。

    因此,当采场覆岩破断至第i层岩层时,裂隙区外边界距该侧煤柱水平距离$ {F_{os}} $和$ {F_{od}} $可用式(11)进行计算,具体如下:

    $$ \left\{ {\begin{array}{*{20}{l}} {{F_{{\rm{os}}}} = {H_i}\cot \;{\beta _s}} \\ {{F_{{\rm{od}}}} = {H_i}\cot ({\beta _d} + A)\cos \;A + {H_i}\sin \;A} \end{array}} \right. $$ (11)

    式中:$ {H_i} $为第i层岩层与煤层的法向距离,m。

    在裂隙区外边界已经界定的基础上,只需确定其宽度即可对缓渗区的外边界进行确定。假设区域内第一层岩层发生周期破断时的岩块长度一致($ \mathop l\nolimits_1 = \mathop l\nolimits_2 = \cdots = \mathop l\nolimits_n = \mathop l\nolimits_{} $),则当第n+1破断岩块呈水平分布时,裂隙区宽度$ {F_w} $可用式(12)进行计算:

    $$ {F_w} = n l $$ (12)

    l1岩块破断后其回转角为$ {\theta _1} $,结合关键块体“S-R”稳定性[35],则:

    $$ \sin\; {\theta _1} = \frac{{{W_1}}}{l} = \frac{1}{l} \left\{M - \left[ {\sum\limits_{r = 0}^{r - 1} {{h_r}\left( {{k_r} - 1} \right)} } \right] - \sum\limits_{}^{} {h\left( {{k_{\textit{z}}} - 1} \right)} \right\}$$ (13)

    式中:W1为第一破断岩块下沉值,m;l为破断岩块长度,$ l = {h_i}\sqrt {{R_T}/3{q_i}} $,m;hr为第r层基本顶岩层的厚度,m;kr为基本顶岩层及其上覆岩层的碎胀系数;Σh为直接顶岩层的厚度,m;kz为直接顶岩层的碎胀系数。

    由“砌体梁”全结构计算得到的位移规律可知,破断岩块的回转角度满足:

    $$ {\theta _2} \approx \frac{1}{4}{\theta _1},{\theta _3} \approx {\left( {\frac{1}{4}} \right)^2}{\theta _1},\cdots,{\theta _n} \approx {\left( {\frac{1}{4}} \right)^{n - 1}}{\theta _1} $$ (14)

    研究表明,当$ {\theta _n} $约为3‰时[33],覆岩离层裂隙和破断裂隙逐渐闭合,第n+1断裂岩块进入缓渗区。此时,位于裂隙区范围内的破断岩块数量n可用式(14)计算:

    $$ n = 1 - \frac{{\ln 0.003 - \ln {\theta _1}}}{{\ln 4}} $$ (15)

    联立式(12)、式(13)和式(15),可得裂隙区宽度$ {F_w} $为:

    $$ {F_w} = {h_i}\sqrt {\frac{{{R_T}}}{{3{q_i}}}} \left\{ {1 - \frac{{\ln 0.003}}{{\ln 4}} + \frac{1}{{\ln 4}}\ln \arcsin \frac{{M - \left[ {\displaystyle\sum\limits_{r = 0}^{r - 1} {{h_r}\left( {{k_r} - 1} \right)} } \right] - \sum\limits_{}^{} {h\left( {{k_{\textit{z}}} - 1} \right)} }}{{{h_i}\sqrt {\dfrac{{{R_T}}}{{3{q_i}}}} }}} \right\} $$ (16)

    因此,通过联立式(8)、式(11)和式(16),即可对覆岩破断至第i层岩层时走向和倾向缓渗区的外边界进行确定。

    根据构建的覆岩瓦斯缓渗区分域准则,结合缓渗区上边界、外边界理论计算公式与物理相似模拟试验研究结果,对覆岩瓦斯缓渗区分域流程进行设计,如图11所示,图11中,Fh为裂隙发育高度;Od为距开切眼距离。

    图  11  覆岩瓦斯缓渗区分域流程
    Figure  11.  Division process of overburden gas slow permeability zone

    为更好的开展覆岩瓦斯缓渗区分形特征研究,依据上节建立的覆岩瓦斯缓渗区分域准则及流程将模型缓渗区沿横向(工作面推进方向)和纵向(垂直于煤层底板向上方向)进行区域划分。其中横向用H(Horizontal)表示,纵向用V(Vertical)表示,通过分形理论以横向和纵向分形维数变化来定量研究缓渗区演化规律。

    首先根据确定出的模型缓渗区边界,以缓渗区底部边界宽度作为长,缓渗区上边界高度作为宽,对所得缓渗区范围进行截取,然后对其沿横向和纵向均按照2 m进行划分,如图12所示。其中横向划分为26个子区域,沿工作面推进方向依次记为H1-H26;纵向划分为19个子区域,沿垂直于煤层底板向上方向依次记为V1-V19。最后按照缓渗区划分结果对每个横向和纵向子区域的离层量、破断裂隙密度、应力集中系数及分形维数进行求解。

    图  12  覆岩瓦斯缓渗区沿横纵向区域划分
    Figure  12.  Division of overburden gas slow permeability zone along horizontal and vertical areas

    由上文研究可知,覆岩瓦斯缓渗区沿横纵向分形维数与离层量、破断裂隙密度及应力集中系数之间的变化存在一定的关联性。因此文章引入灰色理论,通过计算系统中的比较序列与参考序列之间的关联度来度量数据之间的关联强度情况,从而对覆岩瓦斯缓渗区沿横纵向分形维数与离层量、破断裂隙密度及应力集中系数之间的最大关联度进行确定。灰色关联分析的基本步骤如下[37]

    1)根据研究目的对灰色关联分析比较体系进行确定。

    v个方案的w个原始指标值可构成如下原始方案比较矩阵X

    $$ {\boldsymbol{X}} = \left[ {\begin{array}{*{20}{c}} {{X_{^1}}} \\ {{X_2}} \\ \vdots \\ {{X_v}} \end{array}} \right] = \left[ {\begin{array}{*{20}{c}} {{x_{11}}}&{{x_{12}}}& \ldots &{{x_{1w}}} \\ {{x_{21}}}&{{x_{22}}}& \ldots &{{x_{2w}}} \\ \vdots & \vdots & \vdots & \vdots \\ {{x_{v1}}}&{{x_{v2}}}& \ldots &{{x_{vw}}} \end{array}} \right] $$ (17)

    b个方案的w个指标值构成集合${X_b} = \left( {x_{b1}}, {x_{b2}}, \cdots , {x_{bw}} \right)$$\left( {b = 1,2, \cdots ,v} \right)$

    2)确定参考序列:

    $$ {X_0} = \left( {{x_{01}},{x_{02}}, \cdots ,{x_{0w}}} \right) $$ (18)

    3)对指标数据进行无量纲化:

    $$ {x_{bc}}^ * = \frac{{{x_{bc}}}}{{\dfrac{1}{w}\displaystyle\sum\limits_{c = 1}^w {{x_{bc}}} }}(b = 0, \cdots ,v;c = 1, \cdots, w) $$ (19)

    经无量纲化处理的数据序列构成如下矩阵:

    $$ {{\boldsymbol{X}}^ * } = \left[ {\begin{array}{*{20}{c}} {{X_{^0}}^ * } \\ {{X_{^1}}^ * } \\ {{X_{^2}}^ * } \\ \vdots \\ {{X_{^v}}^ * } \end{array}} \right] = \left[ {\begin{array}{*{20}{c}} {{X_{^{01}}}^ * }&{{X_{02}}^ * }& \ldots &{{X_{^{0w}}}^ * } \\ {{X_{^{11}}}^ * }&{{X_{12}}^ * }& \ldots &{{X_{^{1w}}}^ * } \\ {{X_{^{21}}}^ * }&{{X_{^{22}}}^ * }& \ldots &{{X_{^{2w}}}^ * } \\ \vdots & \vdots & \vdots & \vdots \\ {{X_{^{v1}}}^ * }&{{X_{^{v2}}}^ * }& \ldots &{{X_{vw}}^ * } \end{array}} \right] $$ (20)

    4)逐个计算每个被评价对象指标序列(比较序列)与参考序列对应元素的绝对差值,即$\Big | {{X_{0c}}^ * - {X_{bc}}^ * } \Big |$$(b = 1, \cdots ,v;c = 1, \cdots, w)$,并确定$\mathop {\min }\limits_b \mathop {\min }\limits_c \Big | {{X_{0c}}^ * - {X_{bc}}^ * } \Big |$和$\mathop {\max }\limits_b \mathop {\max }\limits_c \Big | {{X_{0c}}^ * - {X_{bc}}^ * } \Big |$。

    5)计算每个比较序列与参考序列对应元素的关联系数:

    $$ {\xi _{bc}} = \frac{{\mathop {\min }\limits_b \mathop {\min }\limits_c \Big | {{X_{0c}}^ * - {X_{bc}}^ * } \Big | + \rho \mathop {\max }\limits_b \mathop {\max }\limits_c \Big | {{X_{0c}}^ * - {X_{bc}}^ * } \Big |}}{{\Big | {{X_{0c}}^ * - {X_{bc}}^ * } \Big | + \rho \mathop {\max }\limits_b \mathop {\max }\limits_c \Big | {{X_{0c}}^ * - {X_{bc}}^ * } \Big |}} $$ (21)

    式中:$ \rho $为区分系数,$ \rho \in $[0,1],通常取值0.5。

    从而可得到关联系数矩阵:

    $$ {\boldsymbol{E}} = \left[ {\begin{array}{*{20}{c}} {{E_1}} \\ {{E_2}} \\ \vdots \\ {{E_v}} \end{array}} \right] = \left[ {\begin{array}{*{20}{c}} {{\xi _{11}}}&{{\xi _{12}}}& \ldots &{{\xi _{1w}}} \\ {{\xi _{21}}}&{{\xi _{22}}}& \ldots &{{\xi _{2w}}} \\ \vdots & \vdots & \vdots & \vdots \\ {{\xi _{v1}}}&{{\xi _{v2}}}& \ldots &{{\xi _{vw}}} \end{array}} \right] $$ (22)

    6)计算每个比较序列与参考序列对应元素的关联系数均值:

    $$ {r_b} = \frac{1}{w}\sum\limits_{c = 1}^w {{\xi _{bc}}} $$ (23)

    从而可得到关联度矩阵为:

    $$ {\boldsymbol{r}} = \left[ {\begin{array}{*{20}{c}} {{r_1}} \\ {{r_2}} \\ \vdots \\ {{r_v}} \end{array}} \right] $$ (24)

    由于分形维数可以很好的反映覆岩裂隙发育程度,因此在研究缓渗区沿横纵向分形维数与离层量、破断裂隙密度及应力集中系数的变化关系时,确定反映系统行为特征的参考数列为距开切眼(煤层底板)不同距离的横(纵)向子域所对应的分形维数,影响系统行为的比较数列为距开切眼(煤层底板)不同距离的横(纵)向子域所对应的离层量、破断裂隙密度及应力集中系数。按照上式进行求解可得关联度如下:

    $$ r{\text{ = }}\left[ {\begin{array}{*{20}{c}} {{r_1}} \\ {{r_2}} \\ {{r_3}} \end{array}} \right]{\text{ = }}\left[ {\begin{array}{*{20}{c}} {0.93} \\ {0.79} \\ {0.87} \end{array}} \right] {r^ * }{\text{ = }}\left[ {\begin{array}{*{20}{c}} {{r_1}^ * } \\ {{r_2}^ * } \\ {{r_3}^ * } \end{array}} \right]{\text{ = }}\left[ {\begin{array}{*{20}{c}} {0.91} \\ {0.77} \\ {0.83} \end{array}} \right] $$

    式中:$ {r_1} $($ {r_1}^ * $)、$ {r_2} $($ {r_2}^ * $)、$ {r_3} $($ {r_3}^ * $)分别为缓渗区沿横(纵)向分形维数与离层量、破断裂隙密度及应力集中系数之间的关联度。

    由计算结果可知,缓渗区沿横纵向分形维数与离层量、破断裂隙密度及应力集中系数之间的关联度排序均为离层量>应力集中系数>破断裂隙密度,即离层量所对应的关联度均最大,缓渗区沿横纵向分形维数变化均与离层量变化相关性最强。

    根据灰色关联分析结果,得到覆岩瓦斯缓渗区沿横纵向分形维数变化均与离层量变化相关性最强。为验证理论计算的准确性,按照覆岩瓦斯缓渗区划分结果对缓渗区沿横纵向分形维数与离层量之间的变化规律进行研究,如图13所示。

    图  13  缓渗区沿横纵向分形维数与离层量之间的变化规律
    Figure  13.  Variation law between horizontal and vertical fractal dimension and separation amount in slow permeability zone

    图13a可知,随着横向子区域与开切眼间距的不断增大,在采动影响下,缓渗区两侧区域(裂隙-缓渗过渡区)易与相邻两侧的裂隙区形成铰接结构,造成缓渗区两侧区域产生大量离层裂隙,而在缓渗区中部(全缓渗区),旧跨落岩层不断被新跨落岩层挤压压实,造成离层裂隙不断发生闭合,使得缓渗区沿横向分形维数与离层量均总体呈现出先减小后增大的变化趋势。由图13b可知,随着纵向子区域与煤层底板距离的不断增大,空洞逐渐发生闭合,岩层垮落空间高度不断降低,在垂直于煤层底板向上方向覆岩垮落范围不断缩小,造成离层裂隙不断减少,且覆岩压实作用也将更加显著,使得缓渗区沿纵向分形维数与离层量均总体呈现出逐渐减小的变化趋势。根据试验分析结果,得到缓渗区沿横纵向分形维数变化均与离层量变化具有一致性,从而验证了灰色关联分析理论计算的准确性。因此可用离层量变化曲线来更好表征缓渗区演化规律,在后期确定卸压瓦斯抽采钻孔(巷道)终孔(巷)时,可通过现场观测覆岩离层量对缓渗区边界进行判断,有利于提升卸压瓦斯抽采效率。

    1)离层量对应工作面侧裂隙-缓渗过渡区的宽度最大为12.6 m,破断裂隙密度对应开切眼侧裂隙−缓渗过渡区的宽度最大为14.9 m,煤层底板应力对应全缓渗区的宽度最大为35.1 m。此外,离层量对应开切眼侧裂隙−缓渗过渡区的宽度小于工作面侧裂隙−缓渗过渡区的宽度,而破断裂隙密度和煤层底板应力对应开切眼侧裂隙−缓渗过渡区的宽度均大于工作面侧裂隙−缓渗过渡区的宽度。

    2)覆岩瓦斯缓渗区在第一次周期来压后初步形成,其后每次周期来压时,覆岩瓦斯缓渗区两侧的垮落角均不断减小而宽度和高度均不断增大。具体表现为从缓渗区初次形成至缓渗区充分发育期间,缓渗区开切眼侧的垮落角从68.3°减小到44.7°,缓渗区工作面侧的垮落角从76.2°减小到53.5°;缓渗区的宽度从16.3 m增大到52.1 m;缓渗区的高度从19.2 m增大到38.4 m。

    3)根据建立的覆岩瓦斯缓渗区分域准则及流程,结合灰色关联分析方法,得到缓渗区沿横纵向分形维数变化均与离层量变化相关性最强,对应关联度排序分别为$ {r_1} $=0.93>$ {r_3} $=0.87>$ {r_2} $=0.79;$ {r_1}^ * $=0.91>$ {r_3}^ * $=0.83>$ {r_2}^ * $=0.77,并通过物理模拟试验验证了灰色关联分析理论计算的准确性。

  • 图  1   经基线校正后煤样的Raman光谱

    Figure  1.   Raman spectrum of coal samples after baseline correction

    图  2   WH煤的Raman光谱分峰拟合

    Figure  2.   Curve fitting Raman spectra of WH coal

    图  3   一级模Raman结构参数随Ro,max的变化情况

    Figure  3.   Correlation of the first-order Raman structural parameters with Ro,max

    图  4   二级模Raman结构参数随Ro,max的变化

    Figure  4.   Correlation of the second-order Raman structural parameters with Ro,max

    表  1   煤样基本属性特征

    Table  1   Basic characteristics of the coal samples

    样品Ro,max
    /%
    工业分析/%元素分析/%
    MadAadVdafCdafHdafOdafNdafSdaf
    WH1.101.060.4127.5786.265.004.541.711.02
    LL1.170.960.1025.0888.454.933.361.600.60
    TL-21.300.980.2223.0087.764.724.131.620.57
    TL-81.440.457.4621.3686.093.916.121.082.71
    DQ1.510.569.9216.6789.924.173.211.261.43
    DEP1.631.050.1715.6289.584.322.981.450.45
    下载: 导出CSV

    表  2   Raman光谱各峰谱带归属[22,24-26]

    Table  2   The assignment of chemical shift in Raman spectrum in coal[22,24-26]

    谱带拉曼位移/cm−1描述
    (2G)L33201670 cm−1处峰的倍频峰,羰基C=O键
    2G3180G峰倍频峰,芳环结构
    (2G)R3060芳香C—H键伸缩振动峰
    D+G2925D峰和G峰的和频峰,大尺寸芳香结构
    (2D)L2810无定形碳结构,甲基和亚甲基的C—H键拉伸振动峰
    2D2670D峰倍频峰,芳环间的C—C键伸缩振动,大尺寸芳香结构
    (2D)R2480大尺寸芳香结构
    2S23001150 cm−1处峰的倍频峰,芳基碳—烷基碳,C=O键
    GL1680羰基C=O键
    G1580石墨特征峰E2g2振动;石墨芳环呼吸振动
    GR1540无定形碳结构,3~5个芳环体系
    VL1500无定形碳结构,例如由有机分子、片段或官能团产生的sp2键和形式的碳;
    非晶碳结构,
    例如亚甲基或甲基及PAHs的类似结构
    VR1465无定形碳结构,甲基结构
    D1350石墨结构中不少于6个环的芳香结构及芳环系统中的缺陷结构
    SL1270芳基—烷基醚;对芳香结构
    S1185sp2-sp3含碳结构,例如芳基碳—烷基碳;芳基-烷基醚;氢化芳环之间的C—C键;
    芳环上的C—H键;钻石六方碳
    SR1060苯环上的C—H键;苯(邻二取代)环
    R960~800芳香环上的C—H键;烷烃以及环烷烃上的C—C键
    下载: 导出CSV
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  • 收稿日期:  2022-05-11
  • 网络出版日期:  2023-07-12
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