真三轴载荷煤体渗流特性及其在瓦斯抽采中的应用

发布时间：2018-09-10 19:49

【摘要】：我国煤层瓦斯赋存以“三高一低”(高应力、高瓦斯压力、高瓦斯含量及低渗透性)为主要特征,煤储层构造复杂,煤层存在许多强烈变形,受载煤体的三向应力受到煤层赋存情况、煤层倾角等地质条件,以及构造运动等力学扰动的影响而有明显差别,所处的应力条件一般为三向不等压状态。另外,瓦斯抽采、煤层开挖等采动行为也会使应力重新分布导致局部应力集中,其应力环境同样为三向不等压状态,即真三轴应力状态(最大主应力σ_1中间主应力σ_2最小主应力σ_3)。与此同时,由于我国成煤历史过程的不均匀性、煤体本身构造、孔隙裂隙发育和变形程度的不同,很大程度上影响着煤体强度和力学变形特征,致使煤层不同层理方向上裂隙扩展特征和渗透率演化有明显差别。因此,开展真三轴条件下不同层理构造(垂直、平行和斜交层理)受载煤体渗流特性的研究迫在眉睫。本文采用理论分析、实验研究、模拟研究和现场应用相结合的研究方法,从细观微观角度分析煤体受载前后裂隙微观结构及不同层理煤样受力前后形态特性的差异,开展真三轴载荷含层理煤体的渗透率实验研究,构建真三轴条件下渗流特性与最大主应力、中间主应力、最小主应力、有效应力、不同层理构造之间的定性与定量关系,建立考虑Klinkenberg效应、瓦斯压力压缩变形、吸附膨胀和有效应力影响的真三轴载荷煤体渗透率动态演化模型及煤岩损伤与渗流耦合模型,并进行煤岩损伤与渗流特性的数值模拟及现场应用,研究成果对优化瓦斯抽采设计具有重要的指导意义。本文的主要研究内容如下:首先,从细观微观角度分析获得了煤体受载前后裂隙微观结构及不同层理煤样受力前后形态特性的差异。通过压汞和工业分析测试,得到实验煤样的基本物性参数,如:煤样的孔隙率、比表面积、孔容、水分、灰分、挥发分、极限吸附常数a/b等,基本物性参数获得为后续的试验和模拟研究奠定了基础。采用扫描电镜对受载前后煤体进行微米级的测试可知,煤样受载后其致密结构遭到破坏,有大量裂隙生成,原始的孔洞也进一步连通,明显观察到大量剪切裂隙。利用高分辨率透射电镜对煤体受载前后的内部微观结构进行测试可知,煤体受到载荷破坏后孔隙、裂隙纹数明显增多,这有利于裂纹的发育、扩展和贯通,最终使煤体的渗透率明显增大。其次,利用TAW-2000 KN电液伺服岩石压力机对不同层理煤样进行单轴压缩实验可知,垂直层理煤样的单轴抗压强度、弹性模量最大,泊松比最小;斜交层理煤样的单轴抗压强度、弹性模量最小,泊松比最大;平行层理的单轴抗压强度、弹性模量和泊松比均介于垂直层理和斜交层理之间。采用SH-II声发射系统对试验过程中的声发射信号进行监测可知,垂直、平行和斜交层理煤样在200s、130s和100s时有明显的声发射信号突变,垂直、平行和斜交层理煤样声发射突变点分别为峰值应力的60%、41%和33%左右。根据真三轴受载煤体渗流特性试验结果,垂直、平行和斜交层理煤样加载初期的渗透率为0.0181、0.1352、0.0822m D,垂直层理煤样加载初期的渗透率仅为平行层理、斜交层理的13.5%、22.2%;垂直、平行和斜交层理煤样在加载末期的渗透率分别为0.00384、0.00635、0.00739m D,其渗透率分别降低了78.9%、95.3%、90.9%。不同层理煤样的渗透率与最大主应力σ_1、中间主应力σ_2、最小主应力σ_3和有效应力均呈指数函数关系,k=a+bexp??-c(σ_i)??(i=1/2/3/e),随着应力的增加渗透率逐渐降低,垂直层理煤样在最大主应力σ_1、中间主应力σ_2和最小主应力σ_3加载阶段渗透率的降低幅度分别为74.1%、16.9%、9.0%;平行层理煤样在最大主应力σ_1、中间主应力σ_2和最小主应力σ_3加载阶段渗透率的降低幅度分别为58.8%、27.8%、13.4%;斜交层理煤样在最大主应力σ_1、中间主应力σ_2和最小主应力σ_3加载阶段渗透率的降低幅度分别为67.4%、21.3%、11.3%。在恒定中间主应力σ_2和最小主应力σ_3条件下,加载最大主应力σ_1直至煤样破坏,不同层理煤样变形破坏过程均表现为应力加载初期渗透率急剧下降,随着应力的不断增加渗透率下降速度明显变缓,但不同层理煤样由压缩向扩容过渡的边界有明显差别,垂直、平行和斜交层理煤样C-D边界对应的最大主应力分别为25、22、19MPa。根据不同层理煤样裂隙演化及渗流特性的模拟结果,垂直、平行和斜交层理煤样在加载的初始阶段,均随着应力的不断加载而被逐渐压密,原始孔隙和层理裂隙面不断闭合,尤其以层理裂隙面附近的压密效果最明显。当最大主应力分别加载到28、24、22MPa时,不同层理煤样原始的孔隙裂隙、层理裂隙面和新萌生的裂隙进一步扩展,局部区域出现贯通,煤样逐渐进入失稳破坏阶段,垂直、平行和斜交层理煤样均以剪切破坏为主,但由于不同层理煤样预制层理面的不同导致裂隙演化最终形态有一定差别。声发射图中垂直层理煤样的抗压、抗拉破坏主要出现在预制层理裂隙面附近,在煤样经历压密阶段、弹性阶段直到煤样破坏的塑性阶段,声发射信号逐渐增加,且声发射信号主要在层理裂隙面的两端和周围增加,但平行层理煤样此阶段没有明显的抗压破坏;斜交层理煤样的3条预制层理中间1条的破坏最明显,声发射信号在这条层理裂隙面上的分布也最多,斜交层理煤样失稳破坏的模拟结果与试验结果基本吻合。不同层理煤样的气体渗流压力场均随着应力场的变化而变化,在应力加载的初期,煤样中气体呈现稳定均匀的渗流,煤样中气体渗流压力场梯度曲线接近于平行直线,随着应力的加载煤样出现零星的裂纹,在裂纹出现的地方气体渗流压力场梯度曲线发生明显变化,气体渗流压力场随着裂隙的变化而不断变化,呈凹凸不平的波浪状分布,煤样出现明显破坏时,其气体渗流压力场变化更加明显,尤其是出现新裂隙的区域。在考虑煤体受到载荷后的变形主要由瓦斯吸附膨胀变形和瓦斯压力压缩变形组成的基础上,建立了载荷煤体的孔隙率动态演化模型;在分析真三轴受载煤体损伤变形的过程中存在三种变形机制的基础上,结合建立的孔隙率动态演化模型,得到基于孔隙率动态变化特征的有效应力公式;根据煤体渗透率和孔隙率立方关系,建立了真三轴载荷煤样的渗透率动态演化模型。最后,根据本煤层顺层钻孔瓦斯抽采的渗流特性,考虑Klinkenberg效应、瓦斯压力压缩变形、吸附膨胀和有效应力的影响,建立了抽采钻孔周围煤体的流固耦合模型并在某矿29031工作面进行现场应用,在抽采钻孔附近,考虑Klinkenberg效应与未考虑Klinkenberg效应相比,考虑Klinkenberg效应影响的瓦斯压力下降更快,而距离抽采钻孔越远,Klinkenberg效应的影响越小。在抽采时间一定的条件下,抽采负压对煤层瓦斯压力的下降影响不明显,抽采负压与有效抽采半径之间满足幂函数关系:y=3.05x~(0.011),相关系数为0.95,结合29031工作面的实际情况,确定29031工作面顺层钻孔合理的抽采负压为15k Pa。抽采钻孔间距对煤层瓦斯压力的下降和抽采效果影响明显,考虑现场的复杂性和不均衡性,需要增加30%的安全余量,结合29031工作面的实际情况进行现场应用,确定该工作面抽采180d后钻孔间距为6m时,瓦斯压力下降幅度及范围最大,同时有效的避免了“空白带”和抽采的无效叠加。根据模拟成果在某矿29031工作面布置顺层钻孔抽采180d后,抽采瓦斯纯量由7.16m~3/min下降到3.58m~3/min,抽采效果十分明显。进一步分析顺层钻孔与层理面夹角对瓦斯抽采效果的影响可知,顺层钻孔与层理面夹角对瓦斯抽采效果的影响比较明显,斜交钻孔的瓦斯抽采效果明显好于平行钻孔,这与真三轴载荷含层理煤体渗流特性的试验结果基本吻合。因此,在煤层瓦斯抽采钻孔布置时应尽量使钻孔方向与层理面垂直,以期达到最佳的瓦斯抽采效果。本文的研究成果不仅对优化瓦斯抽采设计、防治煤与瓦斯突出、减少因瓦斯排放引起的“温室效应”具有重要的实际价值,而且对煤矿的安全高效生产具有重要的现实意义。
[Abstract]:Coal seam gas occurrence in China is characterized by "three high and one low" (high stress, high gas pressure, high gas content and low permeability). Coal reservoir structure is complex, coal seam has many strong deformation. The three-dimensional stress of coal seam is affected by coal seam occurrence, coal seam dip angle and mechanical disturbance such as tectonic movement. In addition, the stress redistribution will also lead to local stress concentration, and the stress environment is also three-dimensional unequal pressure state, that is, true triaxial stress state (maximum principal stress__1 intermediate principal stress__2 minimum principal stress__3). At the same time, because of the inhomogeneity of coal-forming history, the structure of coal body, the development of pore and fracture and the degree of deformation, the strength and mechanical deformation characteristics of coal body are affected to a great extent, and the fracture propagation characteristics and permeability evolution in different bedding directions are obviously different. It is imminent to study the seepage characteristics of coal body loaded with the same bedding structure (vertical, parallel and oblique bedding). In this paper, theoretical analysis, experimental research, simulation research and field application are combined to analyze the micro-structure of coal body before and after loading and the difference of morphological characteristics of different bedding coal samples before and after loading from the micro-view. Experimental study on permeability of bedded coal body under true triaxial load is carried out. The qualitative and quantitative relationships between seepage characteristics and maximum principal stress, intermediate principal stress, minimum principal stress, effective stress, and different bedding structures are constructed under true triaxial load. The dynamic evolution model of coal permeability under true triaxial load and the coupling model of coal-rock damage and seepage flow are studied. The numerical simulation and field application of coal-rock damage and seepage characteristics are carried out. The research results have important guiding significance for optimizing the design of gas extraction. The main contents of this paper are as follows: Firstly, it is obtained from microscopic analysis. Through mercury intrusion test and industrial analysis test, the basic physical parameters of coal samples were obtained, such as porosity, specific surface area, pore volume, moisture, ash, volatile matter, limiting adsorption constant a/b and so on. The results show that the dense structure of coal sample is destroyed, a large number of cracks are formed, the original voids are further connected, and a large number of shear cracks are observed. The results show that the number of pores and crack lines increases obviously after the coal body is damaged by load, which is beneficial to the development, propagation and penetration of cracks, and ultimately makes the permeability of coal body increase significantly. Secondly, the uniaxial compression test of different bedded coal samples by using TAW-2000 KN electro-hydraulic servo rock press shows that the single-axial compression test of vertical bedded coal samples. Axial compressive strength, modulus of elasticity is the largest, Poisson's ratio is the smallest; uniaxial compressive strength, modulus of elasticity is the smallest, Poisson's ratio is the largest; uniaxial compressive strength, modulus of elasticity and Poisson's ratio of parallel bedding are between vertical bedding and oblique bedding. The results show that there are obvious abrupt changes of AE signals in vertical, parallel and oblique bedded coal samples at 200s, 130s and 100s. The abrupt change points of AE signals in vertical, parallel and oblique bedded coal samples are about 60%, 41% and 33% of peak stress respectively. Permeability is 0.0181,0.1352,0.0822 mD, the permeability of vertical bedding coal sample is only parallel bedding at the initial loading stage, and that of oblique bedding coal sample is 13.5%, 22.2%; the permeability of vertical, parallel and oblique bedding coal sample is 0.00384,0.00635,0.00739 mD at the end of loading stage, the permeability is reduced by 78.9%, 95.3% and 90.9% respectively. The maximum principal stress__1, the intermediate principal stress__2, the minimum principal stress__3 and the effective stress all have exponential functions, k = a + bexp?-c (__i)?(i = 1/2/3/e). With the increase of stress, the permeability decreases gradually, and the permeability decreases in the maximum principal stress__1, the intermediate principal stress__2 and the minimum principal stress__3 loading stages, respectively. The permeability of parallel bedded coal samples decreased by 58.8%, 27.8% and 13.4% in the loading stages of maximum principal stress__1, intermediate principal stress__2 and minimum principal stress__3, respectively; and that of oblique bedded coal samples decreased by 67.4% and 2% in the loading stages of maximum principal stress__1, intermediate principal stress__2 and minimum principal stress__3, respectively. Under the condition of constant intermediate principal stress__2 and minimum principal stress__3, the maximum principal stress__1 was loaded until the failure of coal samples. The deformation and failure process of different bedded coal samples showed that the permeability decreased sharply at the initial stage of stress loading, and the permeability decreased slowly with the increase of stress, but the compression direction of different bedded coal samples was changed. The maximum principal stresses corresponding to C-D boundaries of vertical, parallel and oblique bedded coal samples are 25,22,19 MPa, respectively. According to the simulation results of fracture evolution and seepage characteristics of different bedded coal samples, the vertical, parallel and oblique bedded coal samples are gradually compacted with the continuous loading of stress at the initial stage of loading. When the maximum principal stress is applied to 28,24,22 MPa, the original pore fissures, the bedding fissures and the newly sprouted fissures of different bedded coal samples expand further, the local area appears to be connected, and the coal samples gradually enter the instability failure stage. The ultimate shape of fracture evolution is different because of the different prefabricated bedding planes of different bedding coal samples. The compressive and tensile failure of vertical bedding coal samples in AE maps mainly occur near prefabricated bedding fracture planes, and the coal samples undergo compaction and elastic stages. Until the plastic stage of failure of coal sample, acoustic emission signal increases gradually, and acoustic emission signal increases mainly at both ends and around the bedding fracture surface, but parallel bedding coal sample has no obvious compressive failure at this stage; the damage of one of the three prefabricated bedding of oblique bedding coal sample is most obvious, and acoustic emission signal is on this bedding fracture surface. The pressure field of gas seepage in different bedded coal samples varies with the stress field. At the initial stage of stress loading, gas seepage appears stable and uniform, and the gradient curve of gas seepage pressure field in coal samples is close to parallel straight line. The gradient curve of gas seepage pressure field changes obviously when the crack occurs. The gas seepage pressure field changes continuously with the crack change, and the distribution of gas seepage pressure field is uneven and wavy. When the coal sample is obviously destroyed, the change of gas seepage pressure field is more obvious, especially when the crack occurs. A dynamic evolution model of porosity of loaded coal body is established on the basis of considering that the deformation of coal body after loading is mainly composed of gas adsorption expansion deformation and gas pressure compression deformation. According to the dynamic evolution model of porosity, the effective stress formula based on the dynamic variation characteristics of porosity is obtained; according to the cubic relationship between permeability and porosity of coal, the dynamic evolution model of permeability of true triaxial load coal sample is established. Finally, according to the seepage characteristics of gas drainage along seam boreholes, the Klinkenberg effect and gas pressure are considered. Compression deformation, adsorption expansion and effective force influence, the fluid-solid coupling model of coal around the drainage borehole is established and applied in 29031 working face of a mine. In the vicinity of the drainage borehole, considering the Klinkenberg effect is faster than not considering the Klinkenberg effect, and considering the influence of the Klinkenberg effect, the gas pressure decreases faster, while the distance drainage. The farther the drilling hole is, the less the influence of Klinkenberg effect is. Under the condition of a certain extraction time, the negative pressure has no obvious influence on the decrease of gas pressure in coal seam, and the relationship between the negative pressure and the effective extraction radius satisfies power function: y = 3.05x ~ (0.011), the correlation coefficient is 0.95. Combining with the actual situation of 29031 working face, the smoothness of 29031 working face is determined. Reasonable negative pressure of drainage is 15kPa. The interval of drainage boreholes has obvious influence on the decrease of coal seam gas pressure and the effect of drainage. Considering the complexity and imbalance of the site, it is necessary to increase 30% of the safety margin. Combining with the actual situation of 29031 working face, it is determined that when the interval of boreholes is 6m after 180 days of drainage, the gas will be increased. According to the simulation results, after 180 days of layered borehole drainage in 29031 working face of a mine, the gas extraction purity decreased from 7.16 m~3/min to 3.58 m~3/min, and the drainage effect was very obvious. Further analysis of the angle between bedding borehole and bedding plane was carried out. The influence of the angle between bedding borehole and bedding surface on gas drainage effect is obvious. The gas drainage effect of oblique borehole is obviously better than that of parallel borehole, which is basically consistent with the experimental results of seepage characteristics of coal body with bedding under true triaxial load. The research results of this paper not only have important practical value for optimizing gas extraction design, preventing coal and gas outburst, reducing greenhouse effect caused by gas emission, but also have important practical significance for safe and efficient production of coal mine.
【学位授予单位】：中国矿业大学(北京)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TD712.6

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