矿物对无烟煤力学非均质性影响的分子动力学研究——以沁水盆地南部主要黏土矿物高岭石为例

Molecular dynamics study on the effect of minerals on the mechanical heterogeneity of anthracite coal:A case study of kaolinite in the southern part of the Qinshui Basin

煤是由有机显微组分和矿物组成的混合物,矿物的存在会导致煤的大分子结构存在差异,使得煤的力学性质具有明显的非均质性,进而影响区块内整体的煤层气产量、压裂改造效果和成熟技术的规模化应用。以沁水盆地典型的无烟煤和高岭石矿物作为研究对象,通过分子动力学的方法定量研究高岭石及其含量对无烟煤力学性质的影响规律,分析不同矿物含量下无烟煤的分子结构和孔隙特征,从能量和有序化的角度揭示其微观机理。结果表明:①煤有机质与高岭石相互结合过程中同时存在范德华力和氢键作用,且氢键作用强度要高于范德华力,煤有机质周围出现高岭石分子的概率、相互作用强度和有序性均随着矿物质量分数的增加而增大。②无烟煤的孔隙率φ随高岭石质量分数a的增大呈线性规律(φ=−0.2293a+9.6295)降低,而无烟煤的体积模量、弹性模量和剪切模量均随高岭石质量分数的增大呈线性规律增强,泊松比随高岭石质量分数的增大呈二次多项式规律降低。③无烟煤的力学非均质程度整体上随着高岭石质量分数增加而增大,其中不同高岭石质量分数下体积模量、弹性模量和剪切模量的变异性可分为很低、低、中等、高和很高5个部分,而泊松比的变异系数均小于0.1,变异性很低。④随着高岭石质量分数的增加,煤有机质与高岭石的相互作用能呈线性规律(Einteraction=4.5585a+443.929)增大,与纯有机质相比,当高岭石质量分数达到20%时,无烟煤的键能和范德华能分别增加了63.86%和48.06%。⑤高岭石增强煤力学性质的内在原因是高岭石进入煤有机质后占据一定的孔隙空间,且与煤有机质的芳香碳骨架和不同官能团进行相互作用,高岭石质量分数越大充填煤大分子结构孔隙的程度越高,煤大分子结构的能量越大,导致煤大分子结构的孔隙率降低,有序性增大,使得力学强度和抵抗变形的能力增强。

Coal is a mixture of organic macerals and minerals.The presence of minerals can lead to differences in the macromolecular structure of coal,resulting in significant heterogeneity in its mechanical properties,which in turn affects the overall coalbed methane production,fracturing and transformation effects,and the large-scale application of mature technologies within the block.Taking typical anthracite and kaolinite minerals from the Qinshui Basin as the research object,the influence of kaolinite and its content on the mechanical properties of anthracite is quantitatively studied through molecular dynamics methods.The molecular structure and pore characteristics of anthracite under different mineral contents are analyzed,and its microscopic mechanism is revealed from the perspective of energy and ordering.The results indicate that:①There are both van der Waals forces and hydrogen bonding forces during the binding process between coal organic matter and kaolinite,and the intensity of hydrogen bonding is higher than that of van der Waals forces.The probability,interaction strength,and orderliness of kaolinite molecules appearing around coal organic matter increase with the increase of mineral content.②The porosity of anthracite decreases linearly with the increase of kaolinite content,withφ=−0.2293a+9.6295,while the bulk modulus,elastic modulus,and shear modulus of anthracite all increase linearly with the increase of kaolinite content.The Poisson’s ratio decreases exponentially with the increase of kaolinite content.③The degree of mechanical heterogeneity of anthracite increases overall with the increase of kaolinite content.The variability of bulk modulus,elastic modulus,and shear modulus under different kaolinite contents can be divided into five parts:very low,low,medium,high,and very high.The coefficient of variation of Poisson's ratio is all less than 0.1,indicating very low variability.④As the content of kaolinite increases,the interaction energy between coal organic matter and kaolinite shows a linear increase(Einteraction=4.5585a+443.929).Compared with pure organic matter,when the content of kaolinite reaches 20%,the bond energy and van der Waals energy of anthracite increase by 63.86%and 48.06%,respectively.⑤The internal reason why kaolinite enhances the mechanical properties of coal is that it occupies a certain pore space after entering the coal organic matter and interacts with the aromatic carbon skeleton and different functional groups of the coal organic matter.The higher the content of kaolinite,the greater the degree of filling the pores of the coal macromolecular structure and the energy of the coal macromolecular structure,resulting in a decrease in the porosity of the coal macromolecular structure,an increase in orderliness,and an increase in mechanical strength and resistance to deformation.