黏土矿物类型对煤力学非均质性影响的分子机制

Molecular mechanism of influence of clay mineral types on coal mechanical heterogeneity

  • 摘要: 煤层气是我国重要的非常规天然气资源,但无机矿物影响下的煤储层具有强烈的力学非均质性,存在已成功使用的工程技术在其他煤层气井不适用的情况,深入研究煤中无机矿物与有机质的相互作用及其对煤力学非均质性的影响机制具有重要的意义。利用分子模拟对比分析了高岭石、伊利石和蒙脱石3种典型黏土矿物与无烟煤有机质的相互作用范围、位置和强度及差异性,阐明了不同黏土矿物对煤孔隙率的影响规律,揭示了煤力学非均质对黏土矿物类型的响应特征。结果表明:高岭石或伊利石与煤有机质的径向分布函数呈双峰特性,同时存在氢键作用和范德华力,而蒙脱石的径向分布函数呈单峰分布,相互作用以范德华力为主,占总能量变化的89.36%,不存在氢键作用;矿物分子进入到煤有机质后会相互堆叠填充,形成具有一定的非均质性且形状不规则的纳米孔隙结构,相同矿物含量下,含不同矿物类型煤的孔隙率大小顺序为:蒙脱石>伊利石>高岭石;黏土矿物与煤有机质的相互作用强度主要取决于黏土矿物分子的尺寸,呈反比例关系;不同高岭石含量下,无烟煤的体积模量、弹性模量和剪切模量非均质性可分为5个部分,存在变异性极高的区域,而含伊利石和含蒙脱石无烟煤除了剪切模量存在高变异区域,其他力学参数均处于低变异范围以下,高岭石对无烟煤力学非均质的影响程度要大于伊利石和蒙脱石的影响。研究成果将为预测煤储层力学性质的变化规律且根据煤中不同黏土矿物特征有针对性的制定储层改造措施和开发方案提供力学依据。

     

    Abstract: Coalbed methane is an important unconventional natural gas resource in China, but coal reservoirs under the influence of inorganic minerals have strong mechanical heterogeneity, and there are situations where successfully used engineering technologies are not applicable to other coalbed methane wells. In depth research on the interaction between inorganic minerals and organic matter in coal and their impact on coal mechanical heterogeneity is of great significance. Molecular simulation was used to compare and analyze the interaction range, location, strength, and differences between three typical clay minerals, kaolinite, illite, and montmorillonite, and anthracite organic matter. The influence of different clay minerals on coal porosity was elucidated, and the response characteristics of coal mechanical heterogeneity to clay mineral types were revealed. The results showed that the radial distribution function of kaolinite or illite with coal organic matter exhibited a bimodal characteristic, with both hydrogen bonding and van der Waals forces present. In contrast, the radial distribution function of montmorillonite showed a unimodal distribution, with van der Waals forces being the main interaction, accounting for 89.36% of the total energy change, and no hydrogen bonding was observed; After entering the organic matter of coal, mineral molecules will stack and fill each other, forming a nano pore structure with certain heterogeneity and irregular shape. Under the same mineral content, the pore size order of coal containing different mineral types is: montmorillonite > illite > kaolinite; The strength of the interaction between clay minerals and coal organic matter mainly depends on the size of clay mineral molecules, which is inversely proportional; Under different kaolin contents, the heterogeneity of the bulk modulus, elastic modulus, and shear modulus of smokeless coal can be divided into five parts, with areas of extremely high variability. However, smokeless coal containing illite and montmorillonite, except for areas of high variability in shear modulus, all other mechanical parameters are below the low variability range. The influence of kaolin on the mechanical heterogeneity of smokeless coal is greater than that of illite and montmorillonite. The research results will provide a mechanical basis for predicting the changes in the mechanical properties of coal reservoirs and developing targeted reservoir modification measures and development plans based on the characteristics of different clay minerals in coal.

     

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