基于细观尺度变形煤体渗透率模型集及预测效果分析

Analysis of coal permeability model set and prediction effect based on cubic law and meso-scale deformation law

  • 摘要: 为准确预测煤体渗透率随气体压力的演化规律,基于立方定律与细观尺度变形规律构建了煤体渗透率演化模型集,验证了模型的准确性。分析了参数影响下模型演化效果,研究了参数变化对渗透率比最大值的影响,确定了参数的敏感性指标,讨论了内膨胀系数与裂隙吸附应变系数的关系。渗透率模型集包含真实应变模型、等基质宽度模型与等体积模型。结果表明:3类模型表征着单元体在细观尺度上的3种差异性变形特征,仅真实应变模型可描述内膨胀效应,且内膨胀系数为仅考虑吸附作用影响时单元体体积应变与基质体积应变之比。对于沁水盆地区域煤体,较小的内膨胀系数和较大的裂隙吸附应变系数对渗透率比的影响最为显著,内膨胀系数和裂隙吸附应变系数的取值变化分别引起渗透率比的最大值增大了34.29、24.60~38.07倍。相比相同工况条件下的真实应变模型,等体积模型与等基质宽度模型都极易高估渗透比最大值,等体积模型的偏差值在Langmuir最大吸附应变与初始裂隙率影响下分别达到最大值28.81与最小值8.31。等体积模型与等基质宽度模型的参数敏感性指标也偏大,说明参数取值的偏差极易造成预测精度降低,尤其是Langmuir最大吸附应变与Langmuir压力常数。仅当3类模型的裂隙体积应变相等时,内膨胀系数与裂隙吸附应变系数满足非线性关系。提出的3类反映细观尺度变形规律、可适用不同工况的渗透率模型,系统揭示了模型关键参数的影响机理,为多孔介质弹性力学、渗流力学等基础科学以及煤层气开发、矿井瓦斯防治与碳封存等工程实际提供理论指导。

     

    Abstract: To accurately predict the evolution of coal permeability under varying gas pressures, a permeability evolution model set was developed based on the cubic law and meso-scale deformation principles, and its accuracy was validated. The effects of parameter variations on model evolution were analyzed, the influence of parameter changes on the maximum permeability ratio was investigated, sensitivity indices were identified, and the relationship between the internal swelling coefficient and the fracture adsorption strain coefficient was discussed. The permeability model set includes the true strain model, the equal matrix width model, and the equal volume model. The results indicate that these three types of models characterize three distinct deformation features of representative elements at the meso-scale. Among them, only the true strain model can describe the internal swelling effect, where the internal swelling coefficient represents the ratio of the volume strain of the representative element to the matrix volume strain when only adsorption effects are considered. For coal in the Qinshui Basin, smaller internal swelling coefficients and larger fracture adsorption strain coefficients have the most significant impact on the permeability ratio, with changes in the internal swelling coefficient and fracture adsorption strain coefficient causing the maximum permeability ratio to increase by 34.29 and 24.60~38.07 times, respectively. Compared with the true strain model under identical conditions, the equal volume and equal matrix width models tend to significantly overestimate the maximum permeability ratio. The deviation of the equal volume model reaches a maximum of 28.81 and a minimum of 8.31 under the influence of the Langmuir maximum adsorption strain and initial fracture porosity, respectively. Additionally, the parameter sensitivity indices of the equal volume and equal matrix width models are notably high, which means that deviations in parameter values can easily lead to reduced predictive accuracy, particularly for the Langmuir maximum adsorption strain and Langmuir pressure constant. The internal swelling coefficient and fracture adsorption strain coefficient exhibit a nonlinear relationship only when the fracture volume strain is equal across the three models. The proposed three types of permeability models, which reflect meso-scale deformation principles and are applicable to various operating conditions, systematically reveal the influence mechanisms of key model parameters. This provides theoretical guidance for fundamental sciences such as the elasticity mechanics of porous media and seepage mechanics, as well as practical engineering applications, including coalbed methane development, mine gas control, and carbon sequestration.

     

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