抽采钻孔孔周破裂煤体瓦斯径向渗透流态转捩特征试验研究

Experimental study on the flow regime transition characteristics of gas radial seepage in fractured coal bodies around extraction boreholes

钻孔孔周煤体瓦斯径向渗流流态是反映抽采瓦斯渗流运动的重要因素之一。为探究钻孔孔周煤体的不同孔隙结构和瓦斯压力对瓦斯径向渗流运动特征的影响,分别制作了5种不同级配的含孔试样,利用自主设计的瓦斯径向渗流试验系统,采用稳态渗流法对含孔试样进行径向渗流试验研究,分析Forchheimer数和非Darcy误差随渗流速度与级配的变化规律,运用Hazen-Dupuit-Darcy方程和雷诺数理论得到了不同流态的流速区间和雷诺数范围。结果表明:(1)通过对Forchheimer数和非Darcy误差随渗流速度变化曲线的分析可知,Forchheimer数随渗流速度呈二次多项式曲线增加,而非Darcy误差呈线性增加,Forchheimer数和非Darcy误差与孔隙结构密切相关,2者都随级配的增大而变大,说明渗流速度和孔周煤体的孔隙结构是瓦斯渗流偏离线性渗流的重要因素;(2)利用Hazen-Dupuit-Darcy方程对瓦斯径向渗透流态进行表征和划分,随着渗流速度的增大,瓦斯径向渗流运动依次出现前达西、达西和Forchheimer流态,且n=0.2与n=0.4的小级配含孔试样易发生达西层流,而n=0.6、n=0.8和n=1.0的含孔试样却更易于发生Forchheimer流态;(3)通过对达西定律应用时压力梯度的相对误差曲线分析可知,相对误差随渗流速度的增大呈现减小、保持稳定、再增大的变化规律,与瓦斯渗流流态随渗流速度的变化规律相吻合;(4)在双对数坐标中分析了瓦斯渗流不同流态与雷诺数的关系,得出了不同流态的雷诺数区间,发现瓦斯径向渗流运动符合达西层流的雷诺数为0.0012~0.0025。研究结果可为煤层瓦斯预抽工作中依据钻孔孔周煤体瓦斯渗流运动流态变化规律确定钻孔抽采影响范围,从而设计钻孔布孔参数提供重要的理论依据。

The radial gas seepage flow regime of the coal body around borehole is one of important factors reflecting the seepage movement of extracted gas.In order to investigate the effects of different pore structures and gas pressure on the radial seepage motion of gas in the perimeter of borehole,five kinds of pore-bearing coal samples with different gradations were made,and the radial seepage test was carried out on the pore-bearing coal samples by using the self-designed gas radial seepage test system and the steady-state seepage method to analyze the changes of the Forchheimer number and non-Darcy error with the seepage velocity and gradation,and the flow velocity intervals and the Reynolds number ranges of different flow regimes were obtained by using the Hazen-Dupuit-Darcy equation and the Reynolds number theory.The results show that:①Through the analysis of the Forchheimer number and non-Darcy error with the change curve of seepage velocity,the Forchheimer number increases with seepage velocity in a quadratic polynomial curve,while non-Darcy error increases linearly,the Forchheimer number and non-Darcy error are closely related to pore structure,and both become larger with the increase of gradation,indicating that seepage velocity and pore structure of the coal body around the pore are important factors for gas seepage deviating from linear seepage;②The Hazen-Dupuit-Darcy equation was used to characterize and delineate the gas radial permeability flow regime.With the increase of seepage velocity,the gas radial seepage motion appears in order of pre-Darcy,Darcy and Forchheimer flow regimes.The small-graded porous specimens with n=0.2 and n=0.4 are more prone to the Darcy laminar flow,while the porous samples with n=0.6,n=0.8 and n=1.0 are more prone to the Forchheimer flow regime;③Through the analysis of the relative error curve of pressure gradient when Darcy’s law was applied,the relative error with the increase of seepage velocity shows the change law of decreasing,keeping stable,and then increasing,which is consistent with the change law of gas seepage flow pattern with seepage velocity;④In double logarithmic coordinates,the relationship between different flow regimes of gas seepage and Reynolds number was analyzed,and the Reynolds number intervals of different flow regimes were derived,and it was found that the radial seepage motion of gas conforms to the Reynolds number range of 0.0012~0.0025 for Darcy laminar flow.The results of the study can provide an important theoretical basis for the design of drilling parameters by determining the influence range of drilling extraction based on the flow regime of coal seepage movement around the borehole in coal seam gas pre-drainage work.