抽采钻孔孔周煤体渗流阻力分布特征试验研究

Distribution of coal seepage resistance around extraction boreholes:an experimental study

瓦斯抽采钻孔孔周煤体的孔隙结构变化是影响其渗流阻力的重要因素之一。为研究钻孔孔周粉碎区和裂隙区煤体渗流阻力的演化特征,采用稳态渗流法,研究了三轴压实状态下初始孔径大小对破碎煤样内部孔隙压力的影响规律,并对渗透过程中试样分层界面对孔隙结构属性参数的影响进行了分析。结果表明:单一孔径试样在同一渗流速度下,随着孔径的增加,渗透状态逐渐由层流过渡到紊流,渗流阻力由黏滞阻力为主发展成惯性阻力为主,双重孔径组合试样最小孔径大小是影响孔压梯度变化的先决条件;双重孔径组合煤体平均颗粒粒径介于0.1~1.025 mm,孔喉比分布在2~4,且渗透压降随孔喉比的增加呈二次曲线规律增长;孔径大小的变化对渗透率的影响较大,随着初始孔径的增大,其影响瓦斯通过煤体的能力增强;单一孔径试样与双重孔径组合试样非达西流因子分别与粒径的倒数和粒径差的倒数呈线性关系,且随着非达西流因子的增加,非达西渗流效应越来越明显,说明非达西流β因子增长趋势受孔径大小的影响,且孔径大小突增导致流体与渗透骨架的相对接触面积减小,非达西流β因子进一步增加;可见,在瓦斯运移过程中,钻孔孔周煤体的破碎程度与煤层渗透性具有良好的一致性,结合渗透率与非达西影响因子,可以对试样孔隙几何结构参数作出定量描述,从而揭示孔隙结构变化对渗流阻力的影响。

Pore structure change was one of the important factors affecting the coal seepage resistance around gas extraction boreholes.In order to study the evolution characteristics of the coal seepage resistance in the crushed and fractured zones around the borehole.Impact of initial pore size on internal pore pressure within crushed coal samples under triaxial compression was investigated,using the steady⁃state seepage method.Additionally,the influence of specimen layering interfaces on pore structure property parameters during the permeation process was analyzed.Following conclusions were obtained in this study.The percolation state of single pore size specimen with same percolation velocity experienced gradual transition from laminar flow to turbulent flow as the pore size increased,and the percolation resistance state also changed from viscous⁃resistance dominance to inertial⁃resistance.The minimum pore size of the double pore size specimen was a prerequisite for the change of pore pressure gradient.The average particle size of coal consisting of double pore sizes ranging from 0.1 to 1.025 mm,and the porethroat ratio was from 2 to 4.The permeability pressure demonstrated a quadratic curve⁃like behavior,increasing with the increase of the pore⁃throat ratio.The change of the pore size greatly impacted the permeability,and the gas penetration through the coal body was enhanced with the increase of the initial pore size.The non⁃Darcy flow factor of single⁃pore⁃size specimens and double⁃pore⁃size combined specimens expressed linear relation with the inverse of the particle size and the inverse of particle size difference,respectively.Moreover,the non⁃Darcy percolation effect became apparent as the non⁃Darcy flow factor increased,which indicated that the growth trend of non⁃Darcy flowβfactor was influenced by its pore size.Sudden increase of the pore size led to a reduction of the relative contact area between the fluid and the permeable skeleton,then the non⁃Darcy flowβfactor could be further increased.Therefore,favorable consistency,as gas mitigated,could be pronounced between the fragmentation degree of the coal around the borehole and the permeability of the coal seam.Quantitative description of geometric pore parameters of the specimen could be established by combining permeability and non⁃Darcy influence factor,so as to reveal the influence of pore structure changes on seepage resistance.