构造作用对煤岩孔隙结构及煤层气扩散方式的影响

摘要

构造作用不仅对煤岩微观孔隙结构具有控制作用,同时影响煤层气在孔隙中的扩散方式.应用扫描电镜、低温氮吸附试验对海拉尔盆地呼和湖凹陷南部煤岩孔隙结构特征进行系统研究,剖析构造作用对孔隙系统及甲烷扩散方式的影响.结果表明,相对稳定的洼槽中部主要发育原生孔和内生裂缝;背斜翼部构造变形强度增加,外生孔取代原生孔,外生裂缝的数量和规模都显著增加.背斜翼部孔隙结构参数变化速率明显高于洼槽中部,证实构造活动具有复杂化孔隙系统的作用.利用吸附、脱附曲线特征对煤岩孔隙结构特征进行识别,洼槽中部以I型曲线为主,背斜翼部以III型为主.在产气伊始阶段,分子扩散占主导地位;随着压力降低,产气后期洼槽中部煤岩中甲烷分子扩散方式以过渡型扩散为主,背斜翼部煤岩中甲烷分子扩散方式变化不大.%The tectonic action not only has a controlling effect on the microscopic pore structure of coal petrography, but also affects the diffusion mode of coalbed methane in the pores.Scanning electron microscope and low temperature nitrogen adsorption test are combined to research on types of reservoir spaces and pore structure characteristics of low region and ramp region in Huhehu depression, to analyze the influence of tectonism on pore system, discussing diffusion mode of CH4 molecule under different pressure.The results show that types of pore and crack dominated by primary pore and cleating in low region.The ramp region has variety of pore and crack by increasing of deformation grade, dominated by fragmented pore and crack of tension and shear which reflect structural feature, permeability enhancement when the amount of crack increasing.We found that surface to volume ratio and pore volume have a good positive correlation but average pore diameter opposite.The microscopic pore growing in number when pores were destroyed and its surface to volume ratio and pore volume becomes time of growth, therefore heighten coalbed methane adsorptive capacity of coal.Using adsorption and desorption isothermal identify characteristic micropore structure, low region dominated by curve of I type, the ramp region dominated by curve of III type.The pore of ramp region conducive to the adsorption and desorption difficult.The main movement of CH4 in pore system is molecular diffusion at early stage of coal reservoir.As the pressure decreases, due to the difference of pore structure, substituting transition zone flow for molecular diffusion at late stage in low region, the diffusion mode changed little in ramp.