低温循环致裂煤体孔隙结构演化规律试验研究

摘要

For the ineffective gas drainage in single low-permeability coal seam in China,the method of cyclic cryogenic fracturing coal is proposed.ln the process of cyclic cryogenicfluid injection,the coal freezes.Then the frozen coal thaws bv absorbing heat from surrounding coals.So the freezing and thawing cycles emerge in coal mass and the pore structure and permeability of coal will also be changed.The experiment which coal samples were treated by freezing and thawing in a total of 20 cycles was carried out.The nuclear magnetic resonance test was used to the coal samples.The evolution of pore structure characteristics at different freeze-thaw cycles was researched.The results show that the maeropores and fractures' proportion,the total porosity,the effective porosity,the permeability is rising with increasing freeze -thaw cycles.This proves that the pores of coal samples have bigger quantities,larger size and be more connected after multiple freeze-thaw cycles,so the pore-fracture network forms.Finally,the gas permeability of coal seam increases signifieantly.%针对我国单一低透气性煤层常规钻孔瓦斯抽采困难的问题,提出了低温循环致裂增透方法.该方法将液氮等低温介质注入煤体,实现煤体的降温冻结;之后冻结煤体从周围煤岩吸热融化,循环地注入低温流体可使煤体产生交变的冻结-融化现象,从而改变煤体的孔隙结构和渗透性.采用冻融机对饱水煤样进行了共20次循环的冻融试验,并利用核磁共振测试方法,分析了低温循环冻融作用下煤体孔隙结构特征的演化规律.研究结果表明:随着冻融次数的增加,煤样的中、大孔占比、总孔隙度、有效孔隙度和渗透率显著增加,表明多次冻融能使煤体孔隙数量增加,尺寸变大,连通增强,从而形成交织贯通的孔裂隙网络,使煤体透气性大幅增强.