基于低温液氮吸附法的煤岩孔隙分形特征

Fractal Characteristics of Coal and Rock Pore Based on Low Temperature Liquid Nitrogen Adsorption Method

为研究沁水盆地中高煤级煤的孔隙结构特征,采用低温液氮吸附实验测定了不同煤样比表面积及孔径分布数据,依据吸附-解吸曲线和分形维数对煤岩孔隙系统进行分类。结果表明:煤层微小孔较发育,具有比表面积适中(0.418~0.902 m^(2)/g)、平均孔径小(14.6~21.0 nm)、孔容小(0.00186~0.00453 cm^(3)/g)和低温氮吸量适中(1.5~3.0 mL/g)等特征,镜质体反射率为1.51%~2.19%,利于煤层气赋存;基于低温液氮等温吸附曲线,识别出半封闭状态狭缝型孔和平行板状孔、开放状态的平行板状孔和墨水瓶型孔,其中开放孔比封闭孔吸附量要大,墨水瓶型孔最利于煤层气解吸开发;结合孔隙分形特征可知过渡孔与微小孔划分标准为60 nm,煤级越高,孔隙表面形态越复杂,但对孔隙直径影响较小。

In order to study the characteristics of the pore structure of medium and high coal rank coals in the Qinshui Basin,the specific surface area and pore size distribution data of different coal samples were determined by low-temperature liquid nitrogen adsorption experiments,and the coal rock pore system was classified according to the adsorption-desorption curve and fractal dimension.The results show that the coal seams have relatively well-developed micropores,with moderate specific surface area(0.418~0.902 m^(2)/g),small average pore size(14.6~21.0 nm),small pore volume(0.00186~0.00453 cm^(3)/g)and moderate low-temperature nitrogen absorption(1.5~3.0 mL/g),the vitrinite reflectance is 1.51%~2.19%,which is conducive to the occurrence of coalbed methane.Based on the low-temperature liquid nitrogen adsorption isotherm curve,semi-closed slit-shaped pores and parallel plate-shaped pores were identified,open parallel plate pores and ink bottle pores,of which open pores have a larger adsorption capacity than closed pores.Ink bottle pores are most conducive to the desorption and development of coalbed methane.Combined with the fractal characteristics of the pores,it can be seen that the transition pores and micro pores are divided into 60 nm.The higher the coal rank,the more complex the pore surface morphology,but the smaller the impact on the pore diameter.