湖相页岩液态烃对页岩吸附气实验的影响——以鄂尔多斯盆地延长组页岩为例

Effect of liquid hydrocarbons on gas adsorption in alacustrine shale:A case study of the Yanchang Formation,Ordos Basin

通过全岩和黏土矿物X-衍射、索氏抽提、离子抛光、扫描电镜、低温氮气吸附、等温吸附等实验分析,研究了鄂尔多斯盆地延长组湖相页岩储层中的液态烃对吸附气含量的影响。结果表明,延长组湖相页岩储层正处于中成岩阶段A期;有机质演化程度相对较低,不足以形成大量的有机质孔隙,却足以形成大量的液态烃,液态烃占据了页岩中直径约4 nm左右的孔隙。由于氮气不溶于液态烃,而甲烷易溶解于液态烃,使得抽提前后样品低温氮气吸附实验得到的比表面积和吸附量变化大,但抽提前后样品等温吸附实验测得的甲烷"吸附"量变化不大,这表明等温吸附实验中有一部分甲烷以溶解态赋存于页岩样品中。因此在用等温吸附实验研究低成熟度页岩吸附气含量的过程中,必须注意液态烃的影响。

The effect of liquid hydrocarbons on the gas adsorption capacity of lacustrine shales from the Yanchang Formation in the Ordos Basin has been studied using a series of integrated experimental methods including X-ray diffraction of bulk and clay minerals,Soxhlet extraction,focused ion beam scanning electron microscopy（ FIB-SEM）,low-temperature N2 adsorption and high-pressure CH4 adsorption methods. The results show that the lacustrine shale from the Yanchang Formation is in the middle diagenetic stage A. The thermal maturity is relatively low,and contributes to the absence of organic matter pores. However,it is sufficiently high enough forthe production of liquid hydrocarbons which can block up pores with a diameter of 4 nm. Methane can dissolve in these liquid hydrocarbon,whereas nitrogen cannot. As a result,the specific area and adsorbed capacity acquired in low-temperature N2 adsorption analysis show great differences between samples before and after extraction. However,the difference does not exist in the CH4 adsorption analysis. This means that a certain proportion of CH4 exists in these samples in a dissolved form. It is necessary to pay attention to free hydrocarbons during studies of gas adsorption capacity of shales with low thermal maturity usinghigh pressure CH4 adsorption methods.