蜀南地区富有机质页岩孔隙结构及超临界甲烷吸附能力

以蜀南地区龙马溪组下部富有机质页岩为研究对象,通过场发射扫描电镜（FE-SEM）、低压氩气吸附实验和重力法高压甲烷吸附实验,研究页岩孔隙结构特征及超临界状态下页岩储层的甲烷吸附能力,并讨论了页岩孔隙结构对甲烷吸附能力的影响。研究表明,蜀南地区龙马溪组富有机质页岩主要发育有机质孔隙,页岩孔隙结构非均质性强,比表面积为16.846~63.738 m²/g,孔体积为0.050~0.092 cm³/g,微孔和介孔贡献页岩90%以上的比表面积,介孔和宏孔贡献页岩90%以上的孔体积。甲烷在地层条件下处于超临界状态,过剩吸附曲线在约12 MPa时出现极大值,随后开始下降。使用修正过的四元Langmuir-Freundlich （L-F）方程拟合高温甲烷过剩吸附曲线,拟合效果较好,相关系数大于0.997。页岩饱和吸附量为0.067 0~0.220 2 mmol/g,不同页岩样品吸附能力差异明显。海相富有机质页岩中,随着有机质含量的增大,有机质孔隙数量增多,且页岩中微孔比例增大,微孔的吸附能力远大于介孔和宏孔,故页岩吸附能力增强。有机质含量是影响蜀南地区海相富有机质页岩孔隙结构和甲烷吸附能力的主要因素。

Pore structure and supercritical methane sorption capacity of organic-rich shales in southern Sichuan Basin

The lower organic-rich shales in Longmaxi Formation, South Sichuan are taken as research objects in this study. Through field emission scanning electron microscopy (FE-SEM), low-pressure Ar adsorption experiment and gravimetric high-pressure methane adsorption experiment, this paper studies the shale pore structure characteristics and the methane adsorption capacity of shale reservoir under supercritical conditions, and also explores the influences of shale pore structure on methane adsorption capacity. The results show that organic pores are mainly developed in the organic-rich shales, Longmaxi Formation, South Sichuan, where shale pore structure has high heterogeneity, specific surface area of 16.846-63.738 m²/g and pore volume of 0.050-0.092 cm³/g. The micropores and mesopores comprise more than 90% of specific surface area, whereas mesopores and macropores comprise more than 90% of pore volume. Methane is in a supercritical state under formation conditions. The excessive adsorption curve reaches the peak at the pressure of about 12 MPa and then decreases. The modified 4-parameter Langmuir-Freundlich (L-F) equation is used to fit the high-temperature methane excessive adsorption curve, achieving better fitting effects and obtaining the correlation coefficient greater than 0.997. The shale saturated adsorption capacity ranges from 0.067 0 to 0.220 2 mmol/g, and different shale samples have significant differences in adsorption capacity. With the increase of TOC content, the quantity of organic pores and the proportion of shale micropores are increased in marine organic-rich shale, and the adsorption capacity of micropores is rather greater than mesopores and macropores, so that shale adsorption capacity is enhanced. TOC is the key factor affecting the pore structure and methane adsorption capacity of marine organic-rich shales in South Sichuan.