基于重量法的页岩气高压等温吸附研究

Mass method adsorption characteristics of shale gas under high pressure

基于重量法等温吸附仪,开展了页岩气的高压等温吸附测试。实验结果表明,低压下页岩气吸附特征符合Langmuir模型,在实验压力超过10~12 MPa后,页岩吸附表现出了明显的过剩吸附。高压下,样品桶体积以指数形式逐渐降低,这与高压下样品桶的压缩性有关。页岩样品的体积则呈指数形式增加,并在较小的压力范围内趋于平衡,这与页岩对氦气的微量吸附有关。基于最大过剩吸附后页岩过剩吸附量与气相甲烷密度拟合得到的吸附相甲烷密度在不同最大测试压力下呈现动态变化,以最大过剩吸附量后连续2个压力点测得的过剩吸附量与气相甲烷密度拟合获得的吸附相甲烷密度,最接近页岩表面吸附平衡时吸附相甲烷密度。综合考虑样品桶体积、样品体积及最大过剩吸附前后吸附相甲烷体积的动态变化,能够对页岩气绝对吸附量进行准确校正,实现页岩高压等温吸附特征的精细描述,且页岩高压绝对吸附特征符合Langmuir吸附模型。

The isothermal adsorption characteristics of shale gas under high pressure are investigated with a magnetic suspension mass method isothermal adsorption instrument. The experimental results showed that the adsorption characteristics of shale gas at low pressure conform to the Langmuir model. After the experimental pressure exceeds 10-12 MPa,the adsorption of shale shows obvious excess adsorption. Under high pressure,the sample container volume decreases in an exponential manner,which is related to the compressibility of the sample container under high pressure. The volume of shale samples increases exponentially and tends to equilibrate within a relatively small pressure range,which is related to the slight adsorption of He by shales. The densities of adsorbed phase CH4,acquired from the linear fitting with the excess adsorption capacities and the densities of gaseous phase CH4,are also dynamic. The adsorbed phase CH4 density is close to the true one when using the contiguous excess adsorption capacities and the densities of gaseous phase CH4 after the maximum excess adsorption. Combined with the dynamic changes of sample container volume,sample volume and the adsorbed phase CH4 volume before and after the maximum excess adsorption,the absolute adsorption capacities can be calculated,which can present the adsorption characteristics of shale gas accurately. The absolute adsorption of shale gas under high pressure also follows the Langmuir model.