页岩高压解析放气过程中甲烷碳同位素分馏特征

Characteristics of methane carbon isotope fractionation during high-pressure degassing of shale

页岩在放气或解析过程中会出现甲烷碳同位素分馏现象,可以基于该同位素分馏演化特征预测地层压力,并评价页岩储集特征和含气性。研究通过四川盆地寒武系筇竹寺组黑色页岩在不同初始充气压力条件下的解析放气正演物理模拟实验,探讨了放气过程中甲烷碳同位素分馏效应和控制因素。结果表明,模拟体系中释放甲烷的碳同位素组成随放气时间呈现先变轻后变重的特征。解析气甲烷碳同位素存在2个可对比的有效参数:一是碳同位素分馏程度(最小值与初始值的差);二是碳同位素最小值出现的时间。同时,研究发现,放气过程中的甲烷碳同位素分馏特征受初始含气压力和页岩样品地质参数(如TOC、孔隙结构特征、吸附能力等)的影响。总体来说,初始含气压力越高,解析气甲烷碳同位素分馏程度越大,最小同位素值出现的时间越晚。相近压力条件下,随着TOC增大,解析气甲烷碳同位素分馏程度和最小值出现时间先增大后减小;随着页岩平均孔径增大,解析气甲烷碳同位素分馏程度和最小值出现时间先减小后增大。

The effect of isotopic fractionation and the controlling factors in the gas release process were discussed through physical simulation experiments of methane gas degassing in the black shale of the Cambrian Qiongzhusi Formation in the Sichuan Basin under different pressure conditions.The carbon isotope of methane first becomes lighter and then heavier with the degassing time.There are two effective parameters that can be compared for the carbon isotope of methane,one is the degree of carbon isotope fractionation,and the other is the time when the carbon isotope minimum value appears.The experimental pressure and geological parameters of shale samples have an impact on the methane carbon isotope fractionation characteristics of gas,and the correlation between experimental pressure and methane carbon isotope fractionation characteristics is very good.The correlation between the geological parameters of shale samples and methane carbon isotope fractionation characteristics is not significant,and the impact is more complicated.