页岩气析出过程中甲烷稳定碳同位素分馏机制及地球化学涵义

Exploration experiences have showed that during the precipitation and emission of shale gas, the methane stable carbon isotope would appear enormous fractionations, but there is no good explanation to mechanism of this phenomenon currently. It’s generally considered that the isotopes composition of shale gas is fractionated both by adsorption and desorption as well as diffusion. Many scientists have made researches on the diffusive fractionation, but few people pay attention to the fractionation determined by adsorption and desorption. This project is going to use the magnetic suspension balance high pressure and isothermal adsorption instrument which is based on gravimetric method, to precisely characterize the isotope fractionation caused by the adsorption and desorption through step depressurization, and establish the correlation between methane isotope fractionation and the pressure. Based on that we will build the isotope fractionation model which is controlled both by adsorption and desorption as well as diffusion, and then calibrate the model with experimental data. This project is great significant for understanding the mechanism of methane isotope fractionation, rebuilding the beginning accumulation status of shale gas and studying the changes in proportions of free gas and adsorption gas based on the new isotope fractionation model.

页岩气勘探实践表明页岩气析出过程中会出现巨大的甲烷稳定碳同位素分馏效应，而目前对于这种同位素分馏机理的认识尚不完善。现普遍认为页岩气析出过程中的同位素分馏受吸附-解吸和扩散两种作用综合控制，但是这两种作用对同位素分馏的贡献仍存在较大的争议。目前，科学界对扩散作用造成的同位素分馏已经有了相对清晰的认识，而对吸附-解吸过程造成的甲烷稳定碳同位素分馏的定量研究尚未开展。本项目采用磁悬浮天平重量法高压等温吸附仪，通过对高压下饱和吸附的页岩样品逐级降压，精确表征页岩气吸附-解吸过程中产生的甲烷碳同位素分馏效应，并测定甲烷碳同位素的吸附系数。最后建立页岩气吸附-解吸和扩散过程中的同位素分馏模型，并利用页岩气现场解吸实验数据进行标定。本项研究对阐明页岩气析出过程中甲烷稳定碳同位素的分馏机制，从而基于同位素分馏模型确定页岩气赋存状态及析出过程中吸附气和游离气比例的变化具有重要意义。

页岩气勘探实践表明页岩气析出过程中会出现巨大的甲烷稳定碳同位素分馏效应，而目前对于这种同位素分馏机理的认识尚不完善。现普遍认为页岩气析出过程中的同位素分馏受吸附-解吸和扩散两种作用综合控制，但是这两种作用对同位素分馏的贡献仍存在较大的争议。目前，科学界对扩散作用造成的同位素分馏已经有了相对清晰的认识，而对吸附-解吸过程造成的甲烷稳定碳同位素分馏的定量研究尚未开展。针对上述科学问题，本项目开展了上扬子东部下古生界两套页岩气的现场解吸实验，揭示了两套页岩解吸行为及解吸过程中气体组分和同位素组成的变化规律；在此基础上，开展了页岩气吸附-解吸实验，研究吸附-解吸过程中甲烷碳同位素的分馏特征，揭示了吸附-解吸过程中甲烷同位素分馏规律及与温度的关系；建立了一种吸附气体同位素分馏数据测量方法，根据页岩气析出过程中同位素模型，可以预测页岩气析出过程中吸/游比的相对变化。研究结果表明，现场解吸实验表明甲烷碳同位素随解吸过程逐渐变重，其中甲烷碳同位素平均分馏幅度为17.6‰。在等温吸附-解吸实验中收集解析气体碳同位素分馏幅度可达1.4‰，模拟计算显示纯吸附-解吸作用能造成的甲烷碳同位素变重幅度可达到8.5‰。综合扩散和吸附/解吸作用过程中产生的同位素分馏效应，运用双孔隙气体流动模型对页岩气析出过程中同位素分馏数值模拟结果表明，吸附-解吸过程产生中的碳同位素分馏对于整个页岩气析出过程中的同位素分馏效应不可忽视。页岩气开采早期以产出游离气为主，储层原位气体赋存状态以吸附气为主；双孔隙流动模型恢复的不同层位的吸附气/游离气随解吸时间中的占比有所不同。温度、地层压力系数、脆性矿物含量、解吸气含量与甲烷稳定碳同位素值分馏变重幅度呈正相关，粘土矿物含量越高甲烷碳同位素分馏变重幅度越低；研究页岩气解析过程中甲烷稳定碳同位素分馏规律及机理，对于运用甲烷稳定碳同位素分馏精确恢复页岩气含气量、确定页岩气所处产出阶段具有重要意义。