页岩气解析过程中同位素分馏的定量表征及在页岩气关键评价参数确定中的应用

The original gas content and the ratio of adsorbed gas to free gas in shale under geological conditions are two key parameters to assess the shale gas resource potential and recoverable reserves, screen favorable target areas and formations, and formulate a reasonable development program. A variety of methods of assessing the two parameters have been proposed and applied, however, they all have some shortcomings, and therefore, the results are not accurate enough. Recently a method utilizing isotope fractionation effect has come to arouse some interest and attention from people and may provide a new way to solve these problems. However, a quantitative characterization model of isotope fractionation effect during the process of shale gas desorption/diffusion needs to be established first of all. In this study, we plan to establish and calibrate Rayleigh and chemical kinetic mathematical models of carbon and hydrogen isotopic fractionation during the process of shale gas desorption/diffusion basing on laboratory experiments, and then gas loss during shale cores extraction and the original ratio of the adsorbed gas to free gas can be determined by substituting isotopic fractionation data obtained from shale gas desorption/diffusion with seal canisters and grinding machine into the established model. What’s more, we will develop related core software and techniques with independent intellectual property rights. At the same time, the relationship between shale gas isotopic fractionation rate and shale permeability and its application in predicting permeability of shale will also be explored and quantified.

地质条件下页岩原始含气量和吸附/游离气比例是评估页岩气资源潜力和可采储量、筛选有利目标区和层位及制定合理开发方案的2个关键参数。目前评价这2个参数的多种方法都有着各自的不足，结果不够准确。近期逐渐得到关注和重视的同位素法可能为确定这2个关键参数提供了一条全新的途径，但这首先需要对页岩气解析过程中的同位素分馏建立具有理论基础的定量表征模型。本项研究拟通过实验室内页岩气解析及同位素分馏实验，建立并标定基于瑞利方程和化学动力学的页岩气解析过程中碳氢同位素分馏的数值模型，进而将井场密封罐解析、研磨解析及同位素分馏数据带入所建立的模型，恢复岩心提取过程中损失的页岩气量，确定页岩原位吸附/游离气的比例，研发具有自主知识产权的核心技术和软件。同时，进一步利用页岩的渗透性与同位素分馏速率的关系，探索由同位素分馏数据预测页岩渗透性的可能性及表征技术。

我国页岩气资源量丰富，并已在涪陵、长宁、威远、昭通、富顺-永川等区块取得了页岩气勘探开发的重大进展，但还存在众多有待探索和解决的难题。其中，地层条件下页岩原位含气量恢复和吸附气/游离气比例确定，事关页岩气资源潜力、产量、（可采）储量的客观评估、甜点的筛选及开发方案的优化和调整，一直以来是我国油公司及勘探家非常关注、正在探索、攻关中的2个科学难题。现有的评价方法都存在各自的局限性，而近年来逐渐受到关注的同位素方法展现了有效解决上述关键问题的巨大潜力。依托本基金，综合利用各类高分辨率的分析仪器和技术手段，系统、深入地剖析了页岩的矿物组成与孔隙结构特征。在此基础上，开展了系统的现场和实验室观测，揭示了单一过程和多作用耦合过程的同位素分馏效应，结合岩石物质组成与孔隙结构特征，明确了同位素分馏的主控地质因素，揭示了同位素分馏机理，建立了页岩气解析过程同位素分馏的一般模式和定量表征模型，实现了页岩原位含气量和吸附气/游离气比例的准确计算，并成功建立了基于同位素数据的页岩渗透性评价方法。在上述研究的基础上，评价了国内多个区块的页岩气资源潜力和甜点层位，建立了页岩气井生产状态监测和页岩渗透性评价理论和技术，并在焦石坝和武隆页岩气区块进行了实际应用。