Zhai Changbo, Tao Cheng, Zou Yu, Yang Zhenheng, Ye Xin, Nie Haikuan
State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Efficient Development, Beijing 102206, China.
Sinopec Key Laboratory of Shale Oil/Gas Exploration and Production Technology, Beijing 102206, China.
ACS Omega. 2024 Aug 21;9(35):36993-37001. doi: 10.1021/acsomega.4c02492. eCollection 2024 Sep 3.
Unlike conventional natural gas reservoirs, shale gas development involves systematic changes in methane carbon isotopes that cannot be effectively described by existing isotope fractionation models and mechanisms. Therefore, based on fundamental theories such as Rayleigh fractionation, mass transfer flow, and mass conservation, this study established isotopic fractionation equations for methane in adsorbed and free gas. By considering adsorbed and free gases as two end-members and using an isotope mixing model, a fractionation model for methane carbon isotopes during shale gas desorption was constructed. This model quantifies the isotopic fractionation effects during shale gas desorption and elucidates the mechanism of methane carbon isotope fractionation. Using on-site desorbed gas content and isotope data, parameter fitting and model calculations were conducted to characterize methane carbon isotope variations throughout the process of shale core field desorption. The results show a pattern of "initially negative and then turning positive," consistent with those of physical simulation experiments. It was clarified that differences in mixing the two end-members and isotopic fractionation play key roles in the variation of methane carbon isotopic composition in shale gas. By applying the methane carbon isotope fractionation model, the contribution of adsorbed gas during shale gas production was explored. It was found that in the early stage of development, the adsorbed gas in Well JY 1 was negligible. After nearly seven years of development, the contribution of adsorbed gas in the later stage has only reached nearly 15%, indicating that the production contribution of adsorbed gas is still less than 0.3 million cubic feet per day. The open flow of Well JY 6-2 is more conducive to the production of adsorbed gas, but the production capacity is still mainly contributed by free gas, indicating that the shale gas production capacity in the later stage in the Jiaoshiba gas field is still primarily dominated by free gas.
与传统天然气藏不同，页岩气开发涉及甲烷碳同位素的系统性变化，现有同位素分馏模型和机制无法有效描述这些变化。因此，本研究基于瑞利分馏、传质流和质量守恒等基础理论，建立了吸附气和游离气中甲烷的同位素分馏方程。通过将吸附气和游离气视为两个端元，并使用同位素混合模型，构建了页岩气解吸过程中甲烷碳同位素的分馏模型。该模型量化了页岩气解吸过程中的同位素分馏效应，阐明了甲烷碳同位素分馏的机制。利用现场解吸气含量和同位素数据进行参数拟合和模型计算，以表征页岩岩心现场解吸全过程中甲烷碳同位素的变化。结果呈现出“先负后正”的模式，与物理模拟实验结果一致。明确了两个端元混合差异和同位素分馏在页岩气甲烷碳同位素组成变化中起关键作用。通过应用甲烷碳同位素分馏模型，探讨了页岩气生产过程中吸附气的贡献。研究发现，在开发初期，JY 1井的吸附气可忽略不计。经过近七年的开发，后期吸附气的贡献仅达到近15%，表明吸附气的产量贡献仍低于每天30万立方英尺。JY 6 - 2井的无阻流量更有利于吸附气的产出，但产能仍主要由游离气贡献，这表明焦石坝气田后期的页岩气产能仍主要由游离气主导。
