Hu Zhiming, Duan Xianggang, Chang Jin, Zhang Xiaowei, Zhou Shangwen, Xu Yingying, Shen Rui, Gao Shusheng, Mu Ying
Research Institute of Petroleum Exploration and Development, PetroChina, Beijing100083, China.
University of the Chinese Academy of Sciences, Beijing100493, China.
ACS Omega. 2023 Jan 13;8(4):3571-3585. doi: 10.1021/acsomega.2c05789. eCollection 2023 Jan 31.
Shale gas seepage theory provides a scientific basis for dynamically analyzing the physical gas flow processes involved in shale gas extraction and for estimating shale gas production. Conventional experimental techniques and theoretical methods applied in seepage research are unable to accurately illustrate shale gas mass transfer processes at the micro- and nanoscale. In view of these scientific issues, the knowledge of seepage mechanisms and production development design was improved from the perspective of experimental techniques and theoretical models in the paper. First, multiple techniques (e.g., focused ion beam scanning electron microscopy and a combination of mercury intrusion porosimetry and adsorption measurement techniques) were integrated to characterize the micro- and nanopore distribution in shales. Then, molecular dynamics simulations were carried out to analyze the microscale distribution of gas molecules in nanopores. In addition, an upscaled gas flow model for the shale matrix was developed based on molecular dynamics simulations. Finally, the coupled flow and productivity models were set up according to a long-term production physical simulation to identify the production patterns for adsorbed and free gas. The research results show that micropores (diameter: <2 nm) and mesopores (diameter: 2-50 nm) account for more than 70% of all the pores in shales and that they are the primary space hosting adsorbed gas. Molecular simulations reveal that microscopic adsorption layers in organic matter nanopores can be as thick as 0.7 nm and that desorption and diffusion are the main mechanisms behind the migration of gas molecules. An apparent permeability model that comprehensively accounts for adsorption, diffusion, and seepage was developed to address the deficiency of Darcy's law in characterizing gas flowability in shale reservoirs. The productivity model results for a certain gas well show that the production in the first three years accounts for more than 50% of its estimated ultimate recovery and that adsorbed gas contributes more to the annual production than free gas in the eighth year. These research results provide theoretical and technical support for improving the theoretical understanding of shale gas seepage and optimizing shale gas extraction techniques in China.
页岩气渗流理论为动态分析页岩气开采过程中的物理气体流动过程以及估算页岩气产量提供了科学依据。渗流研究中应用的传统实验技术和理论方法无法准确阐明页岩气在微观和纳米尺度上的传质过程。针对这些科学问题，本文从实验技术和理论模型的角度对渗流机理和生产开发设计的认识进行了改进。首先，综合多种技术（如聚焦离子束扫描电子显微镜以及压汞孔隙率测定法与吸附测量技术相结合）来表征页岩中的微孔和纳米孔分布。然后，进行分子动力学模拟以分析纳米孔中气体分子的微观分布。此外，基于分子动力学模拟建立了页岩基质的尺度上推气体流动模型。最后，根据长期生产物理模拟建立了耦合流动与产能模型，以确定吸附气和游离气的生产模式。研究结果表明，微孔（直径：<2 nm）和中孔（直径：2 - 50 nm）占页岩中所有孔隙的70%以上，是吸附气的主要赋存空间。分子模拟表明，有机质纳米孔中的微观吸附层厚度可达0.7 nm，解吸和扩散是气体分子运移的主要机制。为弥补达西定律在表征页岩储层气体流动性方面的不足，建立了一个综合考虑吸附、扩散和渗流的表观渗透率模型。某气井的产能模型结果表明，前三年的产量占其估计最终采收率的50%以上，且在第八年吸附气对年产量的贡献大于游离气。这些研究结果为提高我国对页岩气渗流的理论认识和优化页岩气开采技术提供了理论和技术支持。
