页岩气储层双分形孔隙-裂缝网络模型及气水传输机理研究

The mechanism of gas-water transfer in shale gas reservoir should be identified to make strategies to develop shale gas efficiently with low investment, because the phenomenon of gas-water transfer in reservoir stratum is generally occurred in the development of shale gas reservoir with the wide application of large-scale hydraulic fracturing. Aiming at the investigation of the mass gas-water transfer phenomenon of shale gas reservoir stratum, and based on the pore-fracture structure of the stratum and the characteristics of gas-water transfer, this project is to develop a network model of dual-fractal pore-fracture by applying the fractal theory to characterize the traits of the structure of shale gas reservoir stratum and then conduct a matrix pore model and fracture network model which conform to fractal characterization. Moreover, the constitutive equations of gas-water transfer is developed based on multiple gas-water transfer mechanism and the network model of dual-fractal pore-fracture is solved by adopting the method of finite element explicit iteration combined with gas-water transfer in the matrix porosity model and fracture network model. Thirdly, what this project also aims to do is to investigate the relationship between the capability of gas-water transfer of reservoir stratum, the initial water saturation and pore-fracture structure of shale gas reservoir stratum, and the temperature and pressure of transfer. Also, the fluctuation of the water film thickness of pores and fractures as well as water saturation distribution of reservoir stratum with respect to time is analyzed during the process of gas-water transfer to identify the mechanism of this process. The research achievement will lay a solid foundation for the development of theory and technology of exploiting the shale gas efficiently with low investment.

由于大规模水力压裂工艺的广泛应用，页岩气储层在开发过程中普遍存在气水传输现象，需要明确页岩气储层气水传输机理，以制定页岩气低成本、高效开发策略。本项目以页岩气储层多尺度气水两相传质问题为研究目标，从页岩气储层孔缝结构与气水传输特征两方面入手，利用分形理论表征页岩气储层孔缝结构特征，生成符合分形表征的基质孔隙模型与裂缝网络模型，建立页岩气储层双分形孔隙-裂缝网络模型；基于气水多重传输机制，构建气水传输本构方程，利用有限元显式迭代方法，耦合基质孔隙模型与裂缝网络模型中的气水传输，求解页岩气储层双分形孔隙-裂缝网络模型；研究页岩气储层初始含水饱和度、孔缝结构、传输压力温度等与储层气水传输能力的关系，分析气水传输过程中，孔隙、裂缝水膜厚度，储层含水饱和度分布随传输时间的变化，明确页岩气储层气水传输机理。研究成果为低成本、高效页岩气开发理论与技术的发展提供支撑。

页岩气储层具有孔隙细小、微裂缝发育的特点，储层内孔隙、裂缝结构复杂。气水流体在页岩气储层内由纳米级孔隙传输至毫米级微裂缝，气水由多重传输机制控制，受扩散效应、滑脱效应、渗吸效应等效应以及含水饱和度的影响，且存在微裂缝内窜流，呈现非线性渗流现象，导致对气水传输机理的认识存在困难，影响页岩气的高效开发。.本项目针对目前影响我国页岩气高效开发的基础理论问题，以页岩气储层多尺度气水两相传质问题为研究目标，从页岩气储层孔缝结构与气水传输特征两方面入手，通过开展页岩气储层双分形孔隙-裂缝网络模型研究，揭示页岩气储层气水传输机理，为我国页岩气储层高效开发等工程问题解决提供基础理论支撑。主要成果和认识如下：.1）基于页岩气储层孔缝结构扫描实验结果，利用孔缝结构分形表征方法，分析基质孔隙与裂缝网络在各尺度上的形态、分布等特征，得到基质孔隙与裂缝网络的各分形参数，生成符合页岩气储层分形表征的基质孔隙模型与裂缝网络模型，建立页岩气储层双分形孔隙-裂缝网络模型。.2）基于连续流动、滑脱流动和扩散模型，结合分子物理学方法，研究多重传输机制下孔隙、裂缝的气水传输，分别建立基质与裂缝气水传输本构方程，结合页岩气储层双分形孔隙-裂缝网络模型，利用有限元显式迭代方法，耦合基质孔隙模型与裂缝网络模型中的气水传输，实现了对页岩气储层双分形孔隙-裂缝网络模型的求解。.3）基于页岩气储层双分形孔隙-裂缝网络模型，分析储层初始含水饱和度、孔缝结构、传输压力温度等与页岩气储层气水传输能力的关系，研究气水传输过程中，孔隙、裂缝水膜厚度，储层含水饱和度分布随传输时间的变化，明确了页岩气储层气水传输机理。