页岩基质复杂纳米孔隙的扩散-吸附-固溶耦合传质机理研究

Shale gas is an important resource for the economic development in China, which has been proved to have a great potential in exploitation. The methane gas is storaged in shale matrix complex nanopores, whose exploitation procedure consists of solid solution gas dissolving out, gas desorption on nanopore surfaces, diffusion in matrix nanopore networks and permeation flow in the natural or hydraulic fractures. A further understanding on mechanism of the multi-scale and multi-physics coupling mass transfer in shale matrix porous medium is required for shale core analysis and production curve forecast. However, there are few reports on the diffusion-adsorption-solution coupling mass transfer models in the literature. This project aims to reveal the nanopore surface mass transfer process in shale organic matters with molecular simulation techniques, and set up an integrated mass transfer model for the characteristic pore-throat structure. The mass transfer model is then applied in the pore network provided by reconstruction of 3D SEM data to derive the effective mass transfer coefficients. The volume-averaging method will be used to set up the apparent mass transfer model for organic matters under the frame of organic solid-stylolite. This project will also directly measure the effective permeability and diffusion coefficients for shale powders with a pressure decay method. The mass transfer model derived with numerical simulation and theoretical analysis will be applied in explanations of the experimental pressure curves.

我国页岩气勘探开发潜力巨大，对国民经济发展具有重大战略意义。甲烷气体赋存在页岩的基质复杂纳米孔隙中，在开采过程中依次经历干酪根有机质的气体固溶析出、纳米孔隙表面脱附、基质纳米孔隙网络中的扩散和天然或压裂裂缝中的渗流等过程。深入认识页岩基质多孔结构的多尺度、多物理耦合传质机理是页岩气岩心分析技术和产量曲线模拟的关键之一，而目前综合考虑页岩基质纳米孔隙中扩散-吸附-固溶耦合传质机理的岩石渗流模型尚少见报道。本项目运用分子模拟技术揭示页岩有机质纳米孔表面传质过程，针对纳米孔-吼特征结构建立综合传质模型，并应用于由三维扫描电镜数据重构的孔隙网络，获得有机质表观扩散系数。在有机质-缝合线双重介质模型的框架下，运用体积平均方法建立基质表观传质模型。本项目将运用压力衰减法测量页岩颗粒样品表观渗透率和扩散系数，将由数值模拟和理论分析建立的传质模型应用于实验压力曲线分析，并对基质传质模型做出验证。

非常规天然气的勘探和开发已成为我国清洁能源利用中的一项重要环节。本项目围绕页岩气勘探开发过程中的岩心渗透率测试技术，在微观孔隙尺度利用分子动力学技术开展页岩有机质中的扩散-吸附-固溶耦合传质过程的模拟研究，获得了有机质狭缝中自由气区域和吸附层内的甲烷分子扩散系数。建立了基于化学势梯度的传质模型，可用于对页岩有机质孔隙内的耦合传质过程进行表征。利用计算流体力学和格子玻尔兹曼方法开展了分形结构和Voronoi结构的多孔形貌中的传递过程模拟，探索了复杂孔隙几何结构对流动、扩散等传递现象的影响。系统地发展了适用于颗粒形低渗（纳达西级）多孔样品的快速渗透率/扩散系数测定方法，明确了测试气体种类、气体可压缩性、气体吸附性对渗透率测试的影响，提出了从测试结果中修正上述效应的技术途径，制作了相关数据处理用工程图表。系统地阐明了系统泄露、温度波动对颗粒样渗透率测试过程的压力衰减曲线的影响，提出了基于卡尔曼滤波的压力衰减曲线预处理方法。通过实验研究证明了样品脱附处理程度对测试结果有重大影响。相关结果对于完善此类渗透率测量方法具有重要指导意义，为制定相关测试标准奠定了基础。提出了一种基于双活塞联动的多孔样品孔隙度/真密度测定方法，相关成果可用于研发新型低渗多孔材料的物性测定、表征仪器及指定相关测试标准。