页岩气吸脱附扩散多尺度传输机理研究

Shale gas is a kind of unconventional natural gas resource stored in the dark shale with adsorption, free or dissolved states. The staggered multi-scale pore network systems for gas reservoir are composed of matrix-natural fractures-artificial cracks. The production process of shale gas includes multi-scale effects of desorption-diffusion-seepage with scale varying from nanometer to micrometer and meter. This project intends to reconstruct the microscopic-mesoscopic-macroscopic structure by multi-scale numerical approach and establish a self-coupled model applicable to different scales. The effects of microscopic characteristics, multicomponent competitive adsorption-desorption properties, and real gas are fully taken into consideration in micro/nanopores of shale matrix. The interfacial coupling operators to transmit interfacial information at different scales are proposed and the multi-scale numerical methods coupled with microscopic (Grand Canonical Monte Carlo, GCMC and molecular dynamics, MD)-mesoscopic (Lattice Boltzmann Method, LBM) -macroscopic method are also developed. The multi-physics coupling transport mechanisms which coexist of multi-scale, multi-component, multiphase flow, diffusion, desorption and seepage in shale development processes are formulated. The impact of the actual morphology and formation environment of the shale reservoir on absorbed mass transfer and convective diffusion processes are revealed. The achievements of this project can provide theoretical guidance for shale gas development and plays important roles for production efficiency augmentation of shale gas.

页岩气是以吸附、游离或溶解状态赋存于暗色泥页岩等的非常规天然气资源。气藏地质是由基质-天然裂缝-人造裂缝相互交错的多尺度孔隙网络构成。页岩气在开采过程中经历了从纳米到微米、米级别的解吸-扩散-渗流多尺度效应。本项目拟通过数值重构再现岩层内微观-介观-宏观多尺度空间结构，建立适用不同尺度自耦模型，充分考虑微纳基质孔隙中传输的微观效应、多组分竞争吸脱附效应、实际气体效应等因素的影响，提出不同尺度界面信息传递耦合算子，开发微观巨正则系综蒙特卡洛(GCMC)、分子动力学（MD）方法-介观格子玻尔兹曼（LBM）方法-宏观多尺度耦合的数值方法。本项目可清楚阐明页岩气在开采过程中多尺度、多组分、多相流、扩散、解吸和渗流并存的多物理场耦合输运机理，揭示储层地质特征对吸脱附传质和对流扩散的影响规律。项目研究成果可为我国页岩气的开发设计提供理论指导，为提升页岩气的开发效率发挥重要作用。

页岩气藏由于地质条件复杂、多种岩石矿物共存、气体赋存状态多样（包括游离气、吸附气、溶解气），气体传输过程中涉及多种机理的相互耦合且空间尺度跨度大（纳米孔到宏观孔），因此，传统渗流模型很难准确揭示气体耦合传输机理以及传输能力。本项目采用理论分析、数值模拟、实验相结合的研究手段，开展页岩储层内气体多尺度多物理耦合传输机理的研究。通过高分辨率电镜扫描和低压氮气吸附实验获悉了页岩孔隙形貌。针对单个纳米有机质孔隙，建立气体滑移效应和表面扩散相耦合的解析模型，评估了吸附气体对页岩气总产量的贡献。考虑有纳米多孔机物内的多重扩散，提出局部扩散率的概念，建立了孔隙尺度局部有效扩散模型，揭示了随储层压力增加气体传质从努森扩散到分子扩散的过渡机制。考虑吸附气体分子的表面扩散和分子体积效应，建立了局部耦合扩散模型，明晰了吸附气体在不同孔隙内的贡献大小。考虑岩石基质组成的多样性和空间各项异性，对岩石3D结构进行了数值重构和表征，开发相应的多组分扩散模型，获悉了岩石几何结构、组成等对气体扩散能力的影响规律，并提出预估气体扩散能力的通用公式。此外，考虑到气体多尺度耦合输运特征，发展了气体跨尺度输运方法体系，实现了气体多尺度输运能力的精准预测。本研究成果为页岩气开发方案的制定以及传输能力的预测提供了强有力的理论支撑。