页岩微纳米孔隙-裂隙系统内油水两相的流动机理

Fluid flow in shale formations is of the complex multiphase and multiscale characteristics. However, most of the current studies only concentrate on the single-phase flow in micro/nano- pore-microfracture systems of shale and the investigations on oil/water two-phase flow are very limited. Therefore, the physics of multiphase flow in shale formations is still ambiguous, and the effectiveness of some techniques, e.g., numerical simulation, performance evaluation, and production optimization, are challenged, which has been the bottleneck in the shale oil industrialization exploration. In this proposal, on the basis of molecular dynamics simulation and microfluidic experiment, the principles of water/oil two-phase flow in single micro/nano- pores and slits are studied, from which the variations of contact angle and interfacial tension against the size of nanopores are elucidated. The influence of microscale effect on the two-phase flow behaviors is also discussed and the corresponding mathematical model is built. Then the shale digital core model is constructed through the pore structure characterization and its network connectivity is quantified. Finally, the simulation method for oil/water two-phase flow in micro-/nano- pore-microfracture systems of shale is proposed. Based on the simulation results, the two-phase flow mechanism is explored, the microscopic distributions of water/oil are also characterized, and the prediction technology for relative permeabilities is put forward. These achievements will provide significant guidance for the scientific and efficient development of shale oil.

页岩储层内流体的流动具有多相、多尺度的复杂特征。然而目前针对页岩微纳米孔隙-裂隙系统内流动机理的研究主要集中于单相流体，而对其中油水两相流动机制的研究却鲜有报道。这使得页岩储层的多相渗流理论尚不清楚，宏观数值模拟、产能评价和开发动态优化等技术的准确性面临重大挑战，这已经成为制约页岩油工业化探索的瓶颈。本课题将综合利用分子动力学模拟方法和微流动实验开展页岩储层单个微纳米孔缝内油水两相的流动机制研究，阐明纳米尺度下接触角和界面张力随孔隙尺寸的变化规律，揭示微尺度效应对两相流体流动规律的影响并建立其数学表征模型。进而通过对页岩储集空间特征的分析构建数字岩心，定量描述孔隙-裂隙网络的连通性。在此基础上，建立页岩微纳米孔隙-裂隙系统内油水两相流动的模拟方法，探究其多相流动机理，表征油水的微观赋存状态，形成相渗曲线的预测技术，为页岩油科学有效的开发提供指导。

页岩基质内广泛发育的纳米级孔隙使得传统的渗流力学理论无法有效描述页岩油的流动规律；同时，储层的多尺度、多矿物特征也使得不同化学组成纳米孔内流体的赋存状态和输运规律存在显著差异。但相关研究主要集中于单相流体，有关油水两相流动机理的研究还鲜有报道。阐明页岩微纳米孔隙–裂隙系统内油水两相的渗流机制是页岩油宏观数值模拟和开发优化的基础，对页岩油的科学有效开发具有重要意义。本研究首先基于分子动力学模拟和微流动实验，深化了页岩基质内单相原油的流动机理认识，发现了页岩油纳米孔内存在流动边界层的直接证据，探讨了多组分烷烃的竞争吸附作用和水对烷烃赋存状态的影响；在此基础上，明确了页岩纳米孔内接触角和界面张力随孔隙尺寸的变化规律，校正了毛管力的计算方法，阐明了压差驱动下页岩单纳米孔内油水两相的流动机理，分析了流动型态随驱替压力梯度、表面润湿性和含水饱和度等因素的变化规律，并建立了数学表征模型。进而，基于济阳坳陷页岩油样品，综合利用氮气吸附、高压压汞、FIB-SEM等方法，对夹层状页岩的储集空间和渗流能力进行了表征，明确了夹层状、纹层状和块状页岩孔隙结构特征的差异，构建了夹层状页岩各向异性渗透率的理论模型；并针对当前数字岩心构建方法难以同时满足分辨率和样品尺寸要求的问题，基于生成对抗神经网络模型，结合扫描电镜和CT的优势，提出了页岩多尺度数字岩心的重构算法。在此基础上，充分考虑页岩纳米孔内流动的特殊性，如边界层和孔隙内毛管压力的变化等，提出了压差驱动下页岩微纳米孔隙–裂隙系统内单相原油和油水两相流动的孔隙网络模拟方法，厘清了页岩油的非线性流动机理，明确了孔隙尺度上油水两相的赋存状态和流动规律，实现了相对渗透率曲线的预测。研究成果不但为页岩油的高效开发提供了基础模型和数据，而且也能够为其他领域中多孔介质输运问题的解决提供借鉴。