基于颗粒物质力学的储层参数变化流固耦合模拟方法

The interaction between fluid and reservoir formation particles is of the complex non-linear mechanical characteristics of multi-physics, multiphase and multi-scale. Although the conventional coupling simulation scheme, which is based on phenomenological theory, is easy to be applied in engineering, it cannot be readily employed to disclose the intrinsic interaction mechanism. Therefore, its application has been severely limited with the gradually smaller scales of investigations. In this project, on the basis of reconstructing the model of formation porous media represented by discrete particles, a direct fluid-solid interaction numerical simulation framework will be formulated and implemented for water-oil flowing in the formation on the mesoscopic scale. Taking into account both contact forces and non-contact forces, the fluid flow obtained by Lattice Boltzmann is coupled with solid physics response acquired by discrete element method through proper boundary condition. And the classical Drafting-Kissing-Tumbling process and Ergun equation will be reproduced by simulation to substantiate the effectiveness of this proposed algorithm. Using this approach, the detachment, migration and trapping of particles, which will lead to the formation parameter variation, will be simulated, and the intrinsic physical mechanisms of the interaction between fluid and particles will also be analyzed from the perspective of force chain network. This investigation will contribute to understanding the interaction between porous media and fluid in essence more effectively, and clarifying the principles and rules of formation parameter variation in water flooding.

流体与储层颗粒的相互作用具有多物理场、多相、多尺度的复杂非线性力学特征。传统的流固耦合模拟方法基于唯象理论，虽易于工程应用，但却不利于揭示其相互作用的内在物理机制，因此随着研究尺度的细化，应用越来越受到限制。本课题将通过建立储层的离散颗粒模型，在考虑颗粒之间接触力和非接触力的情况下利用边界条件将格子Boltzmann方法得到的流体流动与离散元方法获得的固体力学响应相耦合，在介观尺度上建立储层内油水两相渗流的流固耦合直接模拟方法。通过模拟对经典的Drafting-Kissing-Tumbling过程和Ergun方程进行复现，以验证算法的有效性。进而对水驱油藏开发中颗粒脱落、运移、捕集所引起的储层参数变化过程进行模拟，通过力链网络分析流体作用下储层参数变化的力学机制，从而更有效地从本质上揭示多孔介质与流体之间的相互作用，阐明水驱开发过程中储层参数变化的机理及规律。

在油田开发过程中，注入水的长期冲刷会使砂粒从岩石骨架上脱落、运移进而引起储层孔隙结构和渗流性质的变化从而形成优势通道。目前国内外研究主要集中在优势通道的识别、描述和治理方面，优势通道形成的力学机制一直尚未明确。通过直接数值模拟方法对水驱作用下储层参数变化的力学机制和基本规律进行研究具有重要的指导意义。本课题首先基于实际岩心的骨架特征和颗粒粒径分布，通过改进的球体充填算法建立多孔介质模型的重构方法。然后在表征单元体尺寸估算的基础上，计算多孔介质离散颗粒模型的孔隙度、比表面积、绝对渗透率、相对渗透率曲线和毛管力曲线等参数，以此验证并完善多孔介质离散颗粒模型的重构算法。基于球形颗粒接触力学建立了多孔介质离散颗粒间的接触力与非接触力的计算模型，并对该模型进行了检验和校正。通过格子 Boltzmann 方法对多孔介质内单相和油水两相流体的流动进行模拟，并利用离散元方法获取颗粒间的固体力学响应；在此基础上建立了考虑颗粒间接触力和非接触力的多孔介质内油水两相渗流的流固耦合模拟方法，并依据该算法编制了数值计算程序。利用所建立的流固耦合模拟方法对单个球形颗粒在粘性流体中的沉降过程进行模拟，并与文献中的研究结果作比较；通过复现经典的 Drafting-Kissing-Tumbling (DKT)现象和马格努斯效应对方法进行了验证。利用所建立的流固耦合模拟方法，从力链的角度阐明了水驱作用下成岩颗粒脱落、运移和捕集等现象的机理，揭示了水驱作用下储层性质变化的基本规律，并建立了渗透率的定量演化模型，为储层伤害的预测和治理、优势通道的识别和描述及进一步提高采收率提供了理论和技术支持。