Plugging shale pores and throats with nanoparticles is an efficient and economic method of preventing water invasion to sustain shale wellbore stability. The nanoparticles size distribution is the key factor affecting plugging performance. Currently, the selection of nanoparticle size is usually based on empirical plugging rules and qualitative evaluation experiments. Experimental evaluations only can reflect the macroscopic plugging performance, but cannot reveal the microscopic plugging mechanism. In addition, the experiment for shale pores plugging is complicated and costly. Thus, it is very meaningful to propose a new method for sizing plugging nanoparticles based on simulation.

The essence of particles plugging process actually is the interactions of particle-particle, particle-rock and particle-fluid. In this paper, the thought of discrete element method is adopted and the plugging nanoparticles are treated as discrete elements. A coupled computational fluid dynamics-discrete element method (CFD-DEM) model is constructed to describe the motion behaviors of nanoparticles (discrete elements) in the process of plugging shale pores. In this model, taking into account the flow action of drilling fluid and the nanoscale effect of nanoparticles, the major considered forces include inter-particle forces (i.e. elastic contact force, friction force and Van der Waals force), hydrodynamic forces (i.e. drag force, Magnus force, buoyancy force and pressure gradient force) and gravitational force. Furthermore, a plugging criterion is established to check the particles plugging status through monitoring the microscopic movements of nanoparticles. Moreover, three evaluation indexes (i.e. porosity reduction of shale, permeability and porosity of external plugging layer, plugging ratio) are proposed to evaluate the macroscopic plugging performances of nanoparticles.

Based on the constructed computational model, microscopic plugging criterion and macroscopic plugging performance evaluation indexes, a novel method of sizing plugging nanoparticles to prevent water invasion for shale wellbore stability is developed. In addition, the validation of the method is conducted and the results are similar to the previous results of plugging experiments. The proposed method can give support to the selection of plugging nanoparticles size through obtaining the microscopic plugging behaviors and macroscopic plugging performances of nanoparticles. And it can also be regarded as a fundamental method to study other kinds of particles plugging problems, such as formation damage and optimization of lost circulation materials.

用纳米颗粒堵塞页岩气孔和喉咙是防止水入侵以维持页岩井眼稳定性的有效且经济的方法。纳米颗粒的尺寸分布是影响堵塞性能的关键因素。当前，纳米颗粒尺寸的选择通常基于经验性的堵塞规则和定性评估实验。实验评估只能反映宏观的堵塞性能，而不能揭示微观的堵塞机理。另外，页岩孔堵塞的实验是复杂且昂贵的。因此，提出一种基于仿真的纳米颗粒尺寸测定方法非常有意义。

颗粒堵塞过程的实质实际上是颗粒-颗粒，颗粒-岩石和颗粒-流体之间的相互作用。本文采用离散元法的思想，将堵塞纳米颗粒作为离散元。建立了耦合计算流体动力学离散元方法（CFD-DEM）模型，以描述页岩孔堵塞过程中纳米颗粒（离散元）的运动行为。在该模型中，考虑到钻井液的流动作用和纳米颗粒的纳米级效应，主要考虑的力包括颗粒间力（即弹性接触力，摩擦力和范德华力），流体动力（即阻力） ，马格努斯力，浮力和压力梯度力）和重力。此外，建立堵塞标准，通过监测纳米颗粒的微观运动来检查颗粒堵塞状态。此外，提出了三个评价指标（即页岩孔隙率降低，外部堵漏层的渗透率和孔隙率，堵漏率）来评价纳米颗粒的宏观堵漏性能。

基于所建立的计算模型，微观堵漏判据和宏观堵漏性能评价指标，开发了一种确定堵漏纳米颗粒尺寸的新方法，以防止页岩井眼稳定性的水分入侵。此外，进行了该方法的验证，结果与先前的堵塞实验结果相似。通过获得纳米颗粒的微观堵塞行为和宏观堵塞性能，该方法可以为堵塞纳米颗粒尺寸的选择提供支持。研究其他种类的颗粒堵塞问题，例如地层破坏和漏失材料的优化，也可以被视为一种基本方法。