介观尺度下裂缝内堵漏颗粒封堵层形成与破坏机理CFD-DEM模拟

堵漏材料进入地层裂缝后,通常期望其能够在漏失通道内快速形成稳定的高强度封堵层,从而实现井筒与漏失层的有效隔离,这对控制漏失程度和强化井筒至关重要。当前,普遍从以封堵层为整体的宏观角度以及从单个堵漏颗粒性质的微观角度来分析研究封堵层的降低漏速能力和承压能力,而从介观尺度研究堵漏颗粒封堵层在裂缝内的演化过程较少。为进一步揭示裂缝内堵漏颗粒封堵层的形成与破坏机理,基于计算流体力学和离散元耦合模拟方法,研究了堵漏颗粒在楔形裂缝内滞留、架桥、封堵和失稳破坏过程,分析了介观尺度下堵漏材料性质及钻井液性能对裂缝封堵层形成影响机理。结果表明,堵漏颗粒进入裂缝后经过滞留、堆积、封堵过程形成封堵层,且封堵层前端是封堵层稳定的关键部位。堵漏颗粒尺寸越小,封堵层位置越接近裂缝出口。堵漏颗粒浓度增加会显著缩短封堵层形成时间,当浓度由2%增加至30%时,封堵层的初始形成时间由0.090 s降低至0.036 s,降低了60%,此外,堵漏颗粒几何和物理性质以及钻井液参数对封堵层形成及破坏均具有较为明显的影响。研究结果对进一步优化堵漏颗粒并快速形成高效封堵层具有一定的参考价值。

CDF-DEM Simulation of the Formation and Failure Mechanisms of Plugging Layers Formed by Plugging Particles in Fractures at Mesoscale

Plugging particles entering the formation fractures are generally expected to quickly form stable high strength plugging layers inside the channels through which mud is lost, thereby effectively isolating the borehole and the loss zones. This characteristic of the plugging particles is critical to controlling the severity of mud losses and strengthening the borehole walls. Presently, the general practice is to study the ability to reduce rate of mud losses and the pressure bearing capacity of a plugging layer from the macro point of view, in which the plugging layer as a whole is studied, and from the micro point of view, in which the properties of a single plugging particle are studied, while studies from the mesoscale point of view on the changing process of a plugging layer formed by the plugging particles inside a fracture are seldom conducted. To further reveal the formation and failure mechanisms of the plugging layers formed by the plugging particles inside formation fractures, based on computational fluid dynamics and discrete element coupling simulation method, the retention, bridging, plugging and destabilization of the plugging particles inside wedge-shaped fractures are studied, and the effects of the properties of the lost circulation materials and the drilling fluids on the formation of plugging layers at mesoscale level are analyzed. The study and the analyses show that after entering the fractures, the plugging particles form plugging layers through a series of processes: retention, accumulation and plugging, and the front end of the plugging layer is key to the stability of that plugging layer. The smaller the particle size is, the closer the plugging layer is to the outlet of the fracture. Increase in the concentration of the plugging particles effectively reduces the time for a plugging layer to form. When the concentration of the plugging particles is raised from 2% to 30%, the time for the plugging layer to initially form is reduced from 0.090 s to 0.036 s, or reduced by 60%. Furthermore, the geometry and physical properties of the plugging particles, as well as the mud properties all play important roles in the formation and failure of a plugging layer. The study results have certain reference values for optimizing plugging particles to rapidly form efficient plugging layers.