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.