A coupled CFD-DEM numerical simulation of formation and evolution of sealing zones

Lost circulation can be one of the most troublesome situations during drilling, especially in naturally fractured formations. This situation may lead to serious economic losses because of the loss of expensive drilling fluid into the formation and nonproductive time spent on regaining drilling ROP. Granular lost circulation materials (LCMs) are the most commonly used to prevent and cure lost circulations. However, poor understanding of how granular LCM works at a micro-scale has limited their effectivity. In this paper, we developed a coupled CFD-DEM model by combining computational fluid dynamics with discrete element methods to simulate the sealing process of LCM in a wedge-shaped fracture by tracking the motion of each individual particle. Formation and evolution of both the sealing zone and the resulting force chain network were investigated by combining micro process visualization and analysis of flow characteristic curves of LCM suspension injection. The results show that injected LCM particles eventually result in three situations after transport in the fracture, depending on their size and concentration: 1) no bridging no sealing, 2) bridging without sealing, and 3) bridging with sealing. The fracture sealing zone is formed by either single-particle bridging or dual/multi-particles bridging of LCMs. A successful sealing comprises four stages: 1) LCM suspension uniform flow, 2) unstable bridging, 3) sealing zone formation and growth, and 4) fluid flow through the porous sealing zone. These situations and evolutionary stages are clearly reflected in the morphological changes of the flow curve. The formation and evolution of the force chain network also include four stages: 1) discrete force chain dynamic initiation, 2) small force chain network unstable formation, 3) discrete force chain network aggregation, 4) force chain network stable propagation. Strong force chains are in the front section and weak force chains are in the middle and end section of the sealing zone. The formation and collapse of force chains within bridging particles determines if a bridging structure and a sealing zone can be formed and stable. Dual-particles bridging formed sealing zone is weaker than single-particle bridging one because of the weak point in the strong force chain in the fracture width's direction. This research provides a better insight into the process of fracture sealing by granular LCMs, which contributes to improve efficiency of lost circulation control jobs.