Study on proppant flowback mechanism during post-fracturing stage based on CFD-DEM

Proppant flowback in the post-fracturing flowback period not only reduces the fracture conductivity but also damages equipment. Due to the current lack of experimental or numerical simulation methods for proppant flowback in partial closure fracture, the mechanisms and patterns of proppant flowback remain unclear. This makes it difficult to predict the risks of proppant flowback, leaving flowback program design without theoretical guidance and resulting in high uncertainty in prevention effectiveness. This paper has further modified the CFD-DEM (Computational Fluid Dynamics-Discrete Element Method) coupling interface by introducing fracture closure pressure into the particle motion equation. Based on the dynamic mesh, the fracture width in the CFD model is adjusted in real time to establish a numerical simulation method that considers fracture closure and synchronous changes in the flow field. By establishing flow similarity at the perforations, a near-wellbore flow field is created in the scaled model that is representative of field conditions, ensuring the practical value of the experimental results. Based on proppant particle force analysis during flowback, we investigated the impact of closure pressure, friction coefficient, perforation parameters, fracture dip angle, proppant particle size combination on proppant flowback. The research indicates that the existence of a threshold closure pressure arises from the competition between the lateral force (driving flowback) exerted by fracture closure on particles and the frictional force (resisting flowback) acting on particles. Below this threshold, increasing closure pressure enhances near-wellbore proppant flowback; above this threshold, increased closure pressure reduces proppant flowback. This threshold value is determined to be 1 MPa under the simulation conditions of this paper. The friction coefficient between particles and the fracture wall has greater impact on particle flowback than the friction coefficient between particles. In the vertical direction of the fracture, flowback is more probable for particles above the perforation. There is higher risk of particle flowback in horizontal fractures. The lateral distribution of large and small particles is more effective in preventing flowback than the vertical distribution. In the horizontal direction, particles nearer to the perforation have a higher probability of flowback. Strategies for proppant flowback control: the flow rate should be kept low initially, and then increased after the bottomhole pressure has been appropriately reduced; perforations should be placed in the upper part of the reservoir (vertical well); the sand concentration should not be increased in the later stages of fracturing to reduce the accumulation of proppant above the perforations; different size proppants should be injected in smaller sizes followed by larger sizes, with a slug of clean fluid in between to achieve a side-by-side placement of larger and smaller proppant, thereby mitigating proppant flowback.