Numerical Simulation of Proppant Transport and Backflow in Unconventional Reservoir Hydraulic Fractures Based on CFD-DEM
Authors: Lv Mingkun , Guo Tiankui , Chen Ming , Jia Xuliang , Yang Renjie , Jia Hui , Qu Zhanqing 
Published: August 2024

ABSTRACT:
During the post-fracturing flowback stage, a large amount of fracturing fluid (30%-80% of the injected volume) will flow back to the surface, but it will also carry proppants into the wellbore. This has adverse effects on the fracturing effectiveness and subsequent production. Preventing and controlling the proppant flowback has become one of the key issues in hydraulic fracturing. Based on the CFD-DEM method, we established the near-well model with perforations and the mid-fracture model with fully open outlet, considering the closure of the fracture. The influence of parameters such as fracture width, flowback fluid velocity, fracture roughness, proppant shape, proppant density, perforation size, and proppant size distribution on proppant flowback was investigated. The research results showed that the model with perforations had a significantly lower proportion of proppant flowback compared to the model with fully open outlets. Increasing the flow velocity significantly increased the proppant flowback. Higher proppant density and fracture roughness were beneficial for proppant bridging and stability. Using ellipsoidal proppant and uniform proppant size also reduced proppant flowback. Decreasing the perforation size reduced proppant flowback by bridging at the perforation. This research provides us with a clearer understanding of the process and mechanisms of proppant flowback.

1. INTRODUCTION
The effectiveness of hydraulic fracturing is one of the key factors determining the development and economic viability of unconventional oil and gas (Cao, 2023a; Somnath, 2021). Hydraulic fractures are supported by proppants, which give them a certain conductivity. Therefore, the distribution of proppants within the fractures has a significant impact on the effectiveness of hydraulic fracturing (Cao, 2023b). The complete hydraulic fracturing process includes injecting proppant-free fluid to create fractures, injecting proppant-carried slurry, flowback of the fracturing fluid, and production initiation. The latter three stages all impact the proppant transport and distribution. Currently, extensive research has been conducted on the proppant transport during the injection of proppant-carried fluids, which involves different fracture shape, operation parameters, and pumping schedule. However, the issue of proppant backflow during the fracturing fluid flowback process does not seem to have received sufficient attention. In fact, the proppant backflow after the fracturing process could lead to a series of issues related to fracturing effectiveness and production, including reduced effective fracture area, near-wellbore fracture closure (Potapenko, 2017), damage to underground and surface equipment due to excessive or prolonged proppant backflow (Zhu, 2021), and impacts on the normal production of oil and gas wells, leading to additional costs (Dmitriy, 2019). After conducting statistical analysis of shale oil wells in Changqing Oilfield in China, it was found that the maximum backflow volume from a single well reached 180 m3, accounting for more than 5% of the injected proppant volume. Clarifying the mechanism and key factors of proppant backflow within the fractures is a prerequisite for optimizing operation parameters to prevent and control proppant backflow.