CFD-DEM simulation of proppant pack stability during flowback in a rough fracture using supercritical CO2

Abstract

The destabilization of proppant packs during flowback, leading to undesirable consequences such as choke points (fracture closure near the wellbore) and proppant production, is a critical concern in fracturing operations. Despite its significance, comprehensive investigations into proppant pack stability during flowback remain limited, particularly in supercritical CO2 (Sc-CO2) fracturing. This study addresses this gap by employing an experimentally validated coupled Computational Fluid Dynamics–Discrete Element Method (CFD–DEM) model, integrated with a heat transfer model, to analyze proppant pack stability in Sc-CO2 fracturing near the wellbore. An authentic proppant pack for flowback analysis is generated by simulating proppant pack development during the pumping stage and subsequent compression during the fracture closure stage in a synthetic rough fracture. The findings reveal that the flowback stage can be subdivided into three stages: translation-controlled, rolling-controlled, and stable. Proppant pack stability is intricately governed by factors such as fluid drag force, inter-particle contact force, and pre-flowback pack shape. Due to the reduced fluid drag force, the retained proppant ratios rise under varied conditions: from 47.6% to 95.7% with a decrease in flowback rate from 0.3 m/s to 0.1 m/s, from 50.0% to 67.8% with an increase in fluid temperature from 40 °C to 160 °C, and from 16.5% to 80.8% with an increase in proppant size from 0.4 mm to 0.8 mm. In comparison, due to the increased inter-particle contact force, the retained proppant ratio increases by 16.4% as the closure width increases from 0.15 mm to 0.3 mm. Additionally, a longer proppant pack with a front characterized by a higher inclination angle and curved zone exhibits higher stability during flowback. These insights significantly enhance the understanding of proppant pack stability during flowback, offering crucial guidance for designing Sc-CO2 fracturing processes.