Effects of natural fracture reopening on proppant transport in supercritical CO2 fracturing: A CFD-DEM study

Abstract

The reopening of natural fractures reshapes the proppant beds in hydraulic fractures, especially at fracture intersections, critically influencing subsequent proppant migration. However, studies on the effects of proppant bed shapes on post-reopening proppant migration are limited. To address this gap, a moving wall boundary technique is integrated into a Computational Fluid Dynamics–Discrete Element Method (CFD-DEM) framework to model natural fracture reopening, enabling an in-depth analysis of how proppant bed reshaping impacts subsequent proppant migration during supercritical CO2 fracturing. Results reveal that proppant migration into natural fractures is governed by both drag force-induced and gravity-induced rolling mechanisms, with the latter exerting a more significant effect. The gravity-induced rolling mechanism is enhanced by five key factors: (a) the generation of a well-developed proppant pack in proximity to the intersection; (b) the inadequately filled V-shaped gully; (c) the increased proppant transport capacity within the natural fracture; (d) the reduced bypassing of proppants at the intersection; and (e) the improved ability of proppants to enter the natural fracture. The enhanced gravity-induced rolling mechanism significantly increases the proppants flowing into natural fractures: by 35.6% between the central and rearward section cases, 168.9% from mild to severe plugging cases, 39.1% with a wider natural fracture width (1.5 mm–2.5 mm), 20.3% with decreased hydraulic fracture flow rate (0.35 m/s to 0.25 m/s), and 33% with reduced fluid temperature (160 °C–40 °C). These findings underscore the impact of natural fracture reopening on subsequent proppant migration, offering valuable insights for optimizing fracturing designs.