Mechanisms of proppant transport diversion and confluence in fracture networks of unconventional reservoir

The geometric and topological structures of fracture networks in unconventional reservoir fracturing are core factors controlling the transport of water-sand two-phase flow. At present, there is still a lack of clear understanding regarding the diversion and confluence laws of proppant within fracture networks. In this paper, numerical simulations are conducted to investigate the basic laws of proppant transport, diversion and confluence in fracture networks, as well as the influences of various parameters. The results show that the diversion process is characterized by inertia-dominated asymmetric distribution, and proppant transport exhibits path selectivity, with proppant preferentially entering main channels and eventually forming a stable stratified structure. During confluence, proppant converges and collides to form a high-concentration core zone, which propagates forward over time and tends to reach dynamic stability. For diversion, the slurry velocity is higher than 0.05 m/s to overcome flow resistance and enter branch fractures, but a velocity exceeding 0.2 m/s induces turbulent deposition. The proppant transport distance increases with rising concentration. When the concentration exceeds 0.3, proppant can be delivered deep into the fracture network. The optimal fracture height for diversion ranges from 2.5 mm to 4.5 mm. For confluence, proppant begins to deposit on fracture walls at a velocity of 0.2 m/s, and backflow occurs at 0.3 m/s. The optimal fracture height for confluence ranges from 1.5 mm to 4.5 mm. In complex fracture networks, high proppant concentration in main channels restricts effective diversion into branch fractures, while the confluence effect reduces transport efficiency in branch fractures near the high-concentration core zone. Proppant diversion follows an inertia-dominated asymmetric distribution with path selectivity during transport. Proppant confluence is characterized by collision and confluence at intersections to form a high-concentration core zone, which continuously migrates forward over time. This study provides theoretical guidance for the efficient development of unconventional oil and gas resources.