Numerical simulation on proppant migration and placement within the rough and complex fractures

Hydraulic fracturing is a key technology for the development of unconventional hydrocarbon resources.The proppant placement morphology determines the fracture conductivity, thus affecting the reservoir stimulation effect. In this paper, the proppant migration and placement within complex fractures was studied by considering the fracture wall roughness through computational fluid mechanics-discrete element method(CFD-DEM) in numerical simulation, which is a key approach to study the proppant migration and placement. The results show that the proppant placement non-uniformity, proppant migration capacity, and proppant volume filled in the far-end and the secondary branched fracture are enhanced within the rough fracture compared with those within smooth fractures. The proppant migration capacity is increased within the fracture at low inclination angles(<60°) and low approach angles(<90°), and the proppant placement area is larger in the inclined fracture than that in the vertical fracture. The rise of injection rate and fracturing fluid viscosity causes more proppants migrate to far-end or secondary fractures, resulting in a non-proppant area within the near-wellbore fracture. An increase by 1.3 times in the injection rate and 3 times in the fracturing fluid viscosity leads to a decrease by 26.6%and 27%, respectively, in the proppant placement area within the near-wellbore fracture. The staged injection with small size proppants followed by large size proppants increases the proppant placement area in the primary fracture by 13%-26%, and that with large size proppants followed by small size proppants increases the proppant placement area by 19%-25%, which is due to that the latter method facilitates filling of the secondary branched fracture. The injection location mainly affects the proppant filling degree within the near-wellbore fractures. Compared with the upper injection, the middle and lower injection is not beneficial to filling of proppants within the near-wellbore fracture.