Numerical study of the wall-retardation effect on proppant transport in rough fractures

Proppant transport and distribution in fractures is critical to evaluate the effectiveness of hydraulic fracturing of unconventional reservoirs such as shale and tight sandstone. This study reproduced the process of proppant transport in rough fractures by two fluid method (TFM) and investigated the wall-retardation effect on proppant migration and settlement. Based on our previous work, the solid–liquid drag model considering wall-retardation effect was further discussed by Lattice Boltzmann method (LBM). The characteristics of rough surfaces of shale and tight sandstone samples were analyzed, and the 4 m × 0.3 m (length × height) rough fracture models were constructed based on these characteristics. The proppant transport and distribution in smooth fractures, sheared fractures and joint fractures were compared, and then the effects of fracture morphology, aperture and mated state on the retarded proppant transport were explored. In smooth fractures, the effect of wall retardation will disappear when the eccentric factor is more than 2.5. In rough fractures, the wall-retardation effect on proppant transport is further enhanced by the unevenly distributed fracture aperture. Both vertical and horizontal velocity of proppant decrease in rough fractures, and this phenomenon is strengthened when the wave-shaped texture or stratification is vertical to the flow direction. The none-flow zones formed by the contact of fracture surfaces will even destroy the continuity of flow filed and block up the proppant transport. However, the morphology of rough fractures will produce little impact on proppant transport when the mean fracture aperture overs the critical fracture aperture. For sheared fractures, the critical fracture aperture is around 6 mm, while for joint fractures, the critical fracture aperture is much less than 4 mm.