The Effect of Fracture Wall Surface Roughness on Proppant Transport

Summary Proppant transport into created fractures is crucial in maximizing hydrocarbon recovery in unconventional reservoirs. Injected proppants keep the created fractures open, enhancing final fracture conductivity and propped and effective fracture length. The roughness and width of the created fracture are also important in proppant transport and distribution within induced fractures, affecting fracture conductivity. This study investigates the effect of fracture width on proppant transport into a complex slot system with 3D-printed rough wall surfaces. This study builds upon previous experimental work (Tatman et al. 2022), exploring different fracture roughness profiles and varying fracture widths. The effects of proppant densities, sizes, and concentrations on proppant transport within rough fracture surfaces were also investigated. A laboratory-scaled slot apparatus was used to examine the effects of various parameters. The slot consisted of a 4-ft-long primary fracture intersecting with a secondary fracture at a 90o angle. The wall surface roughness was printed using 3D printing technology and average fracture widths were set at 0.1 in. and 0.2 in. Fresh water (1 cp) with proppant concentrations of 1 ppg and 2 ppg and proppant sizes of 100-mesh sand [2.65 specific gravity (SG)], 40/70-mesh sand (2.65 SG), and 35/45-mesh ultralightweight (ULW) proppant (1.07 SG) were tested. The results show that fracture wall roughness impacts proppant transport behavior. The rough wall surface formed irregular proppant dune shapes and trapped some of the injected proppant at different locations within the slot, which is distinct from smooth wall surfaces. Decreasing the fracture width of a rough wall surface had a significant impact on proppant transport. The majority of the injected proppant was transported away from the injection point due to the increased slurry velocity. This led to improved proppant transport due to redirected flow direction and associated decreased proppant settling velocities, and a decrease in the proppant-dune-building rate near the inlet point, which carried more proppant deep into the secondary slot. Increasing the proppant concentration had a positive impact on proppant transport within both tested fracture widths by increasing the proppant-dune-buildup rate along the fracture slot and increasing the proppant-covered area inside fractures. The slurry injection rate had a great impact on low-density proppant transport. Decreasing the injection rate for lighter proppant (1.07 SG) helped to build a proppant dune near the injection point, increasing the proppant-covered area. Conversely, the higher injection rate carried the majority of the lighter proppant farther and out of the slots. Larger proppant sizes, (i.e., 40/70-mesh sand) resulted in a large proppant-covered area for both tested fracture widths, due to more particle-to-wall interaction, particle-to-particle interaction, and high density, which increased the settling velocity. However, injecting lighter and larger proppant sizes, such as 35/45-mesh ULW proppant, resulted in less settling and more particles suspended during injection due to lower density.