Experimental Research on the Proppant Transport Behavior in Nonviscous and Viscous Fluids

Hydraulic fracturing is the key stimulation treatment for oil and gas development. The fracture conductivity and effectiveness strongly depend on the placement of proppant inside the fracture, so proppant transport behavior is a critical part of hydraulic fracturing. However, a thorough understanding of proppant movement through multientry perforations as well in nonviscous and viscous fracturing fluids has not yet been fully addressed. Coupled with particle image velocimetry (PIV) technology, scaled testing of the proppant transportation in pure water, slickwater, and guar fracturing fluids was conducted with a parallel and smooth slot configuration to deeply investigate on the proppant transport behavior in hydraulic fracturing. PIV was utilized to analyze particle velocity vectors and streamlines. Based on the microscopic particle trajectory and the dune's macroscopic growing process, we took account of the influence of perforation interference on the proppant movement and compared the proppant transport behaviors and mechanics in water, low-viscosity slickwater, and high-viscosity guar fluid. Then the factors influencing proppant placement were discussed. The experimental results showed that proppant transport occurred under a combination of fluidization and sedimentation in both nonviscous fluids and viscous fluids, primarily dragged by fluidization. The proppant neither penetrates the full length of the fracture nor completely fills it but forms a sand dune in the fracture. The formation process is essentially the same in the different types of fracturing fluids and can be divided into four stages: initial dune formation, growth, establishment of the equilibrium state, and piston-like movement. The dune has an equilibrium height, build-up angle, advancing angle, and void space under the actions of fluid erosion, vortexes, and sand-carrying capability, and its shape essentially depends on the velocity of carrying fluids and proppant. The higher the pumping rate and fluid viscosity, the better the sand-carrying capacity, leading to a lower equilibrium height and bigger void space; the higher the proppant concentration, the stronger the particle–particle and particle–fluid interactions, leading to lower velocity, higher dune, and smaller void space. Additionally, in high-viscosity guar fluids, an additional thin and barely flowing fluid layer was formed between the fluidized layer and the immobile dune, which has a lifting and wrapping effect on the particles and can further reduce the resistance during transportation. Due to perforation interference, violent vortexes can form in the low-velocity zones near the wellbore when the fluid is injected through multiple tiny perforations, even in the laminar flow, and these vortexes are beneficial to increase the travel distance of particles. A better understanding of the proppant movement and placement in the narrow fracture can provide a foundation for optimizing the hydraulic fracturing design and for estimating the well production.