Unlike high-viscosity fluids, proppant transport with slickwater inside fractures is complex, but most fracture models do not adequately describe it. These models solve proppant transport based on simplified continuity equations. They assume that the average proppant velocity is the same as the fluid velocity and proppant settling in the vertical direction is always incorporated using Stokes’ law or similar equations. Some models also include proppant bridging and trapping effects, and velocity slippage between proppant and fluid, using empirical correlations. None of these commercially available models consider proppant momentum and interactions of particles with other particles, fluids, and fracture walls. In addition, turbulent flow near wellbore is completely ignored in these models. Slot tests used for proppant transport in laboratory studies are too small and too simplified to upscale properly to realistic field conditions. CFD (computational fluid dynamics) modeling can bridge some of these gaps. The main objective of this study was to use CFD modeling for a comprehensive understanding of proppant transport with slickwater and improve proppant and fluid selections for unconventional fracturing.
Several lab-scale CFD models were constructed and calibrated using experimental proppant transport data from slot flow tests. Field-scale CFD simulations were performed to evaluate the impact of key parameters on proppant transport and placement, including pumping rate, fracture width, fluid viscosity, proppant size and density, fluid leakoff, fracture height growth, perforation orientation and size, multiple fractures near wellbore, and pre-existing dunes, etc. Field-scale CFD model results show that proppant with wider size distribution provided more favorable placement, as larger proppant particles settle quickly to create a proppant dune and generate sufficient fracture conductivity near the wellbore. Smaller proppant particles travel deep into the fracture to increase partially propped fracture surface area in the far-field. CFD model results further show that the distance that larger proppant could reach inside the fracture is small, especially in the vertical direction, and that proppant height coverage can be improved by dune formation.
CFD model results confirm that proppant placement inside the fracture is significantly affected by pumping rate, fracture width, fluid viscosity, proppant size and density. CFD model results suggest that horizontal wells should always be landed close to the bottom section of the expected fracture where the proppant dune is located. Proppant with wider size distribution, for example 40/140- and 40/200-mesh sand, is expected to provide improved proppant placement.