CFD-DEM simulation of proppant transport by supercritical CO2 in a vertical planar fracture

The great potential of supercritical CO2 fracturing technique in the stimulation of unconventional reservoirs requires further insight into the mechanism of proppant transport in the fracture. In this paper, we adopted the computational fluid dynamics (CFD) coupled discrete element method (DEM) scheme to study the proppant transport in supercritical CO2 fluid, and verified this method via previous experiment data. Based on the visual analysis at the particle-level, transport mechanisms of proppant in the fracture were revealed. The difference in the proppant transport capacity between supercritical CO2 and commonly used fracturing fluids were demonstrated, and effects of slurry injection velocity, proppant concentration, injection temperature, and proppant diameter were discussed. Results indicate that the transport and placement process of the proppant can be divided into three stages, i.e., flat laying stage, proppant dune development stage, and proppant bank development stage. The supercritical CO2 slurry is characterized by obvious stratified flow in the fracture: the static bed at the bottom, the fluidization layer above it, and the pure fluid flow region at the top. Viscous drag transport, lifting transport, and fluidization transport are three proppant transport mechanisms for supercritical CO2 slurry flow, among which the fluidization transport plays a more dominant role. This work is expected to provide an in-depth understanding of the transport mechanism of supercritical CO2 slurry in a vertical planar fracture.