Effect of roughness characteristics of hydraulic fractures on the proppant transport using supercritical CO2

The morphology of rough fractures significantly affects proppant transport in hydraulic fractures. The fracture roughness is even more critical in proppant transport using supercritical CO2 since supercritical CO2 produces rougher fractures than other conventional fracturing fluids. In this study, we used roughness characteristics of the fractures generated by using supercritical CO2 and real rock samples determined by direct measurement together with coupled CFD (Computational Fluid Dynamics)-DEM (Discrete Element Method) to investigate the effects of different roughness characterization parameters on the efficiency of proppant transport using supercritical CO2. Laboratory experimental data of proppant transport in supercritical CO2 within the fracture were used to verify the reliability of the model. The results show that as the ratio of proppant diameter to average width of rough fracture increases, the proppant is more likely to bridge or even form sand blockage when transported in rough fractures, while better transport can be obtained when the ratio is less than 0.4. The main form of proppant particle transport within the rough fractures are the scouring and throwing of particles as the slurry flows through the bridging locations. In addition, the variation of root mean square (RMS) of asperity height values and fractal dimensions can have dissimilar effects on the proppant transport in rough fractures. The distance of proppant transport within the fracture decreases with the increasing fractal dimension, but increasing the RMS of asperity height value will yield proppant transport to longer distance. The proppant is transported farther in rough fractures than in planar fractures, but also, the required pumping pressure is higher in rough fractures, and the pumping pressure is positively related to the fracture roughness. Since using supercritical CO2 for fracturing produces rougher fractures than that of water-based fracturing fluids, higher pump power is required for efficient proppant transport using supercritical CO2, otherwise sand plug may occur. The results of this study lead to a better insight into the proppant transport behavior using supercritical CO2 in rough rock fractures and, therefore, would be useful for the improved field design of waterless fracturing operations conducted by using supercritical CO2.