Relative permeability hysteresis and residual trapping in rough-walled fractures: An experimental investigation of the effects of flow rate and saturation history using the steady-state approach

In this work, we use the steady-state measurement technique to characterize two-phase brine-mineral oil relative permeabilities and residual trapping in water-wet rough-walled fractures induced in Eagle Ford shale rock samples. Furthermore, we systematically probe the effects of flow rate and saturation history on these properties. The influences of capillary, gravity, and viscous forces on fracture flow stability and two-phase relative permeabilities are also investigated under different flow conditions represented by varying capillary and Bond numbers. The results demonstrated significant phase interference for the oil–brine flow in rough-walled fractures, which renders the commonly used x-curve and Corey models inadequate to represent the steady-state oil–brine relative permeabilities measured in this study. The saturation history influenced the relative permeabilities of both the wetting (brine) and non-wetting (mineral oil) fluid phases and the residual saturations during waterflooding. Generally, the residual oil saturation and oil relative permeability decreased with the decline in the initial oil saturation. Furthermore, at similar brine saturations, the oil relative permeability during waterflooding improved as the total flow rate increased. This increase was attributed to the high mobility of the connected oil phase within the fracture and the water-wet characteristics of the fracture walls. The brine relative permeability trend followed that of its counterpart measured under the capillary-dominated regime and only exceeded that at very high brine saturations. At higher flow rates, the residual oil trapping was significantly reduced due to the higher efficiency of the viscous-dominated displacement process. The results suggest that a high total flow rate in water-wet fractures maintains a high non-wetting phase relative permeability over a wide range of water-cut values and reduces the residual non-wetting phase saturation in the fracture at the end of waterflooding. Finally, improved correlation models were devised based on a subset of experimental results generated for fractures with various conductivities. They provide a more accurate description of fractures’ relative permeabilities compared to commonly used models. These correlations were successfully tested against relative permeability data measured for a fracture excluded during the fitting process.