Unsaturated hydraulic conductivity measurements with centrifuges; a review

Conducting drainage or imbibition experiments in a centrifugal force field has long been recognized as a valid and efficient way to determine capillary pressure-saturation and relative permeability-saturation relationships. Experiments involving multiphase flow of immiscible fluids in porous media are sped up because the centrifugal acceleration is many times greater than Earth's gravitational acceleration. In addition, the fact that centrifugal force is a body force and that experiments can be performed under well-controlled conditions are considered advantages over other techniques. During the past few decades, transient-flow centrifuge methods have been developed and applied in petroleum geosciences, while in the soil and environmental sciences the focus has been on steady-state centrifuge methods. To inform both groups of each others' work, the different instrumental approaches used are described. The theoretical background for modeling multiphase fluid flow of immiscible fluids through porous media in a centrifugal force field is then reviewed. This background forms the basis for understanding analytical and numerical formulations to interpret both steady-state and transient-flow centrifuge experiments. Research on the effects of compaction, menisci deformation, boundary conditions, corrections for early-time production data, the selection of measurable variables, and nonuniqueness of the data interpretation is discussed. A numerical example application of the transient-flow centrifuge method is presented. Finally, the major conclusions that can be drawn from the literature are discussed and potential areas for future research are identified.