Relative Permeability Hysteresis: Water-Alternating-Gas Injection and Gas Storage
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 161827, ’Experimental and Theoretical Investigation of Water/Gas Relative Permeability Hysteresis: Applicable to Water-Alternating-Gas (WAG) Injection and Gas-Storage Processes,’ by S. Mobeen Fatemi, SPE, and Mehran Sohrabi, SPE, Heriot-Watt University, prepared for the 2012 Abu Dhabi International Petroleum Exhibition & Conference, Abu Dhabi, 11-14 November. The paper has not been peer reviewed.
Accurate determination of relative permeability hysteresis is needed to predict water-alternating-gas (WAG) injection reliably. Two series of gas/water relative permeability hysteresis curves were obtained from corefloods under mixed-wet conditions. The results revealed that none of the widely used hysteresis models (e.g., Carlson and Killough models) is able to predict the observed cyclic relative permeability hysteresis for alternating injection of gas and water. The results suggest that, for mixed-wet systems, it is necessary to consider irreversible hysteresis loops for modeling both the wetting and the nonwetting phase.
Introduction
Because of nonwetting-phase trapping, wetting-phase relative permeability for imbibition increased compared with the drainage case and nonwetting-phase imbibition relative permeability was less than that of the drainage case. Therefore, relative permeability to a fluid at a given saturation depends on whether that saturation was obtained by approaching it from a higher or lower value. This behavior in relative permeability is the hysteresis effect.
Much of the hysteresis data in the literature was obtained with saturations starting at endpoint values (i.e., irreducible-water saturation or residual-oil saturations for a water/oil system). These data usually deal with the differences between bounding relative permeabilities. Such data are more applicable to modeling reservoir processes in which phase saturations increase or decrease to an intermediate value, then change in the opposite direction. Examples include enhanced-oil-recovery methods such as WAG injection or cyclic-steam stimulation.
Most of the existing relative permeability hysteresis functions were developed for strongly water-wet porous media. However, it generally is accepted that many oil reservoirs are mixed-wet. In a mixed-wet system, the oil-wet pores correspond to the largest pores in the rock, while the small pores are water-wet. There are only a few relative permeability models developed for mixed-wet porous media, of which none are included in commercial simulators.
The purpose of this study was to further investigate the effect of cyclic hysteresis for water/gas systems under mixed-wet conditions. These two-phase relative permeability data are of interest for reliable simulation of processes involving cyclic changes between imbibition and drainage displacement, including WAG injection. The results of this study can be applied to underground hydrocarbon-gas storage, which usually involves cyclic pressurization (drainage) and depressurization (imbibition) on an annual basis. This is especially true if the underground formation has an active aquifer.