The Effect of Overburden Pressure on Relative Permeability

Abstract

Laboratory relative permeability data on reservoir rock are obtained onsamples which are not subjected to overburden pressure during the permeabilitymeasurements. These data are then used for calculating the performancecharacteristics of reservoirs in which the rock is under the pressure of theoverburden. Recent observations have shown that the permeability and porosityof reservoir sandstones are decreased by application of overburden pressure.These observations led us to a study of the effect of overburden pressure onthe relative permeability characteristics of sandstones. A previous note from this laboratory showed that overburden pressure, simulated by applying hydraulic pressure to the outside of a plastic mountedcore, caused a reduction in permeability of the core by as much as 50 per centat 10,000 psig. In a multi phase fluid flow system, the relative permeabilityat a given saturation will change if the permeability or the effectivepermeability changes but not if both permeability and effective permeabilitychange by the same ratio. The tests described in this note were made todetermine the effect of overburden pressure on permeability and effective gaspermeability in the gas-oil system. The gas-oil system was chosen because theavailable equipment was best suited for studying this system. The resultantdata can be applied to other systems of interest because the gas permeabilityin the gas-oil system is the permeability of the nonwetting phase and, as such, is equivalent to the gas permeability in the gas-oil-water system and to theoil permeability in the oil-water system. Cylindrical test plugs, one in. in diameter and one in. long, were cut fromsandstone core samples from three different reservoirs. After extraction intoluene, the plugs were mounted in Lucite by conventional core mountingmethods. The core properties are given in the legend in Fig. 1. The mountedcores were machined to fit the apparatus shown in Fig 4. Air permeabilities asa function of overburden pressure were measured by connecting lines (1) and (2)to a conventional air permeameter. T.N. 194