Gamma-ray absorption method of measuring gas saturation and its application to three-phase relative permeability studies

Interest in the three-phase relative permeability-saturation relation for porous media has been nearly dormant from the time Leverett and Lewis published their classic work in 1941. The advent of novel recovery methods such as forward combustion and hot fluid injection has generated an acute need for relative permeability data for three-phase flow. Although numerous methods are available for determining laboratory relative permeabilities for two-phase systems, only the capillary method and the steady-state dynamic method may be used for obtaining three-phase relative permeability data. The steady-state method is the less complicated of the two methods and is adaptable to routine laboratory work. The purpose of the present study was to adapt the method of gamma-radiation absorption commonly used in soil densitometry to the measurement of gas saturation in a porous medium and to obtain the three-phase relative permeability-saturation relation for a drainage process in a consolidated sandstone. Oil, water, and gas were flowed at steady conditions through Torpedo sandstone cores. Gas saturations were determined by gamma-radiation absorption and water saturations by electrical conductivity method. Pressure and saturation measurements were made in a section of the core devoid of boundary effects. The effect of wettability was limited by using each core only twice, first for the gamma-ray and conductivity calibrations and second for the two- and three-phase drainage flow tests, where the water saturation was held essentially constant. The saturation ranges studied were 33% to 74% for water, 26% to 67% for oil, and from zero to a maximum of 42% for gas in some of the cores. The three-phase relative permeability to water was found to depend only on the water saturation. The oil isoperms are convex towards the 100% oil saturation point, indicating a dependence of the three-phase oil relative permeability on all three fluid saturations. Relative permeability to gas in three-phase flow was also found to be dependent on all three fluid saturations. In contrast to the convex shape reported by several investigators, the gas isoperms are concave towards the 100% gas saturation apex. In addition, the gas isoperms exhibit considerably greater asymmetry than published results