Effect Of Brine-gas Mixture On Velocity In An Unconsolidated Sand Reservoir

Abstract

Gas in an unconsolidated sand reservoir encased in shale often results in a dramatic increase in amplitude of the seismic reflection from the shale/gas-sand interface. Unfortunately, reflection amplitude appears not to vary linearly with water (brine) saturation, and thus cannot be used to estimate gas quantity. Previously presented theoretical velocity computations, which demonstrate that compressional-wave velocity in an unconsolidated gas sand varies non-linearly with water saturation, qualitatively agree with laboratory velocity measurements on a sand specimen composed of pure quartz grains. However, significant departure of measured and theoretical velocities at high water saturation indicates that assumptions for computing compressibility of the water-gas mixture are no longer valid.