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, for a Tertiary sedimentary section, which demonstrate that compressional-wave velocity in an unconsolidated gas sand varies nonlinearly with brine 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 brine saturation indicates that the technique for partially saturating the sand specimen by flowing a gas-brine mixture through the specimen does not provide a sufficiently uniform distribution. The gas preferentially seeks larger pores. In a subsequent experiment on a specimen composed of spherical glass beads of nearly uniform size, the previous, as well as a modified, fluid injection technique was used. For the latter, brine only was injected into the pore space previously filled with a mixture of gas and brine in nearly equal proportions. This resulted in a more uniform distribution of the gas-brine mixture. For approximately equal brine saturations, this modified technique resulted in a measured compressional-wave velocity approximately one-half of the velocity measured for the previously used fluid injection technique. This result implies that if the gas-brine mixture is uniformly distributed in a reservoir, the fluid compressibility is the weighted-by-volume average of the constituent compressibilities.