Attenuation of P- and S-waves as a method of distinguishing gas and condensate from oil and water

Abstract

Abstract Compressional- and shear-wave attenuation data were calculated from sonic waveforms in three wells. The results show that at similar porosities and at in-situ conditions, gas and condensate sandstone reservoirs exhibit higher P-wave attenuation (lower P-wave quality factor, Q P ) than either fully-water or partially 'oil + water' saturated sandstones. However, S-wave attenuation and quality factor Q S do not show such a strong dependence on pore fluids. Furthermore, Q P /Q S < 1 indicates presence of gas or condensate, while Q P /Q S - 1 indicates 'full water' or 'oil + water' saturation. These field data are consistent with laboratory data and theoretical predictions by various researchers. The crossovers of P-S-wave attenuation and Q P -Q S quality factors proved useful for distinguishing gas and condensate from oil and water reservoirs. They may be used in conjunction with V P /V S and other logs as an aid in formation evaluation for the detection of gas, condensate, and oil. Several case studies in the literature report high P-wave energy absorption at seismic frequencies below gas and condensate pools. This suggests that it is not unreasonable to assume that this method could also be used in seismic exploration as an aid for distinguishing gas and condensate from oil and water formations.