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

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 sandstones reservoirs exhibit higher P-wave attenuation 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 resevoirs. 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. -Authors