Attenuation methods for quantifying gas saturation in organic-rich shale and tight gas formations

Quantitative interpretation of waveform attenuation for determining petrophysical properties remains one of the most challenging problems associated with rock physics. In this study, two effective methods are proposed to compute gas saturation in organic-rich shale and tight gas formations from full-waveform sonic attenuations. We first extract compressional (P)- and shear (S)-wave attenuations from monopole and dipole waveforms by median frequency shift and multi-frequency inversion methods, respectively. Crossplots of the P- to S-wave attenuation ratio (Q(P)(-1)/Q(S)(-1)) and core gas saturation show a positive linear correlation. The Q(P)(-1)/Q(S)(-1) value and the neutron-density porosity difference exhibit an identical log trend across different formations. The coincidence of these two different hydrocarbon indicators implies that Q(P)(-1)/Q(S)(-1) is most sensitive to pore-fluid saturation and is less affected by variations in lithology. In the first method, the core-calibrated Q(P)(-1)/Q(S)(-1) yields an accurate estimate of gas saturation. The second method is suited for the absence of core saturation data, which use the probability distribution of Q(P)(-1)/Q(S)(-1) for the evaluation of gas saturation. Compared to conventional resistivity methods, the proposed attenuation method, as a nonelectric approach, provides a more accurate gas saturation prediction for low-resistivity reservoir rocks. Finally, we analyze the characteristics of attenuation-saturation relations in low-porosity rocks and discover the possible attenuation mechanisms.