Petrophysicists and geophysicists do “fluid substitution” to estimate the density and sonic velocity of porous rocks at saturation conditions such as fully hydrocarbon-saturated or “fizz-bearing” with fluids such as gas, condensate, and oil. However, the Gassmann model implementation in many commercial packages sometimes yields implausible results for the compressional sonic velocity in laminated sand-shale sequences. In particular, the counterintuitive phenomenon of fluid effects increasing with computed shale fraction is often observed, as in the gas-bearing interval of a well drilled with oil-based mud shown in Figure 1. DTC FLUSUB (Gassmann fluid substitution) is the result of substituting DTC WET...

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