Changes

The two most frequently used methods of estimating porosity from sonic measurements are the Wyllie time average method and the Raymer-Hunt-Gardner field observation method. Both methods require determining the sonic response (Δ''t''₁) at the depth of interest and determining the response associated with the matrix (Δ''t''_ma). Typical values for Δ''t''_ma are 55.5 μsec/ft for sandstone, 47.5 μsec/ft for limestone, and 43.5 μsec/ft for dolomite. (A more complete listing of interval traveltimes for common minerals is given in the chapter on [[Difficult lithologies]] in Part 4.)

The two most frequently used methods of estimating porosity from sonic measurements are the Wyllie time average method and the Raymer-Hunt-Gardner field observation method. Both methods require determining the sonic response (Δ''t''₁) at the depth of interest and determining the response associated with the matrix (Δ''t''_ma). Typical values for Δ''t''_ma are 55.5 μsec/ft for sandstone, 47.5 μsec/ft for limestone, and 43.5 μsec/ft for dolomite. (A more complete listing of interval traveltimes for common minerals is given in the chapter on [[Difficult lithologies]] in Part 4.)

The ''Wyllie time average method'' requires an estimate of fluid interval transit time (Δ''t''_fl). Typical values range from 189 μsec/ft for saltwater to 204 μsec/ft for freshwater. This method tends to overestimate porosity in uncompacted sandstones and hydrocarbon-bearing reservoirs. Empirical corrections to lessen this error can be implemented using the compaction factor (Cp) and hydrocarbon correction (Hy) terms. Cp describes the influence of pore pressure on the sonic porosity equation. It is normally estimated from comparison of density and apparent sonic porosity or from the sonic response in nearby shale (Cp = Δ''t''_sh/100.0). Hy is approximate and is set equal to 0.9 for oil and 0.7 for gas. The Wyllie time average equation is

The ''Wyllie time average method'' requires an estimate of fluid [[interval transit time]] (Δ''t''_fl). Typical values range from 189 μsec/ft for saltwater to 204 μsec/ft for freshwater. This method tends to overestimate porosity in uncompacted sandstones and hydrocarbon-bearing reservoirs. Empirical corrections to lessen this error can be implemented using the compaction factor (Cp) and hydrocarbon correction (Hy) terms. Cp describes the influence of pore pressure on the sonic porosity equation. It is normally estimated from comparison of density and apparent sonic porosity or from the sonic response in nearby shale (Cp = Δ''t''_sh/100.0). Hy is approximate and is set equal to 0.9 for oil and 0.7 for gas. The Wyllie time average equation is

:<math>\phi_{\rm s} = \frac{\Delta t_{\rm l} - \Delta t_{\rm ma}}{\Delta t_{\rm fl} - \Delta t_{\rm ma}} \frac{\mbox{Hy}}{\mbox{Cp}}</math>

:<math>\phi_{\rm s} = \frac{\Delta t_{\rm l} - \Delta t_{\rm ma}}{\Delta t_{\rm fl} - \Delta t_{\rm ma}} \frac{\mbox{Hy}}{\mbox{Cp}}</math>