The Gassmann Equation, first published in the 1950?s, relates compressional acoustic velocity to elastic moduli of the porous fluid filled rock and empty rock frame, bulk moduli of the empty rock frame, solids and fluids, density and porosity. From these parameters, compressional velocities can be determined.

Krief (1987 approaches the same rock physics model differently. In addition to elastic moduli of solids and fluids, the model incorporates shear moduli of the solids and the Biot compressibility constants. Thus, both compressional and shear velocities are available from Krief, whereas only compressional velocities are derived from Gassmann. Both models allow for predictions of velocity variations as fluids change (gas vs. water.

In high porosity soft rock (porosity values of 40% or greater both models give comparable results ? velocity slowing in the presence of gas. However, at lower porosities, and particularly for rocks with less than 20% porosity, the Gassmann model predicts a much larger velocity slowing effect than does Krief.

The Krief model is a comprehensive solution to velocity properties of rocks over the complete range of lithologies and porosity. Good agreements between predicted and measured compressional and shear velocities is attained.

Examples from both clastic and carbonate reservoirs, using both models, are presented.

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