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ROCK PHYSICS MODELING

Rock physics provides the link between reservoir properties and the seismic properties that can be observed with a geophysical survey. The reservoir parameters of importance are lithology, porosity, permeability, pore fluid types and saturations, temperature, reservoir effective stress, and pore fluid pressure. The related bulk properties that most impact seismic wave propagation are bulk density, and P- and S-wave velocities.

Bulk density is the easiest to understand and quantify. Density is the simple volumetric average of all mineral (solid) and pore fluid (solid, liquid, and gas) phases in the rocks. Density changes during reservoir production are primarily the result of replacing the initial set of fluids with another set.

Seismic velocity is always related to the ratio of elastic stiffness of the fluid-bearing rocks to the bulk density of the rocks. The elastic stiffness depends on the mineralogy, the stiffness of the pore space, and the stiffness (incompressibility) Kf of the mixture of pore fluids in the rock. This is most effectively quantified with the well-known Gassmann's equation, which relates the bulk modulus of the saturated rock Ksat to the dry rock bulk modulus Kdry, the mineral modulus K0, the fluid incompressibility, and the porosity $\phi $:

\begin{displaymath}
\frac{K_{sat}}{K_0 - K_{sat}} = \frac{K_{dry}}{K_0 - K_{dry}} +
\frac{K_{f}}{\phi (K_0 - K_{f})}\end{displaymath} (4)

Important issues when doing fluid substitution:

The fluid densities and incompressibilities used in the modeling were obtained from PetroTools Version 2.3, the seismic rock properties software by PetroSoft.



 
previous up next print clean
Next: Velocity-porosity regression Up: Geophysics-PE: Reservoir monitoring Previous: Memory and Timing
Stanford Exploration Project
11/12/1997