Gassmann Fluid Substitution

Predict how seismic velocities and densities change when pore fluids are substituted. Essential for "what if" scenarios: gas vs. brine, oil vs. water, or CO₂ injection modeling. Based on Gassmann (1951) theory.

Rock Physics Gassmann Fluid Substitution AVO

Input Parameters

Initial Saturated Rock

m/s
m/s
Shear modulus unchanged by fluids
g/cm³
fraction
Typical sandstone: 0.15-0.30

Mineral Matrix

GPa
g/cm³

Initial Fluid (State 1)

GPa
g/cm³

Target Fluid (State 2)

GPa
g/cm³

Results

Theory & References

Gassmann Equation

Ksat = Kdry + (1 - Kdry/Kmin)² / (φ/Kfl + (1-φ)/Kmin - Kdry/Kmin²)

Key Assumptions:

  • Rock is isotropic and homogeneous
  • Pores are interconnected and saturated
  • Frequencies are low (seismic band)
  • No chemical interaction between fluid and rock

Workflow

  1. Back out Kdry: Invert Gassmann using initial saturated state
  2. Forward model: Calculate Ksat2 with new fluid properties
  3. Update density: ρsat = (1-φ)ρmin + φ·ρfl
  4. Calculate velocities: Vp = √[(K + 4/3·G) / ρ]

Note: Shear modulus G is unaffected by pore fluids (Gassmann assumption).