Multi-phase fluid modulus

$$\frac{1}{\kappa_f (P,T)}= \sum_{i}\frac{S_i}{\kappa_i(P,T)}$$

Fluid bulk modulus of a pore filling fluid it a harmonic sum of it's components. The bulk modulus of any given fluid is a function of Pressure P and temperature T. Si is the volume fraction of fluid i in the pore space (saturation), where $$ \sum_{i}S_i=1 $$