# Multi-phase fluid modulus

${\displaystyle {\frac {1}{\kappa _{f}(P,T)}}=\sum _{i}{\frac {S_{i}}{\kappa _{i}(P,T)}}}$
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 ${\displaystyle \sum _{i}S_{i}=1}$