Bulk modulus:

*Material, ...
*Bulk modulus`
<bulk modulus 1> [, <bulk modulus 2>]

This keyword is used to define the bulk modulus of the pore fluid phases. For a two-phase porous medium, the bulk modulus of the pore water has to be given, for a three-phase porous medium, the bulk modulus of the second pore fluid (e.g. air) has to be provided in addition (in this order).