Isothermal compressibility coefficients are required in solving many reservoir engineering problems, including transient fluid flow problems, and they are also required in the determination of the physical properties of the undersaturated crude oil.

By definition, the isothermal compressibility of a substance is defined mathematically by the following expressions:

- In terms of fluid volume:

- In terms of fluid density:

where V and ρ are the volume and density of the fluid, respectively.

For a crude oil system, the isothermal compressibility coefficient of the oil phase co is defined for pressures above the bubble-point by one of the following equivalent expressions:

where

- co = isothermal compressibility, 1/psi
- ρo = oil density lb/ft^3
- Bo = oil formation volume factor, bbl/STB

At pressures below the bubble-point pressure, the oil compressibility is defined as:

where B

g = gas formation volume factor, bbl/scf

Ahmed (2001)