Disclaimer: Not to be confused with Compressibility factor $\begin{array}{l}Z\end{array}$ .

A measure of resistance of a Continuum body to compression/decompression.

A measure of relative change in density $\begin{array}{l}\rho\end{array}$ or molar volume $\begin{array}{l}V_m\end{array}$ under a unit pressure $\begin{array}{l}p\end{array}$ variation:

(1)	$\begin{array}{l}\displaystyle c = \frac{1}{\rho} \left( \frac{\partial \rho}{\partial p} \right) = - \frac{1}{V_m} \left( \frac{\partial V_m}{\partial p} \right)\end{array}$

Symbol	Dimension	SI units	Oil metric units	Oil field units
$\begin{array}{l}c\end{array}$	M^-1 L¹T²	Pa^-1	kPa^-1	psi^-1

Compressibility depends on the thermodynamic conditions at which it is measured and as such is not a material property.

The two major medium compression/decompression processes are isothermal and isentropic which result in different values of compressibility:

Isothermal Compressibility

Isentropic Compressibility

$\begin{array}{l}T = \rm const\end{array}$

$\begin{array}{l}S = \rm const\end{array}$

(2)	$\begin{array}{l}\displaystyle c_T = \frac{1}{\rho} \left( \frac{\partial \rho}{\partial p} \right)_T\end{array}$

(3)	$\begin{array}{l}\displaystyle c_S = \frac{1}{\rho} \left( \frac{\partial \rho}{\partial p} \right)_S\end{array}$

Both $\begin{array}{l}c_T\end{array}$ and $\begin{array}{l}c_S\end{array}$ are not dependent on the amount of chemical substance and defined under a clear conditions of thermodynamic process and as such are the material properties and properly tabulated for the vast majority of materials.

In engineering practise, when the term Compressibility is used as material property it normally means Isothermal Compressibility: $\begin{array}{l}c=c_T\end{array}$ .

Compressibility is related to Z-factor $\begin{array}{l}Z\end{array}$ and Formation Volume Factor (FVF) $\begin{array}{l}B\end{array}$ as:

(4)	$\begin{array}{l}\displaystyle c(p) = \frac{1}{p} - \frac{1}{Z} \frac{dZ}{dp}\end{array}$

(5)	$\begin{array}{l}\displaystyle c(p) = - \frac{1}{B} \frac{dB}{dp}\end{array}$

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See also