A measure of relative change in density \rho or molar volume V_m under a unit pressure p variation:
(1) | \beta = \frac{1}{\rho} \left( \frac{\partial \rho}{\partial p} \right) = - \frac{1}{V_m} \left( \frac{\partial V_m}{\partial p} \right) |
Symbol | Dimension | SI units | Oil metric units | Oil field units |
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\beta or c | M-1 L1 T2 | Pa-1 | kPa-1 | psi-1 |
It measures its resistance of Continuum body to compression/decompression.
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 | ||||
---|---|---|---|---|---|
T = \rm const | S = \rm const | ||||
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Both
\beta_T and
\beta_S 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: \beta=\beta_T.
Compressibility is related to Z-factor
Z and Formation Volume Factor (FVF)
B as:
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Disclaimer
In Thermodynamics the compressibility is denoted by
\beta while intensive heat capacities are denoted by
c with corresponding subscript.
On the other hand Petroleum Industry is traditionally using c symbol to denote compressibility which often lead to confusion with heat capacity.
See also
Physics / Mechanics / Continuum mechanics / Continuum body
[ Isothermal Compressibility ][ Isentropic Compressibility ]
[Fluid compressibility] [Pore compressibility] [Total compressibility]