BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
First the Specific Gravity (SG) is obtained by:
SG =
Density of Oil
Density of Water
The temperature at which the measurement is taken should also be noted, i.e. at 60°F for both.
141.5
° API = – 131.5
SG
or
SG
=
141.5
131.5 + ° API
Therefore, if the SG is 1.0 for water, the °API value would be 10°. Oil with an °API value greater
than 10° would be lighter than water.
The previously described reservoir fluids have the following ranges API gravity and Specific
Gravity.
Type of Fluid SG °API
Crude Oil 0.82 – 1.07 41.1 – 0.7
Volatile Oil 0.78 – 0.85 49.9 – 35.0
Gas/Condensate 0.70 – 0.78 70.6 – 49.9
Fresh Water 1.00 10.0
It can be seen that lighter mixtures have the lower density, SG, and that SG is a function of
composition.
°API gravity indicates the market value of the oil. The higher the ºAPI gravity, the more valuable is
the oil. This is due to lighter oil containing more valuable products, such as gasoline.
°API gravity is measured with a calibrated hydrometer reading density, specific gravity or °API
gravity directly.
Viscosity is basically the physical property which influences the ability of a fluid to flow. Flow rate
is inversely proportional to fluid viscosity. A high pressure is needed to displace very viscous oil.
Viscosity (μ), like density (ρ), is a function of fluid composition, pressure and temperature. The
unit used for viscosity is centipoises (cP).
1 Poise = force in dynes/cm 2 to move a fluid at a rate of 1 cm/sec through an area of 1 cm 2 ; as a
reference water has a viscosity of 1 cp at 68°F or 20°C.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-9