BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
If only liquid (oil) is present, P A , and reservoir pressure is reduced during production, the
specific volume of oil increases. Then, at some different pressure, P B , a bubble of free gas
appears. This stage is called the “Bubble Point”. Further reduction of reservoir pressure, P C and
P D , causes a change to occur. First, the quantity of gas escaping or separating from the oil
increases continuously; second, the specific volume of gas increases; and third, the specific
volume of reservoir oil decreases with pressure reduction below the bubble point, P B .
The term used for the escape of gas from the reservoir oil is referred to as “gas coming out of
solution”. Basically, the gas is just evaporation of the lighter hydrocarbon compounds; mainly
methane and ethane.
Reservoirs themselves can occur at any stage within the P A – P D system and are related to
pressure and temperature existing within the reservoir section.
If conditions are below the bubble point, geological time would normally have separated the
gas from the oil, and the gas will be situated above the oil section in the form of a gas cap in
the reservoir.
Figure 6 - 5
Oil Reservoir with Gas Cap
2. Volatile Crude Oils – These are very similar to the above described oils, but are generally
lighter in weight and color, lower in viscosity, and have a larger volume change with pressure.
This is entirely due to the volatile aspect, i.e. gaseous solution, therefore, higher
compressibility under higher pressure.
Average viscosity is normally 0.78 to 0.85 cP. Changes due to pressure changes are
demonstrated with the same P A – P D method.
Figure 6 - 6
Volatile Crude Oil
The difference in behavior is that the larger amounts of gas (free gas) undergo much more
shrinkage (compressibility) as pressure declines below the bubble point.
3. Gas Condensates – These are hydrocarbon liquids mixed with natural gas instead of crude oils.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-7