BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
Condensates can exist as a very light liquid phase in a gas accumulation. It is a liquid phase
that develops from condensation of the heavier components as pressure is reduced during
production (or, in small reservoirs, during drilling and/or kick occurrence). The phases are
explained in the following P A – P D demonstration.
Figure 6 - 7
Gas Condensates
During expansion of the compressed gas when produced, condensate forms a drop of liquid
from the heavier hydrocarbon compounds. This point is called the “Dew Point” of a reservoir.
The condensate liquid will reach a maximum depending on the pressure reductions, but then
after further reduction, will return to another droplet within the system. This is referred to as
the dew point pressure at the reservoir. Any further reduction of pressure after the dew point
pressure will result in a single phase gas system with gas expansion in proportion to pressure
reduction.
Any reservoir can be, as with oil reservoirs, anywhere between point P A and P D . With
conditions at or below P C , the reservoir would be called a normal condensate reservoir. If
conditions are above the P C stage, the term retrograde condensate reservoir is applied. This
term implies an increase in liquid (condensate) with a decrease in pressure, rather than a
decrease or evaporation under normal conditions.
The viscosity of a gas condensate is normally 0.05 cP or less, but can be as high as 0.5 cP; with
a specific gravity of 0.7 to 0.78.
4. Dry Gas Reservoir – This refers to a system that contains only light hydrocarbons which exist
only in the vapor phase or gas phase under all reservoir conditions.
A system consisting of only dry gas is usually composed of up to 98% by volume methane. As
a result, such reservoirs show an increase in specific volumes due to reduction of reservoir
pressures, in agreement with the laws of expansion of compressible fluids.
Physical Properties of Reservoir Fluids
Petroleum, or more commonly, oil, has several physical properties. They are; density, viscosity,
shrinkage, and gas in solution.
Density is defined as the weight of oil per unit of volume and is expressed at a particular
temperature and pressure. To eliminate the different units used, the petroleum industry has
generally adopted the API Unit as the measurement of density. The API unit is derived as follows:
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-8