BAKER HUGHES - Drilling Fluids Reference Manual
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BAKER HUGHES - Drilling Fluids Reference Manual

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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

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

RESERVOIR APPLICATION FLUIDS<br />

First the Specific Gravity (SG) is obtained by:<br />

SG =<br />

Density of Oil<br />

Density of Water<br />

The temperature at which the measurement is taken should also be noted, i.e. at 60°F for both.<br />

141.5<br />

° API = – 131.5<br />

SG<br />

or<br />

SG<br />

=<br />

141.5<br />

131.5 + ° API<br />

Therefore, if the SG is 1.0 for water, the °API value would be 10°. Oil with an °API value greater<br />

than 10° would be lighter than water.<br />

The previously described reservoir fluids have the following ranges API gravity and Specific<br />

Gravity.<br />

Type of Fluid SG °API<br />

Crude Oil 0.82 – 1.07 41.1 – 0.7<br />

Volatile Oil 0.78 – 0.85 49.9 – 35.0<br />

Gas/Condensate 0.70 – 0.78 70.6 – 49.9<br />

Fresh Water 1.00 10.0<br />

It can be seen that lighter mixtures have the lower density, SG, and that SG is a function of<br />

composition.<br />

°API gravity indicates the market value of the oil. The higher the ºAPI gravity, the more valuable is<br />

the oil. This is due to lighter oil containing more valuable products, such as gasoline.<br />

°API gravity is measured with a calibrated hydrometer reading density, specific gravity or °API<br />

gravity directly.<br />

Viscosity is basically the physical property which influences the ability of a fluid to flow. Flow rate<br />

is inversely proportional to fluid viscosity. A high pressure is needed to displace very viscous oil.<br />

Viscosity (μ), like density (ρ), is a function of fluid composition, pressure and temperature. The<br />

unit used for viscosity is centipoises (cP).<br />

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<br />

reference water has a viscosity of 1 cp at 68°F or 20°C.<br />

<strong>BAKER</strong> <strong>HUGHES</strong> DRILLING FLUIDS<br />

REFERENCE MANUAL<br />

REVISION 2006 6-9

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