Oil Properties Notes

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Oil gravity is a measure of the density of an oil relative to water. Oil gravity must be between 15°API and 59°API.

Gas gravity is a measure of the density of a gas relative to air. Gas gravity is usually based on a gas composition analysis. Gas gravity must be between 0.55 and 1.10.

Solution GOR is a measure of how much gas is evolved from oil (at or above the bubble point pressure) as it goes from reservoir pressure and temperature to separator conditions. Units are standard cubic feet of gas per barrel of oil (SCF/STB). The is no program limitation on solution GOR. Realistically GOR should not be above 5,000 (SCF/STB)

Separator pressure is the operating pressure of the surface facilities. Separator pressure must be between 30 psia and 535 psia.

Separator temperature is the operating temperature of the surface facilities. Separator temperature must be between 76°F and 150°F.

Reservoir pressure is the average pressure in the reservoir. This value can be determined from pressure build up test, shut in bottom hole pressure or it can be estimated from mud weights during drilling. Bottom hole pressure can be estimated by the following formula: Bottom Hole Pressure (psig)= True Vertical Depth (ft) * Mud weight (ppg) *0.052 Reservoir pressure must be between 15 psia and 9,458 psia.

Reservoir temperature is the average temperature in the reservoir. This value can be estimated from a well log header. Reservoir temperature can also be measured during a shut in bottom hole pressure measurement. Reservoir temperature must be between 70°F and 295°F.

Oil volume factor is defined as the ratio of the volume of oil at reservoir conditions to the volume of oil at stock tank conditions. Units are reservoir barrels per stock tank barrel (RB/STB).

Gas oil ratio is a measure of the volume of gas evolved from one barrel of oil as it goes from reservoir to surface conditions. This GOR only deals with the gas that is in solution not free gas in the reservoir that is produced along with the oil. GOR should not be confused with Producing GOR or Solution GOR. Solution GOR is the GOR at or above the bubble point pressure. Producing GOR includes the gas evolved from the oil as well as free gas produced from the reservoir. Units are standard cubic feet per stock tank barrel (SCF/STB).

Oil viscosity is a measure of the oils resistance to flow. Units are centipoise (cp).

Oil compressibility estimates the change in volume with pressure for oil under conditions of constant temperature. Units are 10-6psi-1.

The bubble point is the pressure at which an oil will begin to evolve gas. Above this pressure the oil will exist as a single fluid phase in the reservoir. Below the bubble point pressure the gas will begin to evolve from the oil in the reservoir. Units are pounds per square inch absolute (psia).


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Last updated 08/20/04