PETE 310-Lecture 4

Fluids which are deficient in intermediate components have critical points to the ________-hand side of the envelope

right

Larger Comoponent's critical points are __________ than smaller component's critical points

larger

The Five Reservoir Fluids

Black oil, volatile oil, retrograde gas, wet gas, and dry gas

Black Oils

Initial producing GOR < 2000 scf/STB

When the reservoir pressure falls below the bubble point pressure, the producing GOR increase

Stock-tank oil gravity < 45 API

often very dark (often black), indicating the presence of heavy hydrocarbons

Laboratory analysis indicate an initial oil formation volume factor (FVF) < 2.0 res bbl/STB (oil FVF is the volume of reservoir liquid in barrels to produce one stock-tank barrel)

Consist of a wide variety of chemical species. Contains higher than 20% (mole/mole) heptanes plus (C7+) which indicates large quantity of heavy, nonvolatile hydrocarbons

Phase Diagram of Typical Black Oil

phase diagram covers a wide temperature range

critical point is well up the slope of the phase envelope

iso-vols are spaced evenly within the envelope

When reservoir pressure lies along line 12, the oil is called "undersaturated"

When the reservoir pressure lies along line 23, the oil is called "saturated"

Additional gas evolves from the oil as it moves from the reservoir to the surface separator. This causes some oil shrinkage. The volume of oil at point 2 shrinks by one-half or less on its trip to the stock-tank

Undersaturated Oil

used to indicate that oil could have dissolved more, if more gas had been present in the reservoir

Saturated Oil

During oil production, when the reservoir reaches its bubble point pressure, the oil contains as much dissolved gas as it can hold

Further reduction in pressure will release gas to form a free gas phase in the reservoir

Volatile Oil

2000 < initial producing GOR < 3300 scf/STB

Stock-tank oil > 40 API and it increases during the production, as reservoir pressure falls below the bubble point

Contains relatively fewer heavy oil molecules and more intermediates (C2-C6) in comparison to black oil

Gas associated with the volatile oil is very rich and releases large quantity of liquid as it moves to the surface

The material balance equations have intolerable inaccuracy

Laboratory analysis indicate an initial oil formation volume factor (FVF) > 2.0 res bbl/STB. They are high-shrinkage crude oils; the produced oil shrinks by more than one-half on the trip to stock-tank

Contains 12.5-20% hepatnes plus

Phase Diagram of Typical Volatile Oil

covered temperature range is somewhat smaller than the black oil case

iso-vols are not as evenly spaced

critical temperature is much lower (to the left) than for a black oil: and is close to the reservoir temperature

Retrograde Gases

are also called gas condensates or condensates

initial producing GOR>3300 scf/STB. An initial GOR ratio less than 5000 scf/STB indicates a very rich retrograde gas, one which will condense sufficient liquid to fill 35% or more of the reservoir volume. However, the liquid would seldome flow in the reservoir; therefore, the retrograde liquids cannot be easly produced

60 API stock-tank oil gravity (>> 40 API) and it increases during the production, as reservoir pressure falls below the dew point pressure

Phase Diagram of Typical Retrograde Gas

phase diagram is smaller than that for oils

contains fewer heavy hydrocarbons relative to oils, hence the critical point is further down the left-hand side of the envelope

phase diagram has a critical temperature which is less than the reservoir temperature, and a cricondentherm which is greater than the reservoir temperature

Wet Gases

the word "wet" does not mean that the gas is wet with water; it refers to the hydrocarbon liquid which condenses at surface conditions

Wet gases produce stock-tank liquids with the same range of gravities as the liquids from retrograde gases. The stock-tank liquid usually looks like water

producing GOR is very high (more than 50000 scf/STB) and remain constant during the life of the wet gas reservoir

Phase Diagram of Typical Wet Gases

Entire phase diagram lies below the reservoir temperature. The reservoir pressure path, during the production, does not enter the phase envelope, therefore no liquid is formed in the reservoir. However notice that the separator conditions lie within the phase envelope, causing some liquid to be formed as the surface

Dry Gases

primarily methane with some intermediates

the word "dry" indicates that the gas does not contain enough of the heavier molecules to form hydrocarbon liquid at the surface