The Five Reservoir Fluids Notes

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Petroleum Engineering 325

Petroleum Production Systems

Week 2 – Class LectureThe Five Reservoir Fluids

Texas A&M University – Spring 2014


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• A good summary ofPetroleum Reservoir Fluidscan be found in the articleat the right

• A pdf of this article can befound on eCampus in theLab section for Week 2

• The 1994 article was

written by Dr. W.D. McCain – a professor in thePetroleum EngineeringDepartment at at Texas

A&M – An expert in Petroleum

Fluids – Author of the book The

Properties of PetroleumFluids


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What Type of Fluid is in My Reservoir

• Reservoir fluid type can be confirmed only by laboratorytesting of the fluid• However, readily available production data usually will

indicate the type of fluid in the reservoir – Initial producing gas-oil ratio (GOR) – Stock-tank liquid gravity – Stock-tank oil color

• Initial producing GOR is most important


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Reservoir Types

• Black Oil• Volatile Oil• Retrograde Gas• Wet Gas

• Dry Gas


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From Laboratory Measurements, Heavy Components inPetroleum Mixtures Have the Strongest Effect on the

Characteristics of Fluids

• Black Oils have many heavy molecules• Volatile Oils contain fewer heavier molecules• Retrograde Gases have even fewer heavy ends• Wet Gases even fewer

• Dry Gas is mostly methane

Black Oils are in the lower right handpart of the graph and Dry Gas is in theupper right corner. Other fluids exist inbetween. Black Oils

Dry Gases


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Phase Diagrams

• Phase diagrams are plots of pressure against temperature that showconditions for which a particular substance will exist as a liquid, gas, orboth

• Phase diagrams show the phases of petroleum fluids in the reservoir

• Bubble Point Pressure, p b , is the

pressure at which the first gas isformed as pressure is reduced atconstant temperature – applies to oilreservoirs

• Dew Point Pressure is the pressure

at which the first liquid is formed aspressure is reduced at constanttemperature – applies to gasreservoirs

• Critical Point is where the properties

of liquid and gas converge


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• Oil Reservoirs have bubble points• Gas Reservoirs have dew points

– Retrograde gas reservoirs that is

• Wet Gas and Dry Gas reservoirs exhibit no phase change in

the reservoir as pressure depletes


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Black Oil• Not necessarily black in color,

but is dark• Line 123 represents the

pressure depletion path atreservoir temperature• If the initial reservoir pressure

is above the bubble point,point 1 on the phase diagramto the right, the oil reservoir isundersaturated – No free gas in the reservoir

• As reservoir pressureproceeds from point 1 to 2, oilremains as a single phase inthe reservoir – Gas is produced at the

surface due to furtherpressure and temperaturedrop through the producing

system getting the oil to thesurface – The GOR at the surface is

constant• As reservoir pressure is

further reduced from 2 to 3,gas now comes out ofsolution in the reservoir andthe oil is now saturated

From: Petroleum Engineer International, April 1994, W.D. McCain


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Black Oil Reservoirs

• Above the bubble point line, only liquid is present in thereservoir – Oil flows through the reservoir as a single phase – Gas may still be produced at the surface along with the oil because

pressure and temperature decrease as the oil is produced to the

surface and the bubble point is crossed somewhere in the process,but after the oil enters the wellbore

• Below the bubble point is when gas is liberated from oil• The bubble point pressure is the pressure at which the

first gas is formed, p b• Reservoirs above the bubble point are called

undersaturated


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Black Oil Reservoirs

• If the initial reservoir pressure is less than or equal to thebubble point pressure OR if the flowing bottomhole pressure(p wf ) is below the bubble point, free gas will form in thereservoir and likely flow in the reservoir.

• When oil and gas are present in the reservoir, this type ofreservoir is known as a two-phase or saturated reservoir


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Volatile Oil


• Similar to a Black Oil, if theinitial reservoir pressure of aVolatile Oil is above thebubble point, point 1 on thephase diagram to the right,the oil reservoir isundersaturated – No free gas in the reservoir

• As reservoir pressure isdepleted below the bubblepoint, large volumes of gasleave solution in the reservoir,which greatly reduces thepermeability to oil such thatthe reservoir flow streambecomes mostly gas

• By the time reservoir

pressure reaches point 3, theeffective permeability to oilbecomes near zero and theflow stream in the reservoir isessentially all gas

• Volatile Oils are sometimescalled high-shrinkage oils


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Black Oils vs. Volatile Oils

• When above the bubble point pressure, both are liquids in the reservoir – no free gas in the reservoir

• Both exhibit bubble points as reservoir pressure is decreased duringproduction

• Both release gas in the reservoir pore space at pressures below the

bubble point• Phase diagrams look similar, so what’s the difference between a BlackOil and Volatile Oil

BlackOil

VolatileOil


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• Gas that comes out of solution istypically a dry gas

• Producing GOR < 1,750 scf/STB• Stock tank liquid gravity 1,750 scf/STB• Stock tank liquid gravity >40°API

• Color of stock tank liquid is ofsome color – Brown, reddish, orange, or green

• Stock tank liquid gravityincreases with time

Black Oil Volatile Oil


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• Gas that comes out of solution istypically a dry gas – What do we know about dry gases –

they don’t produce any liquids aspressure and temperature decrease

• Dry gas moves through the reservoiras a second phase, so we have 2-phase flow in the reservoir

• Above the bubble point pressure,GOR and °API remain fairly constant

• Below bubble point pressure, as gasevolves out of solution over time – GOR increases with time – °API gravity decreases with time

• Lighter ends are removed from the oil

• Gas that comes out of solution istypically a retrograde gas – What do we know about retrograde

gases – they do release additionalcondensate at the surface

• With the additional condensate – °API increases over time• As reservoir pressure decreases

below the bubble point and gas isreleased in the reservoir, thepermeability to oil get very small andmuch of the liquids produced at thesurface (gas and condensate) arefrom the retrograde gas

Black Oil Volatile Oil


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Retrograde Gas


• Initial a gas in the reservoir, asthe pressure depletion path inthe reservoir (line 123) goesthrough a dew point

• Continuing from point 2 to point3 (below the dew point),condensate appears in thereservoir pore spaces

• Throughout most of thereservoir, the permeability tothis condensate is near zero, somuch of this condensate nevergets produced – Near the wellbore, the velocity

of the gas may be high enoughthat some of the condensatenear the wellbore may beproduced


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Wet Gas• Similar to a Retrograde Gas, a

Wet Gas is a gas in thereservoir

• Unlike a Retrograde Gas, as thereservoir pressure is depleted,the reservoir gas does not gothrough a dew point in thereservoir, therefore throughoutthe life of the reservoir,condensate does not fall out in

the reservoir • As the gas is produced up the

tubing and through the surfacefacilities, pressure andtemperature are reduced andcondensate is produced

• The word “Wet” gas refers tohydrocarbon liquids, whichcondense at the surfce, notwater


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Dry Gas


• Dry gas is almost all methane• The two-phase envelope is

small and lies below thereservoir and surface conditions

• The fluid is solely gas in thereservoir and at the surface

• Condensate is not produced• The word “Dry” gas refers to

hydrocarbon liquid

– Quite often, liquid water iscondensed at the surface aspressure and temperatures arereduced


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Retrograde Gases, Wet Gases, and Dry Gases

• All are initially a gas in the reservoir • Retrograde Gases exhibit dew points as reservoir pressureis reduced and release increasing volumes of liquidcondensate into the pore space as pressure is reducedbelow the dew point – This condensate seldom flows and is lost to production


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Retrograde Gases, Wet Gases, and Dry Gases

• All are initially a gas in the reservoir • Wet Gases and Dry Gases remain gaseous in the reservoirthroughout depletion – neither releases condensate into thereservoir – The difference between the two is Wet Gases release condensate

as pressure and temperature are reduced to separator conditions• In other words, as gas is produced up the tubing and into the surface facilities,

condensate “falls out”

– Dry Gases do not produce any condensate• NOTE: in this context, “wet” and “dry” does not refer to the presence or

absence of water or water vapor. Water is always present in petroleumreservoir, and all gases normally are saturated with water vapor


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Identify Reservoir Fluid Type withField Information



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Identify Reservoir Fluid Type withLaboratory Information



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Generalized Producing Trends for theFive Reservoir Fluid Types



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The Five Reservoir Fluids as Defined by their Initial Producing GOR and C7+ Content

From: Petroleum Engineer International April 1994 W D McCain

The Five Reservoir Fluids Notes

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