Physics of Oil and Gas Reservoirs

Physics of Oil and Gas Reservoirs The Five Petroleum Reservoir Fluids

Khazar University / Petroleum Engineering

Rovshan JAVADZADE

Reservoir Engineer

[email protected] 12-Oct-14 Week 4

Agenda

Introduction

The Five Petroleum Reservoir Fluids

Black oil

Volatile oil

Retrograde condensate gas

Gas cycling

Wet gas

Dry gas

Properties of Black oil

Properties of Volatile oil

Properties of Dry gas

Properties of Wet gas

12-Oct-14 Physics of Oil and Gas Reservoirs 2


Introduction


Black oil

Volatile oil


Gas cycling

Wet gas

Dry gas





Introduction

The behavior of a reservoir fluid during production is determined by the shape of its phase diagram and the position of its critical point


Relative positions of phases envelopes

Multi-component hydrocarbon


Phase Diagrams for Multicomponent Systems

Reservoir fluids

There are five types of reservoir fluids:

Low-shrinkage oil (heavy oil - black oil)

High-shrinkage oil (volatile oil)


Wet gas

Dry Gas


Phase diagram for reservoir fluids


Phase diagram for reservoir fluids


Introduction


Black oil

Volatile oil


Gas cycling

Wet gas

Dry gas





Black Oil

Black oils consist of components with large,

heavy and nonvolatile

molecules

The phase diagram for Black oils covers a wide

temperature range and

the critical point is well

up the slope of the

phase envelope


Phase diagram for a Black Oil

Field Identification of Black Oil

Black oils are characterized with producing gas-oil ratios of 500 scf/STB or less

The stock-tank oil usually will have a gravity below 30 API or heavier

The stock-tank oil is very dark, indicating the presence of heavy hydrocarbons, often black, sometimes with a greenish cast, or brown

Laboratory analysis will indicate an initial oil formation volume factor of 2.0 res bbl/STB or less

Laboratory determined composition of heptanes plus will be higher than 30 mole percent, an indication of the large quantity of heavy hydrocarbons in black oils



Introduction


Black oil

Volatile oil


Gas cycling

Wet gas

Dry gas





Volatile Oil

Volatile oils contain relatively fewer heavy

molecules and more

intermediates (defined

as ethane through

hexanes) than black oils


Phase diagram for a Volatile Oil

The critical temperature of volatile oils is greater than its reservoir temperature

Volatile oils are characterized with producing gas-oil ratios of less than 8000 scf/STB

The stock-tank oil gravity is usually 40 API or higher

The stock-tank oil is colored: usually brown, orange, or sometimes green

According to laboratory analysis, initial oil formation volume factor is greater than 2.0 res bbl/STB

Compositions of volatile oil contain 12.5 30 mole percent heptanes plus


Field Identification of Volatile Oil


Introduction


Black oil

Volatile oil


Gas cycling

Wet gas

Dry gas





Retrograde Gas

The phase diagram of a retrograde gas is smaller

than that for oils, and the

critical point is further

down the left side of the

envelope

For retrograde gases, the critical temperature is

less than the reservoir

temperature and the

cricondentherm is

greater than the reservoir

temperature


Phase diagram for a Retrograde Gas

Field Identification of Retrograde Gas

The producing gas-oil ratio for a retrograde gas is approximately up to 70000 scf/STB; the upper limit is not well defined

Stock-tank liquid gravities are between 40 and 60 API and increase as reservoir pressure falls below the dew-point pressure

The liquid can be lightly colored, brown, orange, greenish, or water-white

According to laboratory analysis, the heptanes plus fraction is less than 12.5 mole percent



Introduction


Black oil

Volatile oil


Gas cycling

Wet gas

Dry gas





Gas cycling process for Retrograde gas reservoirs


Gas cycling process


Introduction


Black oil

Volatile oil


Gas cycling

Wet gas

Dry gas





Wet Gas

The entire phase diagram of a Wet Gas will be

below reservoir

temperature

A wet gas exists only as a gas in the reservoir

No liquid formed in the reservoir

Liquid is formed only at the surface


Phase diagram for a Wet Gas

Field Identification of Wet Gas

The stock-tank liquid is usually water-white

Wet-gases have very high producing gas-oil ratios; producing gas-oil ratios will remain constant during the life

of a wet gas reservoir. A gas which produces more than

50000 scf/STB can be treated as a wet gas

Condensate liquid > 50 API



Introduction


Black oil

Volatile oil


Gas cycling

Wet gas

Dry gas





Dry Gas

Dry gas is primarily methane with some

intermediates

Hydrocarbon mixture is only gas in the reservoir

and normal surface

separator conditions are

outside the phase envelope

No liquid is formed at the surface

GOR > 100,000 SCF/STB


Phase diagram for a Dry Gas

Comparison of the Phase Diagrams of Reservoir Fluids



Introduction


Black oil

Volatile oil


Gas cycling

Wet gas

Dry gas





Properties of Black Oil

The following physical properties are required for the reservoir

engineering calculations (material balance calculations):

Formation volume factor of oil

Solution gas-oil ratio

Total formation volume factor

Coefficient of isothermal compressibility

Oil viscosity

(Interfacial tension)


Specific Gravity of a Liquid

Liquid specific gravity is defined as the ratio of the density of the liquid to the density of water, both taken at the same temperature

and pressure:

Specific gravity is nondimensional; but in the English system the units are:


=

=

=

/( )

( )/( )

The American Petroleum Institute gravity, or API gravity, is a measure of how heavy or light a petroleum liquid is compared

to water:


=.

. (degree)

RD oil=

API gravity


API gravity

Crude Oil Viscosity

(cP)

API Gravity

(degree) Examples

Tar, Bitumen and

Kerogen 10-10 6-10 Alberta, Canada-Peace River

Very Heavy Oil 10-10 10-12 Venezuela-Boscan

Heavy Oil 10-10 14-22 California-Kern River

Medium-Light Oil 10-10 25-30 Saudi Arabia-Arab Heavy

Light Oil 1-10 31-40 Azerbaijan-ACG

Ultra-Light Oil 10-1 41-50+ Texas-Eagle Ford

Crude Oil Classification

Formation Volume Factor of Oil

Oil formation volume factor is defined as the volume of reservoir oil required to produce one barrel of oil in the stock tank:

The unit is: res bbl/STB

The reciprocal of the formation volume factor is called the shrinkage factor:


= +

=


Formation Volume Factor of Oil

Oil Formation Volume Factor

Solution Gas-Oil Ratio

The quantity of gas dissolved in an oil at reservoir conditions is called solution gas-oil ratio:

Units are standard cubic feet per stock-tank barrel: scf/STB


=

Total Formation Volume Factor

Total Formation Volume Factor (Bt)

is:

Bt = Bo + Bg(Rsb Rs)

Bo Oil Formation Volume Factor

Bg Gas Formation Volume Factor

Rsb the solution gas to oil ratio at

the bubble point

Rs the quantity of gas remaining in

solution at the lower pressure


*Unit for gas formation volume factor is res bbl/scf

*Unit for total formation volume factor is res bbl/STB.


Total Formation Volume Factor

Total and oil formation volume factor

Oil compressibility above Pb

Pressures above the Bubble-point pressure:

In terms of formation volume factors this equation yields:

Oil compressibility can be written in terms of oil density:


Typical shape of the coefficient of isothermal compressibility of oil as a function of pressure at constant reservoir temperature at pressures above the bubble-point:


Oil compressibility above Pb

At pressures below the Bubble-point pressure, the total change in volume is the sum of the

change in liquid volume and the change in free

gas volume:

Consequently, the fractional change in volume as pressure change is:


Oil compressibility below Pb


Oil compressibility below Pb

Oil Viscosity

The coefficient of viscosity is a measure of the resistance to flow exerted by a fluid

The unit of viscosity is centipoise



Introduction


Black oil

Volatile oil


Gas cycling

Wet gas

Dry gas





Properties of Volatile Oil



Introduction


Black oil

Volatile oil


Gas cycling

Wet gas

Dry gas





Ideal Gases

An ideal gas is one where the following assumptions

hold:

Volume of the molecules is insignificant with respect to the total volume of the gas.

There are no attractive or repulsive forces between molecules or between molecules and container walls.

There is no internal energy loss when molecules collide.


Boyles Law

At constant temperature the pressure of a given weight of a gas is inversely

proportional to the volume of a gas:

Charles Law

At constant pressure, the volume of a given weight of gas varies directly with the

temperature:

Avogadros Law:

Under the same conditions of temperature and pressure equal volumes of all ideal

gases contain the same number of molecules. That is, one molecular weight of any

ideal gas occupies the same volume as the molecular weight of another ideal gas at a

given temperature and pressure.


Ideal Gases

The Equation of State for an Ideal Gas

By combining the three laws, an equation of state relating pressure, temperature and volume of a gas is obtained:

For n moles the equation becomes:

PV = nRT

R is Universal Gas Constant

To find the volume occupied by a quantity of gas when the conditions of temperature and pressure are changed from state 1 to state 2 we note that:


=

The Density of an Ideal Gas

Density is defined as the weight per unit volume:

where is the gas density.

For 1 mole, m = MW MW = Molecular weight

Hence,

=


=

Standard Conditions

It is common practice to relate volumes to conditions at surface, ie 14.7 psia and 60F:

*This relationship assumes that reservoir properties behave as ideal.


Apparent Molecular Weight

A mixture does not have a molecular weight although it behaves as though it had a molecular weight. This is called the apparent

molecular weight, AMW:

If yj represents the mole fraction of the jth component:

AMW for air = 28.97, a value of 29.0 is usually sufficiently accurate.


Specific Gravity of a Gas

The specific gravity of a gas, is the ratio of the density of the gas relative to that of dry air at the same conditions:

Mg = AMW of mixture, Mair = AMW of air.


Real Gases

Compressibility Factor for Natural Gases:

The correction factor z which is a function of the gas composition, pressure and

temperature is used to modify the ideal gas law to:

PV = znRT

z - compressibility factor and is an expression of the actual volume to what the ideal volume would be:


=

Reduced temperature and reduced pressure:

=

=

Where, Tc and Pc are the critical temperature and pressure.

For mixtures the compressibility factor (z) has been generated with respect to natural gases, where z is plotted as a function of pseudo

reduced temperature, Tpr and pseudo reduced pressure Ppr where:


Real Gases


Compressibility factors for natural gas (Standing & Katz)

Pseudocritical Properties of Natural Gases


Properties of Dry Gas Gas Formation Volume Factor

The Gas Formation Volume Factor is

defined as the volume

of gas at reservoir

conditions required to

produce one standard

cubic foot of gas at the

surface (unit: rb/scf or

res cu ft/scf)

The reciprocal of the formation volume

factor sometimes

called Gas Expansion

Factor


Gas formation volume factor as a function of

pressure at constant reservoir temperature

Gas Formation Volume Factor

Gas Formation Volume Factor volume occupied by the gas at reservoir temperature and pressure divided by the volume occupied by the same mass of

gas at standard conditions:

The volume of n moles of a gas at reservoir conditions is obtained with the compressibility equation of state:

T and p are reservoir temperature and pressure, consequently

The volume of the same number of moles of the gas at standard conditions, Tsc and psc, is:


=

=

=

Thus, the formation volume factor for the gas:

Tsc = 520 R, Psc = 14.65 psia and zsc = 1

Hence:

Also,

where, temperature must be in R and pressure in psia.


Gas Formation Volume Factor

=

=(. )

= .

= .

. = .

Compressibility of Gas

The coefficient of isothermal compressibility is defined as the fractional change of volume as pressure is changed at constant temperature:

Vm is the specific volume or volume per mole.

Unit is 1/psi



Compressibility of Gas

Coefficient of isothermal compressibility of a gas as a

function of pressure at constant reservoir temperature

Viscosity of Gas

The coefficient of viscosity is a

measure of the

resistance to flow

Gas viscosity decreases as

reservoir pressure

decreases


Viscosity of ethane


Introduction


Black oil

Volatile oil


Gas cycling

Wet gas

Dry gas





Properties of Wet Gas formation volume factor

The Formation Volume Factor of a Wet Gas is defined as the volume

of reservoir gas required to produce one stock-tank barrel of liquid at

the surface:


=

Thank you for your attention!


Documents

Physics of Oil and Gas Reservoirs