Chapter 3 reservoir fluid properties and pvt analysis

Behavior of ideal gases Based on the above kinetic theory of gases, a mathematical equation called equation-of-state can be derived to express the relationship existing between pressure

Vietnam National University - Ho Chi Minh City

University of Technology

Faculty of Geology & Petroleum Engineering

Department of Drilling - Production Engineering

Chapter 3

Reservoir fluid properties and PVT

analysis

References

Tarek Ahmed, Reservoir Engineering Handbook, 4th edition Gulf

Professional Publishing, 2010

Contents

 Reservoir fluid properties

 Classification of reservoir fluids

Reservoir fluid properties

To understand and predict the volumetric behavior of oil and gas reservoirs as a function of pressure, knowledge of the physical

properties of reservoir fluids must be gained

These fluid properties are usually determined by laboratory

experiments performed on samples of actual reservoir fluids

In the absence of experimentally measured properties, it is

necessary for the petroleum engineer to determine the

properties from empirically derived correlations

Properties of natural gases

Properties of natural gases

A gas is defined as a homogeneous fluid of low viscosity and density that has no definite volume but expands to completely fill the vessel in which it is placed

Generally, the natural gas is a mixture of hydrocarbon and

nonhydrocarbon gases The hydrocarbon gases that are normally

found in a natural gas are methanes, ethanes, propanes, butanes,

pentanes, and small amounts of hexanes and heavier The

nonhydrocarbon gases (i.e., impurities) include carbon dioxide,

hydrogen sulfide, and nitrogen

Behavior of ideal gases

The kinetic theory of gases postulates that gases are composed of

a very large number of particles called molecules For an ideal gas, the volume of these molecules is insignificant compared with the

total volume occupied by the gas It is also assumed that these

molecules have no attractive or repulsive forces between them, and that all collisions of molecules are perfectly elastic

Behavior of ideal gases

Based on the above kinetic theory of gases, a mathematical

equation called equation-of-state can be derived to express the

relationship existing between pressure p, volume V, and temperature T for a given quantity of moles of gas n This relationship for perfect gases is called the ideal gas law and is expressed mathematically by the following equation:

pV = nRT where p = absolute pressure, psia

V = volume, ft3

T = absolute temperature, °R

n = number of moles of gas, lb-mole

R = the universal gas constant, which, for the above units, has the value 10.730 psia ft3/lb-mole °R

Behavior of ideal gases

The number of pound-moles of gas, n, is defined as the weight of the gas m divided by the molecular weight M, or:

where m = weight of gas, lb

M = molecular weight, lb/lb-mol

Since the density is defined as the mass per unit volume of the substance,

where ρg = density of the gas, lb/ft3

m n

Apparent Molecular Weight

where Ma = apparent molecular weight of a gas mixture

Mi = molecular weight of the ith component in the mixture

yi = mole fraction of component i in the mixture

Standard Volume

In many natural gas engineering calculations, it is convenient to

measure the volume occupied by l lb-mole of gas at a reference

pressure and temperature These reference conditions are usually

14.7 psia and 60°F, and are commonly referred to as standard

conditions The standard volume is then defined as the volume of

gas occupied by 1 lb-mol of gas at standard conditions

where Vsc = standard volume, scf/lb-mol

scf = standard cubic feet

sc sc

sc

RT V

p

Density

The density of an ideal gas mixture is calculated by simply replacing the molecular weight of the pure component with the apparent molecular weight of the gas mixture to give:

a g

pM RT

 

Specific Gravity

The specific gravity is defined as the ratio of the gas density to that of the air Both densities are measured or expressed at the same pressure and temperature Commonly, the standard pressure psc and standard temperature Tsc are used in defining the gas specific gravity:

g g

Example

A gas well is producing gas with a specific gravity of 0.65 at a rate

of 1.1 MMscf/day The average reservoir pressure and temperature are 1,500 psi and 150°F Calculate:

a Apparent molecular weight of the gas

b Gas density at reservoir conditions

c Flow rate in lb/day

Example

A gas well is producing a natural gas with the following composition:

Assuming an ideal gas behavior, calculate:

a Apparent molecular weight

b Specific gravity

c Gas density at 2,000 psia and 150°F

d Specific volume at 2,000 psia and 150°F

Behavior of real gases

In dealing with gases at a very low pressure, the ideal gas

relationship is a convenient and generally satisfactory tool

At higher pressures, the use of the ideal gas equation-of-state may

lead to errors as great as 500%, as compared to errors of 2–3%

at atmospheric pressure

Behavior of real gases

Numerous equations-of-state have been developed in the attempt

to correlate the pressure-volume-temperature variables for real gases with experimental data In order to express a more exact

relationship between the variables p, V, and T, a correction factor called the gas compressibility factor, gas deviation factor, or simply the z-factor, must be introduced into Equation 2-1 to

account for the departure of gases from ideality The equation has the following form:

pV = znRT

Behavior of real gases

where the gas compressibility factor z is a dimensionless quantity and is defined as the ratio of the actual volume of n-moles of gas at T and p to the ideal volume of the same number of moles at the same T and p:

Studies of the gas compressibility factors for natural gases of various compositions have shown that compressibility factors can be generalized with sufficient accuracies for most engineering purposes when they are expressed in terms of the

following two dimensionless properties:

z

Behavior of real gases

These dimensionless terms are defined by the following expressions:

where p = system pressure, psia

ppr = pseudo-reduced pressure, dimensionless

T = system temperature, °R

Tpr = pseudo-reduced temperature, dimensionless

ppc, Tpc = pseudo-critical pressure and temperature, respectively, and defined by the following relationships:

pr

pc

p p

Behavior of real gases

Based on the concept of

Standing and Katz (1942)

presented a generalized gas

compressibility factor chart as

shown in Figure 2-1 The chart

factors of sweet natural gas as a

function of ppr and Tpr This

chart is generally reliable for

natural gas with minor amount

of nonhydrocarbons It is one of

the most widely accepted

correlations in the oil and gas

industry

Example

A gas reservoir has the following gas composition: the initial reservoir pressure and temperature are 3,000 psia and 180°F, respectively

Calculate the gas compressibility factor under initial reservoir conditions

Behavior of real gases

Equation pV = znRT can be written in terms of the apparent molecular weight Ma and the weight of the gas m:

Solving the above relationship for the gas specific volume and density, give:

Example

A gas reservoir has the following gas composition: the initial reservoir pressure and temperature are 3,000 psia and 180°F, respectively

Calculate the density of the gas phase under initial reservoir conditions Compare the results with that of ideal gas behavior

Behavior of real gases

In cases where the composition of

a natural gas is not available, the

pseudo-critical properties, i.e.,

ppc and Tpc, can be predicted

solely from the specific gravity of

the gas Brown et al (1948)

presented a graphical method for

a convenient approximation of

the pseudo-critical pressure and

pseudo-critical temperature of

gases when only the specific

gravity of the gas is available

The correlation is presented in

Figure Pseudo-critical properties

of natural gases

Behavior of real gases

Standing (1977) expressed this graphical correlation in the following mathematical forms:

Case 1: Natural Gas Systems

Example

A gas reservoir has the following gas composition: the initial reservoir pressure and temperature are 3,000 psia and 180°F, respectively

Calculate the density of the gas phase under initial reservoir conditions by calculating the pseudo-critical properties

Effect of nonhydrocarbon components on the z-factor

Natural gases frequently contain materials other than hydrocarbon components, such as nitrogen, carbon dioxide, and

hydrogen sulfide Hydrocarbon gases are classified as sweet or

sour depending on the hydrogen sulfide content Both sweet and

sour gases may contain nitrogen, carbon dioxide, or both

The common occurrence of small percentages of nitrogen and carbon dioxide is, in part, considered in the correlations previously cited Concentrations of up to 5 percent of these nonhydrocarbon components will not seriously affect accuracy

Errors in compressibility factor calculations as large as 10 percent

may occur in higher concentrations of nonhydrocarbon components in gas mixtures

Effect of nonhydrocarbon components on the z-factor

Natural gases frequently contain materials other than hydrocarbon components, such as nitrogen, carbon dioxide, and

hydrogen sulfide Hydrocarbon gases are classified as sweet or

sour depending on the hydrogen sulfide content Both sweet and

sour gases may contain nitrogen, carbon dioxide, or both

The common occurrence of small percentages of nitrogen and carbon dioxide is, in part, considered in the correlations previously cited Concentrations of up to 5 percent of these nonhydrocarbon components will not seriously affect accuracy

Errors in compressibility factor calculations as large as 10 percent

may occur in higher concentrations of nonhydrocarbon components in gas mixtures

Nonhydrocarbon Adjustment Methods

There are two methods that were developed to adjust the pseudocritical properties of the gases to account for the presence

of the nonhydrocarbon components These two methods are the:

• Wichert-Aziz correction method

• Carr-Kobayashi-Burrows correction method

The Wichert-Aziz Correction Method

Natural gases that contain H2S and or CO2 frequently exhibit different compressibility-factor behavior than do sweet gases Wichert and Aziz (1972) developed a simple, easy-to-use calculation procedure to account for these differences This

method permits the use of the Standing-Katz chart, by using a

pseudo-critical temperature adjustment factor, which is a function

of the concentration of CO 2 and H 2 S in the sour gas This

correction factor is then used to adjust the pseudo-critical

temperature and pressure according to the following expressions:

1

pc pc pc

pc

p T p



The Wichert-Aziz Correction Method

where

T′pc = corrected pseudo-critical temperature, °R

p′pc = corrected pseudo-critical pressure, psia

B = mole fraction of H2S in the gas mixture

ε = pseudo-critical temperature adjustment factor and is defined mathematically by the following expression

where the coefficient A is the sum of the mole fraction H2S and

CO2 in the gas mixture, or:

Example

A sour natural gas has a specific gravity of 0.7 The compositional analysis of the gas shows that it contains 5% CO2 and 10% H2S Calculate the density of the gas at 3,500 psia and 160°F

The Carr-Kobayashi-Burrows Correction Method

Carr, Kobayashi, and Burrows (1954) proposed a simplified procedure to adjust the pseudo-critical properties of natural gases when nonhydrocarbon components are present The

method can be used when the composition of the natural gas is

not available The proposed procedure is summarized in the

following steps:

Step 1: Knowing the specific gravity of the natural gas, calculate the pseudo-critical temperature and pressure by applying Equations

Tpc = 168 + 325γg − 12.5γg2

ppc = 677 + 15.0γg − 37.5γg2

The Carr-Kobayashi-Burrows Correction Method

Step 2: Adjust the estimated pseudo-critical properties by using the following two expressions:

where T′pc = the adjusted pseudo-critical temperature, °R

Tpc = the unadjusted pseudo-critical temperature, °R

yCO2 = mole fraction of CO2

yH2S = mole fraction of H2S in the gas mixture

yN2= mole fraction of nitrogen

p′pc = the adjusted pseudo-critical pressure, psia

ppc = the unadjusted pseudo-critical pressure, psia

The Carr-Kobayashi-Burrows Correction Method

Step 3 Use the adjusted pseudo-critical temperature and pressure to calculate the pseudo-reduced properties

Step 4 Calculate the z-factor from Standings-Katz chart

Example

A sour natural gas has a specific gravity of 0.7 The compositional analysis of the gas shows that it contains 5% CO2 and 10% H2S Calculate the density of the gas at 3,500 psia and 160°F

Correction for high-molecular weight gases

Sutton proposed that this deviation can be minimized by utilizing the mixing rules developed by Stewart et al (1959), together

with newly introduced empirical adjustment factors (FJ, EJ, and

EK) that are related to the presence of the heptane-plus fraction in

the gas mixture The proposed approach is outlined in the following steps:

Correction for high-molecular weight gases

Step 1 Calculate the parameters J and K from the following relationships:

where J = Stewart-Burkhardt-Voo correlating parameter, °R/psia

K = Stewart-Burkhardt-Voo correlating parameter, °R/psia

yi = mole fraction of component i in the gas mixture

K   y T p 



Correction for high-molecular weight gases

Step 2 Calculate the adjustment parameters FJ, EJ, and EK from the following expressions:

where yC7+ = mole fraction of the heptanes-plus component

(Tc)C7+ = critical temperature of the C7+

(pc)C7+ = critical pressure of the C7+

2 0.5

Correction for high-molecular weight gases

Step 3 Adjust the parameters J and K by applying the adjustment factors EJ and EK, according to the relationships:

J’ = J – EJ

K’ = K – EK

Correction for high-molecular weight gases

Step 4 Calculate the adjusted pseudo-critical temperature and pressure from the expressions:

 ' 2 '

'

pc

K T

J



' '

'

pc pc

T p

J



Correction for high-molecular weight gases

Step 5 Having calculated the adjusted Tpc and ppc, the regular procedure of calculating the compressibility factor from the Standing and Katz chart is followed

Sutton’s proposed mixing rules for calculating the pseudo-critical

properties of high-molecular-weight reservoir gases, i.e., γ g > 0.75, should significantly improve the accuracy of the calculated

z-factor