Crude Oil Density

The crude oil density is defined as the mass of a unit volume of the crude at a specified pressure and temperature, mass/volume. The density usually is expressed in pounds per cubic foot and it varies from 30 lb/ft³for light volatile oil to 60 lb/ft³for heavy crude oil with little or no gas solubility. It is one of the most important oil properties, because its value substantially affects crude oil volume calculations. This vital oil property is measured in the laboratory as part of routine PVT tests. When laboratory crude oil density measure- ment is not available, correlations can be used to generate the required density data under reservoir pressure and temperature. Numerous empirical correlations for calculating the density of liquids have been proposed over the years. Based on the available limited meas- ured data on the crude, the correlations can be divided into the following two categories:

• Correlations that use the crude oil composition to determine the density at the pre- vailing pressure and temperature.

• Correlations that use limited PVT data, such as gas gravity, oil gravity, and gas/oil ratio, as correlating parameters.

Density Correlations Based on the Oil Composition

Several reliable methods are available for determining the density of saturated crude oil mixtures from their compositions. The best known and most widely used calculation methods are the following two: Standing-Katz (1942) and Alani-Kennedy (1960). These two methods, presented next, calculate oil densities from their compositions at or below the bubble-point pressure.

Standing-Katz’s Method

Standing and Katz (1942) proposed a graphical correlation for determining the density of hydrocarbon liquid mixtures. The authors developed the correlation from evaluating experimental, compositional, and density data on 15 crude oil samples containing up to 60 mol% methane. The proposed method yielded an average error of 1.2% and maximum

γ_g γ_gi ^b

b

b

a p p

= ⎧ + −p p

⎨⎩

⎫⎬ 1 [1 ( / )] ⎭

( / )

error of 4% for the data on these crude oils. The original correlation had no procedure for handling significant amounts of nonhydrocarbons.

The authors expressed the density of hydrocarbon liquid mixtures as a function of pressure and temperature by the following relationship:

(4–4) where

ρ_o= crude oil density at pand T, lb/ft³

ρ_sc= crude oil density (with all the dissolved solution gas) at standard conditions, that is, 14.7 psia and 60°F, lb/ft³

Δρ_p= density correction for compressibility of oils, lb/ft³ Δρ_T= density correction for thermal expansion of oils, lb/ft³

Standing and Katz correlated graphically the liquid density at standard conditions ρ_sc with the density of the propane-plus fraction, ρ_C

3+, the weight percent of methane in the entire system, (m_C

1)_C

1+, and the weight percent of ethane in the ethane plus, (m_C

2)_C

2+. This graphical correlation is shown in Figure 4–1. The proposed calculation proce- dure is performed on the basis of 1 lb-mole, that is, n_t= 1, of the hydrocarbon system. The following are the specific steps in the Standing and Katz procedure of calculating the liq- uid density at a specified pressure and temperature.

Step 1 Calculate the total weight and the weight of each component in 1 lb-mole of the hydrocarbon mixture by applying the following relationships:

m_i= x_iM_i m_t=

Σ

^xiMi

where

m_i= weight of component iin the mixture, lb/lb-mole x_i= mole fraction of component iin the mixture M_i= molecular weight of component i

m_t= total weight of 1 lb-mole of the mixture, lb/lb-mole

Step 2 Calculate the weight percent of methane in the entire system and the weight per- cent of ethane in the ethane-plus from the following expressions:

(4–5)

and

(4–6) where

(m_C

1)_C

1+= weight percent of methane in the entire system m_C

1= weight of methane in 1 lb-mole of the mixture, that is, x_C

1M_C

1

(m ) x M

m

m m_t m

C C

C C

C

C C

2 2

2 2

2

2

1 + 100

+

=⎡

⎣⎢

⎢

⎤

⎦⎥

⎥ =

−

⎡

⎣⎢

⎢⎢

⎤

⎦⎥

⎥100

(m ) x M

x M

m

i i

i C C n

C C₁ C

1 1

1 1

1 + = 100

⎡

⎣

⎢⎢

⎢⎢

⎤

⎦

⎥⎥

⎥⎥

=

∑

= ^mmt

⎡

⎣⎢ ⎤

⎦⎥100 ρ_o=ρ_sc +Δρ_p−Δρ_T

FIGURE 4–1 Standing and Katz density correlation.

Source: GPSA Engineering Data Book, 10th ed. Tulsa, OK: Gas Processors Suppliers Association, 1987. Courtesy of the Gas Processors Suppliers Association.

(m_C

2)_C

2+= weight percent of ethane in ethane-plus m_C

2= weight of ethane in 1 lb-mole of the mixture, that is, x_C

21M_C M_C 2

1= molecular weight of methane M_C

2= molecular weight of ethane

Step 3 Calculate the density of the propane-plus fraction at standard conditions by using the following equations:

(4–7) with

(4–8) (4–9) where

ρ_C

3+= density of the propane and heavier components, lb/ft³ m_C

3+= weight of the propane and heavier components, lb/ft³ V_C

3+= volume of the propane-plus fraction, ft³/lb-mole V_i= volume of component iin 1 lb-mole of the mixture m_i= weight of component i, that is, x_iM_i, lb/lb-mole

ρ_oi= Density of component iat standard conditions, lb/ft³(density values for pure components are tabulated in Table 1–1 in Chapter 1, but the density of the plus frac- tion must be measured)

Step 4 Using Figure 4–1, enter the ρ_C

3+value into the left ordinate of the chart and move horizontally to the line representing (m_C

2)_C

2+, then drop vertically to the line representing (m_C

1)_C

1+. The density of the oil at standard condition is read on the right side of the chart.

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

(4–10)

with:

(4–11)

where ρ_C

2+= density of the ethane-plus fraction.

Step 5 Correct the density at standard conditions to the actual pressure by reading the additive pressure correction factor, Δρ_p, from Figure 4–2:

ρ_p,60= ρ_sc+ Δρ_p

The pressure correction term Δρ_pcan be expressed mathematically by

(4–12) Δρ =[ .0 000167+( .0 016181 10) ^{−0 0425. ρsc}]p−(10⁻⁸⁸)[ .0 299+(263 10) ^{−0 0603. ρsc}]p²

0 0042 2

2 2

+ . (mC )C₊

ρ_C2+=ρ_C3+⎡⎣1 0 01386− . (m_C2)_C2+ −0 000082. (m_C2)_C²²₂₊⎤⎦ +0 379. (m_C2)_C2+

++ 0 00058 +

1 2

. (m_C)2_C

ρsc=ρC ⎡ − C C − C C

⎣

+ + +

2 1 0 012. (m 1) 1 0 000158. (m 1)²1 ⎤⎤

⎦+0 0133 ₊ . (mC₁)C1

V V m

i i

i i oi C

C C

3

3 3

+= =

= =

∑ ∑

_ρ

m x M_{i i}

i C

3 C 3 +=

∑

=

ρ

ρ

C C C

C

C 3 C

3 3

3

3 +

+ +

= = ₌ = −

=

∑

∑

m V

x M x M

m m

i i

i n

i i

i oi n

t _{11 2}

3

C

C

−

∑

=

m x M_{i i}

i oi n

ρ