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Review of Crude Oil Properties

Specific gravitySolution gas-oil ratio

Density of oil Bubble point pressure Formation volume factor Isothermal compressibilityViscosity

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Solution Gas-Oil Ratio, RSO

Definition: volume of gas per unit volume ofcrude oil produced under standard conditions

Units: Standard Cu Ft (SCF) / Stock TankBarrel (STB)

Volume of gas produced at surface at standard condition

Volume of oil entering stock tank at standard conditionso

R

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Black Oil

Pressure pathin reservoir

Separator

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Solution Gas-Oil Ratio, RSO

Solubility ~ f(T,P,composition of gas andoil)- T- P

- Similarity between oil and gas

- infinitely soluble

Gas solubility (isothermal condition):increase in volume of gas per unit volume ofcrude oil per unit increase in pressure

(SCF/STB/psia, or dRso/dp)

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Solution Gas-Oil Ratio, RSO

Saturated oil: with a slight reduction in pressuregas is released from solution. Unsaturated oil (no gas cap): Bubble point pressure: the pressure at which freegas starts to appear.

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Black Oil

A

B C

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Density of Oil Under Reservoir Conditions

(o) Importance: Often required in various reservoirrelated calculations. Calculation of o:- From composition, with only C3+ compounds

- From composition, with C1, C2, C3+ compounds

- If composition is unknow (from porpertiesmeasured under surface conditions)

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Ideal Solution Principles

Ideal liquid solution: A hypothetical mixture ofliquids in which there is no special force of attractionbetween the components of the solution and for

which no change in internal energy occurs onmixing.

Assumption: nothing happens during mixing exceptdilution (no shrinkage, no expansion, no heatgeneration, no reaction etc.)

Vt = V1+V2 Mt= M1+M2 = Mt/ Vt

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Ex. 2.7: Use ideal-solution principles to calculatethe density at 14.7 psia and 60 deg F of ahydrocarbon liquid of the following composition.

Comp xi MWi xiMWi SC,i Liq V at sc

C3 0.2 44.09 8.82 31.66 0.2786C4 0.3 58.12 17.43 35.78 0.4871

C5 0.5 72.15 36.1 38.51 0.9374

62.35 1.703

1. Calculate mass2. Calculate V

3. sc= 62.35/1.703=36.61

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Correction at high P (Compression)

d

lb / cu ft

sc (lb / cu ft)

d= -sc=d + sc

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Correction at high T (Thermal Expansion)

d

lb / cu ft

sc (lb / cu ft)

d= sc-

= sc- d

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Ex. 2.8: what are the density of the oil in Ex. 2.7under reservoir condition of 2000 psia and

180F.

Comp MF MW xiMWi SC Liq V at sc

C3 0.2 44.09 8.82 31.66 0.2786C4 0.3 58.12 17.43 35.78 0.4871

C5 0.5 72.15 36.1 38.51 0.9374

62.35 1.7031. sc= 62.35/1.703=36.61

2. Correct for pressure, chart #15sc= 36.61, d

= 1.2, sc, p = 36.61+1.2 = 37.83 lb/ft

3. Correct for temperature, chart #16d

= 5.3 lb/ft3,

= 37.81-5.3 = 32.5 lb/cu ft

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Review of Crude Oil Properties

Specific gravity

Solution gas-oil ratio

Density of oil- Calculation from composition (C3+)- Calculation from composition (C1~ C3

+)

- Calculation if composition is not known

Formation volume factor

Bubble point pressureIsothermal compressibility

Viscosity

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What if We Have Methane and Ethane?

Apparent liquid density: A fictitious density thatrepresents the contribution of methane and ethaneto the overall density of the liquid.

How is it obtained?

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Ex. 2.9

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Correction at high P (Compression)

d

lb / cu ft

sc (lb / cu ft)

d= -sc=d + sc

Chart #15

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Correction at high T (Thermal Expansion)

d

lb / cu ft

sc (lb / cu ft)

d= sc-

= sc- d

Chart #16

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Ex. 2.9 solution

+ 33

3

+ 2 22 2

2

Steps:

1. Calculate mass and volume contribution of each component.

(122.1748 7.3897) lb/mole2. Density of C 52.34 lb/cu ft

2.193 cu ft/mole

(1.93. wt% of C in C

n

i i

i

n

i i

i i

n

i i

i

x M

x M

x M

x M

1 11

1

425) lb/mole(100)1.66%

(122.1748-5.4472) lb/mole

(5.4472) lb/mole (100)4.wt% of C in total = 4.46%

122.1748 lb/mole

5. look up density value in chart #14, 49.4 lb/cu ft

6. Pressure correc

n

i ii

x M

x M

tion (Chart #15). d =1.0, 49.4+1.0=50.4 lb/cu ft

7. Temperature correction (Chart #16). d =3.78, 50.4 3.78=36.62 lb/cu ft

46.62

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What If Gas Composition is Unknown

Calculation from gas gravity, stock tank oil gravity,and gas-oil solution ratio.Ex. 2.10:

The producing gas-oil ratio of a well is 768 scf/STB, andthe specific gravities of the gas and stock-tank oil

are 0.786 and 40.7API, respectively. The liquid inthe reservoir is at its bubble point at reservoir

conditions of 2635 psia and 220F. Calculate the

density of this liquid at reservoir conditions.

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Chart #17

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Ex. 2.10 solution:g air g

g

Let's say we have one barrel of stock-tank oil.

1. Calculate the mass of gas and oil

MW (surface condition) = MW =28.95 0.786 22.8 lb/lb mole

scf lb mole lbm = (768 )( )(22.8 ) 46.0 lSTB 379.4 scf lb mole

STO

o w o

o

b gas/STB

141.5 lb oil /cu ft oil0.822

131.5 lb water/cu ft water

lb water lb oil /cu ft oil

(surface condition) = =62.37 0.822cu ft water lb water/cu ft water

51.27 lb oil /cu ft oil

m 51.2

API

g

o

cu ft7 lb oil /cu ft oil 5.615( ) 287.9 lb oil/STB

STB

2. calculate the total volumelook up in chart #17, liquid density of gas is 24.2 lb/cu ft

V 46.0lb/STB / (24.2 lb/cu ft)=1.901 cu ft/STB

V 5.615cu

tot

sc

ft/STB

V 7.516 cu ft/STB

3. Calculate pseudoliquid density under standard condition

(46.0 287.9) lb/STB44.43 lb/cu ft

7.516 cu ft/STB

4. Compressibility adjustment, chart #15

44.43 0.93 45.36 lb/ p

ft at 60 F and 2635 psia

5. Thermal expansion adjustment, chart #16

45.36 4.83 40.53lb/ft at 220 F and 2635 psiaTp

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Good Numbers to Remember

Molecular weight of air: 28.95 lb/lb mole

Under standard conditions, 1 lb mole of

gas occupies 379.4 SCF. Under standard conditions, water density

is 62.37 lb water/cu ft water 1 STB = 5.615 SCF

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Oil Density Above Bubble Point

Two steps:1. Calculate oil density at bubble point.2. Adjust compression effect using isothermalcoefficient.

exp[ ( )]o ob o bc p p

o

b

: oil density at desired pressure

: oil density at bubble pressure

c : a constant determined at the average

pressure of p and p (bubble pressure)

o

ob

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Bubble Point Pressure

Chart #21

You need: Gas-oil ratio Gas gravity

Tank oil gravity Temperature

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Formation Volume Factor BO

Definition (FVF, BO):

BO > 1- temperature high in reservoir- Gas is dissolved in oil under reservoir conditions.

Shrinkage factor: 1/Bo

ro

sc

VBV

Vr: Volume of oil at reservoir T and P

Vsc: Volume of oil at standard condition(14.7 psia and 60F)Units: barrels of oil at reservoir conditionsper barrel of stock-tank oil (res bbl/STB),

always at 60F

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Formation Volume Factor, Bo

Above bubble point pressure,

Below bubble point pressure,

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Ex. 2.10

A sample of reservoir liquid with volume of 450 ccunder reservoir conditions was passed througha separator and into a stock tank at one

atmospheric pressure and 60 deg F. The liquidvolume in the stock tank was 327 cc. A total of1.41 scf of gas was released. Calculate the oil

formation volume factor.

Solution:450 res cc

1.38

327 ST cc

oB

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Estimation of Bo Using Correlation

1.1750.972 0.000147oB F

.5

,

where,

( ) 1.25

141.5oil specific gravity =

131.5

temperature, F

g

soo

o

o API

F R T

T

exp[ ( )]o ob o b B B c p p

-1

oil formation volume factor at the bubble-point pressure

oil compressibility, psi

ob

o

B

c

for :bp p

for :bp p