This document discusses key properties of crude oil, including: 1) Oil is classified based on properties like specific gravity, viscosity, density, etc. with specific gravity and viscosity most commonly used. Specific gravity is represented by API gravity which ranges from 8 to 58 degrees. 2) Bubble point pressure is the pressure at which a small amount of gas is in equilibrium with oil. When pressure drops below this point, gas is liberated from the oil. 3) Other properties discussed include formation volume factor (ratio of reservoir to surface volumes), solution gas-oil ratio (amount of gas dissolved in oil), and compressibility (change in volume with pressure change).

1. Oil Properties
EG314 Reservoir Rocks & Fluid
Properties

2. Specific Gravity of Oil
• Crude oils are classified according to their physical
properties, such as oil specific gravity, viscosity, surface
tension, density, boiling point, and freezing point, etc.
• The most commonly used properties for classification
are viscosity and oil gravity.
• The crude oil gravity is represented in degrees API
(oAPI), ranging from 8 oAPI to 58 oAPI. The higher the
API gravity, the lighter the crude, and vice versa.
• The API gravity and crude oil specific gravity (water =
1) are related by:
횼o = 141.5
131.5 + oAPI
and
oAPI = 141.5 – 131.5
Ύo

3. Bubble Point Pressure
• Bubble point as defined by Standing is the state at which an
infinitesimal quantity of gas is in equilibrium with a large quantity
of fluid.
• Bubble point pressure, symbol pb, is the fluid pressure in a system
at its bubble point (saturation pressure?)
• When the pressure is above the bubblepoint pressure
(undersaturated), the fluid is capable of holding additional gases
or liquid at the existing pressure and temperature.
• When oil & gas are produced and the reservoir pressure drops
below the bubble point pressure, gas is liberated from oil.
• The following equation is used to estimate the bubble point
pressure:
pb = 18.2 {[Rs/Ύg]0.83 x 10 [0.00091(TR) – 0.0125(oAPI)] - 1.4}
Where Rs is the produced gas/oil ratio in SCF/STB and TR is the
reservoir temperature in oF.

4. Above bubble point pressure.
Oil is undersaturated
Solution GOR is constant
At and below bubble
point pressure two
phases produced in the
reservoir as gas comes
out of solution.
Solution GOR reduces
Solution Gas-Oil Ratio, Rs

5. Reservoir Fluid Study
• Below bubble point gas released and mobility
effected by relative permeability
considerations.
• Gas separation in the production tubing is
different and considered to remain with
associated oil.
• Two basic liberation mechanisms.
Flash liberation
Differential liberation

6. Reservoir Fluid Study
• Flash Liberation (vaporization)
The gas is evolved during a definite reduction in
pressure and the gas is kept in contact with the liquid
until equilibrium has been established.
• Differential Liberation (vaporization)
The gas being evolved is being continuously
removed from contact with the liquid and the liquid is
in equilibrium with the gas being evolved over a finite
pressure range.

7. Solution Gas-Oil Ratio, Rs
Above bubble point
All gas in solution
At bubble point
All gas in solution
Below bubble point
Free gas and solution gas
At surface conditions
No gas in solution

8. Oil Formation Volume Factor
• Oil formation volume factor (Bo), is defined as the
ratio of the liquid volume at reservoir conditions to
the liquid volume at stock-tank (standard) conditions.
• This factor is used to convert reservoir barrels to
stock-tank barrels.
• The oil formation volume factor can be expressed
mathematically as:
Bo = (Vo)p,T /(Vo)sc
Where Bo = oil formation vol. factor, bbl/STB
(vo) p,T = vol. of oil under reservoir conditions, bbl
(Vo)sc = vol. of oil is measured under standard conditions, STB

9. Total Formation Volume Factor, Bt
• Sometimes convenient to know volume of
the oil in the reservoir by one stock tank
unit of oil plus the free gas that was
originally dissolved in it.
• Total formation volume factor is used, Bt.
• Sometimes termed two-phase volume
factor.

10. Total Formation Volume Factor, Bt
The total formation volume factor is the volume
in barrels (cubic metre ) that 1.0 stock tank barrel
( cubic metre ) and its initial complement of
dissolved gas occupies at reservoir temperature
and pressure conditions.
Bt  Bo Bg Rsb Rs 
Rsb = the solution gas to oil ratio at the bubble point.

11. Total Formation Volume Factor, Bt
Bt  Bo Bg Rsb Rs 
P = Pb
OIL
Hg
Bob
P < Pb
GAS
OIL
Hg
Bo
Bg(Rsb-Rs)
Bt
Sometimes used in the material balance equation
Does not have volume significance in the reservoir.

12. Total Formation Volume Factor, Bt
Bt  Bo Bg Rsb Rs 
Above Pb, Bt = Bo

13. Oil Compressibility
• Volume changes of oil above the bubble point are very
significant in recovering undersaturated oil.
• Oil formation volume factor reflects these changes
• More fundamentally in the coefficient of compressibility of
the oil.
• or oil compressibility
Pb
o
1 V
  
   
T
c
V P
  
  
o
o
1 B
   
o T
c
B P
  
In terms of Bo
Assuming compressibility
does not change with
pressure, between
conditions 1 & 2.
V
  2
o 2 1
1
c P P ln
V
  