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SPE-172833-MS
Black Oil Property Correlations - State of the Art
Muhammad Ali Al-Marhoun
Reservoir Technologies, Saudi Arabia
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 2
Outline
Introduction
Literature Review
Black Oil Properties Bubblepoint Pressure & Solution Gas Oil Ratio
Oil Formation Volume Factor
Oil Density at Reservoir Conditions
Oil Compressibility above & below bubble point
Oil Viscosity
Adjustment of Differential Data to Separator Conditions
Conclusions
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 3
This paper gives the best correlations to estimate black oil properties based on statistical accuracy and physical behavior.
Hundreds of reservoir-fluid studies of black oil samples representing all areas of the world producing black oils were gathered from different published and unpublished sources to perform statistical error analysis.
All black oil property correlations available in the petroleum literature were compared with this world wide database.
Introduction
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 4
Black oil reservoirs consist of large, heavy, nonvolatile hydrocarbon molecules and the fluid is a liquid at reservoir conditions. They are roughly identified as having
Initial solution gas oil ratio of less than 2,000 scf/STB
Very dark green or brown to black in color Stock-tank oil gravities below 45°API
C7+ composition greater than 20 mole percent
Oil formation volume factor of less than 2.00 bbl/STB
Identification of black oil reservoirs
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 5
For the PVT database used in this study, the best correlations are presented for the following Black Oil Properties:
Solution Gas Oil Ratio
Bubblepoint Pressure
Oil Formation Volume Factor
Oil Density at Reservoir Conditions
Oil Compressibility above bubble point
Oil Compressibility below bubble point
Oil Viscosity at bubble point
Oil Viscosity above bubble point
Dead Oil Viscosity
Black oil properties
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 6
Solution gas oil ratio is the ratio of the volume of the liberated gas from solution to the volume of the remaining stock tank oil both volume corrected to 14.7 psi and 60°F.
Fig. 1 - Typical Solution Gas Oil Ratio Curve.
5432 )460(1
aa
o
a
b
a
gs TpaR
where a1 = 1.4903 x10+3
a2 = 2.626
a3 = 1.3984
a4 = -4.3963
a5 = -1.86
Solution gas oil ratio, Rs
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 7
Oil formation volume factor is the volume of the reservoir liquid at conditions under consideration per unit volume of stock tank oil at 14.7 psi and 60°F. It is usually expressed as bbl/STB.
Fig. 2 - Typical Oil Formation Volume Factor Curve.
60160/1 4321 TaTRaRaRaB osogssob
where a1 = 0.177342 x10-3
a2 = 0.220163 x10-3
a3 = 4.292580 x10-6
a4 = 0.528707 x10-3
Oil formation volume factor, Bo
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 8
The specific gravity of oil is defined as the ratio of density of the oil to that of water both at the same specified pressure and temperature.
An equation for oil relative density at any pressure is expressed as
Fig. 3 - Typical Oil Relative Density Curve.
ogsoop BRx /)1018.2( 4
w
o
o
Oil density at reservoir conditions, ρo
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 9
By definition, the single phase isothermal compressibility or the reciprocal bulk modulus of elasticity is defined as the unit change in volume with pressure. It is usually expressed as 1/psi.
Fig. 4 - Typical Oil Compressibility Curve above Pb.
)460/(/)(/ln 4
3
321 Tappaaac obbobo
where a1 = -14.1042
a2 = 2.7314
a3 = - 56.0605 x 10-6
a4 = 0
Correlations of Co above bubblepoint
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 10
Introduction and definitions
Perrine definition of Co below Pb
Paradox of Perrine definition
New definition of Co below Pb
The constant composition curve
Consistency of Co definitions
Estimation of Co below Bubblepoint
Correlations of Co below bubblepoint
Different Definitions of Oil Compressibility Curves
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 11
According to Perrine definition, the Co below Pb should include
1. The change in liquid volume
2. The change in the amount of dissolved gas is
Therefore, the Perrine defining equation of oil compressibility below Pb is
Typical oil compressibility curve
in petroleum literature.
T
o
P
B
T
s
P
R
T
sg
T
o
o
op
RB
p
B
Bc
1
Perrine definition of Co below Pb
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 12
Below Pb, Experimental data shows that live oil density increases as pressure decreases.
Using Perrine definition co increases with the increase in density
Such behavior cannot happen in nature; observed physical behavior is density and compressibility have opposite trend.
Oil compressibility below Pb increases with
density according to Perrine definition!!
Paradox of Perrine definition
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 13
The new definition of Co
below Pb is
The slope dBo/dp should be
taken along the constant
composition curve at the
pressure of interest Locus of Co below Pb (Green
curve)
New definition of Co below Pb
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 14
Bo curve below the original Pb is a changing composition curve.
Therefore, the slope of the Bo below the original Pb at any pressure is negative if taken along Bo curve.
To calculate Co, the slope of Bo should be taken along a constant composition curve at pressure of interest as shown in figure.
The pressure of interest where Co is to be evaluated is a new bubble point pressure.
The constant composition curve is Bo curve above the new bubble point pressure with a trend similar to Bo curve above the original bubble point pressure.
The slope of dBo/dp below Pb is
taken along constant composition
curve at pressure of interest.
The constant composition curve
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 15
Perrine definition
Co is not consistent with the observed physical behavior.
The new definition
Co is consistent with the observed physical behavior.
Co increases with the decrease of density below as well as above bubble point pressure
Co is continuous from atmospheric pressure to infinity.
Co function is differentiable everywhere except at bubble point where it forms a cusp
Consistency of Co definitions
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 16
3
1 2 3 4
1 2 4
ln / ( ) / /( 460)
ln / /( 460)
o ob b b ob
o ob
c a a a P P a T
c a a a T
)460/(/)(/ln 43
321 TaPPaaac obbobo
At original bubble point pressure, oil compressibility could be
estimated by replacing p by pb
Above original bubble point pressure, oil compressibility could
be estimated the following correlation:
21ln o
ob
ac a
Estimation of Co below bubblepoint
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 17
2
1 2
2
2
(1/ 1/ )
ln /
ln ln (1/ 1/ )
ln( / ) (1/ 1/ )
op ob
o ob
op ob op ob
op ob op ob
a
op ob
c a a
c c a
c c a
c c e
Below the original bubble point pressure, oil compressibility
could be estimated in terms of oil compressibility at the original
bubble point pressure
Estimation of Co below bubblepoint
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 18
The oil viscosity measures the oil’s resistance to flow.
Fig. 8 - Typical Oil Viscosity Curve.
Oil viscosity, μo
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 19
The bubblepoint viscosity can be estimated to an accuracy of 30% with
where
and a1 = 10.715
a2 = 100
a3 = -0.515
a4 = 5.44
a5 = 150
a6 = -0.338
odob
3
21
a
S aRa
6
54
a
S aRa
Oil viscosity at bubblepoint pressure, μob
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 20
The viscosity above bubblepoint can be estimated to an accuracy of 2% with
where a1 = = 0.151292 x10-3
b
2
1oblnln ppa obo
Oil viscosity above bubblepoint pressure
Fig. 8 - Typical Oil Viscosity Curve.
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 21
The dead oil viscosity can be estimated to an accuracy of 35% with
where a1 = 54.56805426
a2 = -7.179530398
a3 = -36.447
a4 = 4.478878992
)ln(ln)(ln)ln(lnlnln api4api321od TaaTaa
Dead oil viscosity, μod
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 22
The adjusted differential solution gas-oil ratios at pressures below bubblepoint are evaluated from the following equation:
The adjusted differential oil formation volume factor at pressures below bubblepoint pressure are evaluated from the following equation:
At atmospheric pressure, if equation yields a value for Boi < 1, then Boi for all pressures are calculated by
sbdsbfsdisi RRRR /
obdobfodioi BBBB /
1/1 obdobfodiobdobfoi BBBBBB
Adjustment of differential to separator test
Paper # SPE-172833-MS• Black Oil Property Correlations - State of the Art • Muhammad Ali Al-Marhoun
Slide 23
The following conclusions were drawn from this evaluation study:
The bubblepoint pressure and solution gas oil ratio exhibited high errors with original coefficients, but when new coefficients are recalculated an improvement occurred.
All correlations available in literature to estimate the oil formation volume factor at bubblepoint pressure show low errors and a good degree of harmony towards the data used.
The selected correlation of isothermal oil compressibility gives an accurate and unique value independent of different separator tests or consistent field data.
Bubblepoint oil viscosity and dead oil viscosity correlations exhibited very high errors for all correlations available in literature. Therefore more research is needed in this area.
The performance of most of the correlations for viscosity above bubblepoint pressure are adequate.
The adjustment of differential liberation data to separator conditions successfully gives the expected values for all the PVT properties at both bubble point and atmospheric pressures.
Conclusions