RELATIVE PERMEABILITY Definition: Measure of the ability of the porous system to conduct one fluid in presence of other fluids It is the composite effect of • pore geometry • fluid distribution • wettability • saturation history • applied forces equilibrium Capillary Number, Ca = µV / IFT ratio of viscous to capillary forces Bond Number, Bo = rgh / (2 IFT / r ) Ratio of gravity to capillary forces
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RELATIVE PERMEABILITY
Definition: Measure of the ability of the porous system to conduct one fluid in presence of other fluids
It is the composite effect of
• pore geometry • fluid distribution • wettability • saturation history • applied forces equilibrium
Capillary Number, Ca = µV / IFT ratio of viscous to capillary forces
Bond Number, Bo = rgh / (2 IFT / r ) Ratio of gravity to capillary forces
RELATIVE PERMEABILITY
Wettability: Tendency of one fluid to spread or to adhere to a solid surface in presence of other immiscible fluids
Waterwet, Oilwet, Intermediatewet
mixedwet or fractionallywet
RELATIVE PERMEABILITY
Drainage: Decrease of the wetting phase saturation Imbibition: Increase of the wetting phase saturation
But most of the reservoirs are not strongly « waterwet » nor « oil wet ». Thus the « wetting » and « nonwetting » fluids cannot be defined.
By convention! Drainage: Decrease of the water saturation Imbibition: Increase of the water saturation
In petrophysics and in reservoir simulators the notion of reference fluid is used
water in oil/water displacements liquid in gas/liquid displacements
RELATIVE PERMEABILITY
Drainage: oil displacement by water oilwet sand
Imbibition: oil displacement by water waterwet sand
Craig, 1971
STEADYSTATE IMMISCIBLE FLOW Kr
Fluid 1
Fluid 2
Fluid 1
Fluid 2
∆P 1
∆P 2
Q 1
Q 2
L P A K Q
L P A K Q
2
2
2 2
1
1
1 1
∆ =
∆ =
µ
µ
L
K1, K2 effective permeabilities
Kr1=K1/K relative permeabilities
Kr2=K2/K
TWOPHASE RELATIVE PERMEABILITY
Base permeability • Kair • Kwater • Koil @ Swi
Krw < Krnw
Krw + Krnw < 1
Kr = ƒ(contact angle)
TWOPHASE RELATIVE PERMEABILITY: HYSTERESIS
Fluid distribution at the pore level
Strong for nonwetting phase Less important for wetting phase
TWOPHASE Kr AND WETTABILITY
TWOPHASE Kr AND WETTABILITY
TWOPHASE Kr AND WETTABILITY
Craig’s rules of thumb:
Waterwet Oilwet
Connate water saturation
Usually > 2025 % PV Generally < 15%PV Frequently < 10%PV
Cross point of Kr curves
> 50% water saturation < 50% water saturation
Krw @ max water saturation
Generally < 0.3 0.5 may be up to 1
Most of formations are of intermediate wettability
TWOPHASE Kr AND WETTABILITY
Important notice when measuring wateroil Kr:
Reproduce within the sample wettability preference of the formation
Nativestate
• Wettability altered by exposure to oxygen
• Wettability altered through loss of light ends in oil by evaporation (role of asphaltenes)
Restoredstate
• Avoid drying of core that can result in dehydration and collapse of clays (effect on rock permeability)
TWOPHASE Kr AND WETTABILITY
Fluid selection for wateroil and gasoil rel perm tests
Wateroil Kr
• reservoir oil
• formation or synthetic brine (saturated with gas)
• reservoir P,T
Gasoil Kr
• refined or reservoir oil
• saturated gas
• reservoir or lab P,T
CHARACTERISTICS OF WATEROIL REL PERMS
Krw end point Cross point
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
0 0,2 0,4 0,6 0,8 1
Sw
Kr
Krw
Kro
Swi Sor
CHARACTERISTICS OF WATEROIL REL PERMS
For homogeneous rocks: Extrapolate smooth Krw curve to low Sw values Assess Kro values at low So values
0,0001
0,001
0,01
0,1
1 0 0,2 0,4 0,6 0,8 1
Sw
Kr
Krw
Kro
Swi Sor
CHARACTERISTICS OF WATEROIL REL PERMS
Kro vs normalized saturation
Son =normalized oil saturation So =oil saturation Sw =water saturation Swi =irreducible water saturation Sor =residual oil saturation
0,001
0,01
0,1
1 0,1 1
Son
Kro
or wi or
or wi or o
on S S S Sw
S S S S S
− − − −
= − −
− =
1 1
1
o n on ro S K =
n o =Corey exponent
CHARACTERISTICS OF WATEROIL REL PERMS
Krw vs normalized saturation
0,0001
0,001
0,01
0,1
1 0,1 1
Swn
Krw
nw=4
Bump flow to get true Krwend point value
or wi wi w
wn S S S S S − −
− = 1
w n wn po end rw rw S K K * int − − =
n w =Corey exponent
CHARACTERISTICS OF WATEROIL REL PERMS
Corey exponents
0 0,1
0,2 0,3
0,4 0,5
0,6 0,7
0,8 0,9
1
0 0,2 0,4 0,6 0,8 1
Swn
Kr
Krw Krw Kro Kro
no=3
no=4 nw=3
nw=5
CHARACTERISTICS OF WATEROIL REL PERMS
Corey exponents should be consistent with wettability measurements
Wettability Oil Corey exponent, no
Water Corey exponent, nw
Water wet 2 to 4 5 to 8
Intermediatewet 3 to 6 3 to 5
Oilwet 6 to 8 2 to 3
MEASURING WATEROIL REL PERMS
Wettability issues
• intermediate wettability • need for appropriate representation in lab tests • tests at reservoir P,T with reservoir fluids
Restoring reservoir wettability
• thorough cleaning (solvents) • mild drying • saturation with water • establishing Swi
•injection of viscous oil (plugs or composite) •porous plate (plugs or composite) •centrifuge (plugs)
• injection of crude oil • aging (at T for several weeks) • displacement of crude oil with live oil
WATEROIL Kr: MEASUREMENT METHODS
• Steady state
• Unsteady state (dynamic displacement)
• Centrifuge
Fluid 1
Fluid 2
Fluid 1
Fluid 2
∆P 1
∆P 2
Q 1
Q 2
WATEROIL Kr: MEASUREMENT METHODS
SteadyState
L P A KK Q
L P A KK Q
r
r
2
2
2 2
1
1
1 1
∆ =
∆ =
µ
µ
WATEROIL Kr: MEASUREMENT METHODS
• STEADYSTATE
Advantages
• Direct determination of Kr @ large range of S • No mathematical developments • Reservoir conditions (P,T)
Shortcomings
• Time consuming, expensive • Capillary end effects
•long composite cores • Representativity of flow in the reservoir questionable
Fluid 1
Fluid 2
Fluid 1
Fluid 2
∆P 1
∆P 2
Q 1
Q 2
WATEROIL Kr: MEASUREMENT METHODS
w nw Non uniform saturation
Fluid 1
Fluid 2
Fluid 1
Fluid 2
∆P 1
∆P 2
Q 1
Q 2
Solution 1
Numerical Simulations if Pc known Solution 2
WATEROIL Kr: MEASUREMENT METHODS
Unsteadystate method
Water Vr
Oil Vnr
∆P
Q
Analytical calculation only for Pc = 0
•Welge Kro/Krw •JBN Kro, Krw
water oil
Swi
WATEROIL Kr: MEASUREMENT METHODS
Unsteadystate method: experimental procedure
•Core initially @ Swi •Measurement of Keo @ Swi •Water injection @ constant flow rate •Measurement of the produced oil volume, water volume and pressure drop with time •Measurement of Kew @ Sorw •Calculations
Welge Kro/Krw JBN Kro, Krw
WATEROIL Kr: MEASUREMENT METHODS
• UNSTEADYSTATE
Advantages • Fast • Representativity of flow in the reservoir • Reservoir conditions (P,T)
Shortcomings • Calculations not straightforward • Strong assumptions for analytical calculations
•homogeneous samples •Pc=0
• Narrow range of saturations (only after BT)
To overcome shortcomings • Perform in situ saturation measurements (Xray or gray
absorption, CTscanner) • Numerical interpretation of the displacement data
WATEROIL Kr: MEASUREMENT METHODS
UNSTEADYSTATE
• Numerical interpretation of the displacement data •Fluid production •Pressure drop •Saturation profiles
• Reliable capillary pressure •representative of rock/fluids/process
WATEROIL Kr: MEASUREMENT METHODS
UNSTEADYSTATE Numerical Interpretation (1)
0
5
10
15
20
25
30
35
40
0 50 100 150 200 250 300 350 400 450 500
Time (mn)
Cum
ulative oil volum
e (cm3)
V oil exp V oil mod
Fluid production
0
20
40
60
80
100
120
140
160
180
200
0 50 100 150 200 250 300 350 400 450 500
Time (mn)
Overall pressure drop (m
bar)
DP exp DP mod
Pressure drop
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 5 10 15 20 25 30
Core length (cm)
Sw
Swj@40mn Swj@80mn
Swj@120mn Swj@160mn
Swj@200 mn calc40
calc80 calc120
calc160 calc200
Saturation profiles
WATEROIL Kr: MEASUREMENT METHODS
UNSTEADYSTATE Numerical Interpretation (2)
0
5
10
15
20
25
30
35
40
0 50 100 150 200 250 300 350 400 450 500
Time (mn)
Cum
ulative oil volum
e (cm3)
V oil exp O mod new15
Fluid production
0
20
40
60
80
100
120
140
160
180
0 50 100 150 200 250 300 350 400 450 500
Time (mn)
Ove
rall pressu
re drop (m
bar)
DP exp DP mod new15
Pressure drop
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 5 10 15 20 25 30
Core length (cm)
Sw
Swj@40mn
Swj@80mn
Swj@120mn
Swj@160mn
Swj@200 mn
calc40 new15
calc80 new15
calc120 new15
calc160 new15
calc200 new15
Saturation profiles
0.15
0.1
0.05
0
0.05
0.1
0.15
0 0.2 0.4 0.6 0.8 1
Water saturation
Wateroil ca
pillary press
ure (bar)
Pcow old Pcow new
WATEROIL Kr: MEASUREMENT METHODS
UNSTEADYSTATE
Two different Pc Two different Kr sets
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90
Sw
Kr
krw new krow new krw old
krow old
WATEROIL Kr: MEASUREMENT METHODS
• CENTRIFUGE
Advantages • Fast • One speed step sufficient • Stable displacement • Kro over wide saturation range
Shortcomings • Calculations not straightforward • Strong assumptions • Kr of only the produced phase (Kro) • Plugs only @ T but not P (representative wettability?)
GASLIQUID RELATIVE PERMEABILITIES
•GASOIL Kr needed when: •expansion of gas cap •gas injection •solution gas drive •gas storage •gas condensate •WAG
•GASWATER needed when: •active aquifer in gas reservoir •WAG
GASLIQUID REL PERM: CRITICAL GAS SATURATION
Sgc : Gas saturation above which gas is continuous
•Very important for solution gas drive (P < Pbubble) •Less important for gas injection
Depends on: • Pore size distribution, aspect ratio, connectivity • Displacement mechanism (higher for pressure depletion than for gas injection
GASLIQUID REL PERM MEASUREMENTS
•Steadystate gasfloods •Core Plugs or Composite cores •Saturation determination end effects •Room or reservoir conditions (refined oil & inert gas or reservoir oil and gas)
•Unsteadystate gasfloods •Core Plugs or Composite cores •End effects local saturation measurements •Room or reservoir conditions (refined oil & inert gas or reservoir oil and gas)
Son =normalized oil saturation So =oil saturation Sgn =normalized gas saturation Sg =gas saturation Swi =irreducible water saturation Sor =residual oil saturation Sgc =critical gas saturation
n o =Corey exponent for oil (4 to 8) n g =Corey exponent for gas (2 to 4)
GASOIL RELATIVE PERMEABILITIES
• Unsteady state measurements on composite or long cores are the most reliable
• Reduced problems with wettability and end effects • Still experiments at reservoir P, T should be given preference
• Obtain KroKrg curves under representative displacement mechanisms
EQUATIONS 2D DISPLACEMENT
) (
0 0
1
1 1 2
2 2 1 1
2 1
2 2
2
2 2 1
1
1
1 1
S P P P
t S
x u
t S
x u
S S
g x P kk u g
x P kk u
c
r r
= −
= ∂
∂ +
∂ ∂
= ∂
∂ +
∂ ∂
= +
−
∂ ∂
− =
−
∂ ∂
− =
ϕ ϕ
ρ µ
ρ µ
EQUATIONS 2D DISPLACEMENT
( )
2
2
1
1
2 1
2
2
1
1
2
2
1
1 1
1 1 1
1
1
1
1
0
r r
r
c
r r
k k
U g k
k
S P
k k U k
t S
x S
x x S
S U
µ µ
ρ ρ µ
µ µ
ϕ
+
− +
= Φ
∂ ∂
+ = Ψ
= ∂
∂ +
∂ ∂
Ψ ∂
∂ +
∂ ∂
∂ Φ ∂
CASE OF NEGLIGIBLE CAPILLARY PRESSURE
( )
2
2
1
1
2 1
2
2
1
1
1 1
1
1
0
0
r r
r
k k
U g k
k
t S
x S
S U
µ µ
ρ ρ µ
ϕ
+
− +
= Φ
= Ψ
= ∂
∂ +
∂ ∂
∂ Φ ∂
BuckleyLeverett equation
Kr unsteady state @ constant flow rate
'
* '
'
* '
'
*
*
) (
tP P P
Q V K
tP P P
Q V K
V tV V S S
AK LQ P
AK LQ P
o o ro
w w rw
P
w w wi w
o o
w w
− =
− =
− − =
=
=
µ
µ
P*: pressure during flow of only water or only oil at the injection rate Q