fractured reservoirs part 4
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Naturally Fractured Reservoirs
Part 4 Recovery process
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Part 4 - Recovery process
Gravity Drainage
Reimbibition
Diffusion
Imbibition
WOC
GOC
Water drive
Gas drive
Flow mechanisms in fractured reservoirs
AA AWater
Injection
GOCWOC
Depletion Gas Injection
Segregat ion
+Convect ion
within fractures
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Part 4 - Recovery process
WOC in fractures
Single-phase expansion
P=Pb
Sg=Sgc
Expansion and gas drive +
Gas segregation within fractures
Two-phase expansion
(immobile gas)
GOC in fractures
Solution ga
diffusion
to fracture
Convectionin the oil phase
within fractures
Sg= Gas saturation
Sgc= Critic saturation gas
P = Pressure
Pb= Bubble pressure
DIFFUSION
IMBIBITION
WOC
GOC
GRAVITY
DRAINAGEInitial GOC
Init ia l WOC
DIFFUSION
IMBIBITION
WOC
GOC
GRAVITY
DRAINAGEInitial GOC
Init ia l WOC
Main flow mechanism during depletion
Flow mechanisms in fractured reservoirs
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Part 4 - Recovery process
1. WATER DRIVE RESERVOIRS (active aquifer, injection)
Spontaneous (capillary) imbibition and/orgravity effects.
2. GAS DRIVE RESERVOIRS (gas cap expansion, secondary gas-cap, injection)
Gravity drainage.If (Non-equilibrium) GAS INJECTION:
Compositional effects: thermodynamic transfers(swelling, vaporization),diffusion.
Transfers associated to fluid contact movements
Flow mechanisms in fractured reservoirs
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Part 4 - Recovery process
DPmf DPcmf DPfDr.g.c
EXPANSION CAPILLARITY GRAVITY VISCOUS DRIVE
negligible
positive/
negative
effects
positiveeffects
sometimessignificant
W
W O
W
O
Water-Oil matrix-fracture transfer mechanisms
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Part 4 - Recovery process
Drive mechanisms invo lved inwater-oi l matr ix-f racture trans fers:
CAPILLARITY : Pcm>>Pcf DPc # Pcm = (IFT/rp).f(Sw)f(Sw)= dimensionless Pc (Leverett function;
rp = characteristic pore dimension # (8k/f)0.5 : rp varies as k0.5
GRAVITY : Dr.g.H (gravity head)H= c (b lock height)if small blocks or rapid water rise in fractures
H< c if high blocks (or vertical capillary continuity of matrix) or slow water
rise in fractures
VISCOSITY : Pressure gradient in fractures viscous drive in matrix
Water-Oil matrix-fracture transfer mechanisms
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Part 4 - Recovery process
* Water-wet matrix: Po-Pw > 0
- water spontaneously sucked
(imbibed) by matrix
- gravity forces do not much affect
the matrix oil recovery process
* Oil-wet matrix: Po-Pw < 0
- oil trapped within pores (no waterimbibition)
- only gravity forces enable matrix
oil recovery
* Intermediate (mixed) wettability:
Po-Pw > and < 0
- water imbibes partially- gravity forces enhance oil
recovery
Sw i= irreducible water saturat ion Sorw= residual oi l saturat ion (forced dis placement)
-Drgc Sw01-Sorw
oil
Po-Pw
Sw0
1-Sorw
-Drgc Sw01-Sorw
water
oil
Po-Pw
Po-Pw
Capillarity in water-oil systems
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Part 4 - Recovery process
Predominantly
COCURRENTimbibition
Cocurrent and
countercurrent(TOTAL) imbibition
W
W O WW+O
W+O
W+O
W W+O
COUNTERCURRENTimbibition
1D laboratory tests on a waterwet sandstone - Effect of boundary conditions
Ref. :B. Bourbiaux and F. Kalaydj ian: "Experim ental study of c ocurrent and coun tercurrent flows in natural porous media",
SPE Reservoir Engin eering, Aug. 1990.
Specific studies on spontaneous imbibition
W t Oil t i f t t f h i
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Part 4 - Recovery process
Average water saturat ion of the matr ix block at the equi l ibr ium between
gravitat ional and capi l lary forces the end of im bibi t ion (and d rainage) bywater:
--
0
)()(
)(
1=
gc w
ow
weqwo
cwcw dPPSgc
Srr
rr
-Drowgc
Po-Pw
Sw0
Block fully immersed in water
z
c
0
trapped
oil
water
Oil recov ery from matr ix block= (Sweq- Sw i)/(1-Swi)
Ultimate recovery from a block immersed in water
Water-Oil matrix-fracture transfer mechanisms
W t Oil t i f t t f h i
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1Part 4 - Recovery process 1
Driving forces: capillarity (if matrix is water-wet) and gravityCountercurrent and cocurrent flows
Main parameters: - rock-fluid wettability properties- block dimensions (a,b,c) and matrix permeability (km)
- block boundary conditions (speed of WOC rise)
Ultimate recovery = f(Pc curve, gravity head Drgc)Production kinetics = f((a,b,c), km, Pc, Mobilities kr/)
NECESSARY Lab. information:
+Complete Pc curve (Pc's > and < 0)+Relative permeability curves -Drgc
Po -Pw
Sw0
Assessment of oil production from water-drive
Water-Oil matrix-fracture transfer mechanisms
W t Oil t i f t t f h i
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Part 4 - Recovery process
Important reservoir information:
+ For non-water-wet rocks, block height estimation and capillary continuity
assessment is a prerequisite to evaluate oil recovery
+ Assessment offracture-to-matrix permeability ratio: if not very high, viscous
pressure drops in fractures will enhance oil recovery (forced displacementwithin matrix)
Concerning field process implementation: the production scheme and
constraints have to be optimized to synchronize the rate of water-oil contact
rise within fractures and the rate of matrix desaturation.
Assessment of oil production from water-drive
Water-Oil matrix-fracture transfer mechanisms
I fl f bl k h
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1Part 4 - Recovery process
-12-10
-8
-6
-4
-2
0
2
4
6
0 0,2 0,4 0,6 0,8 1
PC wat
PC int
PC oil
WATER/OIL - MATRIX CAPILLARY PRESSURE CURVES
Influence of block shape on recovery
I fl f bl k h
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1Part 4 - Recovery process
0
10
20
30
40
50
60
70
80
0,01 0,1 1 10 100 1000 10000
1phi_wat
2phi_wat
1phi_int2phi_int
1phi_oil
2phi_oil
Water wet
Intermediate
wet
Oil
wet
WATER/OIL - SINGLE VS DOUBLE POROSITY MODELS
One matrix block
Influence of block shape on recovery
Influence of block shape on recovery
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1Part 4 - Recovery process
0
10
20
30
40
50
60
70
80
0,1 1 10 100 1000 10000
10x10x10 1md
20x20x20 1md
10x10x10 4md
4 1 1
T50 = 55
T50 = 27 T50 = 220
WATER/OIL - K AND BLOCK SIZE - WATER WET
Influence of block shape on recovery
Influence of block shape on recovery
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1Part 4 - Recovery process
0
10
20
30
40
50
60
70
0,1 1 10 100 1000 10000
10x10x10
20x20x2
2x20x10
5x5x50
WATER/OIL - BLOCK SHAPE - MIXT WET
Influence of block shape on recovery
Influence of block shape on recovery
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1Part 4 - Recovery process
0
10
20
30
40
50
60
70
0,1 1 10 100 1000 10000
10x10x10
20x20x2
2x20x10
5x5x50
WATER/OIL - BLOCK SHAPE - MIXT WET
5 x 5 x 50
2 x 20 x 10
20 x 20 x 2
10 x 10 x 10
Influence of block shape on recovery
Influence of block shape on recovery
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1Part 4 - Recovery process
0
10
20
30
40
50
60
70
80
0,1 1 10 100 1000 10000
20x20x20
40x40x12.6
SHAPE FACTOR = 4 * (1/a2 + 1/b2 + 1/c2 ) a = b = c = 20, = 0.03 a = b = 40, c = 12.6, 0.03
WATER/OIL - SHAPE FACTOR - WATER WET
Influence of block shape on recovery
Water drive in fractured reservoirs
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1Part 4 - Recovery process
Favourable criteria
Isotropic horizontal fracturepermeability; good knowledgeof
this anisotropy if any
Water wettability
Small blocks (horizontaldimensions)
Long matrix block
Low mo
Good matrix permeability
Water drive in fractured reservoirs
Unfavou rable cri ter ia
Non-ident i f ied ho rizontal fracturepermeabi l i ty anisotro py
Oil wettabil i ty
Large matr ix block s (ho r izontal
d imens ions)
Short matr ix block
Highmo
(Very) low matr ix permeabi l i ty
Gas-Oil matrix-fracture transfer mechanisms
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1Part 4 - Recovery process 1
DPmf DCmfPfr.g.c
EXPANSION GRAVITY VISCOUS DRIVE DIFFUSION
significant
if P
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2Part 4 - Recovery process
- Liquid (oil) always wetting
- Gravity alone is the driving mechanism ofoil recovery
- capillary forces always counteract gravity
forces
- There exists a minimum pressure
(displacement pressure Pd) below whichgas cannot enter the matrix
- Matrix blocks having a height lower than
cmin= Pd/Drg cannot be drained at all. gas
Pg-Po
Sg0
1-Sorg-Swi
Pd
Drgc
Sorg= residual oi l saturat ion
Sw i= irreducible water saturat ion
Capillarity in gas oil systems
Ultimate oil recovery by gravity drainage
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2Part 4 - Recovery process 2
Average gas saturat ion of the matr ix block at the equi l ibr ium between
gravitat ional and capi l lary forces at the end o f gas-oi l drainage:
-
-
gc
gg
go
geqgo
ccg dPPSgc
S)(
0)(
)(
1=
rr
rr
Block fully immersed in gas
z
c
0Gas
Oil recovery from matr ix block = Sgeq/(1-Sw i)
Ultimate oil recovery by gravity drainage
Sg
0
DroggcPg- Po
Pd
Trapped oil
Capillary continuity and gas-oil gravity drainage
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2Part 4 - Recovery process
Existence ofporous bridges between superposed blocks (= capillary
continuity) has two effects:
1. The oil can flow from one block to another: the effective block
height controlling the final oil recovery from the stack of blocks
is equal to the height of the stack;
2. The interfaces between superposed blocks form flow restrictions:
the kinetics of drainage of the stack is less than that of a singleblock of equivalent height.
Capillary
continuity
c
c
Effective
height=2c
(c= vert ical dis tance between 2 fractu res)
Capillary continuity and gas oil gravity drainage
Impact of heterogeneities on gravity drainage
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2Part 4 - Recovery process
Flow barriers may exist within the matrix medium (impermeable beds),
with the following consequences:
the effective block height is reduced (lower matrix oil recovery)
the oil produced from the upper blocks into the fractures justabove the barrier(s) may reimbibe the lower blocks if already drained
by gas (this reimbibition phenomenon will not be significant if
matrix desaturation follows GOC movement in fractures).
Flow
barriers
(km=0)
c
Effective block
height= c/4
Oil
reimbibition
(c= vert ical dis tance between 2 fractures)
p g g y g
Impact of capillary continuity on gravity drainage
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2Part 4 - Recovery process 2
Final Sg profile
(cmin : height of capillary holdup zone)
flow-restricted
porous bridges
Experiment on a stack of 3 blocks: c1> cmin; c2 < cmin ; c3 > cmin
p p y y g y g
cmin
Sgmax
GasDynamicsof drainage
cmin
Oil
1
2
3
cmin
cminc1
c2
c3
Assessment of oil production from gravity drainage
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2Part 4 - Recovery process 2
Driving force: gravity (adverse effect of capillary forces)
Main parameters: - block height (c) and cross-section (if lateral gas invasion)
- matrix permeability (km)
- capillary curve
Ultimate recovery = f(Pc curve, gravity head Droggc)Production kinetics = f(c, (a,b), km, Pc, Oil Mobility kro/o)
NECESSARY Lab. information:
+ PVT data, including IFT versus P
+Pc curve (depending on IFT)
+Relative permeabilities (end-points, shape)
DroggcPg-Po
Sg
0
p g y g
Assessment of oil production from gravity drainage
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2Part 4 - Recovery process
Important reservoir information:
+ Assessment of the effective block height (vertical capillary continuity of thematrix, horizontal flow barriers) is a prerequisite to evaluate oil recovery by
gas-oil gravity drainage
+ Horizontal flow barriers or restrictions within the matrix also control the
recovery kinetics (lateral dimensions of matrix blocks may then play a
significant role)
+ The field production strategy (pressure maintenance or depletion) controls
gas drive efficiency.
+ The contribution ofconvection and diffusion phenomena has to beestimated (from production history, PVT,...) for well-fractured reservoirs and
for small-blocks reservoirs subjected to an external gas drive.
g y g
Influence of block shape on recovery
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2Part 4 - Recovery process
0,00
10,00
20,00
30,00
40,00
50,00
60,00
0,01 0,1 1 10 100 1000 10000 100000
Kinetics improves with
Matrix permeability increase
Final recovery depends
on Block height
here c < cmin
GAS/OIL - SENSITIVITY RUNS : BLOCK HEIGHT and MATRIX PERMEABILITY
Influence of block shape on recovery
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2Part 4 - Recovery process
0,00
5,00
10,00
15,00
20,00
25,00
30,00
35,00
40,00
45,00
0,01 0,1 1 10 100 1000 10000 100000
20 x 20 x 5
5 x 5 x 50
2 x 20 x 10
GAS/OIL - INFLUENCE OF THE MATRIX BLOCK SHAPE
Influence of Block size and Re-imbibition on Recover
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2Part 4 - Recovery process
Impact of vertical block size and block to block re-imbibitionratio on gas oil contact
SPE 93760 Iranian giant field
Oil
reimbibition
ab
c
Oil
Gas
0
1000
2000
3000
4000
5000
6000
1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002Date
Depth
(ft)
GOC Measured Depth
Vertical Block Size = 20 ft
Vertical Block Size = 50 ft
Vertical Block Size = 100 ft
Vertical Block Size = 150 ft
Vertical Block Size = 200 ft
0
1000
2000
3000
4000
5000
6000
1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002Date
Depth
(ft)
GOC Measured Depth
No Re-imbibition
20% Re-imbibition
40% Re-imbibition
60% Re-imbibition
80% Re-imbibition
Full Re-imbibition
Influence of Block size and Re-imbibition on Recover
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3Part 4 - Recovery process
SPE 93760 Iranian giant field
Matrix oil saturation in a selected cell
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1937 1947 1957 1967 1977 1987 1997 2007 2017 2027 2037 2047 2057 2067 2077 2087 2097
Date (year)
OilSaturation(Fraction)
History Match Forecast
Fast Kinetic Slow recovery
C= 80 ft = 20%
C= 20 ft = 85%
So = 1-Swi (14%)
So = Sorg (25.6%)
Gas drive in fractured reservoirs
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3Part 4 - Recovery process
Favourable criteria Unfavou rable cri ter ia
- High fracture permeability - Low fracture permeabi l i ty
(especially vertically);
- Thick oil column - Thin oi l colum n
- Large block height - Small block heigh t and/or presence
(capillary continuity, no barriers) of ho rizon tal permeabi l i ty restr ict ion s
(high m atr ix permeabi l i ty aniso tropy )
- Low mo - Highmo- Good matrix permeability - (Very) low matr ix permeabi l i ty
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