Fractured Reservoirs Part 4

7/15/2019 Fractured Reservoirs Part 4

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Phone: +33 1 47 08 80 00

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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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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



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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



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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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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



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* 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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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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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




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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




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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


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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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




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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




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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




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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




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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


Water drive in fractured reservoirs


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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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DPmf DCmfPfr.g.c

EXPANSION GRAVITY VISCOUS DRIVE DIFFUSION

significant

if P


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- 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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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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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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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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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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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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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



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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



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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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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





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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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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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

Documents

Fractured Reservoirs Part 4