Download - New Method for Simulation Of Fractures
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SIMULATION OF DISCRETE FRACTURE NETWORKS USING FLEXIBLE VORONOI GRIDDING
Zuher SyihabDavid S. Schechter
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Outline
• Problem Statement
• Objectives
• Introduction to Gridding Techniques
• Modeling Of Discrete Fracture Network (DFN) Using Voronoi Gridding
• Conclusion
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PROBLEM STATEMENT
• Complex reservoir geometry (faults, fractures, etc)
• Limitation of the existing approach
• Fracture aperture measurements using X-Ray CT Scan.
• Capabilities of existing reservoir simulators
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DUAL POROSITY MODEL
Idealization of fractured reservoirs (Warren and Root, 1963)
• Highly fractured media• Connected fractures• No flow occurs between matrix blocks
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DUAL POROSITY MODEL - LIMITATIONS
• Not applicable for disconnected fractured media• Not suitable to model a small number of large-scale
fractures
Discrete Fracture Network (DFN)
Model
After SPE 79699 Karimi-Fahd, M., Durlofsky, L. J., and Aziz, K
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Fracture
Matrix
• Fractures are represented explicitly.
• Disconnected and isolated fractures
• Complex fractured porous media
• Difficult to be modeled with conventional rectangular grid system
• Current DFN model assumes the fracture apertures are uniform
DISCRETE FRACTURE NETWORK (DFN)
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OBJECTIVES• Develop a general flexible mesh generation
technique based on Voronoi diagram algorithm.• Developing a black-oil reservoir simulator to model
fractured and unfractured systems.• Honoring experimental work by incorporating
fracture aperture distribution into simulation model.• Performing simulation of a system with complex
intersecting fractures and fracture networks generated using fractal approach
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Gridding Techniques
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• Globally Orthogonal Grid
• Corner Point Grid
• Locally Orthogonal
Grid (PEBI/VORONOI)
(a) point-dristributed (b) block-centered
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History & Application of Voronoi Grid
• It was first applied by Heinrich into the reservoir simulation (1987).
• Heinmann named this grid as a PEBI grid (1989)
• Economides et. al applied PEBI grid to model horizontal wells (1991)
• Palagi studied the PEBI grid generation method (1994).
• Chong et. al had also shown that the kind of grid is able to reduce grid orientation effect (2004)
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VORONOI AND DELAUNAY TRIANGULATION
For a set S of points in the Euclidean plane, the unique triangulation DT(S) of S such that no point in S is inside the circumcircle of any triangle in DT(S).
The Voronoi grid is formed by the perpendicular-bisectors of the edges of the Delaunay triangles.
Delaunay Edges
Voronoi Edges
Circumcircle: a unique circle that passes through each of the triangles three vertices
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MODELING DFN Workflow
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Fracture Geometry & Characterization
1. Geometry- Number of fracture- Size and orientation
2. Apetrure distribution
Populate points to generate fracture
(FRACTURE BLOCKS)
Populate points surrounding fractures
(MATRIX BLOCKS)
Add Points for fracture blocks in the
computational domain
Connection list and its properties
. . . . . . . .
. . . . . . . . . . . . . . . .
9w
matrix
matrix
w = fracture width
Flow Connection
AdditionalNodes forFracture
- Neural network- Outcrop- Kim’s network (fractal)
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Geometrical domain Computational domain
matrix
No Flow connection
w
matrix
matrix
Flow connection
MODELING DFN(Fracture Gridding)
(Line = fracture)
w = fracture width
Flow Connection
AdditionalNodes forFracture
Voronoi edge
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2wk f
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A B C A’ B’ C’
D E F D’ E’ F’
Geometrical domain Computational domain
APERTURE DISTRIBUTION AND VOLUME CORRECTION
d1 d1’ ‘
d2 d2’ ‘
df
d2’ ‘
The bulk volume of fracture segments can be computed based on given fracture apertures.
The bulk volume of the matrix block adjoining with the fracture should be corrected due to the volume taken by the fractures
Aperture distribution
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Fracture Network & Voronoi AlgorithmMultiple-FractureSingle-Fracture
Voronoi Edges
Voronoi nodes
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1 2
3
4 5
6
7
8
F(1,2) Fracture-2
Fracture-1
F(2,1)
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FRACTURES AND VORONOI DIAGRAM/PEBI (Example)
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Kim’s Fracture Network(Fractal Geometry)
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Implemented using Visual basic (GUI for pre & post processor) and C++ (processor)
Fully implicit numerical method.
3D, 3-Phase black oil simulator.
Structured and unstructured grid systems.
Grid refinement features.- Radial-like grids- Hexagonal grids- Rectangular grids- Radom
Validated with analytical solution (Pressure Transient Analysis)
Compared with IMEX (CMG) for homogeneous & heterogeneous cases. (25 and up to 150,000 grid blocks in desktop PC).
DFN Simulator
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MATERIAL BALANCE EQUATIONS
Rate of accumulation =
n
e
p
n
e
p
B
SV
B
SV
t
)1(
1
Net flow rate =
NCon
ieiListConq
1)(..
,
e = Evaluated cell NCon = Number of connection of cell#e. Con. List(i) = the ith element in the connection list of cell#e
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nneiei eiei
MTq cei pZpei
Ncells
e
n
e
p
n
e
pNCon
i
neiListCon
neiListConeiListCon B
SV
B
SV
tMT
1
)1(
1
1)(.
1)(.)(.
1
11,
1,
1, n
ewn
egn
eo SSS
e
Con. List(i)
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RESIDUAL FUNCTIONS
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Ncells
eosc
n
eo
op
n
eo
opN
i
n
eion
eioein
o qB
SV
B
SV
tMTr
1
)1(
1
111 1
Ncells
e
n
ew
wpnsw
n
eo
opnso
n
eg
gpN
i
n
eign
eigein
g B
SVR
B
SVR
B
SV
tMTr
1
1
1
1
1
1
1
111 ...1
scg
n
ew
wpnsw
n
eo
opnso
n
eG
Gp qB
SVR
B
SVR
B
SV
...
Ncells
ewsc
n
ew
wp
n
ew
wpN
i
n
eiwn
ewiein
w qB
SV
B
SV
tMTr
1
)1(
1
111 1
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Wellbore ModelingCartesian Grid Block
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)( wfi PPJq
B
k r
w
or
rkh
Jln
2
)k/k( + )k/k(
k/k + k/k 0.28 = r 4/1
yxxy4/1
2yx
2xy
2/1yx
o
X
Peaceman’s Well model
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Wellbore ModelingArbitrary Polygon
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Palagi’s Well model
ij
ij b
i
j
ij d
Center of grid
Ndb ij /tan/
NNdr ijo /tan
2exp
Regular Polygon
j ij
ijij
ijj
o
d
b
dd
b
r
)ln(
exp
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THE SIMULATOR(Implementation Technique)
jiwjigjio aaa ,,, ,,
Create Control Volume Objects
Connection List
Calculate flow coef.
PVT ID
Rock ID
etc
Connection typeVectorization
Solve the matrix usingSparse Matrix Solver (SparseLib++)
(BICG-STAB/GMRES/RI/BICG/CG)
Residual Error Checking
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SIMULATION WORKFLOW
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GRID MODEL
Chapter 2 PVT Table(s)- Oil, Gas, Water
Rock Properties - Relative Permeability Curves - Capillary Pressure
Wells - Locations - Well type (producer/Injector)
PROPERTIES ASSIGNMENT - Constrains (Rate, BHP, etc)k L , k v and
INITIALIZATIONp i , s wi , s gi , s oi
DATA COLLECTION
t = 0
START
A
k = 0, n = 0
CONSTRUCT RESIDUAL FUNCTIONSFOR OIL, GAS & WATER
Eq.3.32. Eq.3.33 & Eq.3.34
PERTURB THE RESIDUAL FUNCTIONAND COMPUTE THE JACOBIAN, J
Eq.3.44
SOLVE THE LINEAR EQUATIONUSING SparseLib++ Solver
Eq.3.45
TEST THE SOLUTIONS
p k+1 , s wk+1 , s g
k+1 & s ok+1
Eq.3.41, Eq.3.42 & Eq.3.43
IS ABS(rok+1 ) <=TOL ?
IS ABS(rwk+1 ) <=TOL ?
YES IS ABS(rgk+1 ) <=TOL ? NOnext time step update fluid & rock properties
iter, k = 0 increase of iterations
time = end of simulation (tMAX)
A
n = n + 1t = t + tk = 0
k = k + 1
Update PVT and Rock properties
(k=k+1)
Update PVTand Rock
properties(n = n+1)
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VALIDATION AND COMPARISON STUDY
DFN simulation and analytical model (Pressure Transient Solution)
DFN simulation and IMEX on modified SPE-1 comparative study
DFN simulation and IMEX on heterogeneous case
DFN simulation and IMEX on unstructured grid case
DFN simulation and dual-porosity shape factor
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DFN SIMULATOR & ANALYTICAL MODEL(Constant Pressure Boundary)
p = 4790 psia
p =
479
0 ps
ia
p =
479
0 ps
ia
p = 4790 psia
k = 215.0 mdh = 100.0 ftpi = 4790 psia
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DRAWDOWN & BUILDUP DERIVATIVE PLOTS(Constant Pressure Boundary)
Pressure and Pressure Derivative Plot
0.1
1
10
100
0.1 1 10
t, hour
p-p(
0) a
nd p
' (ps
ia)
Pressure and Pressure Derivative Plot
0.1
1
10
100
0.1 1 10 100 1000
t, hour
p-p(
0) a
nd p
' (ps
ia)
Radial Flow Regime
Boundary Effect
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DFN & IMEX ON MODIFIED SPE-1 COMPARATIVE STUDY
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DFN & IMEX ON HETEROGENEOUS CASE
Comparison on Block Pressure Profile
CMG and DFN with 2,500 Gridblocks (25 X 25 X 4)
(Heterogeneous Case)
4550
4600
4650
4700
4750
4800
4850
0 20 40 60 80 100 120 140 160 180 200
Time, days
Pre
ss
ure
, ps
ia
DFN(1,1,1)
CMG (1,1,1)
DFN(25,25,1)
CMG(25,25,4)
Relative Error
Maximum : 0.087%
Average : 0.025%
KX / KY
(1,1,1)
(25,25,4)
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Block Pressure Profile
4680
4700
4720
4740
4760
4780
4800
0 20 40 60 80 100 120Time, days
Pre
ss
ure
, ps
ia
CMG
RZ-REGULAR
REGULAR
HEXA
TRIANGLE
IRREGULAR
DFN & IMEX ON UNSTRUCTURED GRID CASE
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DFNSIM AND DUAL-POROSITY SHAPE FACTOR
“shape factor” is the value to quantify the matrix-fracture drainage in the dual-porosity model.
Matrix-fracture drainage in the dual-porosity: *fm
m
b
ppk
V
Fracture width, w 0.001 ft
Fracture spacing, Lx=Ly 100 ft
Fracture permeability, kf 10,000 md
Matrix Permeability, km 0.0001 md
Formation Volume Factor, Bo 1.0 RB/STB
Viscosity, o 0.7 cp
Bulk Volume, Vb 100x100x20 = 200,000 cu-ft
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DFNSIM AND DUAL-POROSITY SHAPE FACTOR
2.28
2L
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DFNSIM AND FRACTURE APERTURE DISTRIBUTION
Descriptions CASE 5.A1 CASE 5.A2
Grid dimension 33x33x1 33x33x1
Fracture spacing 1,220 ft 1,220 ft
Model width/ Length 5,380.4 ft 5,380.4 ft
Model thickness 100 ft 100 ft
Matrix permeability 50 md 50 md
Fracture
permeability
Constant
9,055 md
Log-normally distributed
24 md – 300 D (mean = 9,055 md)
Matrix porosity 0.25 0.25
Fracture porosity 0.5 0.5
Fluid properties SPE-1 SPE-1
Initial conditions SPE-1 SPE-1
Other rock
propertiesSPE-1 SPE-1
Producing rate Oil, 15,000 STB/D Oil, 15,000 STB/D
Minumum produce
BHP1,000 psia 1,000 psia
Injection rate Gas, 50 MMSCF/D Gas, 50 MMSCF/D
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0.0
2,000.0
4,000.0
6,000.0
8,000.0
10,000.0
12,000.0
14,000.0
16,000.0
0 1 2 3 4 5 6 7 8 9 10
BHP,
psi
a
Time, Years
Uniform fracture apertures
Log-Normally Distributed Fracture apertures
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
0 1 2 3 4 5 6 7 8 9 10
Reco
very
Fac
tor,
%
Time, Years
Uniform fracture apertures
Log-Normally Distributed Fracture apertures
DFNSIM AND FRACTURE APERTURE DISTRIBUTION
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DFNSIM AND ISOLATED FRACTURE NETWORK
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0
50
100
150
200
0.00 0.50 1.00 1.50 2.00
GO
R, M
SCF
/ST
B
Time, year
NO FRACTURE
ISOLATED FRACTURES
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.00 0.50 1.00 1.50 2.00
Oil
satu
rati
on, f
ract
ion
Time, year
NO FRACTURE
ISOLATED FRACTURES
DFNSIM AND ISOLATED FRACTURE NETWORK
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SIMULATION ON FRACTAL DISCRETE FRACTURE NETWORK
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2
4
6
30
210
60
240
90
270
120
300
150
330
180 0
Rose Diagram of FDFN
-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
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SIMULATION ON FRACTAL DISCRETE FRACTURE NETWORK
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SIMULATION ON FRACTAL DISCRETE FRACTURE NETWORK
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Numerical ParametersNo fracture, isolated and Connected Fractures
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Numerical Controls No Fracture Isolated Fractures Complex Fractures
Maximum residual error 1.0E-4 1.0E-4 1.0E-4
Max. Newton iteration 25 25 25
Max. linear solver iteration 40 40 140
Linear solver tolerance 1.0E-5 1.0E-5 1E-5
Time step 152 324 2,045
Newton iteration 976 6,576 34,285
Solver iteration 28,315 216,445 1,420,171
Solver failure 0 5 103
Time step cut 7 183 228
Simulation time 458 sec. 8,009 sec. 56,125 sec.
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CONCLUSION
• Dual Porosity models are not applicable for small scale and disconnected fractured media.
• The DFN simulator provides results in good agreement with commercial finite-difference simulators in the cases in which direct comparisons are possible.
• Fracture aperture distribution can be descritized using DFN model.
• DFN model using Voronoi grid system can be used for fractured and unfractured system.
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• The aperture distribution plays very important role reservoir performance.
• Numerically, simulation on fractured systems, whether disconnected or connected, are very challenging. It requires an extensive amount of time to build the grid model and run the simulation.
• DFN simulator capability for multiple reservoir has been tested and it can be a potential tool for sensitivity studies.
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CONCLUSION