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27 May, 2011 — College Station, TX Study of Nonideal and Secondary Fractures
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A Numerical Study of Nonideal and Secondary Fractures in Shale-gas Reservoirs using Voronoi Grids
Thesis Defense 27 May 2011 — College Station, Texas
Olufemi OLORODEDepartment of Petroleum Engineering
Texas A&M UniversityCollege Station, TX 77843-3116 (USA)
+1.803.397.7623 — olufemi.olorode@pe.tamu.edu
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Objectives:
●To present an unstructured mesh-maker that is used in gridding complex and non-ideal fracture geometries
●To study the effects of nonplanar and nonorthogonal fractures on reservoir performance
●To study the interaction between secondary and primary fractures
●To assess the validity of single-fracture representation of multiply-fractured horizontal wells
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Motivation:●Cartesian grids do not provide the
flexibility of modeling irregular fracture geometries.
●Cartesian grids require far more grid-blocks, many of which are unnecessary.
●No consensus on the effect of nonideal fracture geometries on production.
●Very little is known about the interaction between induced and hydraulic fractures (Houze et al. 2010).
Voronoi grids showing nonplanar fractures
Cartesian Mesh showing 4 planar fractures
Unnecessary refinement
X
Y
X
Y
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Approach:
324m260 x Magnification
~100μm
Relative Sandstone Pore Diameter
Relative Shale Pore Diameter
Visualize the grids
Develop Meshmaker
Construct Voronoi grids
Analyze rates using log-log plots
Perform simulation
Provide pressure maps where needed
Any bugs?
Base case?
Debug code
Yes
Yes
No
No
Validate with Ecrin
27 May, 2011 — College Station, TX Study of Nonideal and Secondary Fractures
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xmf = n*xf
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Gridding: Single-fracture Representation
3D ViewSRV
Unstimulated Reservoir Volume
X-axis
Y-axis
1 2 3 4 5
xf
n=6
2D View
Horizontal well
Fractures
Y
Z
X
Y
X
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Results: Log-log Rate Profile
●Discussion: Single-fracture Representation of Multiple Fractures■ Fracture interference is absent in single fracture case■ Boundary-dominated flow is not seen in the single fracture case
Single fracture Representation
10 multi-stage fractures
1 month 5 years1 year
30 years
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Results: Distinguishing between kf and wf
●Conductivity is kept constant at 492 md-ft (1.5x10-10 mm-m2).
●Do we see distinct trends at early times?
wf, ft kfrac, md modified
0.010 50,000 0.33
0.049 10,000 0.066
0.098 5,000 0.033
0.328 1,500 0.0099
Table 1—Fracture parameters in field units
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Results: (after porosity modification)
●Porosity modification keeps mass accumulation constant
●Bad news: we cannot distinguish
between kf and wf.
●Good news: we can represent very
minute fracture cells with much bigger cells.
fified mod
newref wwf /
where,
Porosity Modification:
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Background: Nonplanar & Nonorthogonal Fractures
Nonorthogonal fracturelt = ala = a sin θwhere, lt is total length, andla is apparent length
f
e
d
b
c
θ
a
h = a sin θ
Nonplanar fracturelt = b+c+d+e+fla = lt sin θwhere, lt is total length,la is apparent length,
All segments are inclined at angle θ to the horizontal.
3D Schematic of a Nonplanar Fracture
2D Schematic of a Nonplanar Fracture
3D and 2D Schematics of a Nonorthogonal Fracture
Illustration of “Total” and “Apparent” Lengths
27 May, 2011 — College Station, TX Study of Nonideal and Secondary Fractures
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Gridding2D Aerial View of Nonplanar Fractures
2D Aerial View of Nonorthogonal Fractures
Y
X
Y
X
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Results: Nonorthogonal and Nonplanar Fractures
●Discussion:■ Irregularities in the fracture geometry limits flow-regime analysis with
diagnostic rate plots
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Results: Nonorthogonal and Nonplanar fractures
●Discussion:■ The cumulative production initially matches that of a planar fracture
with xf=lt, but drops gradually over time.
xf=lt
xf=la
Nonplanar frac
Nonorthogonal frac
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Gridding: Secondary Fractures
■ Three secondary fracture configurations are studied:
—A secondary fracture that intersects the primary fracture at height, h/4.
—A centered secondary fracture.
—Two secondary fractures at heights h/4 and 3h/4, respectively.
Y
X
Z
h/4
h/2
h/4
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Results: Secondary Fracture Flow Profile
●Parallel half-slope lines depict linear flow into the SRV.
● Increase in rates correspond to the increase in the SRV that is drained into the wells.
●Change in slope at late times indicate outset of boundary-dominated flow.
●NB:Secondary fractures were modeled with infinite conductivity, and are 0.05 mm (0.00016 ft) wide.
2 secondary fracsCentered secondary frac
Secondary frac at h/4
Primary fracs only
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Results: Effect of Secondary Fracture Conductivity
●Discussion:■ Dimensionless rate profiles show a reduction in the linear half-slope
when the dimensionless conductivity of the secondary fractures becomes less than 10 (finite conductivity)
■ This may be useful in optimizing fracture design
kfrac, md Cf, md-ft CfD
3x106 4.92x102 1.67x104
2x106 3.28x102 1.11x104
1x106 1.64x102 5.56x103
2x105 3.28x101 1.11x103
1x105 * 1.64x101 5.56x102
1x104 1.64x100 5.56x101
2x103 3.28x10-1 1.11x101
1x103 1.64x10-1 5.56x100
2x102 3.28x10-2 1.11x100
Table 2—Secondary fracture conductivity parameters
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Results: Effect of Primary Fracture Conductivity
●Discussion:■ Dimensionless rate profiles show a drop in production as the primary
fracture conductivity drops■ Results match those published by Freeman et al. (2010)
kfrac, md Cf, md-ft CfD
5.00x106 4.92x104 1.67x106
5.00x105 4.92x103 1.67x105
5.00x104 4.92x102 1.67x104
5.00x103 4.92x101 1.67x103
5.00x102 4.92x100 1.67x102
1.67x102 1.64x100 5.56x101
3.33x101 3.28x10-1 1.11x101
1.67x101 1.64x10-1 5.56x100
3.33x100 3.28x10-2 1.11x100
Table 3—Primary fracture conductivity parameters
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Conclusions:
●Irregularities in fracture geometry can limit the analysis of these reservoirs with diagnostic plots.
●Production increases as SRV increases for infinite-conductivity secondary fractures.
●All infinite-conductivity secondary fractures with the same SRV have identical flow behavior, while finite-conductivity secondary fractures show a reduction in magnitude of the half-slope line.
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A Numerical Study of Nonideal and Secondary Fractures in Shale-gas Reservoirs using Voronoi Grids
End of Presentation
Thesis Defense 27 May 2011 — College Station, Texas
Olufemi OLORODEDepartment of Petroleum Engineering
Texas A&M UniversityCollege Station, TX 77843-3116 (USA)
+1.803.397.7623 — olufemi.olorode@pe.tamu.edu
27 May, 2011 — College Station, TX Study of Nonideal and Secondary Fractures
O.M. Olorode
Questions?
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Parameters SI Unit Field Unit Frac half-length, xf 90 m 300 ftFrac width, wf 3 mm 0.00984 ftFrac spacing, df 100 m 328 ftWell length, Lw 1200 m 4000 ftNumber of fracs 12 12
Reservoir thickness, h 100 m 330 ftPermeability, kshale 1.0x10-19 m2 1.0 x10-4 ft
Frac permeability, kfrac 5.0x10-11 m2 5.0 x104 ftPorosity, 4 % 4 %Frac porosity, frac 33 % 33 %Temperature, T 93.33 0C 200 0FWell radius, rw 0.1 m 0.32 ft
Reservoir pressure, pi 3.45x107 Pa 5000 psiaWell pressure, pwf 3.45x106 Pa 500 psia
Table 1.1—Representative Barnett Shale-gas Parameters
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wfiD ppkh
2.141
2
0002637.0
ftD xc
ktt
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