Net Reservoir and Net Pay are required for petrophysical averaging and reservoir modelling.

1. The Difference between Net Pay and
Net Reservoir, how to pick them
and their Application to Reservoir Modelling
Steve Cuddy

2. Outline
• Definitions of Net Pay and Net Reservoir
• How to determine Net Pay and Net Reservoir from
Core and Electrical Logs
• How to correctly upscale Net Reservoir properties for
the 3D reservoir model

3. High Sw
• Common definitions of Net:
• Net Sand removes the shaly intervals
• Net Reservoir removes the low porosity intervals
• Net Pay removes the intervals of high water saturation

4. What is Net?
• Net Reservoir
– The portion of reservoir rock which is capable of storing
hydrocarbon
– Required for upscaling and reservoir modelling
– Relatively easy to pick
• Net Pay
– “The portion of reservoir rock which will produce commercial
quantities of hydrocarbon”- SPWLA
– or The portion of reservoir rock which will produce or help
support production of hydrocarbon over field development
timescales
– Sometimes required to select perforation intervals
– More difficult to pick

5. How can Net Pay be Determined?
• The microlog identifies mudcake
which suggests movable fluids
• When Sxo > Sw indicates
moveable hydrocarbon
• Mud losses – esp. for fractures
• Formation pressure
• Borehole gas chromatography
• Production logs
• NMR
• DST

6. Net Pay
• Usually defined using a Sw and/or permeability cutoff
• But it doesn’t include:
– The ratio of horizontal to vertical permeability (Kh/Kv)
– Standoff distance from the FWL
– Shape of the transition zone
– Gas and water drive
– Draw down
– Water cut
– Fractures
• Most of hydrocarbon above the FWL is potentially producible
• The amount of hydrocarbon produced depends on how hard
we try
• Is Net Pay therefore a function of the oil price?
• Net Pay is difficult to define

7. Net Reservoir
• Net Reservoir is much easier to define than Net Pay
– As it is defined as
– the portion of reservoir rock which is capable of storing
hydrocarbon
• Knowledge of Net Reservoir is essential for:
– Upscaling for reservoir averages
– Reservoir modelling
• Net Reservoir is used to calculate Net/Gross

8. Net/Gross is required calculate the hydrocarbon in place

9. Net Reservoir from Core
• Better vertical resolution than
electrical logs
– 2 inches compared to 2 feet
• Net from core analysis
– Useful where there are sharp
boundaries
– Can be combined with lithology
analysis
• Net from ultraviolet fluorescence
– Only useful for oil that hasn’t been
lost or evaporated
– Gives an upper limit

10. Net Reservoir from Core
• Identify intervals with fluorescence from U/V
photos
– Only for intervals above the Free Water Level
• Porosity histogram picks the porosity cutoff
0.0
0.2
0.4
0.6
0.8
1.0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
Histogram of Core Porosity
Well: E3-P4P SIE-5 SIE-6
Sarvak (Mishrif) Formation
Core Porosity
Intervals with
no fluorescence
Intervals with
fluorescence
0 Porosity (p.u) 30

11. Net Reservoir from Sw vs. Porosity Xplot
• Plot water saturation vs.
porosity above the Free
Water Level (FWL) for all wells
in the field
• This xplot shows how Sw
increases as the porosity
decreases
• Defines the porosity cutoff
where the reservoir rock is
capable of storing
hydrocarbon 0.000.00
0.140.14
0.290.29
0.430.43
0.570.57
0.710.71
0.860.86
1.001.00
0.000 0.000
0.020 0.020
0.040 0.040
0.060 0.060
0.080 0.080
0.100 0.100
0.120 0.120
0.140 0.140
0.160 0.160
0.180 0.180
0.200 0.200
CALC.SW / CALC.PHI Crossplot
Well: 47/2-1 47/3A-11 47/3A-11Z 47/3A-7 47/3E-8 47/3E-9
- - - FEET
Filter: CALC.SW>0&CALC.SW<1&CALC.VSH<.25
CALC.PHI(V/V)
CALC.SW ()
Well Legend:
47/2-1 47/3A-11 47/3A-11Z
47/3A-7 47/3E-8 47/3E-9
1431
1445
14
0
0
0Porosity(p.u)20
0 Water Saturation (%) 100
Each well is shown by a different colour

12. • Calculate the hydrocarbon pore height (HCPH) for each porosity
cutoff for 1 p.u, 2 p.u, 3 p.u etc.
• Plot the HCPH as a function of the porosity cutoff
• This gives you the porosity cutoff where the hydrocarbon kicks in
• It also tells you how much hydrocarbon the reservoir model loses
from by changing this cutoff
HCPH = (1-Sw) * Porosity * Net
Net sand sensitivity to Porosity
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20 25 30 35
Porosity Cut-off
%changeinHCPH
Porosity cutoff
0%changeinHCPH100
0 Porosity (p.u) 35
Net Reservoir from Hydrocarbon Pore Height
HCPH = Net * (1-Sw) * porosity

13. Reservoir high above the FWL has
low saturations of capillary bound
water and hydrocarbon enters
the smaller pores
Reservoir just above the FWL,
with higher porosities, contains
high saturations of capillary
bound water and there is a no
room available for hydrocarbons
Consequently, the Net Reservoir
cut-off varies as a function of the
height above the FWL
Net Reservoir from Water Saturation
Net
Porosity
25 pu 0
Sand
Shale
Gas
FWL
Log derived Sw
100 su 0

14. Net
Porosity
25 pu 0
Sand
Shale
Gas
FWL
Log derived Sw
100 su 0
• The Net Reservoir cut-off varies as a
function of the height above the FWL
• Sw varies as a function of porosity
• BVW = % volume of water in a unit
volume of reservoir
Net Reservoir from Bulk Volume of Water
• BVW varies only as a function of
height above the FWL
• BVW is what is measured by electrical
logs and by core analysis
BVW

15. • Where BVW < Porosity is Net Reservoir
- Hydrocarbon bearing intervals
• Where BVW = Porosity is Non-Net Reservoir
- Shales and tight intervals
• In this example: porosity > 9 p.u
Net Reservoir from Bulk Volume of Water

16. • The SwH Function tells us how water saturation varies as a
function of the height above the Free Water Level (FWL)
• It tells us how the formation porosity is split between
hydrocarbon and water
Net from the Saturation Height Function
0 Water Saturation (%) 100
HeightaboveFWL(Feet)
FWL >
Water
Hydrocarbon
• It tells us the shape of the
transition zone
• It is used to initialize the 3D
reservoir model

17. BVW vs. Height Swh Function
• BVW is independent of facies type,
porosity and permeability
- confirmed by xplots
Free Water Level
• 𝐵𝑉𝑊 = 𝑎𝐻 𝑏
0 Bulk Volume of Water (%) 20
400
Height
Above
FWL
(feet)
0
z-axis colour = well
400
Height
Above
FWL
(feet)
0
0 Bulk Volume of Water (%) 20• Tells us how net varies as a
function of height
Where:
𝐵𝑉𝑊= Bulk Volume Water (Sw*Phi)
𝐻 = Height above FWL
𝑎, 𝑏 = Constants

18. Upscaling
• From ½ foot to the cell size of the reservoir model
• Upscaling porosity and permeability requires net
reservoir to be identified
• Sw-Height functions are used to initialize the reservoir
model. It is essential that the SWHF predicted water
saturations upscale accurately
• This is done by integrating the Sw-Height function
• Unlike other parameters, water saturation must be
pore volume averaged

19. Upscaling Water Saturations
 
 21
2211



SwSw
Sw
= average water saturationSw
 = average porosity
Sw = average bulk volume of water
• Averages are over net reservoir sections
• “A function that predicts BVW from height is especially
appropriate to this application” Paul Worthington
Pore volume averaged

20. Upscaling Permeability
• Log and core permeabilities represent typically 2 feet
• To be used in a reservoir model, the predicted
permeabilities must upscale correctly
• Predicted permeabilities must have the same dynamic
range (standard deviation s) as the core data
Core Permeability Predicted Permeability
0.01 (mD) 1000 0.01 (mD) 1000
s s

21. Core permeability upscaling
Core distribution Linear Regression Fuzzy logic prediction
Frequency Histogram of CORE.CKHL_NC
Well: 15 Wells
Range: All of Well
Filter: CKHL_NC>0.001
0.0
0.2
0.4
0.6
0.8
1.0
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.001
0.01
0.1
1
10
100
1000
Wells:
1. 2/5-1
2. 2/5-12A
3. 2/5-13Z
4. 2/5-17
5. 2/5-2
6. 2/5-3
7. 2/5-4
8. 2/5-6
9. 2/5-8B
10. 2/5-9
11. 2/5-H01
12. 2/5-H02
13. 2/5-H04
14. 2/5-H18
15. 2/5-H34
Statistics:
Possible values 1123
Missing values 0
Minimumvalue 0.00109
Maximumvalue 2159.16626
Range 2159.16517
Mean 59.71616
Geometric Mean 2.78804
Harmonic Mean 0.02935
Variance 15340.71661
Standard Deviation 123.85765
Skewness 6.10523
Kurtosis 80.35227
Median 5.14044
Mode 100.00000
1123
1122
0
1
Frequency Histogram of PERM.KFL
Well: 15 Wells
Range: All of Well
Filter: CKHL_NC>.001
0.0
0.2
0.4
0.6
0.8
1.0
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.001
0.01
0.1
1
10
100
1000
Wells:
1. 2/5-1
2. 2/5-12A
3. 2/5-13Z
4. 2/5-17
5. 2/5-2
6. 2/5-3
7. 2/5-4
8. 2/5-6
9. 2/5-8B
10. 2/5-9
11. 2/5-H01
12. 2/5-H02
13. 2/5-H04
14. 2/5-H18
15. 2/5-H34
Statistics:
Possible values 1120
Missing values 0
Minimumvalue 0.00015
Maximumvalue 926.65411
Range 926.65397
Mean 64.44684
Geometric Mean 3.08146
Harmonic Mean 0.01087
Variance 14082.12021
Standard Deviation 118.66811
Skewness 2.73982
Kurtosis 12.40389
Median 7.58578
Mode 100.00000
1120
1094
26
0
Frequency Histogram of PERM.KLIN
Well: 15 Wells
Range: All of Well
Filter: CKHL_NC>.001
0.0
0.2
0.4
0.6
0.8
1.0
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.001
0.01
0.1
1
10
100
1000
Wells:
1. 2/5-1
2. 2/5-12A
3. 2/5-13Z
4. 2/5-17
5. 2/5-2
6. 2/5-3
7. 2/5-4
8. 2/5-6
9. 2/5-8B
10. 2/5-9
11. 2/5-H01
12. 2/5-H02
13. 2/5-H04
14. 2/5-H18
15. 2/5-H34
Statistics:
Possible values 1120
Missing values 0
Minimumvalue 0.00016
Maximumvalue 3626.16382
Range 3626.16366
Mean 45.29200
Geometric Mean 0.96455
Harmonic Mean 0.01464
Variance 44699.46287
Standard Deviation 211.42247
Skewness 9.67622
Kurtosis 121.92938
Median 1.20226
Mode 6.30957
1120
1088
19
13
0.001 mD 1000• Permeability frequency plots
- Colour represents data from 15 cored wells
• Least squares regresses towards the mean
• Fuzzy logic predicted permeabilities preserves the dynamic
range and matches the core distribution
• Regression techniques are poor at predicting the extremes and
therefore will be incorrect when upscaled

22. Conclusions
• Net Pay
– Is difficult, if not impossible, to define
– Depends on the oil price
• Net Reservoir is reservoir rock capable of storing hydrocarbon
• Net Reservoir can be determined
– Using core, logs and Sw-height functions
– Net reservoir depends on the height above the FWL
• Upscaling requires:
– Net reservoir cut-off for porosity, Sw and permeability
– Correct upscaling for the 3D reservoir model is essential