av Per Avseth

1. Using rock physics
to reduce seismic exploration risk
on the Norwegian shelf
Per Avseth
Adjunct Professor, NTNU
Geophysical Advisor, Odin Petroleum
Lunch seminar, Oslo, 22/5-2012

2. Rock physics – the bridge between
geology and geophysics!
Seismic data Reservoir geology
Qualitative
interpretation
Rock physics analysis
Constant Contact
Cement Cement
Elastic Modulus
Quantitative interpretation
of physical rock properties,
Friable
Initial
Sand lithologies and pore fluids
Pack
0.30 0.35 0.40
Porosity

3. Outline
• The rock physics link = the rock physics
bottleneck
• Seismic fluid sensitivity and geological processes
• Snap-shot examples from the Norwegian shelf
• The issue of scale
• The future of rock physics

4. 3 big challenges in seismic reservoir
characterization using rock physics!
• More unknown variables than known observables!
• Fluid (and stress) sensitivity can vary drastically, not only
from one field to another, but within a given field!
• What is valid at microscale is not necessarily valid at
seismic scale!

5. The Rock Physics Bottleneck
From seismic data we can obtain only 3 (possibly 4) acoustic
properties: Vp, Vs, density, (and Q). Very often we have reliable
estimates of only 1 or 2 (AI and Vp/Vs).
Seismic Reservoir
Rock Physics Properties
Attributes
Properties
Traveltime Porosity
Vnmo Vp Saturation
Vp/Vs Vs Pressure
Ip,Is Density Lithology
Ro, G Q Pressure
AI, EI Stress
Q Temp.
anisotropy Etc.
etc

6. The rock physics bottleneck: Example from Barents Sea
Challenge: More unknowns than independent measurements.
 We need to constrain by local geology!
Increasing burial
(compaction) Increasing porosity
Increasing clay volume Increasing HC saturation

7. Rock Physics Templates (Ødegaard and Avseth, 2004)
1) Increasing shaliness 1) Decreasing effective pressure
2) Increasing cement volume 2) Increasing gas saturation
3) Increasing porosity

8. Seismic fluid sensitivity
- controlling factors
• Grain contacts (pressure and cement)
• Poreshape and pore stiffness (e.g. cracks)
• Porosity
• Mineralogy
• Saturation pattern and scale (patchy vs. uniform)
• Viscoelastic effects of fluid movement
• Relative contrast (cap-rock properties)

9. Press and guess!
Whats inside the container?
Compressibility of dry rock:
1 = 1 φ
+
K dry K mineral Kφ
Compressibility of pore space
1 = 1 ∂v pore
K φ v pore ∂σ
Compressibility of saturated rock:
1 ≈ 1 φ
+ K +K
K sat K mineral φ fluid

10. Grane versus Glitne reservoir sands
Constant Contact
Cement Cement
Elastic Modulus
Constant
Constant Contact Cement
Contact Cement
Cement Fraction (2%) Line Line
Cement Fraction (2%) Line
Line
3.53.5
Initial
Friable Sand
Vp (km/s)
Pack
Vp (km/s)
0.30 0.35 0.40
3.0 3 Well #2
Grane sst Porosity
2.52.5
Unconsolidated
Unconsolidated Glitne sands
Well #1
Line
Line
0.25 0.3
0.30 0.35
0.35 0.4
0.25 Porosity
0.40
Porosity

11. SEM images and XRD reveal quartz cement
Unconsolidated Cemented
Cement rim
(Glitne) (Grane) 4000
Si
Counts
Well #1 Uncemented Well #2 Cemented
2000
C
O
0.25 mm 0.25 mm 0
0 2 4
Energy (keV)
Back-scatter light Cathode lum. light Grain
SEM back-scatter image: Well #2 SEM cathode-luminescent image:
4000 Si
Well #2
Counts
2000
O
0.1 mm C
0.1 mm
0
0 2 4
Energy (keV)
Qz-grain
Qz-cement rim

12. North Sea compaction trends of
sands and shales

13. Couppled rock-physics and diagenesis
modeling (Helset et al., 2004)
0
Exemplar modelling
20
0 (Lander and Walderhaug)
40
0
60
0 Porosity Contact cement model
80
0 Cement volume 4.00
10 0
0
Vp (km/s)
12 0
0
14 0
0
16 0
0
3.00
18 0
0
ep )
D th(m
20 0
0
22 0
0
Friable sand model
24 0
0
26 0
0
2.00
28 0
0 0.100 0.200 0.300 0.400
30 0
0 phi (frac)
0 5 10 15 20 25 30 35
R c F ctio s(%
o k ra n )
CrePro (%
o o sity ) Ma CrePro (%
e s. o o sity ) Qa ce e t (%
u rtz mn )

14. Couppled rock-physics and diagenesis
modeling (Helset et al., 2004)
0
Exemplar modelling
20
0 (Lander and Walderhaug)
40
0
60
0 Porosity
80
0 Cement volume
10 0
0
12 0
0
14 0
0
16 0
0
18 0
0
e th )
D p (m
20 0
0
22 0
0
24 0
0
26 0
0
28 0
0
30 0
0
0 5 10 15 20 25 30 35
R c F ctio s(%
o k ra n )
CrePro (%
o o sity ) Ma CrePro (%
e s. o o sity ) Qa ce e t (%
u rtz mn ) Note decreasing fluid sensitivity with depth and diagenesis
Ma Qa ce e t (%
e s. u rtz mn )

15. Using rock physics to estimation of cement volume
(Example from Alvheim Field)
Constant Contact
Cement Cement
4500 10
Elastic Modulus
4000
Qz 9
8
3500 Constant cement
trends
Vs
7
3000 Initial
6 Friable Sand
Increasing cement volume
Cement volume
Pack
2500 5
0.30 0.35 0.40
Vs (m/s)
Dvorkin-Nur Porosity
4
2000
contact cement
3
Qz-cement
1500 Picture 2
2
1000
1
500 Shale 0
0 -1
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
Porosity
Porosity

16. Cement estimation vs. depth

17. Bayesian lithology and fluid prediction constrained by spatial
coupling and rock physics depth trends
(Rimstad, Avseth and Omre, 2012)
(Rimstadi

18. Estimated depth trends (well 1)

19. Rock physics model w/uncertainties
estimated from Well 1 (depth integrated)
shale
Shale
Brine sand
Oil sand
Gas sand

20. 3-D seismic prediction results
With depth trends Without depth trends
Red=gas Green=oil

21. From loose sediments to consolidated rocks – what
happens to fluid and stress sensitivity?
Porosity
Porosity
Loose sands:
• Large fluid sensitivity
(Gassmann theory works well)
• Large stress sensitivity
(Hertz-Mindlin theory applies)
Gullfaks (loose sands)
Statfjord (consolidated)
Consolidated sandstones:
• Reduced fluid sensitivity
(Gassmann theory works as long as pores are connected)
• Reduced stress sensitivity
(Hertz-Mindlin theory does not apply to cemented grain contacts. Dvorkin-
Nur ignores stress-sensitive grain contacts)

22. 4D anomalies; Gullfaks vs. Statfjord
Picture 72
Before Water injection After water injection
(Duffaut and Landrø, 2007)
σ diff ~6 MPa  σ diff ~0-1 MPa
Water injector offline Water injector online
Top Target
σ diff ~15 MPa  σ diff ~6-7 MPa

23. Fluid and pressure sensitivity in Gullfaks versus Statfjord Fields
(Duffaut, Avseth and Landrø, 2011)

24. Troll East time shift analysis
(Avseth, Skjei and Skålnes, 2012)
Seismic observations
(courtesy of Åshild Skålnes, Statoil)
Base Tertiary
Top Draupne
Cretaceous overburden Top Sognefjord
Top Fensfjord
Gas coloumn

25. Geologic overview (schematic), Troll East
Well A Well B
Compaction and
Compaction
depositional trend
trend
Draupne Fm
GWC
Sognefjord Fm
Fensfjord Fm

26. Shear modulus versus porosity
Sognefjord Formation
Contact cement model
9
x 10
15
Well B
Shear modulus (Pa)
Diagenesis (east)
10
5 Well A
(west)
Friable sand model
0
0.1 0.2 0.3 0.4
Porosity

27. Timeshift at GOC
Seismic observations
Modelled time shifts (courtesy of Åshild Skålnes, Statoil)
4 6
x 10 1.5
6.745 31/3-S-41 1.2
1.1
6.74 31/3-1
3.5 1
6.735 0.9 1
31/6-6
6.73 dTWT
0.8
UTM-Y
3
6.725
Well A
31/6-1
31/6-A-37
(ms)
0.7
Well B
31/6-5 0.6
0.5
6.72 0.5
2.5 31/6-2
31/6-B-6H 0.4
6.715
31/6-8 0.3
25.35
6.71
5.4 1.4 5.45 1.6 5.5 5.55
0.2 0
1 1.2 1.8 2
UTM-X 5
x 10

28. Barents Sea; a challenging area due complex
tectonic and uplift episodes
(Ohm et al., 2008)

29. Compaction trends – 7120/1-2
CC
MC
MC
Torsk
Transition
Kolmule
zone
CC
7120/1-2

30. Skalle fluid and facies classification results
(Lehocki, Avseth, Buran and Jørstad, 2012, EAGE Copenhagen)
Fluids Facies
Pre-drill
(Myrsildre
well only)
Post-drill
(Skalle well)

31. Be aware of scale effects!
0.63 mm
2µm

32. Future of rock physics
(as I see it…)
• More integration with basin modeling
• Using rock physics trends to constrain
migration and full waveform inversions
• Rock physics of EM, gravity and seismic
integrated.
• Rock physics of source rocks and
unconventionals (practical recipes and
computational revelations).

33. Rock physics modeling of geological processes:
From granular rocks to cracked media (Avseth and Johansen, 2012)
Elastic modulus
DEM HSUB CCT
Mineral
point alpha=1.0
0.01 0.1
Decreasing
aspect ratio
Initial
contact
cement
Porosity
Critical
porosity

34. Ksat and Kdry versus Porosity
10
x 10
6
5 α = 1.0
4
α = 0.1
K (Pa)
3 Wet rock
α = 0.01
2 5% contact
Dry rock cement
1
0
0 0.1 0.2 0.3 0.4
Porosity

35. RPT analysis of tight gas sandstone w/cracks
(Bakhorji, Mustafa, Avseth and Johansen, 2012)
2.2 0.8
2
0.6
Vp/Vs
Brine rock
Swt
1.8
0.4
1.6
0.2
Dry rock
1.4
4 6 8 10 12 14 16
AI

36. Conclusions
• Rock physics is both a bridge and a bottle-neck between
geophysics and geology.
• Better integration with geology can help us constrain the
non-uniqeness in quantitative interpretation.
• Be aware of the rock type and associated rock stiffness
before you look for hydrocarbons using seismic data.
• If rocks are well cemented, it can be hard to detect oil
from seismic. The oil-window seems to be located around
the depth where reservoir sands start to be cemented. In
the Barents Sea, the oil window is probably within stiffer
rocks than in the North Sea and the Norwegian Sea.
• At the end of the day, remember that seismic is the sound
of geology!

37. Let’s rock together!
Geologist Geophysicist

38. Acknowlegdements
• Thanks to Geoforskning.no for the invitation
• Thanks to Spring Energy for sponsoring this event
• Thanks to Statoil and Lundin-Norway w/licence partners for
data on various fields on the Norwegian shelf.
• Thanks to everybody who has inspired me!
• Thanks to everone who has contributed!