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Using rock physics to reduce seismic exploration risk
on the Norwegian shelf
Per AvsethAdjunct Professor, NTNU
Geophysical Advisor, Odin Petroleum
Lunch seminar, Oslo, 22/5-2012
Rock physics – the bridge between geology and geophysics!
Seismic data Reservoir geology
Qualitativeinterpretation
Rock physics analysis
Quantitative interpretationof physical rock properties,lithologies and pore fluids
0.30 0.35 0.40
Ela
stic
Modulu
s
Porosity
ContactCement
InitialSandPack
Friable
ConstantCement
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
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!
The Rock Physics Bottleneck
Seismic AttributesTraveltimeVnmoVp/VsIp,IsRo, GAI, EIQanisotropyetc
Rock PhysicsProperties
VpVsDensityQ
ReservoirProperties
PorositySaturationPressureLithologyPressureStressTemp.Etc.
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).
The rock physics bottleneck: Example from Barents SeaChallenge: More unknowns than independent measurements.
We need to constrain by local geology!
Increasing burial (compaction) Increasing porosity
Increasing clay volume Increasing HC saturation
Rock Physics Templates (Ødegaard and Avseth, 2004)
1) Increasing shaliness2) Increasing cement volume3) Increasing porosity
4) Decreasing effective pressure5) Increasing gas saturation
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)
Press and guess!Whats inside the container?
1Kdry
= 1Kmineral
+K
1K
= 1v pore
v pore
1Ksat
1Kmineral
+
K + K fluid
Compressibility of dry rock:
Compressibility of pore space
Compressibility of saturated rock:
Grane versus Glitne reservoir sands
2.5
3
3.5
0.25 0.3 0.35 0.4
Vp
(km
/s)
Porosity
Contact CementLine
UnconsolidatedLine
ConstantCement Fraction (2%) Line
Well #1
Well #2
UnconsolidatedLine
Constant Cement Fraction (2%) Line
Contact CementLine
0.25 0.30 0.35 0.40
2.5
3.0
3.5
Porosity
Vp
(km
/s)
Glitne sands
Grane sst0.30 0.35 0.40
Ela
stic
Modulu
s
Porosity
ContactCement
InitialSandPack
Friable
ConstantCement
SEM images and XRD reveal quartz cement
Well #2 Cemented
0.25 mm
Well #1 Uncemented
0.25 mm
SEM cathode-luminescent image: Well #2
0.1 mm0.1 mm
SEM back-scatter image: Well #2
Unconsolidated(Glitne)
Cemented(Grane)
Back-scatter light Cathode lum. light
Qz-cement rim Qz-grain
4000
2000
00 2 4
Co
un
ts
Energy (keV)
CO
Si
Cement rim
4000
2000
0
Co
un
ts
0 2 4Energy (keV)
Si
OC
Grain
North Sea compaction trends of sands and shales
Couppled rock-physics and diagenesis modeling (Helset et al., 2004)
Core Porosity (%) Meas. Core Porosity (%) Quartz cement (%)Meas. Quartz cement (%)
Rock Fractions (%)35302520151050
Dept
h (m
)
3 000
2 800
2 600
2 400
2 200
2 000
1 800
1 600
1 400
1 200
1 000
800
600
400
200
0
2.00
3.00
4.00
0.100 0.200 0.300 0.400
phi (frac)
Vp
(km
/s)
Cement volumePorosity
Exemplar modelling
(Lander and Walderhaug)
Friable sand model
Contact cement model
Couppled rock-physics and diagenesis modeling (Helset et al., 2004)
Core Porosity (%) Meas. Core Porosity (%) Quartz cement (%)Meas. Quartz cement (%)
Rock Fractions (%)35302520151050
Dept
h (m
)
3 000
2 800
2 600
2 400
2 200
2 000
1 800
1 600
1 400
1 200
1 000
800
600
400
200
0 Exemplar modelling
(Lander and Walderhaug)
Cement volumePorosity
Note decreasing fluid sensitivity with depth and diagenesis
Using rock physics to estimation of cement volume
(Example from Alvheim Field)
0.30 0.35 0.40
Ela
stic
Modulu
s
Porosity
ContactCement
InitialSandPack
Friable
ConstantCement
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.450
500
1000
1500
2000
2500
3000
3500
4000
4500
Porosity
Vs
(m/s
)
-1
0
1
2
3
4
5
6
7
8
9
10
Increasing cement volume
Cem
ent
volu
me
Shale
Qz
Dvorkin-Nurcontact cement
Constant cementtrendsVs
Porosity
Qz-cement
Cement estimation vs. depth
Bayesian lithology and fluid prediction constrained by spatial coupling and rock physics depth trends
(Rimstad, Avseth and Omre, 2012)(Rimstadi
Estimated depth trends (well 1)
Rock physics model w/uncertainties estimated from Well 1 (depth integrated)
shale
Brine sand
Oil sand
Gas sand
Shale
3-D seismic prediction results
Red=gas Green=oil
With depth trends Without depth trends
From loose sediments to consolidated rocks – what happens to fluid and stress sensitivity?
Porosity
Loose sands: • Large fluid sensitivity (Gassmann theory works well)• Large stress sensitivity (Hertz-Mindlin theory applies)
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)
Statfjord (consolidated)
Porosity
Gullfaks (loose sands)
4D anomalies; Gullfaks vs. Statfjord (Duffaut and Landrø, 2007)Before Water injectionAfter water injection
diff ~6 MPa diff ~0-1 MPa
Water injector offline Water injector online
Top Target
diff ~15 MPa diff ~6-7 MPa
Fluid and pressure sensitivity in Gullfaks versus Statfjord Fields(Duffaut, Avseth and Landrø, 2011)
Troll East time shift analysis(Avseth, Skjei and Skålnes, 2012)
Base Tertiary
Top Draupne
Top Sognefjord
Top Fensfjord
Gas coloumn
Cretaceous overburden
Seismic observations(courtesy of Åshild Skålnes, Statoil)
Sognefjord Fm
Fensfjord Fm
Draupne Fm
Well A
Compaction trend
Compaction and depositional trend
Geologic overview (schematic), Troll East
GWC
Well B
Shear modulus versus porositySognefjord Formation
0.1 0.2 0.3 0.40
5
10
15x 10
9
Porosity
Sh
ea
r m
od
ulu
s (
Pa
)
Well B(east)
Well A (west)
Contact cement model
Friable sand model
Diagenesis
Timeshift at GOC
5.35 5.4 5.45 5.5 5.55
x 105
6.71
6.715
6.72
6.725
6.73
6.735
6.74
6.745x 10
6
UTM-X
UT
M-Y
31/3-S-41
31/3-1
31/6-B-6H
31/6-1
31/6-2
31/6-5
31/6-6
31/6-8
31/6-A-37
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1 1.2 1.4 1.6 1.8 22
2.5
3
3.5
4
0
0.5
1
1.5
dTWT(ms)
Modelled time shifts
Seismic observations(courtesy of Åshild Skålnes, Statoil)
Well AWell B
Barents Sea; a challenging area due complex tectonic and uplift episodes
(Ohm et al., 2008)
7120/1-2
Compaction trends – 7120/1-2
MC
Torsk
KolmuleTransition
zone
CC
MC
CC
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)
Be aware of scale effects!0.63 m
m2m
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).
Rock physics modeling of geological processes:From granular rocks to cracked media (Avseth and Johansen, 2012)
Mineralpoint alpha=1.0
0.10.01
Decreasing aspect ratio
Initial contact cement
DEM HSUB CCT
Critical porosity
Elastic modulus
Porosity
0 0.1 0.2 0.3 0.40
1
2
3
4
5
6x 10
10
Porosity
K (
Pa
)
Ksat and Kdry versus Porosity
Dry rock
Wet rock
5% contact cement
= 1.0
= 0.1
= 0.01
RPT analysis of tight gas sandstone w/cracks(Bakhorji, Mustafa, Avseth and Johansen, 2012)
4 6 8 10 12 14 161.4
1.6
1.8
2
2.2
AI
Vp
/Vs
0.2
0.4
0.6
0.8
Dry rock
Brine rockSwt
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!
Let’s rock together!
Geologist Geophysicist
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!