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!