HAGI

Bandung - 23 to 24 June 2014

Presenter : Jacques BONNAFE

1

Velocity Model Building

2

Outline

Introduction Principles

Types of velocity models

Velocity model building methodology and tools

Anisotropy

Example

Conclusions

3

Depth Imaging

Prestack surface

gathers

Velocity Model

Depth Migration

Algorithm PSDM image

Preprocessing required (decon, demult, )

Acquisition geometry is determinant (mon/multi/wide

azimuth, maximum offset)

Velocity Model

Major concern of a PSDM project

4

High Performance

Computing

Integrated Depth Imaging Toolkit

Velocity Model

Building

Interpretation

Residual Moveout

Computation

Depth

Migration

Algorithm

5

Outline

Introduction Principles

Types of velocity models

Different types of models

How to make the initial model

Velocity model building methodology and tools

Anisotropy

Example

Conclusions

LAYERED MODEL

6

7

GRID MODEL

Smooth Models

With constraints

8

Hybrid parameterization

No velocity contrasts between layers

V0(x,y) and k(x,y) within each layer

Horizon not necessary geological

9

Outline

Introduction Principles

Types of velocity models

Velocity model building methodology and tools

reflection tomography, principle, gamma, updates

Other techniques: refraction tomography, scan, ....

Well ties

Anisotropy

Example

Conclusions

10

Velocity

Model

PreStk

Depth Mig.

Are

CIPS

Flat?

NO

YES

Auto Pick

Residual

Moveout

& Dip in z

3D Ray-trace

Linear

Tomography

Equations

Iterate

PreStk.

Depth Mig.

Image

Tomo Solver

for Smooth

Update To

Interval Vels

Principle of tomographic updating

11

offset

dep

th

RMO section

x,y

dep

th OK

Too fast

Too slow

CRP Gather

RMO definition

12

Gamma definition The ratio of the migration velocity to the true (geological) velocity

13

Comparison of RMO before and after iteration on maps

0-1

km

1

-2 k

m

Iteration #2 isotropic Iteration #3 VTI anisotropic

-4 0 +4

-4 0 +4

-4 0 +4

-4 0 +4

RMO Statistic Initial VTI

Interval 0 1

Km

Interval 1 2

Km

Interval 2 3

Km

Interval 3 4

Km

Interval 4 5

Km

Interval 5 6

Km

Interval 6 7

Km

Interval 7 8

Km

Comparison of RMO before and after iteration on histograms

Iteration 3 Iteration 4

PSDM Stack Overlaid with Gamma: inline

PSDM Stack overlaid with Gamma : depth = 500m

GAMMA ON HORIZON

17

18

Example of cdp gathers evolution

Iteration 1 Iteration 2 Iteration 3 Iteration 4

EXAMPLE OF STACK AND VELOCITY MODEL

19

Iteration 1 Iteration 5

20

2 km 2 km

EXAMPLE OF STACK AND VELOCITY MODEL

Iteration 2 Iteration 3

21

One iteration illustrated: Initial RMO residuals

22

One iteration illustrated: Inversion + New PSDM + RMO residuals

23

One iteration illustrated: Initial PSDM

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One iteration illustrated: New PSDM

25

Other tools

Refraction tomography

Velocity scan

Information from wells of geologists

FWI

26

Refraction tomography

First Break Time Low High

Shallow Gas

FB in Offset Domain

FB in Shot Domain

Manual picking done

FB in various Offset Planes

200 262.5 m 1600 1762.5 m

2000 2162.5 m 2400 2562.5 m

First Break picking:

Automated picks in good seismic area

Manual picks in degraded seismic area

Full offset used

Application:

For shallow anomalies

Down to depth approx 25% of maximum offset

First Break picks

First Break Time Low High

Late arrival

27

Iterative refraction tomography updates

Smoothed PSTM velocity model as initial model

Several iterations of calculations run with

decreasing grid size

QCs performed:

Comparison with previous model with greater grid size

Visual observation by overlying the model with depth-

stretched PSTM section

Target line migration using final shallow model

Final Shallow Model

Update is effective < 400m (~1/6 cable length)

400 m

400 m 400 m

400 m

Main elements of refraction tomography:

Initial velocity model

Forward modeler module for refracted wave

First Break picks

Refraction tomography equation builder and

solver

100 x 100 m

400 x 400 m 200 x 200 m

50 x 50 m

4 km

4 km

4 km

4 km

500 m

model for scanning

-28-

Carbonate

flooding

Carbonate V=2750 m/s

-29-

-30-

Carbonate V=2900 m/s

-31-

Carbonate V=3100 m/s

Map of reefs

-32-

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VELOCITIES CONTROL AT WELL

Initial velocity

Iteration 1 velocity

Iteration 2 velocity

Iteration 3 velocity

Iteration 4 final velocity

TWT TVD

1400 5000 m/s

Horizon A

Horizon B

34

Basic Depth Imaging Workflow

Velocity model may be iteratively built using a layer stripping approach

Simplifies velocity update process

May rather build downwards

PSDM migration performed at each iteration

Moveout extracted from the Gathers for velocity update

Workflow and tools adapted to study

Objectives : Exploration/Reservoir imaging

Geological environment

Data : Marine/land acquisition, mono/multi/wide azimuth

35

Interpretations have to be very consistent with the seismic information

Used for ray-tracing

Used for velocity contrasts positioning, e.g. salt bodies, carbonate build-ups

The accuracy of the inversion result depends on these interpretations

The RMO extracted (along interpretations) have to represent reliable

seismic information:

If multiples are picked by RMO, velocity model will be updated wrongly

Use interpretation to discriminate

In area of poor signal to noise ratio, RMO analysis can be erroneous

If maximum offset is not long enough, RMO can be inaccurate

In very complex areas or when initial model is very wrong, the moveout can be far from

hyperbolic; sometimes gathers lack coherence

Direct interpretive input into the model

Importance of seismic interpretation in the VMB

36

Outline

Introduction Principles

Types of velocity models

Velocity model building methodology and tools

Anisotropy

Example

Conclusions

37

Anisotropy

39

Velocity Issue: isotropic, anisotropic STI or VTI?

Salt Salt Salt

True Model (STI) Velocity is increasing with angle

to structural dip normal

Isotropic Migration Velocity does not vary

with angle of propagation

VTI Migration Velocity is increasing

with angle of propagation

Fast

Slow Slow

Fast

40

Thomsens VTI parameters estimation

Delta estimation (relates vertical and imaging vels.)

Comparison of depth markers on seismic and on wells

Requires accurate calibrations on wells

Epsilon estimation (relates vertical and horizontal vels.)

Measurements based on hockey sticks on gathers

Then Epsilon averaged to obtain single epsilon functionvalue per

layer

Sometimes set to a constant percentage of delta (150% or 200%)

Iter.2 isotropic Iter.2: delta applied Iter.3 VTI anisotropic

Averaged Epsilon function (z)

d,

z

Averaged Delta function d (z)

41

Anisotropic model Building

In the Isotropic case:

Vnmo = Vvertical by layer

Vnmo = Vrms multi-layer

Isotropic model/migration, anisotropic earth:

Vertical velocity over estimated

Mismatch between horizon and well marker (typically, horizons deeper)

Mismatch between sonic and seismic velocities

In the anisotropic case : Vnmo Vvertical or Vrms

Match horizon and well marker

Match between sonic and seismic velocities Calibrated PSDM image

Very delicate and time consuming work

3D Depth Migration

ANISOTROPIC ISOTROPIC (stretched to well)

Vertical stretch is not sufficient !!

43

Outline

Introduction Principles

Types of velocity models

Velocity model building methodology and tools

Anisotropy

Example: BEKAPAI VMB

Step 1. First Break picking

Step 2. Refraction tomography

Step 3. Initial PSDM velocity building

Step 4. Reflection tomography

Results

Conclusions

44 - IPA11-G-121 Combined Refraction Tomography and Reflection Tomography for PSDM Velocity Model Building

Step1. First Break picking and QC

First Break Time Low High

Shallow Gas

FB in Offset Domain

FB in Shot Domain

Manual picking done

FB in various Offset Planes

200 262.5 m 1600 1762.5 m

2000 2162.5 m 2400 2562.5 m

First Break picking:

Automated picks in good seismic area

Manual picks in degraded seismic area

Full offset used

QC performed:

Visual observation of picked events in shot domain,

offset domain and various offset planes

Edit manually anomalous picks

First Break picks

First Break Time Low High

Late arrival

45 - IPA11-G-121 Combined Refraction Tomography and Reflection Tomography for PSDM Velocity Model Building

Step 2. Iterative refraction tomography updates

Smoothed PSTM velocity model as initial model

Four iterations run with decreasing grid size

(400m, 200m, 100m, 50m)

QCs performed:

Comparison with previous model with greater grid size

Visual observation by overlying the model with depth-

stretched PSTM section

Target line migration using final shallow model

Final Shallow Model

Update is effective < 400m (~1/6 cable length)

400 m

400 m 400 m

400 m

Main elements of refraction tomography:

Initial velocity model

Forward modeler module for refracted wave

First Break picks

Refraction tomography equation builder and

solver

100 x 100 m

400 x 400 m 200 x 200 m

50 x 50 m

4 km

4 km

4 km

4 km

500 m

46

Merge at Z = 400 m

Step 3. Initial PSDM velocity building

Final Shallow Model

Smoothed PSTM velocity*

Initial PSDM velocity model

Model is ready to be updated through

iterative reflection tomography

* PSTM velocity field was first converted to interval velocity and

then smoothed spatially

3 km

3 km

2 km

500 m

1500 m

1500 m

47

Step 4. Iterative reflection tomography updates

Main elements of reflection tomography:

Initial velocity model

Pre-Stack Depth Migration

Automated RMO and Dip picking

Reflection tomography equation builder and solver

Four iterations carried-out:

Two isotropic model followed by two VTI

anisotropic updates

Kirchhoff PSDM to depth-migrate the

gathers

On 50 x 50m grid size

(Woodward, et. al., 2008, A decade of tomography)

Best model was selected based on comparison with previous model using:

Gamma map & histogram

Gamma/stack section overlay

Gathers display

Velocity model/stack overlay

Next slide

Velocity needs to be slower

Velocity is correct

Velocity needs to be faster

Gamma = ratio of the migration velocity to the true geological velocity

48 - IPA11-G-121 Combined Refraction Tomography and Reflection Tomography for PSDM Velocity Model Building

RMO picking and QC

Too slow Too fast

Vel. error map (1-2km): iter.2 Vel. Error map (1-2km): iter.3

-4 0 +4

Automated RMO pick (50 x 50m):

Precondition gathers with de-multiple and offset

muting prior to picking

QC done through:

Visual observation of picked events on gathers

Visual observation of RMO attribute in section

and map views

Histogram plot of RMO attribute

-4 0 +4

-10%

Gamma section: initial model Gamma section: 1st update

Gamma = 0.9 (10% too slow) Gamma = 1.1 (10% too fast)

+10%

Gamma QC: iteration 1

50

0 m

4 km

10

00

m

6 km 6 km

Significant improvement of Gamma after 1st iteration

0

0.8

1.0

1.2

Significant improvement of Gamma after 3rd iteration (VTI)

49

Thomsens VTI parameters estimation

Delta estimation (relates vertical and imaging vels.)

Computed based on time-picked horizons at nine wells

Workflow consists of:

1) Seismic to well calibration to obtain calibrated vertical

velocity field

2) Extraction of horizons TWT and the corresponding depth

TVDSS (Zo)

3) Convert the horizons TWT to depth (Ziso) using isotropic

velocity field

4) For each horizon, compute the delta

Single delta function (depth-variant) was used in 3rd VTI iteration

Delta set inactive (0.0001) at last iteration; considering quite

important depth errors in deep interval after 3rd VTI iteration

Epsilon estimation (relates vertical and horizontal vels.)

Automated based on far offsets at gathers located in nine wells

Individual Epsilon function at well was averaged to obtain single

epsilon function

QC done by comparing gathers before and after application of

Delta and Epsilon

Iter.2 isotropic Iter.2: delta applied Iter.3 VTI anisotropic

Averaged Epsilon function (z)

z

Step 5. Introducing anisotropy

50

VTI anisotropy model: improvement of depthing & focusing

Initial model 1st iteration model 2nd iteration model 3rd iteration VTI model

Isotropic Isotropic Anisotropic

Improvement of gather flatness = more focused stack

60

0 m

Multiple

51

Final PSDM velocity model

2 km

2 km

Initial model for reflection tomography

Final model

Initial velocity

Iteration 1 velocity

Iteration 2 velocity

Iteration 3 velocity

Iteration 4 final velocity

1400 m/s 5000 m/s

Measured interval velocity

Velocity reversal well captured thanks to reflection tomography

Low velocity anomaly captured by refraction tomography

Converging RMO (projected) G

am

ma r

eduction

Iteration #

1000 m

1000 m

600 m

s

Velocities in time domain (TWT)

52

Improvement of image at shallow interval Enable the delineation of shallow gas anomaly

Shallow slice 2002 dataset

Shallow slice 2010 PSDM dataset

2002 Post-STM: IL20600 converted in depth

2010 PSDM: IL20600

Low velocity anomaly imaged properly

1.5 km

500 m

500 m

1.5 km

2.5 km

2.5 km

53

Improvements of image at target interval

2002 Post-STM: IL20500

COHERENCY @ BETA

50

0 m

s

2 km

2.5 km

Note: AGC was applied on the dataset

54

Improvements of image at target interval COHERENCY

@ BETA

2 km

60

0 m

2010 PSDM: IL20500

More focused fault image

Higher S/N ratio

More preserved amplitude

Clearer shallow gas image

Push-down and statics not fully solved

2.5 km

55

Operational added-values brought by PSDM dataset More accurate post-mortem evaluation of recently drilled wells

Well G drilled before PSDM (targeting Central Panel)

Well G results (sedimentology, fluid, etc) are more coherent with the West Panel consistent with the new structural scheme on PSDM

Structural interpretation on PSDM performed independently from well G results

Two other wells drilled after PSDM showed acceptable consistency in term of structural scheme and expected HC column

SW SW NE NE

BETA old map

Well G Well G

Well G Well G

1 km 1 km

750 m 750 m

BETA new map

WEST CENTRAL EAST WEST CENTRAL EAST

Old interpretation overlaid on

PSDM

56

Conclusions of Bekapai PSDM example

Reliable full-field velocity model was obtained by combining refraction

tomography at shallow depth and reflection tomography at deeper depth

Refraction tomography proved to be a solution to build a reliable shallow

velocity model that would not be achieved by using reflection tomography

Efficiency of refraction tomography relies on the quality of first break picks;

QC and editing are therefore fundamental, but time consuming

PSDM final product has improved drastically structural image and

understanding of the field

57

Outline

Introduction Principles

Types of velocity models

Velocity model building methodology and tools

Anisotropy

Example:

Conclusions

58

Conclusions on VMB

Great variety of tools

Strong involvements of interpreters

The survey should be big enough to allow proper tomography

Time consuming process

A good model is the key to the success of the PSDM project