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Geohazard interpretation
Truth or Myth?
Stavanger 6th December 2011
1 - Classification: Internal 2010-05-03
Extract from a Site Survey Report anno 2011
Shallow gas detection is normally made, in the first instance, by recognition of
anomalously high amplitude, usually phase reversed reflectors ('bright' spots), allied
to a number of other characteristics, such as acoustic masking, velocity pulldown,
structural closure, edge effects, frequency reduction, and basal 'flat' spots.
Extract from a Site Survey Report anno 2011
Risk Model Probability Typical Characteristics
High Gas most probable
model
High amplitude with 3 or 4 other well defined
features (positive AVO, closure, phase
reversal etc)
Modera
te Gas a likely model
High amplitude with 2 other subsidiary gas
related features
Slight Gas a possible
model
1 or 2 gas related features or high amplitude
alone.
Reasons Why Predicted Gas is not seen when drilling
• Well Moved outside an anomaly
• Countermeasures makes it very difficult to detect
Gas
• Wrong Interpretation
«Avoiding Anomalies «-Problems
A B
C
Well A: Warning at 550 ms
Well B: No Warning
Well C: Warning at 700 ms
«Avoiding Anomalies «-Problems
A1
A2 A3
Real Case studies
Chimneys
Chimneys
Chimneys
11
Chimneys
S180
S182
S185
Gas Kick Case 1
Gas Kick Case 1
Anomalies
Gas Kick Case 1
Anomalies
Blanking
Gas Kick Case 1
Anomalies
Blanking Structure
Gas Kick Case 1
Gas Kick Case 2 (Spot the Gas….?)
Gas Kick case Case 2 (Spot the Gas….?)
Classif
ication:
Interna
l
Shallow Gas Case 1 (AVO Anomaly) E W
Classif
ication:
Interna
l
Shallow Gas Case 1 (AVO Anomaly)
1250m
Some Technology Advances
1986 2011
Single Channel Echo Sounder Multichannel Echo Sounder
Analogue Paper records Digital Files
48 Channels Streamers 96-120 Channels Streamers
140 Cu In Air Gun Sleeve Gun Clusters
Single Channel Multielectrode Sparker Multichannel mini airgun
Seismic Interpretation on paper records Seismic Interpretation on Work Stations
No 3D Seismic Used 3D Seismic Used
Reports on paper PDF-Files
21 - Classification: Internal 2010-05-03
Always think geology when you look at
seismic!
Believe it or not… This reflects real geology!
The art of Geohazard Interpretation
24 - Classification: Internal 2010-05-03
The art of Geohazard Interpretation
25 - Classification: Internal 2010-05-03
Actions to Improve Prediction
• Systemise well information
• Get an understanding for the local amplitude/Gas relationships
• Try and optimize acquisition parametres
26 - Classification: Internal 2010-05-03
27 - Classification: Internal 2010-05-03
Shallow Gas interpretation
Truth or Myth Dag Lundqvist
Principal Geologist, Statoil ASA
www.statoil.com
Thank you
