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Principles and Survey aspects of Geophysical data acquisition (3D, 4D & OBC)
Rob Kok Senior Offshore Surveyor Shell UIE (NAM Assen)
Workshop Hydrographic Society Benelux Amsterdam 7th December 2011
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Geodesy ?
“Geodesy” probably ranks as one of the top 25 words of which nobody
knows the meaning.
Helmert (1880):
“the science of the measurement and mapping of the earth’s surface”
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Introduction – New Energy Future
2D / 3D seismic
Binning / Data Management
4D seismic
OBC
High Resolution seismic
Seafloor Geodesy – Taking Land Survey Techniques Offshore
Q&A
Content
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New Energy Future
The world in 2050
9 billion people 2.5 billion more than today
4-5 times richer with most extra wealth coming from developing countries
Double the energy using twice as much energy as now; emerging markets (China)
Twice as efficient using half the energy as now to produce each dollar of wealth
6-10 times more energy from renewable sources
Challenges as well as Opportunities
Moving to lower carbon intensity including energy efficiency and carbon capture and storage Ensuring safety and environmental responsibility more difficult surface and subsurface conditions Cost reduction for wide variety of development projects technology as key lever
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Our Operating Environment
Conventional Hydrocarbons Enhanced
Oil Recovery
Gas Shale Tight Oil & Gas
In-situ Heavy Oil Production
Coal Bed Methane
Difficult Reservoirs
More Mega Projects
Increasing Complexity
Increasing Pace of Technology Development
Cost Pressure
More Integrated Technology & Engineering Solutions
More „difficult‟ Hydrocarbons
Minimising Environmental Footprint
We are expected to deliver: MORE ENERGY, SECURE ENERGY, RESPONSIBLE ENERGY
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2D / 3D seismic
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2D / 3D seismic
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Basics 2D
Basic
1 streamer
“Cheap”
Reconnaissance
Speculative
3D
2 -16 streamers
Careful planning of acquisition parameters
Multiple sail lines (swaths) ... multiple streamers recording source data
2D / 3D seismic
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2D / 3D seismic
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Considerations / Issues Vessel selection
Instrument room (location, recording equipment)
Back deck facilities
Streamers (number, spacing, length, towing depth, hydrophone grouping)
Compressors / airgun arrays
Positioning of in-sea equipment
Tailbuoy (active DGPS or RTK)
Source floats
Streamers (compasses, acoustic ranging units)
Multi vessel operation
2D / 3D seismic
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2D / 3D seismic
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Operational Constraints Survey location
Time or year
Survey size, line length
Acquisition parameters
Excessive feathering
Currents
Other activity in area (fishing)
Time sharing
Marine mammals
Equipment breakdown
2D / 3D seismic
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Binning / Data Management
Binning
• Processing of 3D seismic based on grouping seismic traces
• Referenced to a point (bin node) which form a matrix
• Bin Nodes are referenced to its location in the real world
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Binning / Data Management
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4D seismic is time-lapsed 3D seismic
Acquired at different times (in productive life of reservoir)
Additional data required to optimise value (time-lapse logs, cores,
VSP‟s, pressures etc.)
REPEATABILITY
4D seismic
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How are we influencing projects ?
•Making better decisions on redevelopment
•Exploiting unforeseen possibilities
•Avoiding unforeseen problems
•Avoiding unnecessary infill
4D seismic
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4D seismic
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4D seismic
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4D streamer seismic
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OBC (Ocean Bottom Cable)
OBC is an advanced method of acquiring 4D seismic data
Cables in much more stable position than when towed
Cables may be trenched for positioning repeatability
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Picture courtesy of WesternGeco
Seismic Acquisition (ocean bottom cable) Retrievable normally for 3D imaging issues Platform obstructing streamer acquisition
Permanent (often trenched) normally for 4D Better positioning repeatability than streamer Large upfront costs evened out
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Acquisition of low fold, but highly repeatable data, resulting in relatively small data volumes. Sparse OBC concept minimizes the number of receiver cables deployed. Viable for permanent systems.
OBC – sparse acquisition
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High Resolution seismic
Acquiring the highest quality data to provide exceptional interpretation
Dedicated survey vessels
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High Resolution seismic
• Shallow Gas (High Resolution) Surveys are undertaken to minimise the risk of encountering shallow gas
•Executed ahead of drilling operations
•Grid of 2D lines, normally 1-1.5 km long
•Small - high-frequency – source
•Weather sensitive operation (tow depth)
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High Resolution seismic
Survey Area – differences HiRes and 3D
• SHALLOW GAS SURVEY
Discrete lines, 100 m spacing
Survey area 1000 x 1000m (UKOOA Guidelines)
Acquired to 2 Secs
• 3D EXPLORATION SEISMIC
Grid of data typically 25 x 25m bin spacing
Survey area 20 x 20km
Acquired to circa 7 Secs
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High Resolution seismic
3D EXPLORATION SEISMIC SITE SURVEY
SOURCE TYPE AIRGUN SLEEVE GUN
SOURCE VOLUME 3000 cu in 160 cu in
BANDWIDTH (-6dB) 4 - 90 Hz 5 - 250 Hz
SHOT SPACING 25m 6.25m
CABLE LENGTH 4000m+ 600m
GROUP LENGTH 25m 12.5m
CMP SPACING 12.5m 6.25m
NEAR TRACE OFFSET 150m+ <50m
TOW DEPTH 7m 2.5m
SAMPLE RATE 2 - 4ms 1ms
ANTI-ALIAS FILTER 180 Hz 306Hz
Acquisition Parameters
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High Resolution seismic
3D Exploration Seismic Data 2D High-Resolution Seismic Data
Approx. 550m
Vertical Resolution
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High Resolution seismic
The cost of getting it wrong is very high (HSE !)
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High Resolution seismic
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Taking Land Survey Techniques Offshore
Seafloor Geodesy
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Seafloor geodesy – aims
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The proven land techniques …
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What can we measure?
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Displacements
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Seafloor geodesy – aims
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Business Challenges
To monitor depletion and determine compartmentalisation
Enhanced reservoir management
Future development planning
Operational Challenges
Network Design
Seabed Conditions
The challenges offshore
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GIS Analysis for Network Design
Fishing Data + No compatts shallower than max. fishing depth
Slopes + 25 cm Bathy Peaks Line of Sight Inclination
Seabed Conditions Hard Soft Dubious
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Operational – 1/2
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Operational – 2/2
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Q & A
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