2011-12-07 - Rob Kok - Geophysical Data Acquisition

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

Shell UK Ltd 6 November 2010 RESTRICTED

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



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




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


How are we influencing projects ?

•Making better decisions on redevelopment

•Exploiting unforeseen possibilities

•Avoiding unforeseen problems

•Avoiding unnecessary infill

4D seismic


4D seismic


4D seismic


4D streamer seismic


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



Acquiring the highest quality data to provide exceptional interpretation

Dedicated survey vessels



• 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)



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



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



3D Exploration Seismic Data 2D High-Resolution Seismic Data

Approx. 550m

Vertical Resolution



The cost of getting it wrong is very high (HSE !)




Taking Land Survey Techniques Offshore

Seafloor Geodesy


Seafloor geodesy – aims


The proven land techniques …


What can we measure?


Displacements


Seafloor geodesy – aims


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


Operational – 1/2


Operational – 2/2

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Q & A

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Documents

2011-12-07 - Rob Kok - Geophysical Data Acquisition