Wellbore Survey Quality Considerations

Wellbore Survey Quality

Considerations

Presentation for APSG

Houston 6 May 2011

Neil S. Bergstrom P.E.

Halliburton Sperry Drilling

Copyright © Halliburton Company 2010 – All rights reserved

Wellpath as Surveyed (most likely path)


Wellpath Showing Error Ellipses


Why Wellbore Survey accuracy?

Wellbore surveys are used for the life of the asset – and beyond.

Regulation / Property Lines

Geologic Targets

Plan Section

Prevent Collisions

Replacement Wells - EOR

Relief Wells

Abandoment


Wellbore Surveying Methodology

“ Dead Reckoning”

Starting Position (uncertain)

Direction

Inclination (uncertain)

Azimuth (uncertain)

Distance (measured Depth) ( uncertain)

Ending Position (even more uncertain)


Minimum Curvature Calculation Algorithm

 Currently industry standard – obsoleting other methods such as

 Average angle, balanced tangential, or radius of curvature

 Equivalent to a “Great Circle” on a sphere

 Assumes a smooth curve connecting the end survey points

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Curvature is DLS = Dogleg Severity usually in Degrees / 100 ft.

Apparent DLS depends upon survey spacing.

Not the same as “Motor Yield”


SAG (Misalignment Error)

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SAG is the error in inclination measurements caused by the misalignment of the directional sensor in relation to the borehole.

SAG correction is normally provided as an office support function on request at extra cost.

Misalignment Angle

MWD Sensors

Stabiliser Stabiliser

Bit


Uncertainty Ellipses from Monte-Carlo Simulation

1 σ ellipse = 1 standard deviation ellipse calculated hole bottom location

2 σ ellipse = 2 std.dev.ellipse

3 σ ellipse = 3 std.dev.ellipse


Error Ellipse size is specified by the IPM

• IPM = Instrument Performance Model (ISCWSA)

• Also called a toolcode

• Represents a model of the performance of the tool and the way it was run and processed.

• Can be provided by instrument vendor, service company, or operating company.

• The IPM is the technical specification of the advertised survey accuracy


The IPM is the mathematical description of the expected Errors


Other Errors

• Surface Positioning Errors

• Can be minor or major (wrong site)

• These must be added to Ellipse of Uncertainty

• Declination Errors – correlate between adjacent wells

• This includes Grid Convergence Errors

• Wrong sign of declination or grid convergence

• These are Gross Errors or Blunders (> 3 S.D.) and are not modeled by the EOU


Ellipses of Uncertainty Overlap (Bad)

Reference Well min.distance

overlap

Comparison Well reported separation


Anti-Collision Risk

Minor Risk

(Economic Only)

Major Risk

(HSE)


Magnetic and Gyroscopic measurements

• Magnetic (MWD or EMS)

• Azimuth from Earth’s field

• Simple and reliable

• Can’t use in casing

• Affected by nearby steel

• Well-known algorithms and error models

• Raw data can be verified and recomputed

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Gyroscopic

Spinning mass

SRG Surface Referenced

NS North-seeking

Gyrocompassing

Continuous Mode

Inclination limitations

Vendor error models

Can’t be verified by 3 rd party


Magnetic Survey Tools

 Major Error Sources

 Uncertainty in Magnetic North

 Nearby Steel

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Drillstring (internal)

External (adjacent casing or fish)


Accelerometers

Magnetometers

Three sources of Earth’s Magnetic Field

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Internal (Iron Core)

 Predictable mathematical model

Crustal minerals

 Constant

 Can be measured (one time)

Solar wind

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Short term variations

Geomagnetic storms are predictable but effects are not

 Can be measured (ongoing)


Improvements to Magnetic Surveys

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Correct for Z-axis magnetic interference (From Drillstring)

 Single-station corrections (Sperry term = Short Collar Correction)

Measure actual magnetic field (IFR = In-Field Referencing)

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Ground Shots (on Surface)

Downward Continuation from Aeromagnetic survey

 Source of anomalies is typically basement rocks

Monitor changing magnetic field

 Local or interpolated geomagnetic observatory (IIFR)

Apply SAG corrections

 Compensates for bending of BHA under its own weight

Apply Multi-Station analysis

 Compensates for some sensor errors


Other Survey Improvements / QC

(in addition to IFR1, IFR2, Multistation Analysis)

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Benchmark Surveys – Tripping in/out (memory data)

Rotation check shots (cluster shots) at same MD

 Rig time required; may be difficult in Horizontal wells

Repeated surveys with a different tool / technology

 Especially good for catching gross errors

 Overlap surveys if a trip is needed

Store the raw data for re-computation with higher accuracy

 (such as a relief well or P&A)


Error Ellipse Examples

Plan view showing error ellipses


Error Ellipses for Typical Barnett Shale Well

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Profile: vertical to 6000 ft, curve 10 deg/100 to 7000, TD 12,000 ft MD

5000 ft lateral section

 North direction: Standard MWD ellipse at TD is ~ 118 ft x 48 ft (radius)

 IFR1 (crustal) correction: 71 x 48 ft.

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IFR2 (crustal + realtime + MS + dual sensor SAG): 57 x 19 ft

Error reduction of 50% through advanced processing

 East direction: Standard MWD ellipse at TD is ~ 184 x 48 ft

 IFR1 (crustal) correction: 158 x 48

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IFR2 (crustal + realtime + MS + dual sensor SAG): 73 x 19

Error reduction of 60% through advanced processing


Example travelling cylinder plots


Example Ladder Plot


Example Separation Factor Plot


Archiving Survey Information

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Every survey needs an associated error model.

All useful positional data should be stored.

All corrections (declination, grid correction, magnetic interference) should be documented.

Raw data, check shots, and QC information are valuable for future checks.

This data may be needed at well plug and abandonment many years in the future.

Archiving survey data is the well owner’s responsibility.


Questions and

Discussion


Wellbore Survey Quality Considerations

Wellbore Survey Quality

Considerations

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Wellbore Survey Quality Considerations