Wellbore Survey Quality Considerations -...

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Wellbore Survey Quality Considerations

Presentation for APSG Houston 6 May 2011

Neil S. Bergstrom P.E. Halliburton Sperry Drilling


Wellpath as Surveyed (most likely path)


Wellpath Showing Error Ellipses


Why Wellbore Survey accuracy?

Relief WellsAbandoment

Plan Section

Geologic Targets

Prevent CollisionsReplacement Wells - EOR

Regulation / Property Lines Wellbore surveys are used for the life of theasset – and beyond.


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

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) 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.

MWD SensorsStabiliser Stabiliser

Bit

Misalignment Angle


Uncertainty Ellipses from Monte-Carlo Simulation

1σ ellipse = 1 standard deviation ellipse

2σ ellipse = 2 std.dev.ellipse

3σ ellipse = 3 std.dev.ellipse

calculated holebottom location


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 Comparison Wellmin.distance

reportedseparation

overlap


Anti-Collision Risk

Minor Risk (Economic Only)

Major Risk(HSE)

http://www.keyposters.com/buy.php?item=3531792�


Magnetic and Gyroscopic measurements Gyroscopic Spinning mass SRG Surface Referenced NS North-seeking Gyrocompassing Continuous Mode Inclination limitations Vendor error models Can’t be verified by 3rd party

• 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


Magnetic Survey Tools

Major Error Sources

Uncertainty in Magnetic North

Nearby Steel Drillstring (internal) External (adjacent casing or fish)

Accelerometers

Magnetometers


Three sources of Earth’s Magnetic Field

Internal (Iron Core) Predictable mathematical model

Crustal minerals Constant Can be measured (one time)

Solar wind Short term variations Geomagnetic storms are

predictable but effects are not Can be measured (ongoing)


Improvements to Magnetic Surveys Correct for Z-axis magnetic interference (From Drillstring)

Single-station corrections (Sperry term = Short Collar Correction) Measure actual magnetic field (IFR = In-Field Referencing)

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)

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 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. 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 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 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

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