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Making the Most of Borehole Surveying Prof Angus Jamieson University of the Highlands and Islands Video presentation available at www.uhi.ac.uk/surveying-summary
Making the Most of Borehole Surveying
Prof Angus Jamieson
University of the Highlands and Islands
Video presentation available at www.uhi.ac.uk/surveying-summary
This Presentation Covers ...
►1. Why survey
►2. Coordinate Systems
►3. North References
►4. Survey Tools
►5. Error Models
►6. Correction Techniques
►7. Common Pitfalls
Section 1 Why Survey ?
Prof Angus Jamieson
University of the Highlands and Islands
Video presentation available at www.uhi.ac.uk/surveying-summary
Don’t be in the wrong place at the wrong time !!
Why do we survey at all ?
►Ensure a safe well path to the target
►Ensure you hit the target
►Ensure you don’t hit another well
►Provide good log positions to G&G
►Provide good reserves estimates
►Report data to the regulators
►Conduct ‘forensics’ investigations afterwards
►Prepared for relief well if necessary
Business Case ?
►A shorter gyro run $10,000 +
►A proximity ‘shut in’ $100,000 +
►A plug back sidetrack $1 million +
►A dry well or ‘Dead Zone’ $10 million +
►A deep landing $100 million +
►A minor collision blowout $1 billion +
►A major collision blowout $10 billion +
Poor Surveying costs Production
Poor Surveying costs Production
Poor Surveying costs Production
10% production lost but we saved the cost of a gyro !
How serious is a blowout ?
Blow out with no fire
Very High Pressures
Add Fire and we have disaster
Low probability – High Impact
In Summary
Saving money
on surveying is a high stakes gamble which,
if lost, will make you famous
Section 2 Coordinate Systems
Prof Angus Jamieson
University of the Highlands and Islands
Video presentation available at www.uhi.ac.uk/surveying-summary
Mapping the World
Any Projection distorts the World
Gerardus Mercator 1512
Project from the centre of the Earth
Mercator Projection
Greenland is actually only 10% of the size of Africa
The Equator
The Centre of the World
The Centre of the World
The Worlds Time Zones
UTM Zones
UTM Zones
Section 3 North Reference
Prof Angus Jamieson
University of the Highlands and Islands
Video presentation available at www.uhi.ac.uk/surveying-summary
The World
The Greenwich Meridian
The Equator
Latitude & Longitude
Transverse Mercator
Transverse Mercator
For Any Point on the Earths Surface True North is towards the North Pole
If a TM cylinder is wrapped at another longitude, Map North follows the
cylinder
So unless you’re at the centreline of the map, True and Grid DON’T line up
The True Direction of Grid North is called the CONVERGENCE
Universal Transverse Mercator
Grid Convergence
Grid Convergence
The True Direction of Magnetic North is called the DECLINATION
With three Norths it is easy to get confused
MWD measures from Magnetic North
Gyros usually measure from True
But most surveys are finally reported in Grid
For Example if Declination was -6 degs and Convergence was +2 degs
Section 4 Survey Tools
Prof Angus Jamieson
University of the Highlands and Islands
Video presentation available at www.uhi.ac.uk/surveying-summary
MWD
MWD
(1) Accelerometers– use gravity field vector (0 degrees inclination reference) Several designs are available
Dual axes
Exciter / pick-offs
Torquer permanent magnet
hinge
restoring coil
pendulous arm
G
Single axis
(2) Magnetometers – use magnetic field vector (magnetic North reference)
N
N S
N
N S
Secondary coil
cores
S
S
Primary coil Two identical cores with primary winding around (in opposite directions). Secondary coil around all. Primary current produces magnetic field in each core, equal and opposite so no voltage induced in secondary winding. When placed in an external magnetic field, an unbalance occurs and a voltage is produced in the secondary coil, this is directly proportional to the external magnetic field.
Modern Gravity and Magnetic Sensors
Photo-Mechanical Multishot
A Magnetic ‘Drop’ Tool
Compass v Magnetometer
The Compass Measures Both Inclination and Direction but is less accurate and less robust
The Magnetometer has no moving parts but requires three orthogonal instruments to measure the magnetic field. Accelerometers measure Inclination from vertical.
Gyroscopic Effects
►A gyro does not want to change the orientation of the spin axis.
►Conventional Gyros are lined up on a reference azimuth and remain facing that way for azimuth measurement down hole.
Gyroscopic Principles
Inertia:
when the spinning portion of
a gyroscope (called a rotor)
is set in motion it will
attempt to keep its axis of
rotation continuously
pointing in the same
direction
Precession:
when a force is applied to a
spinning rotor, it will
attempt to compensate by
rotating around an axis that
is perpendicular to the
applied force images © 2002 Encyclopædia Britannica, Inc.
Conventional Gyro
Gyroscopic Effects
►A gyro forced to torque around its X axis when spinning around the Y axis will start to rotate around the Z axis.
►This is known as ‘Precession’ and can be used to measure rate of change of orientation against time.
►Continuous Gyro surveying integrates rate of azimuth change against time to measure its current direction.
Vertical Earth
Rotation Vector
Horizontal Earth
Rotation Vector
Gyro Sensor
Wellbore Direction
Continuous Gyro
North Seeking Gyros
►A North Seeking Gyro is simply a highly sensitive rate gyro which measures the earths rotation and senses the direction to the polar axis.
►This usually takes about 1 – 2 minutes of stationary sensing so is often only used in top hole (up to 15 degrees) after which the survey is run in continuous mode.
Gyroscopic Tools
Section 5 Error Models
Prof Angus Jamieson
University of the Highlands and Islands
Video presentation available at www.uhi.ac.uk/surveying-summary
A Normal Distribution
A Normal Distribution
A Normal Distribution
A Normal Distribution
Probability in two dimensions
The full distribution in 3D
Co-variance determines the orientation
Standard Deviations in 3D
►2 SDs in 1D cover 95%
BUT......
►2.38 SDs in 3D cover approx 95%
►2.79 SDs in 3D cover approx 98.5%
The Magic Formula
?
The Magic Formula
The Magic Formula
The Magic Formula
Building a detailed error model
►Find all error sources affecting Md, Inc & Az
►Find 1 sd values for each coefficient
►Work out affects on Md,inc and az
►Decide whether ‘Random’ or ‘Systematic’
►Covert to errors in North, East & Vertical
►Build a Co Variance Matrix
►Work out Ellipse dimensions and orientation
SPE Paper # 67616 by Hugh Williamson of BP. Accuracy Prediction for Directional Measurement While Drilling
►Processes & procedures are followed
►Tools are properly calibrated
►Survey intervals no greater than 100 ft
►Non-magnetic spacing as recommended
►Individual surveys pass QC checks
Propagation of Errors
The model recognises 4 modes of error propagation:
►Random – uncorrelated from one measurement to the next
►Systematic – correlated from one measurement to the next within one single tool run
►Well-by-well – correlated from one measurement to the next within an entire well
►Global – always correlated, including well to well
The ISCWSA MWD Model
Convert Observation Error to Position Error
Effect of Inclination Error
Azimuth Error only affects horizontal position
Building the Covariance
►For systematic errors
dN = dN1 + dN2 + dN3 ......
►For random errors
dN = sqrt(dN1^2 + dN2^2 + dN3^2 ...)
►For each error source
Add up all effects in a survey station
Add up all survey stations in a ‘leg’
Add up all legs in a survey
The Co-Variance Matrix
In the North East Vertical Reference there may be covariances
Imagine a different set of orthogonal axes that don’t see any covariances
Rotate to a new Covariance Matrix
Covariances are zero
The viewing vectors are Eigen Vectors The ellipse dimensions are Eigen Values
Collision Risk
High Collision Risk
Section 6 Correction Techniques
Prof Angus Jamieson
University of the Highlands and Islands
Video presentation available at www.uhi.ac.uk/surveying-summary
Major Corrections
► Depth Correction There are many sources of error affecting both drill pipe and wireline
length downhole but depth correction will usually address the mechanical and thermal stretch. These account for the major depth errors and can be as much as 0.2%
► Sag Correction This is an inclination correction to allow for the natural bending of the
BHA under its own weight. It increases with inclination and can be 0.5 degrees or more.
► IFR Correction This usually refers to the local correction of magnetic declination and is
derived from an In-Field Referencing survey of the oilfield. This can be up to 1 degree in places
► Magnetic Interference This applies to azimuth only and corrects for the magnetic influence of
the BHA itself. It is particularly important when using short non-mag collars.
5 Sources of Depth Error
►Mechanical Stretch
►Survey Resolution
►Tool Misalignment
►Temperature Effects
►BHA Deflection (Sag)
Mechanical Stretch
Survey Resolution
Normal Minimum Curvature
Tool Misalignment
Temperature Effects
►Steel will stretch by
1.3m / 1000 / 100 degs C
Sag Correction
IFR Correction
The Earth’s Magnetic Vector
1. Secular Variation Long slow changes in the earths magnetic core.
Typical Size: Fractions of a deg/year
Cured By: BGGM or HDGM magnetic model
2. Diurnal Variation Rapid daily variations caused by solar wind and earth rotation.
Typical Size: 0.2 degs (Randomized)
Cured By: Interpolated In Field Referencing (IIFR)
3. Crustal Variation Permanent local effects caused by deep, magnetic basement rock
Typical Size: 1 degree
Cured By: In Field Referencing (described later) IFR
A Powerful Force but subject to three Variations
Declination is on the Move
Rapidly in Geological Time !
Diurnal Variation
Crustal Variation
Crustal Variation
Interactive IFR Map
Magnetic Interference ► The interference created by the collars in the BHA can
influence the observed by several degrees.
► Short Collar solutions only use the X and Y mags to calculate the azimuth
► Multi Station Analysis uses the fact that as the BHA changes toolface and attitude, the background magnetic field is unchanged but interference components rotate with the BHA. We can therefore back out the interference components over several survey stations
Summary
►Reducing error is nearly always possible
►Sag is usually the biggest benefit in Vertical
►IFR is usually the biggest benefit in Horizontal
►Short Collar should only be used with caution
►MSA is only reliable in an accurate mag field
►SC and MSA do not work well when Bz small
►For high accuracy work nothing beats gyros
Section 7 Common Pitfalls
Prof Angus Jamieson
University of the Highlands and Islands
Video presentation available at www.uhi.ac.uk/surveying-summary
Top 10 List of what can go wrong
► Units and conversion factors
► TVD Referencing
► Failure to use sag correction
► Uncertain Connection to Map
► Misapplied Convergence
► Old Declination Values
► Bad Computer Data Unchallenged
► Use of GPS on wrong Datum
► Not Enough Surveys
► Home made software
Conclusion
If we don’t get the message out that wellbore positioning is worth spending money on, we will continue to waste reserves and occasionally risk lives.
