Borehole Surveying

Making the Most of Borehole Surveying

Prof Angus JamiesonUniversity of the Highlands and Islands

Video presentation available at www.uhi.ac.uk/surveying-summary

http://www.uhi.ac.uk/surveyingsummary

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 1Why Survey ?




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



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 2Coordinate Systems




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 3North Reference




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 degsand Convergence was +2 degs

Section 4Survey Tools




MWD

MWD

(1) Accelerometers– use gravity field vector (0 degrees inclination reference) Several designs are available

Dual axes

Exciter /pick-offs

Torquerpermanent magnet

hinge

restoring coil

pendulous arm

G

Single axis

(2) Magnetometers – use magnetic field vector (magnetic North reference)

N

NS

N

N S

Secondary coil

cores

S

S

Primary coilTwo 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 forceimages © 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 EarthRotation Vector

Horizontal EarthRotation 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 5Error Models




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 6Correction Techniques




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 VariationLong slow changes in the earths magnetic core.Typical Size: Fractions of a deg/year Cured By: BGGM or HDGM magnetic model

2. Diurnal VariationRapid daily variations caused by solar wind and earth rotation.Typical Size: 0.2 degs (Randomized) Cured By: Interpolated In Field Referencing (IIFR)

3. Crustal VariationPermanent local effects caused by deep, magnetic basement rockTypical Size: 1 degreeCured 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 7Common Pitfalls




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.

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Borehole Surveying