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Baker Hughes INTEQ

Gyroscopic SurveyingLevel Rotor Gyro Photomechanical Systems

Training Manual

Part Number 750-500-071

Rev. A

May 1997

Baker Hughes INTEQTechnical Publications GroupP.O. Box 670968Houston, TX 77267-0968USA713-625-4415


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The information contained herein is believed to be accurate and, whereappropriate, based on sound engineering principles. However, BakerHughes INTEQ makes no warranties or representations to that effect. Allsuch information is furnished as is, and use of such information isentirely at the risk of the user. Unauthorized copying and/or use of theinformation contained herein is prohibited, and subject to penalties undercopyright and other laws of the United States and other countries.

1997 Baker Hughes INTEQ

All Rights Reserved


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Reference Manual i750-500-071 Rev. A / April 1997 Confidential

Table of Contents

Table of Contents

Chapter 1

Gyroscopic Surveys & EquipmentTraining Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

Instrument Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

The Baker Hughes INTEQ Gyroscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3Orientation of the Gyro Compass Card . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4Example of Orientation Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4Gyro Drift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-5Gyros Used by Baker Hughes INTEQ. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-5

Gyro Multishot Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7Running Gear. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7Warm-Up Box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-9Control Sub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-10Monitor Box. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-10

Chapter 2

Gyro Multishot SurveysTraining Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

Pre-Job Information Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

Running a Gyro Multishot Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2Rigsite Preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2GMS Survey Running Procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4

Chapter 3

Processing and Calculating a GMS SurveyTraining Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

Orientation Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2Orientation Correction Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2Orientation Correction Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3


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ii Baker Hughes INTEQConfidential 750-500-071 Rev. A / April 1997

Table of Contents Gyroscopic Surveying

Drift Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3Example of Drift During a Drift Check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4Drawing a Drift Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

TAC Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

Chapter 4

Survey DocumentationTraining Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

Final Survey Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

Forms, Reports, and Worksheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

Chapter 5

Gyro Single Shot SurveysTraining Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

GSS Instrument Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

Pre-Job Information Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4

Running a Gyro Single Shot Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4Rigsite Preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4GSS Survey Running Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6

Applying Orientation and Drift Corrections to GSS Surveys . . . . . . . . . . . . . . . . . 5-7

Chapter 6

High-Angle Gyro SurveyingTraining Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

Revised Foresight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2Calculating the Revised Foresight for a 2" Gyro . . . . . . . . . . . . . . . . . . . . . . . 6-2

Example Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5Calculating the Revised Foresight for a 1" Gyro . . . . . . . . . . . . . . . . . . . . . . . 6-5

Example Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

Inter-Gimbal Error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6Example Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9Inter-Gimbal Correction for the 1" Gyro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9

Example Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10Correcting Observed Drift Readings for Inter-Gimbal Error . . . . . . . . . . . . . . 6-10

Example Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11


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Reference Manual iii750-500-071 Rev. A / April 1997 Confidential

Gyroscopic Surveying Table of Contents

Chapter 7

In-Hole OrientationTraining Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

Downhole Orientation Using Conventional Multishot System . . . . . . . . . . . . . . . . . 7-1Necessary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1Running Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2Establishing Orientation Correction and Total Drift . . . . . . . . . . . . . . . . . . . . . . 7-3

Appendix A

Tool Axis Correction (TAC)Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1

Survey Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1Rotation Shots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1

Yo-Yo Shots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-2In-Run Versus Out-Run Shots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3

Calculation of the TAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3

Method A: Rotation Shots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3 Method B: Yo-Yo Shots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3 Method C: In-Run / Out-Run Shots . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3

Derivation of Formulae Used in the TAC Program. . . . . . . . . . . . . . . . . . . . . . A-4

Appendix BExercisesExample 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-1Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-2Example 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-3


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Training Manual 1-1750-500-071 Rev. A / May 1997 Confidential

Chapter 1

Gyroscopic Surveys & EquipmentTraining Objectives

Upon completion of this chapter, the trainee should be able to:

Explain the basic principles of the level rotor gyro.

Explain what is meant by gyro drift.

Explain the terms foresight , foresight direction and orientationcorrection.

List and identify the instruments required for Gyro Multishot Surveys(GMS).

List and identify all items of running gear and ancillary equipmentrequired for GMS surveys.

Instrument Overview

A conventional gyroscope consists of a rapidly spinning wheel (called the

Rotor) , which is mounted in a frame (called Gimbals ). The termgyroscope was first used by a French physicist who, in 1852, designed a3-frame gyroscope to demonstrate the Earth's rotation. He called hismechanism a gyroscope from the Greek words GUROS (revolution) andSKOPEEIN (to view). See Figure 1-1 .

The rotor is held in the inner gimbal by rotor bearings. The frame has ballbearings between the inner gimbal and the outer gimbal and also betweenthe outer gimbal and the body of the gyroscope.

When the gyro is running, the spin rotor has a lot of angular momentumand resists attempts to change the direction of its spin vector. In plain

terms, the spinning wheel wants to point its spin axis in the same directionall the time. The function of the gimbal system is to allow the case of thegyroscope to turn to different orientations without disturbing the spin rotor.(In technical terms, the gimballing system isolates the rotor from baserotation).


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1-2 Baker Hughes INTEQConfidential 750-500-071 Rev. A / May 1997

Gyroscopic Surveys & Equipment Gyroscopic Surveying

Figure 1-1 The Baker Hughes INTEQ Gyroscope


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Gyroscopic Surveying Gyroscopic Surveys & Equipment

The Baker Hughes INTEQ Gyroscope

Baker Hughes INTEQ has two sizes of conventional gyroscope, 1" ODand 2" OD. These gyroscopes are known as two-degree-of-freedomlevel-rotor gyros. A compass card is mounted on top of the outer gimbal.

In the 1" gyro, the East-West line on the compass card is aligned with thespin motor axis. In the 2" gyro, the North-South line on the compass cardis aligned with the spin motor axis. When the gyro is running, the spin axispoints in a fixed direction (theoretically) so the compass card maintains a

fixed orientation.

You will be shown a 2" Baker gyroscope. It will be run up in a warm-up box and its property of rigidity demonstrated.

Note: Regardless of how the warm-up box is turned and tilted,the rotor spin axis points in a fixed direction and the

North-South line on the compass card always stays in thesame vertical plane.

The Baker Hughes INTEQ gyros are used simply to provide a compasscard which will maintain its initial orientation.

Note: In practice, the spin axis gradually precesses from itsinitial heading (Gyro Drift) which complicates theinterpretation and calculation of conventional gyrosurveys.

Angle units containing either plumb bob or drift arc inclinometers areattached to the top of the gyros. These angle units have a transparent glassbase so the inclinometer and angle scale are superimposed on the gyrocompass card in the survey picture. The gyro compass replaces themagnetic compass of magnetic type angle units, but otherwise thereadings of inclination and hole direction are obtained by the same basicmethod as for the magnetic type surveys.

Note: The fundamental point to appreciate is that gyroscopesare not significantly affected by the proximity of

magnetized steel, so gyro survey tools are used insituations where magnetic interference prevents the useof any magnetic type survey tools. Gyroscopic surveysare always run on a wireline of some type because thegyro is a delicate instrument.


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Orientation of the Gyro Compass CardThese conventional gyros have no preferred orientation. They willpreserve their initial spin axis orientation. It is, therefore, necessary to gothrough a standard procedure to determine the relationship between the

North of the gyro compass and either True or Grid North.A reference mark (called the foresight ) must exist, which can be seen fromthe rotary table or orientation point on the rig. The direction from the rotarytable to that reference mark must be accurately known. On land rigs, thisreference might be a landmark whose coordinates could be obtained from amap. On offshore platforms in the North Sea, there are usually metal platesfixed to the structure one for each line of slots. The direction from therotary table to the metal plate is determined using a land surveyor's NorthSeeking gyroscope and theodolite. In many parts of the world, foresightsmay not be as readily available; therefore, help may have to be solicitedfrom the company representative.

At the start of the survey, the lower half of the tool is rested on an orientingtable using a support arm. The gyro is stabbed onto the control sub and atelescope arrangement is used to physically align the tool and, hence, aparticular point on the head of the gyro to the foresight (reference mark).When aligned in this way, a line bisecting the head of the gyro and passingthrough the zero on the outer Vernier scale should point directly at theforesight. The Vernier scale is fixed to the body of the gyro, and theVernier 0 is in a fixed orientation relative to the orienting lug and canonconnector on the base of the gyro.

Now, if the North of the gyro compass were actually pointing to North(True or Grid), then the reading on the compass card next to the Vernier 0would be the known direction from the rotary table to the reference point(the foresight direction ). In practice, there will be a difference which isdetermined at this stage in the proceedings. By using the Vernier scale, thisdifference, which is the necessary Orientation Correction , can beaccurately determined.

Examine the 2"gyro again. Look at the Vernier scale. Note therelationship between the zero on the Vernier scale and the position of theorienting lug and Canon connector on the bottom of the gyroscope.

Example of Orientation Correction Known Reference Direction N3.5E (003.5)

Initial Vernier 0 Reading on Gyro N6.8E (006.8)

Orientation Correction 3.3 (3.3 WEST)


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This orientation correction is a constant correction for one particular gyrosurvey. In the case of a gyro multishot, it must be applied to every singlesurvey reading of hole direction.

Gyro DriftWe saw earlier that the two-degree-of-freedom level-rotor gyro will try topoint its rotor spin axis in the same direction all the time. Unfortunately, inpractice the spin axis will slowly precess from its initial heading. Surveyorsrefer to this as Gyro Drift . There are several causes of gyro drift:

1. The gyro spin axis tends to maintain a constant direction relativeto an inertial frame of reference, e.g., a distant star, and not to thesurface of the Earth, which is spinning on its axis. Therefore, aperfect gyro of this type would appear to drift because of therotation of the Earth.

2. The bearings of the gimbal system are not perfectly frictionless.3. It is extremely difficult for a technician to balance the spin motor

housing perfectly to achieve zero drift, even at one particularlatitude.

In practice, the gyros are balanced so that the rate of drift is belowacceptable limits. Technicians work to a limit of 2' drift per hour; surveyorswork to a limit of 6' per hour when checking gyros. Gyros tend to driftmore at high inclinations because they are in a less stable gimbalconfiguration.

In both gyro single shot and gyro multishot surveys, an attempt is made tocorrect for gyro drift. In both types of survey, the alignment of the gyrocompass is checked at the end of the survey as well as the start. Since thegyro is aligned to the same reference mark in both cases, the differencebetween the Vernier 0 readings at the start of the survey and the end of asurvey is the Total Observed Drift .

We shall return to the subjects of orientation and drift corrections later inthis manual.

Gyros Used by Baker Hughes INTEQ

Historically, BHI had five different gyros which were used forconventional gyro single shot or gyro multishot surveys. These were:

Humphrey 2" and 1"

D.K. 2"

Eastman 2" and 1"


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All of these are two-degree-of-freedom level rotor gyros. At present, theBaker Hughes INTEQ gyros are used almost exclusively, but some districtsmay still have Humphrey or D.K gyros on their inventory.

The following tables give some basic information about all of these gyros:

Notice that all of these gyros have a torquer to keep the spin rotor axishorizontal. The torquer is controlled by either a mercury switch mountedon the underside of the spin motor housing (Humphrey gyros), or by anelectrolytic switch on top of the rotor housing. If the spin axis tilts awayfrom the horizontal, fluid moves to one end of the switch and current to thetorquer is switched ON. A small torque is then applied to the outer gimbalwhich causes the spin rotor axis to precess back to the horizontal. In thecase of the Baker Hughes INTEQ gyros, the torque is applied viaelectromagnetic coils.

During an actual survey, the gyroscope is powered by 16 D-cells in abattery barrel. The battery voltage will decrease with time, so thegyroscope is connected to a Control Sub which has the battery voltage asits input and supplies current to the gyro at a constant voltage level of 28V.The control sub is screwed into the battery barrel.

Gyro Size Current Speed Motor Torquer

Humphrey 2" 275 mA 19 - 21 K Induction Mercury Switch / Electromagnetic

Humphrey 1" 315 mA 38 K Synchronous Mercury Switch / Electromagnetic

DK 2" l50 mA 20 - 21 K Induction Electrolytic Switch / Mechanical

Eastman 2" l80 mA 21 K Induction Electrolytic Switch / Electromagnetic

Eastman 1" 220 mA 42 K Synchronous Electrolytic Switch / Electromagnetic

Gyro Size Construction Temp.Limit

Metals Caging Stops

Humphrey 2" Non-Symetrical open motor 250 F Different metals Y/N 55 - 70

Humphrey 1" Non-Symetrical open motor 250 F Different metals Y 28 - 34

DK 2" Symetrical sealed 300 F Same metals N 70

Eastman 2" Symetrical sealed 300 F Same metals N 70

Eastman 1" Symetrical sealed 300 F Same metals N 35


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Gyro Multishot Surveys

Gyro Multishot Surveys (GMS) are taken of sections of eased wellbore.Since the gyroscope is a delicate instrument, GMS surveys are always runon a wireline of some sort. Either the 2" or the 3" system can be used to runGMS surveys. However, the 3" system is normally preferred because the2" gyro is more stable and will have less tendency to drift at higherinclinations. The conventional gyro multishot instrument consists of:

The multishot battery pack (as for DMS).

The multishot instrument body incorporating the camera, motorassembly, switch assembly and electronic timer (as for DMS).

The gyro angle unit. Available ranges are 0 to 2, 0 to 5, 0 to 12,0 to 24, 0 to 34, 5 to 90.

The gyroscope. Either a 1" or a 2" Baker Hughes INTEQ gyromay be used, but as already stated, the 2" gyro is usually preferred.

Running Gear

The running gear for the 3" GMS system consists of:

Wireline cablehead + crossover or spearpoint + wireline sub or ropesocket

2 gyro swivels

Gyro spacer bar with top centralizer and stop collars

3" instrument barrel

3" control sub

2" or 3" battery barrel

Lower spacer bar with centralizer and stop collars

Bottom landing shock assembly

On the next page, make a large diagram of a complete GMS tool.


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Diagram of a Complete GMS Tool


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Warm-Up Box

This is a portable power supply. It can be used to bring the gyro to fullspeed before running the survey. Stab the gyro into the Gyro Cup (seeFigure 1-2 ) on the warm-up box at least 30 minutes before running the

survey.

5-pin lead.............................110 volts only

7-pin lead.............................110 or 220 volts

Note: M is a modified box able to take the initial surge of ECgyros.

Have the warm-up box switched ON and stab the gyro. External powerswitch need only be on when you are charging the battery by an externalpower source. Battery volts should read 11 to 13 volts. The Low light willflash if power is too low. The Charged light will come on when fullycharged.

BATTERY

C HA RG E D LOW

EXT. POW ER

B ATTE RY GYR O

VOLTS

EX TER N A L

VOLTS

VOLTSGYRO

C U R R EN TO FF O N

OFF ONG Y R O

C AGI NG

MeterSimpsonNo. 3323

MeterAdjustmentScrew

CannonPlug GyroConnector

CagingConnector Gyro

CupExternalPowerConnector

BatteryIndicatorDiodes

ExternalPowerIndicatorDiode

ExternalPowerSwitch BAKER

HUGHE SINTEQ

SelectorKnob

2-1/2 in. GyroAlignment PinSocket

GyroPowerSwitch

1-1/2 in. GyroAlignment PinSocket

Figure 1-2 Feature and Control Placement on Warm-Up Box


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Control Sub

This maintains a constant voltage of 28 volts to the gyro. Sixteen D-cellbatteries in the battery barrel provide 24 volts. This is boosted to 28 voltsby the control sub and is maintained at that until the supply is reduced to 8

volts without a load or 14 volts under gyro load.Control Sub pins one (+VE) and three (VE), shown below, can be used tomonitor gyro volts using a multi-meter.

Monitor Box

The Monitor Box is connected to the Control Sub to check electricalfunctions. The Selector Switch has five positions:

OFF . . . . . . . . . . . . . . . . . Power to the gyro is disconnected.

BATTERY VOLTS . . . . . Voltage output delivered to the Control Subfrom the batteries.

GYRO VOLTS . . . . . . . . Stabilized output voltage from the Control Sub,

i.e., the voltage of the DC supply to the gyro.(should be 28V).

GYRO CURRENT . . . . . Electric current supplied to the gyro inmilliamps.

NULL . . . . . . . . . . . . . . . . Torquer-motor current consumption inmilliamps. This switch position is used onlywhen orienting the EC 1" gyro.

Note: When using an BHI 2" gyro, do not use the OFF or NULL settings because, in either case, current to the gyro

will be switched OFF.

1 3


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Chapter 2

Gyro Multishot SurveysTraining Objectives


List all the pre-job information required for GMS surveys.

List the pre-job equipment checks which must be done prior torunning a GMS survey.

List and explain the steps to be followed in running a GMS survey.

Assemble a GMS tool correctly.

Pre-Job Information Required

Most of the following information should be available before you leaveyour base for the rig. However, some of it should be verified with theCompany man or the drilling engineer at the rigsite.

1. The name of the field, the well name, the slot number and the slotcoordinates.

2. The size of casing to be surveyed, OD and ID.

3. The total depth interval to be surveyed and the interval betweensurveys.

4. The type of wireline to be used.

5. The nature and location of the foresight to be used.

6. The foresight direction.

7. The maximum angle (inclination).8. For wells of over 10 inclination, the average hole direction.

9. The maximum downhole temperature.

10. The fluid weight and condition.

11. The tie-on coordinates to be used for the final survey calculation.


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Gyro Multishot Surveys Gyroscopic Surveying

12. The target direction and the origin to be used for calculatingvertical section.

13. The depth of the casing shoe.

14. If the survey is to be run just after the casing has been cemented,

the depth of the top of the cement.

Running a Gyro Multishot Survey

When the kit is made up at the district base, a complete check of every itemshould be carried out. Obviously, only green-labeled instruments will beused.

Rigsite Preparations

1. On arrival at the rigsite, go and see the Company man to let himknow you have arrived and find out how soon you will berunning the survey. Verify all the job information with him.

2. Ensure all equipment has arrived safely. Make sure you have allthe equipment and instruments you need to run the survey. Checkfor visible signs of damage. Store the running gear in aconvenient, safe place. If you are not running on Baker HughesINTEQ wireline, check with the wireline operator to affirm thatyou have the correct cross-over.

3. If the multishot cameras (the back-up instrument included) werenot loaded with film in the office, they should both be checkedand fully loaded.

Note: Under normal circumstances, you should load thecameras in the office when you are making up the kit.

4. Ensure there are no bubbles obscuring the scale of the angleunits. Load ten AA-cells into a battery pack and test the voltageoutput with a multimeter (15 volts). Check the current outputalso, which should exceed 5 amps and is typically 6 to 7 amps.This is done only momentarily, as it is a severe drain on thebatteries.

5. Note the asset numbers of the instruments you will use.

6. See the drilling engineer to double check any job information forwhich you are uncertain. It is vital that you are 100% certainwhat you will use as the foresight and what the foresightdirection is. You should also verify the tie-on coordinates to be


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Gyroscopic Surveying Gyro Multishot Surveys

used for the final survey calculation. For a high angle GMS, it isimportant to obtain the MWD surveys (or single shots) of thesection of well which you will survey so you can calculate yourrevised foresight. (see Chapter 6, Revised Foresight ).

7. Go to the rig floor and ensure you can see the foresight.

8. Make up the lower half of the tool. Put 16 D -cells, tip up, in thebattery barrel. Screw in the control sub hand tight, check thebattery volts and gyro volts using the monitor box, then tightenthe control sub in the battery barrel with pipe wrenches. Thecentralizer may be fixed either on the battery barrel or on aspacer bar, if one is used. Store the bottom half of the toolsomewhere safe, e.g., behind the draw works.

9. Make up the top half of the tool. The top centralizer should befitted on the top spacer bar if one is used, otherwise on the

instrument barrel. Secure the centralizers fixed end which shouldface upwards. Adjust the centralizer to the required size and lockthe stop collars. Store this section of the tool in a safe position,preferably near the cat walk.

10. If there is plenty of time before you will run the survey and ifthere is a safe, dust-free and vibration-free environment (e.g.,your cabin on an offshore platform), drift check both gyros.

11. Plug the warm-up box into a main power source and put it oncharge until the CHARGED light comes on.

Note: Ideally, the internal battery of the Warm-Up Box is fullycharged before you leave your base.

12. Prepare your field sheets and other paperwork. Plan the depths atwhich you will take drift checks.

Steps 1 through 12 should be completed as soon as possible after you havearrived at the rig. Some of these steps may be unnecessary if you made upthe kit yourself and loaded the cameras, charged up the warm-up box, etc.at that stage (which you should have done), particularly if the job is on aland rig and you have transported the equipment yourself. However, whenequipment is freighted to an offshore platform, there is always thepossibility of damage during transit.

The remainder of the rig-up procedure should be undertaken two hoursbefore the survey is due to start.

13. Take the ancillary items you need up to the rig floor, such asorienting table with legs and support arm, the telescope kitincluding monitor box, spare gloves, rags, etc.


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14. Find a convenient, safe place to warm up the gyro, preferablywith main power for the warm-up box. Switch ON the gyro andlet it run up to speed. The gyro should be warmed up for at least30 minutes (preferably about 1 hour) before the survey begins.

Note: Remember to align the gyro compass card to the desiredorientation before you start up the gyro.

15. About 15 minutes before the survey is due, take the gyro in thewarm-up box to the rig floor and place it in the dog house. Linethe warm-up box up with the direction to the foresight and takethe Vernier 0 reading. If the gyro has drifted appreciably fromthe desired orientation, you may at this stage tweak it back tothe desired orientation.

GMS Survey Running Procedures16. As soon as the rig is ready, the wireline operator will rig up the

sheave wheels.

17. Connect the top half of the tool to the wireline. Lift it up to therig floor and set it to one side, e.g., in the mousehole.

18. Lift up the lower half of the tool (using the tugger line attached tothe control subs lifting ring), lower it into the well, and supportit on the orienting table with the orienting arm.

19. Plug in the monitor box and quickly check the battery volts andgyro volts readings. Then, turn the select switch to the GyroCurrent position.

20. In the dog house, assemble the multishot instruments to start thecamera running. If you are going to run a long survey and it isimportant to conserve film, you should not attach the batterypack until you reach step 27.

21. Take the gyro from the warm-up box and stab it onto the controlsub. Watch for the current reading on the monitor box. Securethe gyro to the control sub with one wrap of Kapton tape.

22. Fit the lower telescope onto the control sub. Attach the upper

scope to the head of the gyro. Set the two scribe lines on the glasson either side of Vernier 0.

Note: The Vernier 0 should face towards you and away fromthe foresight.

23. Using the orienting arm, turn the tool until you are sighted onthe foresight through the upper scope. Now adjust the lower


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scope until you are sighted on the foresight through it also (usingthe adjusting thumb screw shown in Figure 2-1 ). Once bothtelescopes are aligned on the foresight simultaneously, lock thelower scope with the locking thumb screw. Then check again thatboth telescopes are still simultaneously aligned on the foresight.

Take off the upper scope and store it away safely. Use a plasticbag to protect the gyro once you have removed the top scope.

Figure 2-1 Three-Inch OD Gyro System


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24. Check the gyro current reading on the monitor box. By now, itshould have stabilized. Also check the battery volts readingagain. Unplug the monitor box and store it away.

25. Ensure the lower telescope is still aligned on the foresight. If not,turn the tool using the orienting arm until it is.

26. Use a single shot reader to take the Vernier 0 reading visually.This is your Visual Start Case .

27. Fetch the instruments. Attach the battery pack to start thecamera, if you have not already done so. Ensure the lights of thecamera are working by holding up the instrument assembly andlooking in through the base of the angle unit. Do not invert theinstruments. This will also allow you to ensure there is nocondensation on the glass of the angle unit and the bubble is notobscuring the inclination scale. Now attach the instrument

assembly to the head of the gyro.28. Simultaneously key the multishot timer and start your

stopwatch. Listen for the motor winding the film on twice duringthe first minute.

29. Ensure the scope is still aligned on the foresight as the cameratakes a picture at minute one. This is not used as a start casepicture, but is useful information nonetheless.

30. Have the instrument barrel lowered over the instrument packageand gyro, and make it up to the control sub hand tight. Use onepipe wrench and the orienting arm held against your leg totighten the connection a little further, but do not use undue force.

31. Now take the Start Case pictures. Line up on the foresight. Taketwo successive pictures aligned on the foresight. Then, turn thetool about 30 using the arm, and wait until a picture has beentaken. Now turn back to the foresight and take a third start casepicture.

Note: When the camera is actually taking the start case pictures, you should be looking through the telescope tomake certain the tool is oriented exactly on the foresight

and that there is no tool movement while the picture isbeing taken.

32. Remove the telescope and store it in the alignment kit case.

33. Signal to the wireline operator to pick up the tool. Remove theorienting arm and table. Lower the tool until the gyro is levelwith the rotary table, then stop the wireline and zero the wirelinedepth counter.


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34. Once the driller has opened the rams, run in hole to the firstsurvey depth.

35. You now proceed to run the survey by stopping the wirelineevery 100' (or designated depth interval between surveys) to letthe camera take a stationary picture. The tool should bestationary from ten seconds before the camera lights come onuntil ten seconds after.

36. Periodically, you will keep the tool stationary for several minutesat certain depths. These stationary periods are called DriftChecks . They allow you to measure the gyro drift accuratelyduring each period by taking the change in the compass cardreading relative to any of the numbers on the Vernier scale. Sincethe tool is not moving, the Vernier scale (which is fixed to thecase of the gyro) is not moving. Therefore, any change in aVernier reading (e.g., Vernier 1) must be due to the compass card

turning, i.e., to the gyro drifting.37. You should either take a five minute drift check every fifteen

minutes or a four minute drift check every twelve minutes (25%of downhole survey time).

38. Take the first drift check quite early in the survey, say fiveminutes after the last start case picture was taken. When runningoffshore, take the first drift check as soon as the tool is below seabed.

39. Even if you have to traverse a large depth interval before you

reach the section you are to survey, you must still take driftchecks at regular time intervals while running in.

40. Plan your drift checks so you reach your deepest survey stationmidway between the last in-run drift check and the first out-rundrift check.

41. While pulling out of hole, stop and take a survey at every fourthin-run station. Continue to take drift checks at regular timeintervals.

42. When the tool returns to surface, check the wireline depthreading when the tool is back at rotary table level. The readingshould be within 10' of zero.

43. As the tool comes out of the well, hose it down with water orwipe it with rags.

44. Support the tool with the orienting arm and table and attach thelower scope.


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45. Turn the tool with the orienting arm until the cross-hairs of thescope are sighted exactly on the foresight. Take the End Casepicture.

46. Carefully break off the instrument barrel and lift it up to revealthe instruments.

47. Remove the instrument assembly from the gyro, cover the gyrowith a plastic bag, and put the instruments in the dog house.

48. Line up on the foresight and take your Visual End Case .

49. Plug the monitor box into the control sub and note readings ofgyro current, gyro volts, and battery volts. Disconnect and putaway the monitor box.

50. Verify the camera is still working, then disconnect the batterypack, noting the final picture time (the last flash).

51. Unstab the gyro from the control sub, put it in the warm-up boxand allow it to run down.

52. Screw the protective cap onto the control sub and use the tuggerto lift the lower half of the tool out of the hole. Remove it to asafe location on the rig floor.

53. Store the orienting table, telescope kit, etc. out of harm's way.

54. Remove the gyro and all instruments from the rig floor.

55. Develop, wash, and dry your survey film. Then proceed to readthe film using the projector.


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Chapter 3

Processing and Calculating a GMSSurvey

Training Objectives


Explain what is meant by an orientation correction.

Calculate an orientation correction, given a start case reading and aforesight direction.

Calculate values of gyro drift from initial and final Vernier readings.

Perform the necessary calculation and draw a drift curve, given therequired data.

Apply both orientation and drift corrections to observed GMS surveydata.

Explain what a Tool Axis Correction (TAC) is.

Explain the running procedures which may be followed to ensuregood data sets for the TAC calculation.

Use a TACCAL/TACCOR program to correct a GMS survey forTAC.

Overview

A total of four corrections are applicable to the raw azimuths as read fromthe film. Depending upon the inclination of the survey, only three willapply at any one time. The corrections must be applied in the followingorder.

Inter-gimbal correction (only if inclination is > 10)

Orientation correction

Drift correction

Tool Axis Correction (only if inclination is < 10)


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Processing and Calculating a GMS Survey Gyroscopic Surveying

Orientation Correction

This was explained briefly in Chapter 1, Orientation of the Gyro CompassCard . This correction compensates for the fact that the North of the gyrocompass card is not perfectly aligned to True (or Grid) North.

If the start case reading (Vernier 0) and the foresight direction areexpressed in azimuth, then the orientation correction may be simplycalculated using:

Orientation Correction = Foresight Azimuth Start Case Azimuth

Orientation Correction ExampleForesight Direction: 147.50Start Case Vernier 0 reading: 142.70Orientation Correction: 4.8 (4.8E)

If the start case reading and the foresight direction are expressed asquadrant bearings, the magnitude of the orientation correction is just theangular difference between the start case and the foresight bearings. Inorder to determine whether this is an East or West correction, imaginerotating the start case reading to the foresight bearing. If this rotation isclockwise round the compass card, the orientation correction is EAST; ifthe rotation is anti-clockwise then the orientation correction is WEST.

W E

S

8 .2 o

Direction of rotation

Start Case Foresigh t

Start CaseS 0 4 7 Wes to

ForesightS03 5 Easto

Foresight Direction = S03 5 East

S tart Case Read ing = S04 7 Westo

o

Figure 3-1 Orientation Correction


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Gyroscopic Surveying Processing and Calculating a GMS Survey

The total angle between the start case bearing and the foresight bearing is4.7 + 3.5 = 8.2.

If we turn from the start case bearing round to the foresight bearing we areturning anti-clockwise. Hence, this is a WEST orientation correction.

Orientation Correction = 8.2W or 8.2.We could also calculate the example shown in Figure 3-1 by expressing thestart case and foresight directions in azimuth.

Orientation Correction = Foresight Azimuth Start Case Azimuth

= 176.5 184.7

= 8.2

Orientation Correction Calculations

Calculate the orientation correction required in each of the following cases.

An East or West orientation correction is applied to the observed directionsin the same way as an East or West magnetic declination correction,although, of course, a gyro orientation correction has nothing whatever todo with a declination correction.

Drift CorrectionsAs explained earlier, the gyro spin axis gradually precesses from its initialheading. This is referred to as Gyro Drift . The rate of precession is sampledperiodically by taking regular Drift Checks . Since the case of the tool isstationary during a drift check, the change in any of the Vernier readingsbetween the beginning and the end of the drift check is the drift which hasoccurred in that time.

Foresight Direction Start Case Reading Orientation Correction

341.0 338.5

S 63 W S 64.3 W

N 89.5 W S 89.5 W

S 83.5 E S 80.7 E

163.5 165.2

270.5 269.5

N 17 E N 17.25 W

S 07.5 W S 04.2 E

N 03.5 E N 08.4 E

138.0 143.9


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Example of Drift During a Drift CheckStart of Drift Check, Vernier 0 reading: N 81.6 EEnd of Drift Check, Vernier 0 reading: N 81.1 EDrift during this Drift Check: 0.5 EAST

Remember that the compass card is actually turning in the opposite senseto that of the apparent rotation of the Vernier (which does not move duringa drift check). So, to decide whether the gyro drift is East or West, imagineturning the final Vernier reading to the initial Vernier reading ( Last toFirst ). If this rotation is clockwise, the drift is EAST; if it is a counter-clockwise rotation, the drift is WEST.

When you read your survey film for a GMS survey, look at the Vernierreadings for each minute of the drift check. Provided there are noanomalous readings, you will take the difference between the initial andfinal Vernier readings as the total drift during the drift check. Notice that

the individual Vernier readings have no significance it is only thedifference between the initial and final readings that matters. You must alsoappreciate that for these measurements of drift during the drift checks. Wecan use any number on the Vernier scale as a reference (Vernier 0, Vernier1, etc.), but obviously we must use the same Vernier to take both readingsfor an individual drift check.

In order to determine the appropriate drift correction for each surveystation of a GMS, we construct a graph of Gyro Drift vs Time . This graph iscalled a Drift Curve . The steps involved in drawing a drift curve areenumerated below.

Drawing a Drift Curve1. Divide the survey into time periods with one drift check per time

period. Calculate the mid-points between the finishing time ofone drift check and the starting time of the next drift check.

2. Calculate the drift during each time period using the drift rate inthe drift check for that time period.

3. The drift at each mid-point will be the sum of the values of driftfor all the preceding time periods. Plot these values of calculateddrift at each of the mid-point times and at the end case. The valueof drift (in degrees) found by summing the values for all the time

Drift during

time period

Drift from start to end of

drift check Number of minutes in

drift check ----------------------------------------------------------- Length of period=


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periods (taking account of sign) is the Total Calculated Drift .This is also the value of drift calculated to have occurred by theend case time, i.e., over the whole survey.

4. Draw the graph of calculated drift by drawing straight linesconnecting the value of drift at each mid-point to the value at thenext mid-point.

Note: The graph starts from the start case time with Degreesof Drift equal to zero. The graph does not pass through

zero time.

5. Calculate the Total Observed Drift which is the differencebetween the start case and end case Vernier 0 readings (on film).Plot this value of drift on the graph at the end case time.

6. Draw a straight line (lightly, in pencil) from the point of zerodrift at the start case time to the point defined by total observeddrift at the end case time. (Plotted in Step 5). This is line (a) inFigure 3-2 , Observed Drift .

7. Draw a second straight line (lightly, in pencil) from the start caseto the point defined by the Total Calculated Drift at the end casetime. This is line (b), Calculated Drift .

8

7

65

4

3

2

1

0

-1

-2

-3

Drift (degrees)

Midpointbetweendrift checks

Calculateddrift curve

Closed (corrected)drift curve

Time (mins)

(a )

(b )

b- a

b- a

Total calculated drift

Total observed drift

Closure

Figure 3-2 Graphing the Drift Curve


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8. At each mid point between drift checks, measure the differencein drift value between lines (b) and (a). (See Figure 3-2 ).

9. Subtract the respective differences (b a) from the calculateddrift curve mid-points and plot the new set of mid-point driftvalues.

10. Join the points plotted in Step 9 to obtain the Closed (i.e.,corrected) Drift Curve . Our convention is that this is drawn in redpencil.

11. The hole direction at each survey station is corrected by applyingthe closed drift curve value for the time in minutes when thepicture was taken.

The example shown in Figure 3-3 shows a proper drift curve drawn for thefollowing data.

Foresight Azimuth:08.5Start Case at Minute 6: Vernier 0 reading N 11.5 E

End Case at Minute 66: Vernier 0 reading N 7.4 E

Drift CheckNumber

Time(Minutes)

Start VernierReading

End VernierReading

Drift(Degrees)

1 12 - 17 N 25.4 E N 25.0 E 0.4 E

2 31 - 36 N 13.7 W N 13.9 W 0.2 E

3 52 - 57 S 56.8 E S 57.5 E 0.7 E

1.04. 0

5. 0

3. 0

2. 0

1. 0

0.0 -3.0

0.0

-1.0

-2.0

Drift (degrees)

Orientation + Drift (degrees)

Start Case

Time (mins)

Total calculated drift = 5.32 E

Total observed drift = 4.1 E

Drift curve closure = 1.2

0 6 10 20 24 30 40 44 50 60 70

(0.4 /5) x 18= 1.44 E

(0.2 /5) x 20= 0.80 E

(0.7 /5) x 22= 3.08 E

= +1.44 = +2.24

Figure 3-3 Drift Curve Example


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TAC Correction

The final correction which may have to be applied to a low angle GMSsurvey is the so-called TAC Correction which corrects for misalignment ofthe tool axis. This is explained in detail in Appendix A . This correction isapplied at the rigsite using the Baker Hughes INTEQ TAC program for aHewlett Packard programmable calculator.


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Chapter 4

Survey DocumentationTraining Objectives


List the standard survey documentation required for GMS surveys.

Complete all the standard post-job reports correctly.

Final Survey Calculation

You must perform the final survey calculation at the rigsite and give copiesto both the Company man and the drilling engineer. However, indicate onthe calculation sheet that it is a field calculation only and will be checkedand corrected in the office.

Forms, Reports, and Worksheets

The survey envelope, which should be clearly labeled, should contain allthe documentation listed here plus the original survey film . Remember, thisis the permanent record of your survey.

Note: Remember to get your job ticket signed before you leavethe rigsite.

Orientation Diagram polar graph paper with foresight, start case,and orientation correction shown clearly.

Survey Report.

Equipment Performance Report.

Gyro Field Sheets.

Field Calculation Sheets (photocopy).

Drift Curve.

Office Check Field Sheets.


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Survey Documentation Gyroscopic Surveying

Copy of Computer Survey Calculation.

Any other documentation required by your district.

Note: You should note all asset numbers before you run in hole.


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Chapter 5

Gyro Single Shot SurveysTraining Objectives


Explain the circumstances in which gyro single shots are taken.

List and identify the components of a gyro single shot instrument andexplain the function of each component.

Operate gyro single shot instruments correctly.

List and identify all the items of running gear required for gyro singleshot surveys.

Assemble a gyro single shot tool correctly.

List the pre-job information required for gyro single shot.

List the pre-job equipment checks which must be done prior torunning a gyro single shot survey.

List and explain the steps to be followed in running a gyro single shot

survey. Calculate the necessary orientation and drift corrections and apply

these to the gyro single shot survey.

GSS Instrument Overview

Conventional gyro single shots are rarely taken today because they havebeen largely replaced by either Seeker or Sigma 175.

The gyro single shot is primarily used to orientate deflection tools in areasof magnetic influence. Of course, it will at the same time give theinclination and direction of the wellbore. The tool is run in through thedrillstring and seats in an orienting sub in the BHA, with the tool itselflying inside a drill collar. The instrument consists of:

1. The Single Shot Battery Pack (as for R single shot).


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Gyro Single Shot Surveys Gyroscopic Surveying

2. A Single Shot Timing Device - usually a 33 minute mechanicaltimer or a 99 minute electronic timer.

Note: 33 minute mechanical timers are officially obsolete, butmay still be encountered in some regions.

3. A single shot camera (R type).

4. A single shot adaptor which screws on to the front of the singleshot camera. This adaptor has a lens and also a light gate(shutter) which can be opened or closed manually. There arethree bulbs on the front of this adaptor which replace the bulbs ofthe single shot camera. The front of this adaptor screws onto thegyro angle unit.

5. A gyro angle unit usually a 0 to 12 unit.

6. A 1" Baker Hughes INTEQ gyroscope.There are special 0 to 1 and 0 to 4Camera and Plumb Bob assemblieswhich replaces items 3 and 5. A clear angle unit, which does not containan inclinometer, is then necessary as a cross-over from the single shotadaptor to the head of the gyro.

Note: Camera and Plumb Bob assemblies are officiallyobsolete, but may still be encountered in some regions

The gyro single shot instruments fit inside either a 2" OD or a 1.75" ODpressure barrel. The 2" OD system is most often used. The gyro is stabbedonto a voltage control sub which is screwed into a battery barrel containing16 D-cells in series. The control sub boosts the battery voltage andmaintains the voltage supplied to the gyro at a constant 28 volts. Thecomplete downhole tool consists of:

Wireline cablehead + crossover or spearpoint + wireline sub or ropesocket

2 Swivels Finger Pin Stabilizer (with finger pins cut to size) 2" Instrument Barrel (containing instruments)

2" Control Sub 2" Battery Barrel Finger Pin Stabilizer Soft-Shock Assembly Adjustable Muleshoe

Draw a diagram of a complete gyro single shot tool and label the diagramcarefully.


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Gyroscopic Surveying Gyro Single Shot Surveys

Complete Gyro Single Shot Tool


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Pre-Job Information Required

You will need all the standard information required for any type of survey,viz, well name, slot coordinates, tie-on coordinates, etc. You must alsoknow the nature and location of the foresight and what the foresightdirection is. Also you must know what type of wireline will be used;usually it is the rig sandline. Gyro single shots are almost exclusively usedto determine tool face orientation of deflection tools in shallow kick-offs,so downhole temperature, fluid condition, etc., should not be aconsideration.

At the rig, note the bottomhole assembly in your tally book. Calculate thedistance from the bit to the muleshoe sleeve. This is necessary so that youknow at what depth the tool will land in the muleshoe when you arerunning surveys. Once you have made up the two halves of the tool,measure and note lengths of each. The sum of the bit to muleshoe

distance plus the length of the lower half of the tool (muleshoe to top ofcontrol sub) gives the distance which must be subtracted from the bit depthto obtain the survey depth. This calculated survey depth should cross-checkwith the wireline depth counter reading. The length of the top half of thetool is useful when calibrating a Cavins depthometer prior to running inhole.

You should also note the minimum ID through which the tool must passand the ID of the collar it will be inside when seated downhole.

Running a Gyro Single Shot Survey

Rigsite Preparations1. At the rigsite, advise the company man of your arrival and find

out when the job will start. Also, see the directional driller whowill know all the details of the job.

2. Verify that you have all the equipment you need and look for anyvisible signs of damage.

3. Verify you have all the required job information.

4. Check the sandline/wireline connections.

5. Ensure that you can see the foresight from the rig floor.

6. Test the gyro single shot instruments (if time permits).

7. Make up the two halves of the tool:

Top Section Wireline crossover, 2 swivels, finger pinstabilizer (with rubber finger pins), 2" gyro instrumentbarrel.


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Lower Section 2" control sub, 2" battery barrel, finger pinstabilizer (optional), soft-shock assembly, adjustablemuleshoe.

Remember to load 16 D-cells into the battery barrel (tip to control sub).Check the battery volts and gyro volts readings with the monitor boxbefore making up the control sub tight.

8. Lay the lower half of the tool horizontal on tool stands and attachbubble levels to the bubble level recess on the 2" control sub andto the muleshoe slot. Turn the whole tool until the control subrecess is facing vertically upwards, then adjust the adjustablemuleshoe until the muleshoe slot is also facing verticallyupwards. Tighten the locking screws on the adjustable muleshoe,then double check that the muleshoe slot is properly aligned tothe recess on the control sub.

Note: The above procedure is vital so that correct tool faceorientation is obtained. When the 1" gyro is attached tothe control sub, the Vernier 0 will be 180 out from themuleshoe slot. Since we effectively orientate the gyrocompass card 180 out, the Vernier 0 reading on ourgyro single shot gives the toolface orientation (gyrotoolface).

9. Put a lead (tell tale) slug in the hole in the muleshoe slot.

10. Put the inserts for the 2" system in the orienting arm and the

lower telescope assembly.11. Take both sections of the tool plus instruments and ancillary

items to the rig floor. As soon as convenient, connect the topsection of the tool to the sandline. Remember, you needdeveloper and fixer fluid and a single shot developing tank. Alsohave a cup of water in the dog house for washing the developedfilm disk.

12. Warm the gyro up for 15 to 30 minutes before the first singleshot run. On subsequent runs, a 10 to 15 minute warm-up shouldsuffice, depending on how frequently you are running the

surveys. If they are drilling very quickly and you are taking aGSS every single, it is probably not worth running the gyro downbetween surveys. (This also depends on the total time the job isexpected to last). Orientate the gyro compass card as requiredbefore you switch on the gyro, following the same procedure asfor GMS.

13. Make up the complete gyro single shot instrument. Close thelight gate (shutter) before you load the single shot camera.


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GSS Survey Running Procedures1. Once the kelly has been broken off, put the lower half of the tool

into the drillstring resting on the orienting table. Remember, theorienting arm fits onto the battery barrel in the 2" gyro system.

2. Stab the gyro onto the control sub and use the monitor box tocheck the electrical readings. Use the support sleeve to protectthe 1" gyro.

3. On the first run only, fit upper and lower telescope assembliesand adjust the lower telescope as described in steps 22-23 of theGMS running procedure enumerated in Chapter 2, GMS SurveyRunning Procedures

Note: On subsequent runs, you can use the lower scope as afixed scope to save rig time, but you must be certain

that it has been firmly locked so that no subsequentadjustment occurs. Wrapping tape around the lockingthumb screw after the first run will help immobilize thescope.

4. After removing the top scope, turn the tool using the orientingarm until you are precisely lined up on the foresight through thelower scope. Use a single shot reader to take your Start Casevisually. Note this in your tally book.

5. Remove the support sleeve and monitor box. Attach theinstruments to the gyro and set the required time on theinstrument timer. Start your surface stop watch and theinstrument watch simultaneously.

6. Open the light gate (shutter).

7. Lower the instrument barrel over the instruments and make up tothe control sub.

8. Take the tool weight on the sandline and remove the orientingarm. Zero the wireline depth counter when the angle unit isapproximately at rotary table level, or attach the Cavinsdepthometer to the sandline and set initial reading based on the

length of the GSS tool.9. Run tool in hole. When the depthometer reading indicates 50' to

go, have the sandline slowed so the tool lands gently in themuleshoe sleeve.

10. Flag the sandline at this point. On the next run, the flag willindicate when the tool has nearly reached bottom.


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11. Wait on bottom until the picture has been taken, then pull the toolout of hole.

12. Check the lead slug for indentation. Rest the tool on the orientingarm and unscrew the instrument barrel from the control sub toaccess the instruments.

13. Close the light gate (shutter).

14. Remove the instruments from the gyro, place a plastic bag overthe gyro, then immediately take the instruments into the doghouse and unload the single shot disc into the developing tank.

15. Attach the lower (fixed) telescope, line up on the foresight, andvisually take the End Case Vernier 0 Reading . Stop yourstopwatch.

16. Quickly take final monitor box readings. When you are taking a

lot of gyro single shots, you should pay particular attention to thebattery volts reading. You should change out the batteries inthe battery barrel when this reading falls to 16 volts.

17. Unstab the gyro from the control sub and transfer it to the warm-up box in the dog house. Unless you expect to run again within20 minutes, you should switch OFF and run down the gyro, butnot until you know you have a good survey

18. Remove the single shot disc from the developing tank, wash anddry it.

19. Read the values of inclination, hole direction, and toolface fromthe disc. Apply the necessary orientation and drift corrections toboth the direction and toolface readings.

20. Store both running gear sections out of the way on the rig floor.Store instruments carefully. Since it may be used many times,have everything accessible.

Applying Orientation and Drift Corrections to GSSSurveys

The orientation correction is calculated from the start case reading and theforesight direction, just as for GMS.

The total drift from start case to end case is calculated as for GMS. There isno possibility of taking drift checks with conventional single shot surveys,so we calculate the drift which has occurred when the survey is taken,assuming the gyro drifts at a constant rate.


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Example 1

Foresight Direction: N 34 E

Start Case reading: N 36 E (minute 0)

End Case reading: N 38 E (minute 18)

Survey readings (taken at minute 10):

Inclination: 6

Hole Direction: N 40 W

Toolface: N 30 W

Orientation Correction: 2.5 WEST

Total Gyro Drift (start case to end case): 2 WEST

Total Correction = Orientation Correction + Drift Correction 1

In this example, total correction = 2.5 WEST + 1.1 WEST, i.e., approx.3 WEST. So the corrected values are:

Hole Direction N 43 W

Toolface N 33 W

1. Added algebraically

Dr if t correction total drift time of downhole surveytime start case end case------------------------------------------------------------------------=

drift correction 2oWest

10 minutes18 minutes ------------------------------ 11 o West = =


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Example 2

Calculate the necessary orientation and drift corrections and apply them tothe observed hole direction and tool face readings.

Foresight Direction: N 19 W

Start Case reading: N 17 W (minute 0)

End Case reading: N 20 W (minute 25)


Inclination: 4

Hole Direction: S 37 E

Toolface: S 45 E


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Example 4

Calculate the necessary orientation and drift corrections and apply them tothe observed hole direction and tool face readings.


Start Case reading: N 01 E (minute 0)

End Case reading: N 01 W (minute 16)


Inclination: 5

Hole Direction: S 47 W

Toolface: S 62 W


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Chapter 6

High-Angle Gyro SurveyingTraining Objectives


Explain why a revised foresight is used in high angle GMS.

Calculate a revised foresight direction.

Explain what is meant by inter-gimbal error.

Correct gyro survey data for inter-gimbal error, including observeddrift readings.

Overview

When gyro surveys are run in wells of over 10' inclination, there are someadditional effects and procedures to be followed. These are:

1. In order to run the gyro in a more favorable gimbal configuration

and to minimize inter-gimbal error (see below), we orientate thegyro so that the spin rotor axis points approximately in averagehole direction. This is done by calculating a Revised Foresight .

2. When the survey tool (and hence, the case of the gyro) is tiltedaway from the vertical, the outer gimbal tilts about the innergimbal axis, and the compass card, which is mounted on theouter gimbal, is tilted from the horizontal. The hole directionobserved on the plane of the compass card must be projectedonto the horizontal plane to obtain the true hole direction. Thiseffect is referred to as inter-cardinal gimbal error or Inter-Gimbal

Error . All the observed hole directions must be corrected for thiseffect.

3. The observed drift readings must also be corrected forinter-gimbal error.

4. The internal Q.C. specifications are stricter and more clearlydefined for GMS surveys at inclinations over 10'. This is becauseuncertainties in inclination and azimuth measurements have a


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High-Angle Gyro Surveying Gyroscopic Surveying

much larger effect on bottomhole position uncertainty as thewellbore inclination increases.

5. Under certain circumstances, a high angle GMS survey of a deephole section can be in hole oriented to a previous survey. Thismay be used instead of, or in addition to, normal surfaceorientation procedures. However, a revised foresight would stillbe used to put gyro spin axis in average hole direction.

Revised Foresight

For wells of inclination over 10, it is preferable to orientate the gyro sothat the spin motor axis points in average hole direction because this is themost stable gimbal configuration. For the 2" gyro, the North-South lineon the compass card is aligned with the spin axis. Hence, for the 2" gyro,

we want the North (or South) of the gyro compass card to point in averagehole direction. We achieve this as follows:

1. Study existing survey data, e.g., MWD surveys of the holesection you are going to survey. Estimate the average holedirection of this section.

2. Note the normal foresight direction.

3. Calculate the revised foresight as explained below.

4. When you start up the gyro in the warm-up box, have the Vernier0 facing 180 out from the foresight direction. Set the revisedforesight direction on the compass card at Vernier 0. Keep yourfinger on the compass card as you switch ON the gyro and forabout 20 seconds thereafter.

5. At the beginning of the survey, when you align the lowertelescope to the foresight as per normal procedures, check thatthe Vernier 0 reading is approximately the revised foresight(within 5, preferably).

Calculating the Revised Foresight for a 2" Gyro Example 1


Average Hole Direction: N 50 E

We want the North of the gyro compass card to point in average holedirection. Therefore, it will point to N50E in this case.


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Gyroscopic Surveying High-Angle Gyro Surveying

If the gyro North faces to N50E, as shown in Figure 6-1 , then the directionto the foresight will be N35W on the gyro compass card. So, N35W is therevised foresight.

If bearings are expressed in azimuth, then the revised foresight azimuthrequired to orient North in hole direction is found simply from:

Revised Foresight = Foresight Azimuth Average Hole Azimuth.

If a negative number obtained, add 360. The previous example becomes:

Revised Foresight = 015 050 = 35

Adding 360 gives 325.

Revised Foresight = 325 o Azimuth.

In this case, we decided to run South in hole direction so we add or subtract180 (to obtain an answer in the range 0 to 360). In the example shown inFigure 6-2 , revised foresight = 201 180 = 021.

1535

50

R.F.S.

NF.S.

S

W E

Figure 6-1 Revised Foresight: Example 1


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Example Calculations

Calculate the revised foresight in each of the following cases assuming a2" gyro is being used.

Calculating the Revised Foresight for a 1" GyroIn the 1" gyro, the East-West line on the compass card is aligned with thespin motor axis. This means that the North-South line on the compass cardis parallel to the inner gimbal axis. If we wish to run the gyro with the spinaxis in hole direction, that means pointing the East or West of the compasscard in hole direction.

When the wellbore inclination is in the range of 10 to 30, this is aperfectly viable option. However, if the inclination is over 30, then there isa strong risk that the spin motor of a 1" gyro will hit the stops and spinout. In this case, you must run the gyro with the inner gimbal axis alignedto the hole direction, i.e., with North or South in hole direction.

When the gyro is orientated with the gimbal axis in hole direction, the spinaxis is perpendicular to the outer gimbal axis and the spin motor, innergimbal, and outer gimbal axes are orthogonal (perpendicular to each other).This is also a stable gimbal configuration. As previously stated for the 1"gyro, this is achieved by running North or South in hole direction.

Note: For simplicity, it is recommended that whenever arevised foresight is calculated for a 1"gyro, North orSouth on the compass card is pointed in average holedirection. The revised foresight for a 1"gyro is thencalculated in exactly the same way as for a 2"gyro.

Foresight Hole Direction Revised Foresight

a. 077 264

b. N 25 W N 65 E

c. N 19 E S 80 W

d. South N 33 E

e. N 27 W South


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Inter-Gimbal Error

As stated previously, when our gyros are used in non-vertical wells, the

compass card will no longer lie in a horizontal plane. In fact, the anglebetween the plane of the compass card and the horizontal is equal to thewellbore inclination. Since the compass card is not horizontal, the observedhole direction on the plane of the compass card is not equal to the anglebetween spin axis direction and hole direction on the horizontal plane. SeeFigure 6-3 , Figure 6-4 , and Figure 6-5 .

For a 2" gyro, the North-South line on the compass card is aligned withspin axis direction. So, if there were no orientation or drift corrections, thenthe angle on the horizontal plane between the spin axis direction and theborehole plane (see Figure 6-3 ) would be the true hole direction.

Hence, we correct for inter-gimbal effect by mathematically projecting theobserved hole direction onto the horizontal plane. This is done using thefollowing formula for the 2" gyro.

tan( True Bearing ) = tan ( Observed Bearing ) cos ( Inclination )

True Bearing = arctan[tan( Observed Bearing ) cos( Inclination )]

Example

Foresight Hole Direction Revised Foresight

a. S 40 W N 10 E

b. N 19 W N 55 E

c. 165 025

d. South 010

Inclination Observed Direction Direction Corrected forInter-Gimbal Error

60.5 N 45.0 E N 26.2 E


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Figure 6-3 Inter-Gimbal Error (IGE)


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Gyro wheel

R

R = High-side projectedonto the horizontal plane

s = D irection of gyrospin axis

Horizontal plane

Figure 6-4 Inter-Gimbal Error: Horizontal Plane

R'

s'

Plane of compass card

R' = Direction of the high side

s' = Spin axis projected ontothe plane of the compass card

Gyro wheel

Figure 6-5 Inter-Gimbal Error: Plane of Compass Card


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Example CalculationsUse the formula to correct the survey readings (below) for inter-gimbalerror (2" gyro).

Of course, if you calculate your revised foresight correctly and set up thegyro orientation accordingly on surface, then your observed hole directionsshould all be close to North or South. This reduces the inter-gimbal effect,which is one reason for using a revised foresight.

Below 10 inclination, the inter-gimbal correction makes little difference tothe values of hole direction so we do not apply it.

Inter-Gimbal Correction for the 1" GyroFor the 1" gyro, the gimbal configuration is the same, but the East-Westline on the compass card is aligned with spin axis direction. The inter-gimbal formula for a 1" gyro is:

So Example


10.0 N 12.5 E

15.0 N 12.5 E

20.0 N 12.5 E

20.0 N 35.0 E

30.0 N 35.0 E

45.0 S 35.0 E

45.0 145.0

60.0 S 35.0 E


30.0 N 60 E N 63.4 E

True Bearing( )tan ObserveBrearing( )tan Incl inat ion( )cos-----------------------------------------------------------=

True Bear ing ar c

ObserveBrearing( )tan Incl inat ion( )cos

-----------------------------------------------------------tan=


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Use the formula to correct the earlier IGE survey readings (1" gyro).

Note: For the 2" gyro, correcting for IGE reduces the bearing(angle from North or South), but for the 1" gyro,correcting for IGE increases the bearing.

Note: The inter-gimbal error correction is the first correctionwhich must be applied to a high angle gyro survey (over10).

Correcting Observed Drift Readings for Inter-Gimbal ErrorOn a high angle GMS survey, the drift observed in the drift checks should

also be corrected for IGE, since this drift is observed on the plane of thecompass card, not on the horizontal plane. However, the Vernier readingsare arbitrary, being determined by the way the tool rotates in the casingwhen it is moving. We use the difference between the start and end Vernierreadings of a drift check to accurately measure the rotation of the compasscard. But, in order to calculate how much the spin axis of the gyro hasprecessed in the horizontal plane, we apply the observed drift to theobserved hole direction. We then inter-gimbal the observed hole direction

and the observed hole direction plus the observed gyro drift. Thedifference between these two inter-gimballed values is the actual gyro drift,i.e., the actual number of degrees which the spin axis has precessed during

the drift check. An example should make this clearer. Example (2" Gyro)

Inclination Observed D irectionDirection Corrected for

Inter-Gimbal Error

10 N 45 E

15 N 75 E

20 N 75 E

20 S 75 E

20 105

30 N 80.5 E

Vernier Readings

Inclination Hole Direction Start End

47.0 N 26.0 E S 86.2 W S 87.3 W


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The observed drift is 1.1W. Now because the Vernier readings are moreprecise than the hole direction readings, we do not read the hole direction atthe end of the drift check, but calculate what it should be using theobserved drift.

Observed hole direction at the first minute of the drift check is N26.0E.

If the gyro drifts 1.1W, then at the end of the drift check the observed holedirection will be N27.lE.

Now we apply the 2" gyro IGE to both these hole direction readings.

N26.0E Inter-gimballed N18 40EN27.1E Inter-gimballed N19 24EThe difference between the inter-gimballed values is the true value of gyrodrift: 19.24 18.4 = 0.84. So the inter-gimballed drift is 0.8W. (Thiscorrection will not alter the direction of the gyro drift).


In the examples below, calculate the drift in the drift check, corrected forIGE. Assume a 2" gyro is being used.

Example a. Vernier Readings


40.0 N 25.0 E N 45.3 E N 44.4 E

Example b. Vernier Readings


61.5 N 14.5 E S 38.4 E S 39.2 E


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Example c. Vernier Readings


10.0 N 39.0 E S 17.7 W S 18.2 W


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Chapter 7

In-Hole OrientationTraining Objectives


Explain what is meant by in-hole orientation.

State the conditions which are necessary before a survey may bein-hole oriented.

State the procedure for running an in-hole oriented survey.

Calculate the correct orientation correction for an in-hole orientedsurvey.

Calculate the total observed drift correctly for an in-hole orientedsurvey.

Downhole Orientation Using ConventionalMultishot System

If you are running a gyro multishot of a deep section of hole (9 5 8" casing or7" liner), then instead of establishing orientation by sighting on surface,you can overlap with the bottom stations of the definitive survey of theprevious hole section. This can be done both when running a conventionalgyro multishot with a camera, an angle unit, etc., or when running a Sigma300 gyro multishot.

The following notes apply specifically to in-hole orienting with theconventional gyro multishot system.

In-hole orientation is only possible if the survey of the previous sectionsatisfies the following conditions:

Necessary Conditions1. The survey must have been certified as a good survey and

accepted as such by the customer. Normally, it should be thedefinitive survey of that section.


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In-Hole Orientation Gyroscopic Surveying

2. There must be a high enough inclination at the bottom of theprevious survey so that directions are accurately established. It ishard to set a definite limit on this, since it depends on the angle-unit used if the previous survey was a conventional gyromultishot. Certainly, the inclinations at the stations where your

survey will overlap with the previous survey must be over 3in any circumstances, even when your survey will be run using a5 angle unit. If you will be using a higher range of angle-unitparticularly a 5 to 90 unit, then it is desirable to haveinclinations of over 10 at least. Really, the technique of in-holeorientation is best used on high-angle wells. Under nocircumstances should you ever in-hole orient in a near-verticalportion of any well (i.e., under 3).

3. The previous survey should be accurate as far as can bedetermined. Obviously, if it is a Ferranti Survey, there should not

be a problem, but if it was a gyro multishot, whether Sigma orconventional, then the in-run/out-run comparison and drift curveclosure should be good. You ought to look carefully at theprevious survey and satisfy yourself that you can validly in-holeorientate to it.

Running ProcedureOn surface, sight on the foresight and take a visual start case (to ensureyour revised foresight is okay). It is good practice to take a picture on theforesight, although it is not absolutely essential.

Now run in hole and proceed without stopping to a point just above the firstoverlap station. (You may, if you wish, take some drift checks whilerunning in to the tie-on point, but this is not really necessary if you aredefinitely going to in-hole orient). Keep the tool stationary for 10 minutesto allow the gyro to settle down (in particular, to permit the torquermotor to erect the spin axis to the horizontal). Do not use this as a driftcheck. You now proceed with the survey starting by taking pictures at theoverlap stations.

Normally, you should have either 4 or 5 of these overlap stations. Four isthe recommended minimum, while more than 5 may give you extraproblem when it comes to deciding on the orientation correction. Thebottom station of the previous survey must often be considered suspect,and in the case of a FINDS, the bottom 3 surveys should be ignored.Another possible technique is to take overlap surveys at 50-foot intervalsand interpolate between the appropriate stations of the previous survey.

The survey is carried out normally with regular drift checks, but take yourfirst drift check quite soon after the overlap stations (say 5 minutes), or itwill be applied over too long a period.


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Training Manual A-1750-500-071 Rev. A / April 1997 Confidential

Appendix A

Tool Axis Correction (TAC)Introduction

This is sometimes know as AEC (axial error correction). When using gyromultishot equipment in eased holes inclined at low angles (less than 5),the effects of imperfect tool centralization are significant in terms ofrepeatability and absolute accuracy.

Although every effort should be made to ensure accurate centralization,biases in centralizers and angle units cannot be completely eliminated.

The error in survey data created by such biases, variously described asoffset center , or axial error , can be measured and recorded dataapproximately corrected. The correction method assumes the following:

The error arises from the misalignment of the tool axis with the truehole axis at any point.

The error is systematic for any specific tool configuration.

Tool weight and wireline tension do not create a distorting effect, andthe centralizers are rigidly attached so that they cannot rotate aboutthe tool pressure barrels.

Survey Procedure

To determine

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