1Schlumberger Anadrill

MWD DIRECTIONAL SURVEY TRAINING MANUAL

Improving our understanding of the Surveying of Boreholeswith M1/M3 and Slim 1 MWD Tools

With a better knowledge of the way in which MWD tools measure direction andinclination, the accuracy of their sensors and an understanding of how thesemeasurements are altered by external influences we can provide the best D & Iservice in the business.

Original - August 1993Andy Ball et al

Revised - October 1996Acknowledgment: Tom Follis

James LeshikarRuss NeuschaeferWayne Phillips

2CONTENTS

SECTION 1

INTRODUCTION Page 3

INCLINATION ERROR Page 5

AZIMUTH ERROR Page 7

SECTION 2

MWD INITS FOR D&I - ADVISOR Page 16MWD INITS FOR D&I - IDEAL Page 21SURVEY ACCEPTANCE CRITERIA Page 234/5 AXIS SURVEY Page 24CONTINUOUS D&I Page 25

SECTION 3

APPENDIX A - MAGNETIC FIELDS Page 27APPENDIX B - THE EARTHS GRAVITATIONAL FIELD Page 38APPENDIX C - MAGNETOMETERS Page 40APPENDIX D - ACCELEROMETERS Page 44APPENDIX E - MAGNETIC INTERFERENCE Page 47APPENDIX F - D&I COMPUTATIONS Page 55APPENDIX G - NON-MAGNETIC REQUIREMENTS Page 65APPENDIX H - MAGCOR PROGRAM Page 67APPENDIX I - COORDINATE SYSTEMS Page 79APPENDIX J - THE GEOMAG PROGRAM Page 90

SECTION 4

D & I EXERCISES Page 95

3Introduction

To : Anadrill MWD Engineers and Directional Drillers.

The purpose of this paper is to review the current status of our Direction and Inclinationmeasurements and the options we have to ensure the best accuracy we can obtain in theprevailing conditions.

This document should be used for reference. It is backed up with a number of appendiceswith more detailed descriptions of the principles and descriptions of D&I measurements.

Section 3 has exercises in Direction and Inclination measurements. It is strongly suggestedthat you run through these exercises. Review the answers with your FSMs.

The importance of obtaining accurate D&I measurements cannot be over-emphasized.This is the core of MWD services. As we diversify into other Formation Evaluationmeasurements we note an increasing lack of attention to the core service of D&I.

4Differences between M1/M3/M10 and Slim 1 MWD D & I Packages

The Inclinometer and Magnetometer packages are essentially the same between Slim 1and MWD SCAs with only the definitions of the axis changed.

These differences are critical when having to input raw data into 3rd party correctionprograms such as SUCOP. The Slim 1 addresses three Magnetometers, threeAccelerometers and Temperature sensors. This data is then processed downhole. The rawreadings of the Magnetometer and Inclinometer sensors are compensated for variations insensitivity due to temperature. A cubic polynomial model of these temperature effects isused. The polynomial readings are then normalized using the calibration data stored in thetools ROM. Thus the operator at surface receives only calculated surveys or pre-processed data from downhole. The Slim 1 tool behaves like it has the perfect D&I sensor.

The MWD M1/M3 system utilizes the SCA. No modifications have been made to thispackage in the changeover from M1 to M3. The SCA D&I package measures only rawdata of the magnetometers, inclinometers and the temperature of the Inclinometerpackage. There is an oven around the Inclinometers which sustains the Inclinometers at135 degrees C. All the data sent by MWD M1/M3 has to be normalized for sensormisalignment and temperature variations at surface. This increased manipulation of data atsurface requires considerable operator input and thus an increased possibility of error. Onthe plus side there is more potential to recover from failed axis situations.

The POWERPULSE transmits the individual axis readings to surface. The tool doeshowever manipulate some of the data downhole. The calibration matrixes are written tothe tool and are applied to the separate axes prior to the data being transmitted thusreducing the data and number handling at surface.

Surveys

Surveys are used to determine the orientation of the well at the point where the MWDtool is lying in the borehole. This is done by measuring size and direction of thegravitational and magnetic fields exerted on the MWD tool.

Details of the way in which the MWD tool measures direction and inclination are coveredin Appendices C & D of Section 3.

The principle of measurement is straight forward. To obtain the best surveys we need agood knowledge of all the different influences that can affect those measurements. For thisreason we will concentrate on sources of errors, how they manifest themselves and how tominimize them.

5Inclination Error

Measurement of Inclination is made by 3 accelerometers mounted in the Sensor Cartridge.The principle of this measurement is described in Section 3, Appendix D.

Inclination measurement errors come from :

a. Movement. If the drill string moves during the time when the survey is takenerroneous surveys will result. The X axis is the axis most affected by drill stringmovement. Movement may occur due to the driller releasing the brake prematurelyor rig heave or torque release in the drill string downhole. Employing correctsurvey procedure is critical in avoiding movements. Good communicationsbetween Anadrill Engineers and Driller is required.

b. Misalignment of the MWD Collar in the Well bore. Currently an angle ofmisalignment is displayed at the bottom of the results of BHA Advisor (BHA A).This angle has to be added to the calculated survey manually. We are currentlyworking on getting a real time correction for this misalignment. Caution should beexercised in using this correction as it assumes that the wellbore is in gauge. Onlyutilize this correction at the request of the client and ensure that you explain thepossible errors it can induce due to out of gauge holes.

c. Misalignment of the Slim 1 Tool within the bore of the Non Mag Drill Collar.Recent testing has shown that this can be nearly 0.1. Future changes to Slim 1centralization should reduce this error.

d. Accelerometer Misalignments. Accelerometers within the Inclinometer packageare not exactly aligned along the physical axes of the tools. These are compensatedfor by pre - job calibrations. This principal is covered in more detail under Azimuthcorrections.

Calibration / Roll Tests create a Matrix to correct for the misalignment and supplyscaling to ensure accurate quantitative readings are measured by the tools.

e. Temperature. The first survey may be erroneous due to the Inclinometer notreaching 135 degrees C. A good baseline of G values can only be obtained aftersome 15 minutes circulating and several surveys.

6D&I WARM UP TIME AFTER PUMPSHUTDOWN

SHUTDOWN TIME -MINUTES

0

3

6

9

12

15

18

21

24

27

0 100 200

125 C

100 C

75 C

50 C

25 C

AMBIENT MUD TEMP.

7Azimuth Error

Measurement of Azimuth is made by 3 magnetometers mounted in the Sensor Cartridge.Inclinometer measurements are also used in calculating Azimuth, therefore the abovesection on Inclination errors is also very relevant to Azimuth error. The principle ofazimuth measurement is described in Section 3, Appendix D.

The greatest possibility for error in MWD surveying is in measuring Azimuth. Magneticanomalies are widespread within the drilling environment. It is our job, as engineerspreparing jobs and working at the rig site, to identify possibilities for anomalies and ensurethat they are either avoided or we have the means to correct for them.

Refer to Section 3, appendix E for a Summary of Magnetic Interference.

In preparing tools for the rig site we have the following checks to make :

a. Magnetic Parts.

Ensure all Modifications to avoid the use of Magnetic parts close to the D&Ipackage are implemented. There are New Style M3 Shock standoffs with Shimsthat are non magnetic for use on the SCA. The original shock sleeves weremagnetic. These are being replaced but there will still be the Magnetic type in thefield for some time. Ref TIR#47.

b. LWD Power.

Ensure the Wiring of the LTB lines for LWD is correct. Is the CDN Batterypowered or powered from the MWD ? Currents flowing through the MWD collarcan have effects on the Accuracy of the magnetometers. Current Anadrill policy isto never power the CDN from the MWD tool.

c. Return 20v Line.

Collar mass must not be used as power return.If WOB is run; be aware that a leakage of the Voltage line will set up a groundloop current which could influence the magnetometers. This is only possible withthe M3 tool.

d. MWD Collar Hot Spots.

Non magnetic collars can develop areas on the collar called magnetic hot spots.The tools developed to measure for magnetic hot spots can detect changes as smallas one Gamma at the center line of a Non Magnetic Collar. The Anadrill Standardfor the acceptable amount of Magnetization of a new Non Magnetic Collar is 0.04micro tesla (40 gammas) per 0.1m in length.

8e. D & I Calibration.

Check the date of the tools last calibration. Check the tools history and its previouscalibration. Ensure that any changes in calibration/roll test results can be explainedclearly. The calibration procedure can be used to track D&I sensor stability.

For Slim 1 the calibration data is written into a ROM in the tool. There is nooperator manipulation of this data. You should still check previous calibrations.In the POWERPULSE, the correction matrix is also written to the downhole tool.The calibration matrix is applied downhole. Note that attention should still bemade to inputs for matrix.

f. Correction Matrix Definition and Functionality.

In order to properly interpret the output voltages of the D&I sensors it isnecessary to obtain very accurate values for the sensor scale factors, biases andorientations.

The scale factor for a given sensor can be defined as the unit conversion factorfor the output of the sensor. For example, the number of millivolts/gamma fora magnetometer sensor is the scale factor for that particular sensor and thenumber of volts/g for a particular inclinometer is the scale factor for thatsensor. The matrix diagonal for the M1/M3 magnetometer (going from top leftto bottom right) will always be near 1.0 for a 5.0v Sensor Cartridge, 1.25 for a4.0v Sensor Cartridge and 1.4 for a 3.57v Sensor Cartridge. The matrixdiagonal for PowerPulse (going from top left to bottom right) will be 1.28.

The Bias can be defined as the sensor output in a zero gravitational ormagnetic field. This output is usually less than or greater than zero due tocircuit imbalances. The bias entered for PowerPulse will be zero. Zero isentered at the surface for the PowerPulse bias because the bias is correcteddownhole.

The sensor orientation is a function of the alignment of the sensors on thephysical axes of the tool.

To calibrate the D&I package, data is taken at 16 different tool orientations. Thescale factors, biases and sensor orientations are determined from the measurementsmade during a 16 point roll test. The scale factors and sensor orientations are thenincorporated into a 3x3 matrix which is used at the rig site along with sensor biasesto provide a more accurate survey. The following is the correction equation for theX, Y, and Z accelerometers. The equations will apply to the magnetometerssubstituting MM for the AA values.

Corr X = AA11(rawX-biasX) + AA12(rawY-biasY) + AA13(rawZ-biasZ)

Corr Y = AA21(rawX-biasX) + AA22(rawY-biasY) + AA23(rawZ-biasZ)

Corr Z = AA31(rawX-biasX) + AA32(rawY-biasY) + AA33(rawZ-biasZ)

9Where : AA11(rawX-biasX) is the scale factor correction. This is the magnitude of the X vector in Figure A. This is responsible for the majority of the correction of the X axis reading.

PhysicalZ Axis

PhysicalX Axis

PhysicalY Axis

YAW

PITCH

Y Sensor

X Sensor

Z Sensor

YAW

YAWPITCH

PITCH

Figure A.

AA12(rawY-biasY) and AA13(rawZ-biasZ) are the alignment correction factorsfor the X accelerometers. This is the yaw and pitch correction to get the X sensorvector to lie on the Physical X axis. These alignment correction factors play moreof a minor role in correcting the axis reading.

Important note for 3.57v SCA calibrated on the SFTK-15 Stand.

There are currently two stands in the field for calibrating the D&I packages of theSCA. The Aberdeen stand holds the complete SCA. The most common stand inuse today is the SFTK-15 (SE-92 Modified Wireline Gyro Calibration stand). Thisstand only accepts the D&I package from the SCA. We have found recently thatwhen calibrating the same 3.57v SCA on both types of stand there arediscrepancies between the two sets of calibration results. The voltage divider is inthe signal path during the calibration on the Aberdeen stand and its effects arecalibrated out. On the SFTK-15 system, the voltage divider is not in the signal pathduring the calibration and a divider ratio of 5/7 is assumed by the software. In factbecause of the 100 ohm output impedance of the magnetometer, the 10 mega-ohminput impedance of the sensor filter and the fact that the divider is made with 20K

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and 50K resistors, the divider ratio is slightly smaller than 5/7 by about 3 parts in1000. The fix in the short term is that you multiply all 9 elements of yourmagnetometer correction matrices by 1.00287 if you are using one of the dividers.MR 38 for the SCA rectifies the problem with the voltage divider. Check thismodification has been made if you are using 3.57V SCA.

g. Temperature.

Specification of M1/M3 magnetometers limits thermal drift. The PowerPulse andthe Slim tools have thermal models in their software.

The full correction matrix is computed with the SCA in the D&I calibration standat four different temperatures. The four matrixes are entered into the Advisorwhich, by interpolation, will obtain the proper value to be used according to thecartridge temperature (measured in the magnetometer assembly). This is theultimate in the precision of the Anadrill MWD and is required only by certainclients. (Shell).

Normally the full calibration matrix is computed for one temperature onlywith the SCA at room temperature in the Calibration stand.

You should be aware that if you intend to use tools in a very hot well, close to thelimit of the MWD tool, then a high temperature calibration and roll test may beperformed to satisfy the client of the tools accuracy at these extremes oftemperature.

h. Magnetometer Over Range.

Is your SCA set up with 3.57v or 5.0v Magnetometers? If there is a question as tothe range of the magnetometers in your SCA, look at the correction matrix. TheMM11, MM22 and MM33 diagonal on the correction matrix will read thefollowing approximate values :

1.00 for a 5.0v sensor or 5.0volts = 50.0 Kgammas.1.25 for a 4.0v sensor or 4.0volts = 50.0 Kgammas.1.40 for a 3.57v sensor or 3.57volts = 50.0 Kgammas.

Ensure that the location of the job does not have a magnetic Field Strength whichwill over range a 5 volt magnetometer. Remember, the tools were originallydesigned for an H value of 50,000 gammas or 50 gammas/count, where 1000counts are equal to 5 volts. If the local H value is greater than 51,200 gammas thenthe magnetometers may over range. The maximum voltage to the ADC in the toolis 5.12 Volts before wrapping occurs.

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i. Non Magnetic Collar Spacing.

Have the client planned to have sufficient NMDC on the well site. Get to knowwhat the expected BHAs are and calculate the requirements for NMDC from theequation by Grindrod & Wolff (Appendix G). Remember that nearly all Azimutherrors are due to Magnetic interference from other parts of the drill string. The lessinterference you have the fewer other techniques you will have to use to correctfor these. The value of the predicted azimuth error (AE) should be less than 0.5degrees. If it is not then continue adding lengths of non magnetic collar above andbelow the MWD until the value (AE) is less than 0.5 degrees. Some clients insistthe calculated error should be less than 0.25

Note: The Grindrod and Wolff equation for estimating the Interfering Field (IF)and Azimuth Error (AE) is an industry standard. Still, it is only an estimate andwill not predict the exact values for each drillstring.

If the client insists on using an assembly with insufficient non magneticcollars then you must inform him, prior to running in the hole, the expectedazimuth error. It is better to be up front about this than be accused ofhaving an inaccurate tool later.

Remember also that in many situations (Horizontal, High Inclination wells) it isimpractical to achieve a predicted azimuth error of less than 0.5 degrees.

j. External Magnetic Interferences.

What is the distance required from casing or from a fish left in the hole before theeffects of magnetic interference are no longer felt ?

This is a difficult question to answer because a distance of 10 to 15 ft away may besatisfactory, but in a different well, 100ft away may yield abnormal surveys :

The magnetic effects are not the same if the well is deviated or straight. The effects change as the distance from the steel in question changes. The latitude of the area has an effect. A casing collar usually produces a higher interfering field. The distance required to be clear of effects from a casing shoe can be

anywhere from 30 to 150 ft away. The same amount may be needed awayfrom a fish, but normally 30ft is acceptable.

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k. Survey Technique.

It is important that the correct procedure is properly and clearly explained to theDriller who has control of the drill string and pumps during surveying. If the drillstring moves during the time when the survey is taken erroneous surveys willresult. Movement may occur due to the Driller releasing the brake prematurely, righeave or torque release in the drill string downhole. The X axis is the axis mostaffected by drill string movement, up and down (i.e Heave). Axes Y and Z aremost affected by torque release in the drill string.

l. If you anticipate Magnetic interference problems.

The client refuses to run more than one Monel and all calculations indicatesignificant azimuth errors will result. Calculate the expected error and inform theclient. Propose options of running a magnetic correction program such asMAGCOR from the Advisor/IDEAL or SUCOP. It is important that the clientrealizes that this service costs an additional 10% to our service as we are obligedto pay a royalty charge to Shell for the use of their patented Procedure. Apassword is also necessary to access the program.

Magcor / Sucop.

These programs are correction routines for magnetic interference from the drillstring. It is carried out in two steps. The first step plots Hy and Hz from aminimum of four up to six check shots at different toolfaces.(At least one shot ineach quadrant). A circle is then drawn through the points and the centerdetermined. Any offset from the origin is then subtracted from the axesmeasurements as a correction. This exercise is designed to correct for anylocalized magnetic Hot Spots. Using the newly calculated values of Hy and Hz,(Hy2 + Hz2) will be constant if a roll test is performed. This is one method tocheck the quality of the correction.

The second step of the program corrects for drillstring interference along the axisof the tool. (Hx direction). The program compares the H from the tool to the Hyou input. (This is normally from GEOMAG and refined with actualMagnetometer values where available or previous experience from a wellcalibrated SCA in BHAs free from magnetic interference). A correction to Hx isapplied so that tool H matches the expected H. (Similar to the 5 axis calculation)The survey is then recalculated with the new Hx value.

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There are certain dangers in running Magcor and Sucop which are important tonote:

1. The input value for H must be correct. Total H is the reference for all thecorrection. If this value is wrong then your corrected survey will be wrong.

2. All SCA, SEA, and MEA must be calibrated to measure H accurately.

3. As with 5 axis corrections, if Hx axis is reading close to zero, then some greatererrors may be introduced.

m. Determining Axes Failures.

The first indications of a failed axis is that the G, H and/or Dip is not within thedefined deviations and/or the survey does not match with the check shot survey.The first thing to identify is whether an accelerometer or magnetometer has failed.

The following guidelines should be used in determining a failed axis :

If a magnetometer fails then the azimuth will be affected. If an accelerometer fails then both the Inclination and Azimuth will be affected. If a failure of a sensor occurs that has a low output and the failed output is

showing 0 counts then only the DIP will be affected, and this could be very slight. If a sensor fails, its sign, as indicated by the sign word may also be affected. The 4/5 Axis option should always be activated. This will help in determining if a

sensor failure has occurred and which axis has failed. If the Advisor/IDEAL 4/5 axis functionality cannot determine the failed axes,

either two axes of the same sensor have failed or the G,H and Dip deviations aretoo tight.

Perform a 5 point Roll Test ensuring that the GTF has changed by approximately90 deg. at each survey point.

Note the values for Gx and Hx during roll test, these values should remain fairlyconstant, +/- a few counts.

GFH HFH DIP DESCRIPTION

Good Bad Bad Magnetometer has failed or magnetic Interference is occurring.

Bad Good Good/Bad Bad Inclination & Azimuth indicates an accelerometer failure.

Bad Bad Bad Bad Inclination &/or Azimuth indicates an Accelerometer andMagnetometer have failed.

Good Good Bad Bad inclinometer or magnetometer, where failed sensor outputis equal to 0 and working sensor output was close to 0.

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The square root of the sum of the squares of the Y and Z axis for each sensor typeshould remain constant. The constant vector is called T.

HY2 + HZ2 = Constant = Th

GY2 + GZ2 = Constant = Tg

Print out the Survey hold File (Advisor) or Downhole surveys (IDEAL) andexamine the raw data.

Perform an offline 4/5 axis analysis if a D&I abort occurs and change G, H and Dipdeviations until a 4/5 axis is calculated successfully.

Change G, H and Dip deviations to values which work offline. Check on Quickstart page (Advisor) or Survey page (IDEAL) that Survey

reference is the correct survey reference.

Note : Usually, accelerometers will have either a 0 voltage or a +/- volt output whena failure occurs. The most common failure mode is a +/-1024 count outputfrom the accelerometer. Magnetometers normally do not have hard failures.The output becomes erratic, it does not usually have a zero or full scalereading when it fails.

n. The potential for large errors in calculating the 5 axis survey are great. Maximumerror occurs where the remaining axes are reading close to maximum and minimumscale. However there are many cases where errors are quite small or almost nonexistent. In fact , for some errors, rotating the tool to another toolface 45 or 135degrees away can produce smaller errors. Appendix F covers the principal and themanual calculation of surveys with a bad axis. Use the uncertainty valuescalculated on the IDEAL survey page to determine the quality of the correction.

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SECTION 2

Quick references for Survey Procedures

1. MWD INITS FOR D&I - ADVISOR

2. MWD INITS FOR D&I - IDEAL

3. SURVEY ACCEPTANCE CRITERIA

4. 4/5 AXIS SURVEYS

5. CONTINUOUS D&I

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MWD INITS FOR D&I - ADVISOR

The following inits refer to Advisor V5.2+. For versions later than this please ensurethat you refer to the corresponding release notes.

1. MWD MODE. Input M for MWD Mode and T for Tool Face mode.

2. TC TYPE. Telemetry cartridge type.

3. SC VOLTS This measurement is critical to the operation of MAGCOR. Thisdefines the Sensor Cartridge Voltage and the Calibration of theMagnetometer. This input is used to convert LOCN H inKGammas.

4. T/F ANGLE Input in Degrees. This is the Angle measured clockwise (lookingD/hole) from the Slick Pin hole to the Scribe Line of the Motor orbent sub.

If a jetting assembly is used then measure around to the largest jet.

5. BIT-SLPN The distance between the Bit and the Slick Pin of the MWD Tool.(On 7 FLS collars this means upper Slick Pin)

6. RES-SLPN The distance between the slick pin and the mid point between the SNResistivity Electrodes. As it is measured downwards from the Slick Pin thismeasurement is positive. The default value is 4.6 ft.

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7. LOCN H. This measurement is critical to the operation of MAGCOR. Thevalue is entered in KiloGammas. It is the true Total Magnetic Fieldstrength for the location. It must be agreed with the Client. Thevalue can be from Geomag. The input goes to calculating theLocation H (in Counts) which appears just below the MWD INITStitle.

8. MAG DIP This is the Magnetic DIP Angle for the given location. This isentered in Degrees. It is used as a reference for survey acceptancecriteria. The DIP Angle is calculated from running GEOMAG ortaken from previous measurement data. (ie MWD Runs) and mustbe agreed with the client. This input is critical for MTFcalculations as an input to the 4/5 Axis D&I corrections.

9. MAG DEC All MWD surveys measure Azimuth to Magnetic North. Werequire the measurements to Grid / True North. The differencebetween the Grid / True North and Magnetic North is theMagnetic Declination. If Measured North is to the East of TrueNorth then the input must be +ve. If Measured North is to theWest of True North then the input must be -ve. Before input thisvalue must be agreed with the client.

10. DWOB This option is used for Zeroing the DWOB sub.11. DTORQ This option is used for Zeroing the DTOR sub.12. FS RES. This defines the full scale reading of resistivity.

13. ERR CRN. This flags the Advisor system to go and look for the errorcorrection matrix. Inputs to this Init are: N for None, S for a singlematrix and M for multiple matrixes made at various temperatures.Following this flag the system will refer to the SC # ( INIT 14) toselect the correct matrix.Powerpulse requires this to be set at S.

14. SC # This is the Serial number of the sensor cartridge in the hole. If thenumber selected does not match an existing matrix file, errorcorrection will not be turned on when the MWD INIITS is exited.It is therefore very important that the Matrix is entered prior toentering 1. Downhole Tool Inits. ( Matrix is entered at 7. DNIError Temp Correction Matrix)

15. COL OD. MWD Collar Outside Diameter.16. HOLE DIA. Borehole/Bit Diameter.17. BARITE. Is Mud Weighing material Barite.18. DEF COND. Default value for Mud Conductivity at the surface.19. DEF TEMP. Default value for Mud Temperature at the surface.20. DEF MWT. Default value for Mud Weight going in the hole at the surface.21. DEF DHT. Default value for Downhole Temperature.

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22. 4/5 AXIS. This input is used in the calculation of TF angles only. It is notused to switch 4/5 axis survey calculations on/off. (This is done atthe quick start page). It is used to define failed tools axes that areto be replaced by calculated values in the Tool face computation.Maximum of 2 can be defined, however, Y and Z cannot bedefined for the same sensor.

23. TOOL G This should be entered as the magnitude of the G in counts. Thevalue is best taken from the G of this tool prior to a G axis failure.It is used both as a reference for survey acceptance criteria and inthe calculation of values for failed axes in 4/5 axis correction. Itmust be input accurately, since a difference of even one count cancause a significant change in results, especially for failed X axissensors in low inclination holes.

24. TOOL H This should be entered as the magnitude of the H in counts. Thevalue is best taken from the H of this tool prior to an H axisfailure. It is used both as a reference for survey acceptance criteriaand in the calculation of values for failed axes in 4/5 axiscorrection. It must be input accurately, since a difference of evenone count can cause a significant change in results, especially forfailed X axis sensors in low inclination holes.

25. G DEV This is the error, in counts, allowable in TOOL G before a tool G-axis is assumed to have failed, meaning that axis correction will berequired i.e it switches 4/5 axis measurements on as long as 4/5axis is enabled at quick start menu.(Note : The Survey Accept/Reject Criteria sets the Good/Bad flagin the survey hold file. This flag is not set by G DEV in the MWDInits.)

26. H DEV This is the error, in counts, allowable in TOOL H before a tool H-axis is assumed to have failed, meaning that axis correction will berequired i.e it switches 4/5 axis measurements on as long as 4/5axis is enabled at quick start menu.(Note : The Survey Accept/Reject Criteria sets the Good/Bad flagin the survey hold file. This flag is not set by H DEV in the MWDInits.)

27. DIP DEV. This is the error, in degrees, allowable in DIP before a tool axis isassumed to have failed, meaning that axis correction will berequired.(Note : The Survey Accept/Reject Criteria sets the Good/Bad flagin the survey hold file. This flag is not set by DIP DEV in theMWD Inits.)

28. GR CART. Serial number of GRA for reference to Calibration.

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29. D&I TEMP. This is used to manually input the D&I temperature in case ofeither a D&I temperature sensor failure or use of a mismatched setof cartridges. If the entered temperature is less than or equal to32F (0C), the display is always forced to zero and thetransmitted temperature is used, otherwise the entered defaulttemperature is used. The temperature is entered and displayed insystem temperature units. This input has no effect unlesscorrection mode M is selected at the error correction init #13.

Note : The D&I error correction will not be applied when 4/5 D&Icorrection is switched on.

30. DC MATRL. This is the type of drill collar material that encases the gamma ray cartridge.31. GR-SLPN. This is the distance from the Slick Pin to the GR measurement point.

32. D&I-SLPN. This is the distance between the slick pin and the mid point of theD&I package. This point is mid way between the accelerometersand magnetometers. This point was always a constant -7.4 ft.However with the introduction of the GRTM without a BHA thisdistance is reduced to -6.96 ft. The value for the PowerPulse is -7.83 ft.

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Special considerations for POWERPULSE.

POWERPULSE INITS REQUIRED ON ADVISOR V5.2+

1 MWD MODE N N/A to Date 17 BARITE N Req for LWD2 TC TYPE N N/A to Date 18 DEF CON N Req for LWD3 SC VOLTAGE Y 5 volts (for Magcor) 19 DEF TEMP N Req for LWD4 T/F ANGLE Y Measured from ROP* 20 DEF MW N Req for LWD5 BIT-SLPN Y Measured to ROP 21 DEF DH TEMP N Req for LWD6 RES-SLPN Y Measured to ROP 22 4/5 AXIS Y As M37 LOC H Y As M3 23 TOOL G Y As M38 MAG DIP Y As M3 24 TOOL H Y As M39 MAG DEC Y As M3 25 G DEV Y As M310 DWOB OFFSET N N/A to Date 26 H DEV Y As M311 DTOR OFFSET N N/A to Date 27 DIP DEV Y As M312 FS RES N N/A to Date 28 GR CART # N N/A to Date13 ERR CORR Y Ref MEA No. See below 29 D&I TEMP N N/A to Date14 SC# Y Input MEA No. 30 DC MTRL N N/A to Date15 COL OD Y MDC Diameter 31 GR TO SLPN Y Gr to ROP16 HOLE OD Y As M3 32 D&I TO SLPN Y D&I to ROP (-7.83ft)

ERROR CORRECTION INPUTS FOR POWERPULSE Note : "SINGLE" Error Correction Matrix should be selected.

AA 1,1 1.00 AA 1,2 0.00 AA 1,3 0.00AA 2,1 0.00 AA 2,2 1.00 AA 2,3 0.00AA 3,1 0.00 AA 3,2 0.00 AA 3,3 1.00

BIAS X 0 BIAS Y 0 BIAS Z 0

MM 1,1 1.28 MM 1,2 0.00 MM 3,1 0.00MM 2,1 0.00 MM 2,2 1.28 MM 2,3 0.00MM 3,1 0.00 MM 3,2 0.00 MM 3,3 1.28

BIAS X 0 BIAS Y 0 BIAS Z 0

The Powerpulse Magnetometers measure up to 64,000 gammas full scale.This is scaled to 5v via the magnetometer matrix input of 1.28MWD Init 3, SC voltage is obsolete and should always be input as 5.

* ROP = Read Out Port on the POWERPULSE tool.As on the M1 and M3 this is the Y axis reference and for all sensor points

21

IDEAL V4.0 D&I INITS

Location HThis value should be be obtained from the "Geomag" program. This value will be used inthe "Magnetic Correction" application when available. This is not a reference value fortool H, the reference value of tool H is entered on the survey panel. The differencebetween local H and tool H is that location H is generated by Geomag calculation is atheoretical value for the given elevation, latitude and longitude for the rig site, wherereference tool H is a value set by user, which is obtained from a tools good readings,which depends many local conditions and the tool itself.

Reference DipThis value should be obtained from the "Geomag" program. This value will be used as on-line criteria for surveys and will be used for "Magnetic Correction" application whenavailable.

Magnetic DeclinationMagnetic declination is an angle between true (geographic) north and magnetic north. It isnot corrected for convergence, the grid convergence angle. This value must be enteredbefore acquisition is started. The value will be used for Azimuth and MTF computations.There is no default value for it and computations will not take place if the value is absent.

22

Azimuth (with respect to the true north) = magnetic azimuth + magnetic declinationWhere magnetic azimuth is the azimuth measured by the tool.

Grid Convergence AngleThis value must be entered before acquisition is started. The value will be used for surveyand toolface computations. The default value is zero deg. If grid correction angle is notavailable, use the default value. If the value is non-zero system will always computeAzimuth and MTF using grid correction angle.

Azimuth = magnetic azimuth + magnetic declination - grid corr angle.

Grid NorthTrue North

M agnetic North

Azimuth

12

3

m

g

1: Azimuth (Magnetic North) m: Magnetic Declination2: Azimuth (True North) (negative in this case)3: Azimuth (Grid North) g: Grid Correction

(positive in this case)

Total Correction (for azimuth) As explained in above section total correction forazimuth is computed from magnetic declination and grid correction angle. It is displayedon D&I init panel and it will update whenever user hit return key in any of those twofields, it will update the value. This value is also displayed on the toolface panel.total correction = magnetic declination - grid correction angle.

TN TN

M N GN

GN M N

G N

M N-8 5

TNM N

-8 -3

TNGN

-9-4 +7

+10

(-8) - 5 = -13 (-3) - (-8) = +5 (-9) - (-4) = -5 10 - 7 = 3

TN = True North : M N = M agnetic North : G N = Local G rid N orth

23

Toolface Correction AngleThis value must be entered before acquisition is started. The value will be used for surveyand toolface computations. There is no default value for it and computations will not takeplace if the value is absent.

D&I TemperatureThis field is optional. If D&I temperature from downhole is incorrect or unavailable forany reason, user can input D&I temperature value here and that value will be used insurvey computations.

Sensor Cartridge SelectionUser must select the sensor cartridge that is used for MWD survey. The correction matricesassociated with that sensor cartridge number will be used for D&I temperature correctioncomputations. The sensor cartridge number can be selected from the "SC# list". This listshows the sensor cartridge numbers for which the correction matrices are entered into thecorrection matrix database. If user enters the sensor cartridge number for which there is nocorrection matrix in the database, a unity matrix with zero bias will be used for thecomputations. Correction matrix for sensor cartridge can be entered or modified usingcorrection matrix panel.

Survey Acceptance Criteria

The initial value for G, H, and Dip should be obtained from the Geomag program or theclient, and agreed with the client. These reference values can be used when the tool is firstrun in the hole. After the tool has had a chance to stabilize with respect to temperature andthe magnetometer is clear of external magnetic interference, an average of the values for G,H and Dip from the last 5 to 10 surveys should be used as a baseline. The default values forG, H, and Dip in IDEAL are:

Reference Deviation UnitsTool G 1000.0 +/- 3.0 Milli GTool H 1000.0 +/- 3.5 KgammaDip 0.0 +/- 0.3 Deg

Note: Tool H should have a deviation of +/- 350 gammas not Kgamma which equals +/- 7 counts. Thedeviations will be hardcoded in IDEAL V5.0.

In the case of computing online 4/5 axis surveys, the tighter the tolerance on G, H, and Dip,the greater chance that the recalculated values of Dip may lie outside the reference values.If this occurs, the deviations must be widened so that the 4/5 axis D&I correction is notaborted.

A 4/5 axis D&I correction abort may also be caused by inputting inaccurate inputs forreference G, H, and/or Dip values. When the axis is solved for and re-substituted into the4/5 axis calculation the calculated G, H, or Dip may be outside of the specified deviations.The accuracy of the 4/5 axis is solution is dependent on the accuracy of the G and H valuesinput.

24

For each mwd survey, maximum uncertainty and statistical uncertainty are computed forinclination, azimuth, dip angle, mtf and gtf and displayed on the IDEAL survey page.Uncertainty of any computations depends on error on each input. Error on inclinationdepends on error on gx, gy and gz. Error on azimuth depends on gx, gy, gz, hx, hy and hz.Error on these axis are computed from bias error, relative scaling error, absolute scalingerror and coupling errors.

For an open survey, statistical uncertainty is displayed in the uncertainty column. Witheach mwd survey statistical uncertainty is displayed for inclination and azimuth. Bothmaximum and statistical uncertainty are stored in database for inclination, azimuth, dip,mtf and gtf.

Uncertainty is also computed for 4/5 axis survey, the uncertainty of 4/5 axis survey will behigher than 6 axis survey. If the 4/5 axis survey uncertainty is very large, user shouldconsider not using the 4/5 axis algorithm because the solution it finds is not dependable. Insuch cases fixing the problem with the tool is recommended.

Note: Uncertainty for inclination, azimuth, and dip are in degrees.

4/5 Axis Correction

The 4/5 axis algorithm in IDEAL will try to correct a failed axis on a bad 6axis survey.A down hole 6axis survey is considered bad if any one of the values of inclination,azimuth, dip angle, tool G or tool H does not match the reference values within thedeviation limits. The algorithm can detect and correct for up to two bad axes as long astwo axes of the same sensor type have not failed.

The correction technique operates by comparing the computed values for inclination,azimuth, magnetic dip, tool G and tool H with the reference values entered by the user.These reference values are entered manually on the survey panel. The reference value ofinclination and azimuth are updated whenever a survey is accepted. Initial values forinclination and azimuth reference are set to zero. The initial value for tool G and tool Hshould be that prior to the axis failure. These are very important numbers because they areused as the starting point for calculating the magnitude of the failed axis. Specially whenthe value of failed axis is small(less than 50 counts). For example if Gy fails in the low drifthole, error of 0.2 in tool G or tool H can cause errors of 100% in the calculated value ofthe failed axis and error of up to 20 degrees in azimuth.

Note: A failed axis survey with high uncertainty values should not be accepted.

Whenever a failed axis survey is computed it is important to look at its uncertaintycalculations for its inclination, azimuth, and magnetic dip angle. A failed axis surveywhose uncertainty value is high should not be considered as a solution and such surveyshould not be accepted. High uncertainty indicates that the algorithm cannot really find thefailed axis value to the require accuracy. Fixing the problem with the tool is recommendedin such cases.

25

If the value computed for a failed Y or Z axis is small compared to the transverse axis onthat sensor, it is worth rolling the tool by approximately 90 degrees to bring the failed axisto a large value. This will minimize the azimuth errors.

Because D&I axes are not completely orthogonal, the correction matrix is applied to theerror corrected axes to back out the raw sensor measurement and then a 4/5 axis solution iscomputed. The error correction matrix is then re-applied and a 4/5 axis survey is computed.See section 3, Appendix F for details on the calculations.

Continuous D&I

Continuous D&I computations are done when Rgx and Rhx are transmitted by the MWDtool. For the computations to take place, at least one accepted MWD survey is required.The computer uses the Rgx and Rhx values in conjunction with the gy, gz, hy, and hz valuesfrom the previously accepted survey to calculate continuous D&I. The results of thecontinuous D&I computations are displayed on the IDEAL toolface control panel. Thecontinuous D&I calculations are not replacements for surveys, but are useful to help predictbuild rates and turns while drilling.

26

SECTION 3

List Of Appendices

Appendix A - Magnetic Fields.

Appendix B - The Earths Gravitational Field.

Appendix C - Magnetometers.

Appendix D - Accelerometers.

Appendix E - Magnetic Interference.

Appendix F - D&I Computations.

Appendix G - Non Magnetic Drill Collar requirements.

Appendix H - MAGCOR Program.

Appendix I - Coordinate Systems.

Appendix J - The GEOMAG Program.

27

Appendix A - MAGNETIC FIELDS

There are several theories to explain the Earths magnetic field:

Theory #1:Rotation of the Earth's solid exterior relative to its liquid iron core is believed toinduce a slow rotation of the core. A magnetic field results from the electricalcurrents generated by the relative motion between the liquid core and the mantle.The conclusion that there is a liquid portion of the core is compatible withavailable data (fig. 1).

Core

(fig. 1)

28

Theory #2:Similar to theory #1. The center portion of the Earth is largely composed of ironand has the mechanical properties of a fluid. These fluids are subjected to internalcirculation currents similar to phenomena observed at the periphery of the sun. Theinternal circulation of these fluids acts as the source of the Earths magnetic fieldaccording to the principle of a self excited dynamo (fig. 2).

(fig. 2)

The total magnetic field is the sum of two fields of different origins:

The principal field which originates within the fluid nucleus of the Earth. The transitory field which is generated outside the Earth. This field is caused by

the rotation of the Earth relative to the sum and by the cycles of the sun's activity.

Aspects of the transitory field:

Secular variations of approximately 15 gammas per year - a minor effect. Diurnal solar variation on the order of 30 to 40 gammas per day - a minor effect. The cyclical "Eleven Years" variation - a minor effect. Magnetic storms which may reach several hundreds of gammas - a major effect.

29

The Earths own magnetic field extends out to approximately 8 times the radius of theplanet. Beyond this prevails the Magneto Pause, a region in space where the Earthsmagnetic field contacts the Solar Wind. On its sunward side, the Earths magnetosphere iscompressed by high energy particles from the solar wind (fig. 3).

(fig 3)

These particles collide with the Earths magnetic field at a speed of 640 miles per secondand are slowed down at the shock front to 400 miles per second. Variations in the solarwind produce changes in the Earths magnetic field. Solar flare particles reach the Earth inapproximately two days. The shock wave preceding the cloud of plasma from the solarflare compresses the magnetosphere and rapidly intensifies the geomagnetic field atground level (fig. 4). This compression takes place over a few minutes and is called theSudden Storm Commencement (SSC). It is followed by the Initial Phase (IP) which lastsfrom 30 minutes to a few hours. The Main Phase (MP) produces a drop in the magneticfield strength due to an opposing field generated by the energized particles in themagnetosphere. This is normally not a problem for locations in the Gulf of Mexico and atlower latitudes.

30

31

The total magnetic field intensity is the vector sum of its horizontal component and itsvertical component (fig. 5). The vertical component of the magnetic field points towardthe ground and therefore contributes nothing to the determination of the direction ofmagnetic north. The horizontal component can be computed from the following equation:

Magnetic Field Strength (HFH) X COS (Dip Angle) = Horizontal Component

In Alaska: 57,510 gammas X COS (80.6) = 9392 gammasGulf of Mexico: 50,450 gammas X COS (59.7) = 25,250 gammas

Only the horizontal component of the Earths magnetic field is desired because thedownward vector contributes only to the magnitude of the magnetic field strength and notto the direction. The expected value can be obtained from the Geomag program.Differences observed between the measured HFH value and the value derived fromGeomag may be due to the following factors:

Uncertainties in chart values and drill string magnetism. Uncertainties induced by temporal variations in the magnetic field. Uncertainty in the measured value of the magnetic field. Temperature sensitivity of the magnetometers. Errors from the A/D converter.

X

Z

Y

HORIZONTAL COMPONENTOF MAGNETIC FIELD STRENGTH

MAGNETIC FIELD STRENGTH

(fig 5)

Common relative values of total magnetic field strength:

Gulf of Mexico East Canada Beaufort Sea North Sea50,000 gammas 54,000 58,500 50,000

32

MAGNETICDIPANGLE

MAGNETIC DIP ANGLE

(fig 6)

MAGNETIC DIP ANGLE

The magnetic dip angle is equal to the angle between the tangent to the Earths surface andthe magnetic field vector (fig. 6). This is also the angle formed between the total magneticfield vector (HFH) and the horizontal vector. Extreme values which you are unlikely tosee for dip angle range from 90 degrees close to the North Pole to almost zero degrees atthe equator (fig 6A). There are also several other points on the Earths surface where thedip is equal to 90 degrees. These are due to local anomalies and are called "dip holes".

Common relative values for dip angle:

Gulf of Mexico East Canada Beaufort Sea North Sea59 degrees 70 degrees 84 degrees 70 degrees

33

EQUATOR

NORTH POLE or Totalmagnetic field vector

DIP = 0

TANGENT at theEQUATOR

ANGLE FORMED WITH MAGNETIC VECTORIS EQUAL TO 0

EQUATOR DIP = 90

TANGENT at the NORTHPOLE

ANGLE FORMED WITH MAGNETIC VECTORIS EQUAL TO 90

NORTH POLE or Totalmagnetic field vector

(fig. 6A)

34

MAGNETIC DECLINATION ANGLE

TRUENORTH

MAGNETICNORTH

ANGLE OFDECLINATION

(fig 7)

MAGNETIC DECLINATION ANGLE

The Earth can be thought of as having a magnetic dipole running through its centre withNorth and South poles at either end. This dipole does not necessarily correspond with theEarths rotational axis. The angle between magnetic North and geographic North (trueNorth) is defined as the magnetic declination or the angle of declination (fig. 7). This isdependant upon the location (both in latitude and longitude) and can vary in areas of highmagnetic activity (such as Alaska). All magnetic surveys require a conversion togeographic direction by adding or subtracting this angle (fig. 8). If magnetic declination isknown, then the direction of the Earths magnetic field relative to true North can becalculated. Angles of declination to the West of geographic North are negative andmagnetic declinations to the East of geographic North are positive. For example, 5 Westcan be written as -5 and 5 East can be written as +5. An angle without a +/- or anEast/West nomenclature is not sufficient.

35

Magnetic declination can vary up to 1.7 and the total magnetic field strength may vary byas much as 770 gammas during extreme sun spot activity. Also remember, the closer tothe equator:

the lower the total field strength the higher the horizontal component and the less the dip angle

Note:At declinations greater than 7 in magnitude, magnetic declination is more unstable.

N N

E

S

WE

S

W

- +

+

+

+

-

--

Easterly Declination Westerly Declination

(fig. 8)The center of the Earth's magnetic field is unstable and difficult to localize. Presently,magnetic center drifts from the center of the Earth at the rate of two kilometers per year(three degrees westward and two degrees northward per year). For comparison, themagnetic declination in Europe during the last twenty centuries has ranged from 27 Eastto 22 West. Magnetic center is currently located about 1200 kilometers from geographicNorth. The Earth's magnetic field is subject to changes in polarity, but this happens over atime scale of millions of years. During Miocene, Pliocene and Quaternary periods, Northand South poles have been inverted several times. Although the cause for the reversal hasyet to be explained, the effects can be seen in the sea floor. Using highly sensitivemagnetometers (developed to detect submarines during WWll), scientists sweep theoceans to record the magnetic retention of the lava rock formed from sea floor spreading.Lava flowing up from the interior solidifies in the crack that follows the crest of mid-oceanridges. The rock then becomes magnetized with either normal or reversed magnetization,depending on the direction of the Earth's field at the time. Newer material fills the crack,continuing the process. In this way, the sea floor acts like a tape recorder that encodes, bymagnetic imprinting, the history of reversals of the geomagnetic field. In this way,scientists have proven the existence of polar shifts, but cannot explain their occurrence.

36

GRID CONVERGENCE ANGLE

In surface surveying operations, field data are measured relative to the irregular andcurved surface of the Earth. In order to map this data onto a flat map, corrections haveto be applied to the field data to account for he Earth's curved surface. In oil fieldoperations, corrections have to be applied both to actual measurements that are plottedand from measurements taken from a well plot. The corrections depend upon the methodused to project the ellipsoid onto a flat surface. The mathematical function of relatingpoints on one surface to points on another surface is called a projection.

There are two major types of projections; the Transverse Mercator Projection (FigureUTM Projection) and the Lambert Conformal Conic Projection (Figure LambertProjection). Both use a secant type of projection, where the cone or cylinder intersects,instead of being tangent, to the datum surface.

Figure UTM Projection

The Lambert Projection provides the closest approximation to the datum surface for arectangular zone greatest in east-west extent. The Universal Transverse Mercator (UTM)projection is a standardized form of Transverse Mercator and provides the closest fit foran area north-south in extent. The positioning of the cone or cylinder with respect to thedatum surface is specific for each location. A central meridian is selected that willminimize the amount of error produced due to projection.

Shapes on the surface of the globe are transferred to the map in a way that may beillustrated by imagining the globe to be made of glass with a source of light at the center.A shadow is thrown onto the paper (ie. A1 becomes A ). The cylinder or cone is thenunwrapped giving a correct scale representation along the central meridian. Each zone isflattened and a square imposed on it. A network is formed with two sets of uniformlyspaced straight lines intersecting at right angles. This network is termed a grid.

One of these grid lines is the central meridian. This is the only line that bears true north.This is caused by the fact that the meridians converge toward the poles while the north-south grid lines are parallel to the central meridian (Figure Grid Convergence) The north-south lines of the Grid defines Grid North. The grid convergence angle is the angulardifference between grid north and true north (shown as angle a in Figure GridConvergence).

37

Figure Lambert Projection

As with magnetic declination, the grid convergence angle is always positive if its to theEast of magnetic North and negative if its to the West of magnetic North. The gridconvergence angle is SUBTRACTED from Azimuth and MTF calculations.

Figure Grid Convergence

38

Appendix B - THE EARTHS GRAVITATIONAL FIELD

Newtons Law of Gravitation:

Every particle of matter in the universe attracts every other particle with a force whichis directly proportional to the product of the masses and inversely proportional to thesquare of the distance between them.

Gravitational force is a function of the distance from the center of the bodies in question(fig. 9).

The gravitational field (G) is primarily a function of:

Latitude (main factor). Depth / Altitude : referenced to mean sea level (MSL). Regional fluctuations in the density of the Earth's crust.

Changes in the measured value of G are attributed to the Earth's rotation. The rotation hasgiven the Earth a slightly flattened shape. Therefore, the equatorial radius is larger than thepolar radius. The G value changes from 0.997 at 0 degree latitude (Equator) toapproximately 1.003 at 90 degree latitude (a 0.006 change).

Another effect is due to the centrifugal force due to Earth rotation.

W

mw2

mgWorst Case is at the equator:

w2g = 0.030m/sec

2 which is 0.003g's

A decrease in G can also be seen with increasing hole depth. The rate of change isapproximately 0.0005 per 10000 feet. You would have to be at 20,000 feet to see a 0.001change. In other words, if the G value was exactly 1.000 on the surface, it would be 0.999at 20,000 feet.

39

Regional fluctuations in the density of the Earths crust are practically negligible.

Other reasons for discrepancies in the measured G value are due to instrumentation errorsin the inclinometer. These can be attributed to:

Temperature sensitivity. Errors due to resolution of the A/D converters. Shifts in the sensor operating parameters which occur when the inclinometer is

exposed to the shocks and vibrations of the drilling environment. (This can beobserved when surveying with "time option").

G m Me

r 2g = g

Mass = m

G = UniversalGravitationalConstant

r = radius between centers

MASS of EARTH = Me

EARTHS GRAVITATIONAL FIELD

(fig. 9)

40

Appendix C - MAGNETOMETERS

Anadrill employs three fluxgate saturation induction magnetometers in the D & I survey packageto measure the Earths magnetic field. In Slim 1, the alignment of these tri-axial magnetometersfollows the convention:

The Z-axis lies along the tool axis.The X and Y-axes are perpendicular to each other in a plane perpendicular to the Z-axis.

In MWD M3/M1, and M10 the alignment of these tri-axial magnetometers follows theconvention:

The X-axis lies along the tool axis.The Y and Z-axes are perpendicular to each other in a plane perpendicular to the X-axis.

A basic fluxgate magnetometer uses a magnetic core material to gate the ambient magnetic fieldflux lines into and out of a pickup coil wound about a primary coil which, in turn, is wound aboutthe core. The design is based on the phenomenon of magnetic hysteresis and saturation: the failureof the magnetized body to immediately return to its original value when the external value isreduced. The field which drives the core through its hysteresis loop is generated by passing acurrent through the primary coil. As the flux lines are drawn into and expelled from the core, theflux in the pickup coil changes and an induced voltage appears across the terminals of the coil (fig.10, 11).

In practice, the core is composed of a highly magnetically permeable material and is placed insideof a pair of primary (Drive) coils. These drive coils are toroidally wound in series and areimbalanced by a resistor. The primary coils are surrounded by a secondary (Pickup) coil. The coreis sharply driven into a saturated state by an AC voltage supply, then slowly returns to itstransition condition (traces A & B of fig. 12). The resulting hysteresis effect produces an outputthat resembles trace D, in the absence of a magnetic field. The Earths magnetic fieldindependently induces a response which resembles trace C. Combined, the resultant pickup signalwould resemble trace E.

The detection circuitry does not directly measure the voltage pulses to determine the magneticfield strength. Instead, a summing op-amp (error amp) seeks to equalize the positive and negativepulses. By adjusting the DC bucking current, the effects of the magnetic field are eliminated. Withthe effects of the field nullified, the positive and negative pulses are equal (trace D). The DCvoltage measured across the reference resistor is thus the true measure of the magnetic fieldstrength along the axis of that magnetometer.

41

Pickup (sensing) coil

REFERENCE AXIS

PICKUP COIL

DRIVE COILS

IMBALANCINGRESISTOR

(fig. 10)

PICKUPCOIL DC BUCKING CURRENT Is

CORE

DC OUTPUT

BLOCKSDC CURRENT

a

Rs

PEAKDETECTOR/AVERAGER

-

DCerror

+

Earths Field Detection and Nulling(fig. 11)

42

OSCILLATOR SIGNAL (5KHZ)

TIME

A

B

C

D

E

DRIVE CURRENT WAVE FORM

TRANSITION TRANSITION

SAT. SAT. SAT.

PICKUP VOLTAGE DUE TOEARTHS FIELD ONLY

PICKUP VOLTAGE DUE TO CORE IMBALANCE ONLY

RESULTANT PICKUP SIGNAL

Pulse sequence generated across terminals of pickup coil in presence ofEarths field.

(fig. 12)

43

CLOCKWISE NO CURRENT COUNTER-CLOCKWISE

INDUCEDVOLTAGE

INDUCEDVOLTAGE

INDUCEDVOLTAGE

(fig. 13)

44

Appendix D - ACCELEROMETERS

A basic accelerometer consists of a simple mass which is constrained to only move linearlywithin a frame work and which is coupled to that frame work by an elastic member. FromFigure 14, if the vehicle (or main frame) and the mass are traveling at the same speed, thenno acceleration is felt (position A). If the main frame is suddenly moved (acceleration), themass tends to maintain its own speed by virtue of its inertia. Therefore, there is a relativemotion between the vehicle and the mass.

By measuring the acceleration force on the known mass, the vehicle acceleration can bedetermined. This can be seen by comparing distance Y and distance Z in Figure 23.

"Accelerometer" is the name given to a transducer which responds to acceleration, while"inclinometer" is the name given to a set of low range accelerometers. The inclinometer isused as a slope detector because it is responsive to small gravitational accelerationchanges. An inclinometer refers to a set of three accelerometers.

The accelerometer currently fulfills two major functions: It is considered a vertical reference device when its function is the proper

alignment of reference axis. It is considered an inertial device when its major function is the detection and

measurement of acceleration.

An accelerometer device cannot distinguish the difference between acceleration andgravity. This is important when discussing the determination of the vertical force. Avertical reference device acting as a plumb line on the rotating Earth assumes the samedirection in which gravity acts on the rotating Earth. Gravity, in this case, is the resultantbetween the Earth's mass attraction force and its rotational centrifugal force. Since theEarth is not a perfect sphere, the geocentric vertical does not coincide with the gravityvertical force.

45

INCLINOMETER PRINCIPLE

A pendulum suspended inside a case attempts to hang in alignment with the gravity vector(fig. 15). A compensating electrical force is applied to the pendulum to restore the proofmass to its original position (horizontal to the case). The sensor imbalance is detected bythe pickoffs and a signal is sent to the servo. This signal causes an electric current to flowthrough the torque coil. The torquer current is designed to oppose the reed displacement.The restoring force is then encoded as an analog signal.

Some single shot instruments or Totco devices that only record drift are sometimesreferred to as inclinometers.

46

47

Appendix E - MAGNETIC INTERFERENCE

There are two types of magnetic interference:

Drill string magnetic interference. External magnetic interference, which can include interference from:

1. A fish left in the hole.2. Nearby casing.3. A magnetic "hot spot" in the drill collar.4. Fluctuation in the Earths magnetic field.5. Certain formations (iron pyrite, hematite and possibly hematite mud).

Any deviation from the expected magnetic field value can indicate magnetic interference.External magnetic interference can occur as the drill string moves away from the casing shoe orfrom the casing window. It can also occur as another cased hole is approached. There arecertain instances where a gyro survey may need to be used if the well requires steering out ofcasing or if a possible collision exists with another well (especially if the well being drilled isstraight). There are also cases where magnetic interference may be corrected or at least takeninto account until a different BHA is used.

DRILL STRING MAGNETIC INTERFERENCE

The drill string can be compared to a long slender magnet with its lower end comprising one ofthe magnetic poles. Even if the components of a drilling assembly have been demagnetized afterinspection, the steel section of the drill string will become magnetized by the presence of theEarths field (fig. 16). (In any case, a non-magnetized steel component would probably affect theD&I package by distorting the Earths magnetic field).

Drill string magnetism can be a source of error in calculations made from the suppliedmagnetometer data. This may happen as the angle builds from vertical (fig. 17) or as theazimuth moves away from a north/south axis (fig. 18). Also, changing the composition of theBHA between runs may change the effects of the drill string.

It is because of drill string magnetism that non-magnetic drill collars are needed. Non-magneticdrill collars are used to position the compass or D&I package out of the magnetic influence ofthe drill string, both above and below the MWD tool. The magnetometers are measuring theresultant vector of the Earths magnetic field and the drill string. Since this is in effect one longdipole magnet with its flux lines parallel to the drill string, only the X-axis of the magnetometerpackage is affected, normally creating a greater magnetic field effect along this axis. Themagnitude of this error is dependent on the pole strength of the magnetized drill stringcomponents and their distance from the MWD tool. The error could appear in the calculatedsurvey as an increased or decreased total HFH value depending on the direction of the borehole.The total H value should remain constant regardless of the tool face orientation or depth as longas the hole inclination, azimuth and BHA remain relatively constant.

48

When drill string magnetism is causing an error on the X-axis magnetometer, only the horizontalcomponent of that error can interfere with the measurement of the Earths magnetic field (seeMagnetic Field Strength section in Appendix A). The horizontal component of the X-axis erroris equal to the X-axis error multiplied by the sine of the hole deviation. This is why experiencehas shown that the magnetic survey accuracy worsens as the hole angle increases (especiallywith drill string magnetic interference). Since the horizontal component of the Earths magneticfield is smaller in higher latitudes, the error from a magnetized drill string is relatively greaterthan that experienced in lower latitudes (fig. 19). Thus, a 50 gammas error has a larger effect ona smaller horizontal component, 0.53% error in Alaska compared to only 0.20% in the Gulf ofMexico.

Inc

Mag

xzy

xzy

Drill String Magnetism

MagneticFlux Lines

MagneticFlux Lines

EarthsMagneticFlux

Fig. 16

49

Inc

Mag

xzy

xzy

Drill String Magnetism

Inc

Mag

x

zy

x

zy

Horizontal Componentof X Axis error smallwith no Inclination.

Horizontal Componentof X Axis error largerwith Increased Angle

Fig. 17

50

Hx drilling North

Hx Componentdue to Drill StringMagnetism

Hx drilling East Hx Componentdue to Drill StringMagnetism

Figure 18

Hx will read the total magnetic field when drilling along the dip line in a North/South direction.Therefore, Hx will be a larger vector in the North/South direction than in the East/West direction.The interference from drill string magnetism affects the x-axis only. Since the drill string magnetismis the same value regardless of the direction of drilling, the drill string magnetism is a largerpercentage of the total Hx value when drilling East/West.

Drill string magnetism azimuth error is largest when drilling a horizontal well in the East/Westdirection. This is why the monel requirements are higher for these wells. See Appendix G.

51

D r il l S t r ing Magn et is m

Inc

Mag

x

zy

x

zy

Fig. 19

5 0 Gammas

9,4 00 Gammas

2 5 ,2 0 0 Gammas

Hor i z ont al Com ponentof Dr i l l S t r i ngMagnet i c F i el d

Hor i z ont al Com ponentof Ear t hs Magn et i cF i el d i n A l as ka

Hor i z ont al Componentof Ear t h's Magnet i c F i el di n Gul f of Mex i co.

The increased value of the X-axis due to drill string magnetism will normally cause all calculatedazimuths to lie closer to north. This error will show up when a gyro is run in the well. All MWDsurveys will be positioned (magnetically) north of the gyro survey stations. (Most gyros derivetrue north from the Earths rotation).

MINIMIZING ERRORS

One way to minimize the error caused by the drill string is to eliminate as much of themagnetism as possible. This is done by isolating the magnetometer package with as many non-magnetic drill collars as possible. The length of the non-magnetic collars implies a uniform andnon-interrupted non-magnetic environment. Obviously the presence of a steel stabilizer or steelcomponent between two non-magnetic collars results in a pinching of the lines of force (fig. 20).

52

This is detrimental to the accuracy of the survey. A steel stabilizer may be satisfactory on theEquator, but not as far north as the North Sea.

NMDC

SteelStabiliser

NMDC

NMDC

NMDC

Fig 20

Length of NMDCimplies a uniform,non interupted,non magneticenvironment.

The following are circumstances where more non-magnetic drill collars are necessary to counterdrill string magnetism effects. These are also examples in which the azimuth accuracy will likelydecrease.

The further away from the Equator (in latitude). The larger the hole angle (drift). The further away from a north/south hole direction.

Note that with 120 feet of non-magnetic material above the magnetometer package the effectsof drill string magnetism in places like ALASKA may still be seen. In fact, Anadrill hasperformed jobs in Alaska with as much as 165 feet of non-magnetic material (Motor - 30 feetMonel - 45 feet M1 collar - 90 feet Monel).

53

Remember:

If magnetic interference is encountered from the drill string, the total H value shouldremain constant regardless of tool face orientation or depth as long as the holeinclination, azimuth and BHA remain fairly constant.

The horizontal component of the X-axis error is equal to:[(X-axis error) x sin(drift)]. This is why magnetic survey accuracy declines as the holeangle increases (especially with drill string magnetic interference).

A higher dip angle results in a lower horizontal component of the magnetic field whichmeans that the same drill string magnetic interference will have a greater effect.

EXTERNAL MAGNETIC INTERFERENCE

When magnetic interference from external sources is encountered (such as from a fish in thehole or from nearby casing), all three axis of the D&I package will be affected. Therefore, thetotal magnetic field will vary. (The total H value will also vary when the D&I package is closeto casing joints). If a hot spot occurs on a non-magnetic collar, our total H value will changewith varying tool face settings, but will be repeatable when the BHA is placed in the sameorientation (fig. 21). Remember, in places such as Alaska, total field strength can routinely varyby 100 gammas.

Do not mistakenly interpret change in total H value as a failed magnetometer sensor. Itmay be caused by magnetic interference.

Do not mistakenly interpret a change in a survey with a failed magnetometer orinclinometer; it may be due to a tool face dependency.

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Effect of Magnetic Hot Spotin MWD Collar

Hx X

Hz

Hy

Z Y

H TotalHot Spot

H Earth

H Measured

Hot Spot

Hot Spot alignedwith Y axis.

Magnetic Hot SpotRotating with MWDCollar.

All 3 axes measurements areeffected. Fluctuation in TotalField is observed when MWDTool is rotated.

Calculated Azimuth will bewrong but will be repeatablewith the same tool face.

Fig 21

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Appendix F - D&I COMPUTATIONS

A. Definition of Terms

Ax, Ay, Az = Raw Accelerometer Data transmitted by tool ("Raw Values").

Gx, Gy, Gz = Accelerometer Data after Sign Word and Scale Factor havebeen applied ("Calculated Values").

Gx, Gy, Gz = Processed Accelerometer Data After normalization ("ProcessedValues").

Mx, My, Mz = Raw magnetometer Data after Sign Word and Scale Factorhave been applied.

Hx, Hy, Hz = Processed Magnetometer Data after normalization.

SW = Sign Word sent up with Raw Survey Data.

GFH = Total gravity vector.

HFH = Total magnetic field vector.

T = Projection of GFH onto YZ plane.

B. Normal Survey Calculations

Processing the Raw Survey Data:

1. Apply the Sign Word to the Raw Data.

2. Subtract 1024 from all values that have a 0 below them.

3. Apply the Scale Factors: -1 for accelerometers.+1 for magnetometers.

* Correction matrices / biases are applied at this point.

4. Using the Calculated Values, compute the GFH and HFH magnitudes.

GFH = Gx2 + Gy2 + Gz2 ...... Equation 1

HFH = Hx2 + Hy2 + Hz2 ...... Equation 2

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5. Compute the "Processed Data" using the equations below:

Gx = Gx/GFH, Gy = Gy/GFH, Gz = Gz/GFH

Hx = Hx/HFH, Hy = Hy/HFH, Hz = Hz/HFH

Computing the Inclination (I) and Azimuth (Az):

1. Calculate the inclination (I) using the equations below:

GY2 + GZ2 = T ...... Equation 3

I = tan-1 (T/Gx) ...... Equation 4

2. Calculate V (the normalized vertical component of H) using the following equation:

V = (GxHx) + (GyHy) + (GzHz) ...... Equation 5

3. Calculate the two Azimuthal projections A1 and A2 using the formulae below:

A1 = Hx - (VGx) ...... Equation 6

A2 = (GyHz) - (GzHy) ...... Equation 7

4. Calculate the Magnetic Azimuth (MA):

MA = tan-1 (A2/A1) ...... Equation 8

5. Correction for magnetic azimuth (MA).

If: A1 < 0, add 180 degrees to MAMA < 0, add 360 degrees to MA calculated.A1 > 0, and MA > 0, no correction

Note: A1 has priority

6. To obtain the final azimuth add magnetic declination to magnetic azimuth and subtractgrid convergence. Remember Westward declinations are Negative and Eastwarddeclinations are Positive.

Final Azimuth = MA + Declination - Grid Convergence ...... Equation 9

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C. Example of Survey Calculation (M1)

Ax Mx Ay My Az Mz SW Comments

67 696 59 816 748 697 25 Raw Data0 1 1 0 0 1 SW Applied

-1024 -1024 -1024-957 696 59 -208 -276 697 Multiply by

-1 1 -1 1 -1 1 Scale Factor

Gx Hx Gy Hy GZ HZ957 696 -59 -208 +276 697 Calculated Data

Gx Hx Gy Hy Gz Hz.959 .691 -.059 -.207 .277 .692 Processed Data

GFH = 9572 + (-592) + 2762 = 997

HFH = 6962 + (-2082) + 6972 = 1007

T = 592 + (-2762) = 282

Inclination Calculation:

I = tan-1 (T/Gx) =16.4 degrees

Azimuth Calculation:

V = (Gx .Hx) + (Gy .Hy) + (Gz .Hz) = .866

A1 = Hx - (V.Gx) = -0.139

A2 = (Gy .Hz) - (Gz .Hy) = 0.0165

MA = tan-1 (A2/A1) = -6.75

A1 < 0 add 180 to MA

Declination 5 E, add +5Grid Convergence 2 W, subtract (-2)

Final Azimuth = -6.75 + 180 + 5 - (-2) = 180.3 degrees

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M3/M10 6 AXIS SURVEY CALCULATION

The ADC in the M3/M10 is a 12 bit converter. In other words:

-5V = 0 Cts 0V = 2048 Cts 5V = 4095 Cts

The Advisor/IDEAL uses the following scheme:

-5V = -2048 Cts 0V = 0 Cts 5V = 2048 Cts

This is necessary because the sensors are centered on 0 Volts and therefore, for example,the counts for -5V and 5V should be equal in magnitude. Therefore, the raw data on theAdvisor/IDEAL is defined as Counts from the SPM - 2048.

Consequently, the raw data sent by the TCA and decoded on the SPM is different fromthat displayed on the Advisor/IDEAL.

Ax Mx Ay My Az MzSPM DATA 1000 3252 1542 3358 422 2976

RAW DATA -1048 1204 -506 1310 -1626 928

The Advisor algorithms are setup to deal with M1 data. Therefore, the Advisor nowneeds to convert the M3 Raw data to the equivalent reading the M1 would send. The M3survey data is a 12 bit word while the M1 survey data is a 10 bit word (11 bits with thesign word). The conversion then is:

(M3 Raw Data) = (M1 Equiv) 2048 1024

The survey calculation would then be as follows:

Ax Mx Ay My Az MzRAW DATA -1048 1204 -506 1310 -1626 928

M1 Equiv -524 602 -253 655 -813 464

Apply SF -1 -1 -1

Calc. Data Gx Hx Gy Hy Gz Hz 524 602 253 655 813 464

The same principle would apply for the vectors in the toolface frame, except that they are8 bit words instead of 12 bit words.

Note: The raw data on the Advisor/IDEAL can be negative, but the tool is transmitting positive values. At this point, the survey calculations are the same as the previousexample.

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D. Survey Calculations with a Bad Sensor

This section is intended to clarify the procedures to be used when one of the D&I sensorsfail. It provides some useful hints on recognizing a failed sensor.

Examination of equations 1 - 9 will lead to the following conclusions:

* If a magnetometer fails, then the azimuth will be affected.* If an accelerometer fails, both azimuth and inclination will be affected.* If a sensor fails, its sign, as indicated by the Sign Word (SW), may also be in error.

Any error here will only affect the bad sensor.

Recognizing a Failed Sensor:

The first indication of a possible D&I failure occurs when the survey is computed andeither GFH or HFH is out of range, or the calculated inclination/azimuth does not agreewith previous surveys.

Identify the following problems:

1. Was this a valid survey? Could the cause of the problem be:

a) Moving the drill pipe while surveying.b) Signal problems during the D&I frame transmission.

2. Which parameters are wrong?

a) Good GFH, good inclination indicate that the accelerometers are functional.b) Bad HFH, bad azimuth indicate that the magnetometer has failed.c) Good HFH, bad GFH, bad inclination and bad azimuth indicate an accelerometer

failure.d) Bad GFH, bad HFH, bad inclination and bad azimuth indicate that both an

accelerometer and magnetometer have failed.

3. Compare current incorrect data to prior surveys.

a) X-axis data will change very little from survey to survey, so Ax, Mx, Gx, Gx, Hxor Hx should all be similar to previous surveys. If there is a major change, suspectthe X-axis sensor.

b) The values for the Y and Z axes will change constantly as the tool rotatesdownhole. Rotate the drill pipe and take another survey. The new values shouldchange; if one axis sensor remains constant, that is the failed sensor.

c) The value T should remain constant as the tool rotates.

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Assumptions

1. Assuming an average for GFH or HFH then solving for the sensor value reveals themagnitude of the sensor, but not its sign.

2. The sign of the X-axis sensors will normally be positive. Gx will be positive if theinclination is less than 90 degree. There is not a simple relationship for understandingwhen Hx is positive or negative. The sign of these sensors can be checked bycomparing the latest processed data to previous good survey processed data.

3. The signs of Gy and Gz do not matter when computing the inclination. See equations3, 4 and 5.

4. The signs of Gy, Gz, Hy and Hz are critical to the calculation of a correct azimuth.Since Y and Z continually change as the drill string rotates, signs from the previoussurvey cannot be used in the azimuth computations. When a defective accelerometeror magnetometer is isolated and its magnitude computed, this value with both possiblesigns must be tested.

Sin-1V = Dip Angle ...... Equation 10

Since Sin-1V is the dip angle, this is a quick check of whether the correct sign has beenused.

5. It is impossible to obtain the sign of the defective sensor from the Sign Word (SW).When a sensor fails, it may have the incorrect sign, as well as an incorrect magnitude.Only the bad sensor will be affected; the signs of the operative sensors will still becorrect.

6. The Advisor/IDEAL software (4/5 axis D*I correction) will identify the failedsensor(s) and compute a corrected drift and azimuth. If the 4/5 axis correction isenabled the Master Processor will check the MWD survey for the failed axes. It candetect and correct up to two bad axes, but they must be of different sensor types (i.e.one G and one H).

The correction technique operates by comparing the computed values for drift,azimuth, magnetic dip, G and H with operator entered references. These references areentered manually on the Master Quickstart menu (Advisor) or the survey page(IDEAL). These values are updated whenever a good survey is received or a goodcorrection is made.

The entered MWD Initialization for tool G and H should be that received from thetool prior to the axis failure. This is a very important number as it is used as thestarting point for calculating the magnitude of the failed axis. An error in the tool G orH value will adversely affect the accuracy of the 4/5 axis survey.

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Advisor: If any axes fail during a T/F run, they must be identified from the D&I framedata and manually specified to the computer via "4/5 Axis" option on the MWD INITpage. Axes thus identified will be recomputed before the T/F calculations are made.

Refer to Advisor System Users Guide for D&I Axis Correction and Toolface With AFailed Axis.

Refer to IDEAL FRM for D&I Axis Correction.

FIVE AXIS D&I PROCEDURE

GFH = Gx2 + Gy2 + Gz2

HFH = Hx2 + Hy2 + Hz2

{(Gx . Hx) + (Gy . Hy) + (Gz .Hz)} / (GFH . HFH) = SIN DIP

The procedure will also utilize the GFH or HFH from previous good surveys dependingupon which type of sensor has failed, that is, accelerometer or magnetometer.

Bad Accelerometer

1. Copy Gx, Gy and Gz; then change signs.

Gx = ________________________ x (-1) = _______________________

Gy = ________________________ x (-1) = _______________________

Gz = ________________________ x (-1) = _______________________

2. Find the average GFH from previous good surveys.

GFH ave = ____________________

3. Make an estimate of gx. (All estimates will be denoted by a small initial, eg. gx, gy andgz).

gx = GFH ave2 - Gy2 - Gz2

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4. Try that estimate in the DIP formula.

Temp 1 = Gy.Hy + Gz.Hz = ____________________

Temp 2 = gx.Hx = ____________________

Dip gx (+) = Temp 1 - Temp 2 = ____________________

Dip gx (-) = Temp 1 + Temp 2 = ____________________

Similarly check for gy and gz.

5. gy = GFH ave2 - Gx2 - Gz2 = _____________________

Temp 3 = Gx.Hx + Gz.Hz = ______________________

Temp 4 = gy.Hy = ______________________

Dip gy(+) = Temp 3 - Temp 4 = _______________________

Dip gy(-) = Temp 3 + Temp 4 = _______________________

6. gz = GFH ave2 - Gx2 - Gy2 = ________________________

Temp 5 = Gx.Hx + Gy.Hy = _______________________

Temp 6 = gz.Hz = _______________________

Dip gz(+) = Temp 5 - Temp 6 = _______________________

Dip gz(-) = Temp 5 + Temp 6 = _______________________

7. Compute the Sin of the actual DIP angle and multiply by the GFH ave.

DIP = Sin (magnetic dip angle) x GFH ave = _____________________

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8. Compare your estimates of DIP with the actual DIP.

DIP - Dip gx(+) = ______________________

DIP - Dip gx(-) = ______________________

DIP - Dip gy(+) = ______________________

DIP - Dip gy(-) = ______________________

DIP - Dip gz(+) = ______________________

DIP - Dip gz(-) = ______________________

Bad Magnetometer

1. Copy Hx, Hy and Hz.

Hx = _____________________

Hy = _____________________

Hz = _____________________

2. Use HFH ave = _______________________

3. Estimate for hx.

hx = HFH ave2 - Hy2 - Hz2 = ______________________

Temp 1 = Gy.Hy + Gz.Hz = _______________________

Temp 2 = Gx.hx = __________________________

Dip hx(+) = Temp 1 + Temp 2 = _______________________

Dip hx(-) = Temp 1 - Temp 2 = _______________________

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4. Estimate for hy.

hy = HFH ave2 - Hx2 - Hz2 = _______________________

Temp 3 = Gx.Hx + Gz.Hz = _______________________

Temp 4 = Gy.hy = ______________________

Dip hy(+) = Temp 3 + Temp 4 = _______________________

Dip hy(-) = Temp 3 - Temp 4 = _______________________

5. Estimate for hz.

hz = HFH ave2 - Hx2 - Hy2 = ________________________

Temp 5 = Gx.Hx + Gy.Hy = _______________________

Temp 6 = Gz.hz = ______________________

Dip hz(+) = Temp 5 + Temp 6 = _______________________

Dip hz(-) = Temp 5 - Temp 6 = _______________________

6. Compare your Dip estimates to the real DIP.

7. Calculate the Sin of the actual DIP and multiply by HFH ave.

DIP - Dip hx(+) = _____________________

DIP - Dip hx(-) = _____________________

DIP - Dip hy(+) = _____________________

DIP - Dip hy(-) = _____________________

DIP - Dip hz(+) = _____________________

DIP - Dip hz(-) = _____________________

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Appendix G - NON-MAGNETIC DRILL COLLAR REQUIREMENTSIt will be necessary to calculate the lengths of NMDC required above and below theMWD tool. Drill string magnetism can cause errors in directional surveys, due to thecoupling that occurs between the Earths magnetic field and the field around the drillstring. This coupling is most severe in highly deviated holes perpendicular to the Earthsmagnetic field direction, that is, wells drilled in an East / West direction at high inclinationswill require the most NMDC.

The engineer requires the following information in order to calculate the number of drillcollars needed to minimize magnetic interference:

1. Declination of the Earths magnetic field at the wellsite.2. Earths magnetic field strength at the wellsite.3. Dip angle of the Earths magnetic field at the wellsite.4. Inclination of wellbore.5. Azimuth of wellbore.6. Bottom hole assembly configuration.7. Magnetic pole strengths of the drill string.

All the above parameters could be determined except the magnetic pole strengths of thedrill string.

NMDC Requirements

LP

c b y z x

NMDCMag Collars or Motor NMDC

MWD / Slim 1 NMDC

SurveyPackage

Pole Strength

IF = 770

(z+x)2 +LP.

(y+b)2 -LP.

(y+b+c)2

57300 * IF * Sin 1 * Sin(Az-MD)H *CosDIP

AE =

IF = The Calculated Interfering FieldLP = Pole Strength of Components below the MWD

AE = Predicted Azimuth error due to Interfering Field.

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For LP. use the following values:Stabilizer and bit below the MWD = 7730 ft or more drill collars or other BHA = 260Mud Motor or Turbine = 860

x = Length of Non-magnetic collar above MWD/Slim 1z = Length of MWD collar above D&I sensor pointy = Length of MWD collar below D&I sensor pointb = Length of Non-magnetic collar below MWD/Slim 1

H = Total magnetic Field Strength in GammasAz = Azimuth of the wellI = Inclination of the wellMD = Magnetic declinationDIP = Dip Angle

NOTE: ALL LENGTH UNITS ARE IN FEET.

This equation was developed assuming an 8 inch collar (BHA). The interfering field for a4 inch collar (BHA) may be different.

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Appendix H - RUNNING MAGCOR

MAGCOR is NOT an alternative to running sufficient non magnetic material in thedrillstring.

Most problems in running MAGCOR are due to a poor understanding of the program andits inputs/outputs. The following guidelines will help :

Magcor corrects for external magnetic interference. It assumes that the LOCN H is thetrue H value for the location. It also assumes that the MWD / Slim magnetometers areproperly calibrated to measure this H accurately.

1) Setting up the program prior to taking a cluster shot.

The program is run in realtime and is accessed at the same time as the survey is read intosurhld.hld. The matrix corrected survey is read into magcor.hld concurrently withsurhld.hld. However, for this to be achieved in the correct manner, a number of inputsneed to be in place correctly prior to this.

a) Magnetic correction must be enabled on the Start Up Page.

b) MWD inits page must be initialized and set up correctly.

IT IS VERY IMPORTANT FOR THE CORRECT OPERATION OF MAGCORTHAT : DIP, Declination, Locn.H and SC volt are INPUT correctly.

Magcor