-
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
-
10
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.
-
11
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.
-
12
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.
-
13
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.
-
14
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.
-
15
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
-
16
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.
-
17
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.
-
18
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.
-
19
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.
-
20
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.
-
54
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
-
55
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
-
56
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
-
57
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
-
58
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.
-
59
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.
-
60
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.
-
61
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
-
62
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 =
_____________________
-
63
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 = _______________________
-
64
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(-) = _____________________
-
65
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.
-
66
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.
-
67
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