Drilling & Evaluation
TRAINING &
DEVELOPMENT
MWD Academy Manual
MWD Class Pre-Read
Directional-Gamma Service
Table of Contents
Pre-Read Overview .......................................................................... 3
Directional Service........................................................................... 3
Introduction ............................................................................................ 3
Objectives ............................................................................................... 4
Resources ............................................................................................... 4
Overview ................................................................................................. 5
Directional Surveying ............................................................................... 6
Toolface Steering ..................................................................................... 9
Survey Corrections ................................................................................. 13
Summary .............................................................................................. 16
Review Questions .................................................................................. 17
Pre-Read Summary........................................................................ 19
Page 2
Pre-Read Overview
The purpose of this pre-read is to prepare you for the Measurement While Drilling
(MWD) Directional-Gamma (DG) Academy. Each section should be completed prior to
attending the relevant instructor-led training. It is designed to further your
understanding of the relevant MWD services by building on the prerequisite web-based
activities that you will complete prior to attending the classroom training.
This document is one of four main sections:
1. NaviTrak Probe Service
2. Surface Hardware
3. Directional Service
4. Telemetry
Each section will contain several topics to read, and will also give some references to
specific TechPubs references that should be read in conjunction with this document.
There are also research and review questions presented throughout the document to
help direct your learning.
You will be tested on each of these sections prior to the coordinating module in the
instructor-led training.
Directional Service
Introduction
This pre-read will build on previous directional and wellbore positioning related training
modules. It introduces the concept of the Measurement While Drilling (MWD)
Directional Service, and will provide a foundation for the topics that will be covered
during the relevant classroom training and practical exercises. You are expected to
familiarize yourself with the contents of this document prior to attending instructor-led
training on the MWD Directional Service.
Page 3
Objectives
After working through this guide, participants will be expected to:
• Define the applications of the MWD Directional Service.
• Identify the main MWD Survey parameters used for wellbore positioning
calculations.
• List the earth’s gravitational and magnetic field characteristics used in survey QC.
• Describe how magnetic and highside toolface are used in wellpath control.
• Explain the importance of the scribeline correction in a steerable motor drilling
assembly.
Resources
The documents listed here can be used as further references, although more
information on this topic is available at TechPubs Online and Baker Hughes Direct.
1. MWD-20-75-0000-01-01 MWD Directional Survey Services Operations
Manual
2. 750-500-072 Wellbore Positioning Training Manual
3. SAG-20-90-0000-00-02 BHA SAG Correction Program User Notes
Page 4
Overview
The basic MWD Directional Service provided by Baker Hughes is used for two main
reasons:
1. To tell which way the wellpath went.
• Permit calculation of well coordinates at a series of measured depths, thereby
accurately specifying the path of the borehole and current location.
• Locate dog legs and allow calculation of dog leg severity values.
2. To tell which way the wellpath is going.
• Measure the inclination and direction at the bottom of the hole and hence
determine where the well is heading.
• Determine the orientation of the toolface of deflection tools or steerable systems.
Directional surveys are taken by the MWD tool for wellbore positioning purposes and
are used in calculations which map the location of the well at the survey station.
Measurements of the wellbore inclination and azimuth allow the drilled wellpath’s
coordinates to be calculated and compared to the planned wellpath. Survey accuracy is
critical to ensure that the location of the hole is known to be within agreed tolerances.
Accurate knowledge of the path of a borehole is necessary to:
• hit geological target areas
• avoid collision with other wells, especially during platform drilling
• define the target of a relief well in the event of a blowout
• provide a better definition of geological and reservoir data to allow for
optimization of production
• fulfill the requirements of government legislation and regulations
As well as taking surveys to map the path of the wellbore, the MWD Directional Service
is also used to help steer the path along the course. A number of deflection tools exist
for directing a wellpath and an MWD tool or other directional survey tool is required to
point them in the right direction as well as confirm that they drill the intended path. The
most common method of directionally controlling a wellbore is to use a steerable motor
which incorporates a fixed bend. The MWD tool is used to orient that bend so that it will
drill the desired trajectory.
Page 5
Directional Surveying
Land survey techniques, such as Global Positioning Systems (GPS), are used to
determine the well’s surface location. Directional surveys are taken at regular intervals
throughout the drilling process and are used to calculate the position of the wellbore
relative to the surface location.
The Baker Hughes MWD tool uses six sensors, three accelerometers and three
magnetometers, fixed on 3 axis’ to make measurements of the gravity and magnetic
fields.
Z - Axially downward through the
tool.
Y - Cross-axial to the tool, positive
in the direction from the tool
center to the tool scribeline. The
scribeline is used as a reference
and is marked on the MWD collar.
X - Cross-axial to the tool,
perpendicular to Z and Y.
The accelerometers and magnetometers measure the components of the earth’s
gravitational and magnetic fields, respectively, along the three orthogonal axes at the
survey point. It is Baker Hughes’ standard procedure to transmit to the surface
directional survey data consisting of these 6 raw values: Gx, Gy, Gz, Bx, By, and Bz. The
tool’s inclination and azimuth and the orientation of the deflection tool (toolface), are
calculated from these measurements. To ensure that the transmitted data is valid, the
raw measurements are used to calculate QC parameters that can be compared to
known reference values:
• GT: Gravitational Field Strength – Generally around 9.81m/s2, this number
does vary according to location, and an accurate local value can be estimated
using a gravity field model and compared to the value calculated from the MWD
tool’s Gx, Gy and Gz readings.
• BT: Magnetic Field Strength – The strength of the earth’s magnetic field at
any location, and at any given time, can be estimated using a geomagnetic
model. The value calculated from the downhole Bx, By and Bz readings can then
be compared to this estimation of a local value.
• DIP: Magnetic Dip Angle – As with field strength, the angle that the magnetic
field lines create with the horizontal plane can be estimated for a given location
and time using a geomagnetic model. All 6 raw directional values from the MWD
tool are used in the calculation of the DIP angle.
Page 6
If the values calculated from the MWD sensor measurements agree with the predicted
values for these three parameters, it can be assumed that the sensor readings are
within specification and, therefore, the inclination, azimuth and toolface are within
specification. If the difference between the measured and reference values exceed
specified QC limits, the survey is “flagged” and must be retaken after resolving the
cause of the QC failure. Tool movement, magnetic interference and sensor failure are a
few of the possible causes of QC failure.
Once the calculated data has been checked for quality, three values are used for
wellbore positioning purposes:
1. Measured Depth: Distance measured along the actual
course of the borehole from the surface reference point
to the survey sensor measure point.
2. Inclination: The angle between vertical and the tangent
to the borehole axis. The vertical reference is the
direction of the local gravity vector. A correction for the Inclination
sag of the drilling assembly at the MWD tool location
may be applied at surface to the calculated value.
3. Azimuth: The direction of the borehole measured as a
clockwise angle on the horizontal plane from the North
reference. The tool uses Magnetic North as a reference
for azimuth. This magnetic azimuth is then corrected to
True or Grid North and a correction for drillstring
magnetic interference might also be applied. Azimuth
Further calculations then use these values, along with the previous known position, to
determine the coordinates of the new survey point. This process occurs for each survey
to plot the trajectory of the wellpath from surface to bottom-hole location, as well as
calculate the amount of dog leg between measurement points
Page 7
Survey Procedure
MWD tools are generally programmed to take the required measurements and transmit
a raw survey on powering up. Most tools use an alternator driven by the flow of drilling
fluid to supply power. This means that taking a directional survey requires stopping and
then restarting mud circulation. Battery-powered tools are able to take surveys while
there is no mud flow, reducing the effect of vibration on the raw measurements,
although these tools mostly still require a power on-off cycle to take a survey.
Detailed procedures for taking an MWD directional survey can be found in the following
document on TechPubs:
MWD-20-75-0000-01-01 MWD Survey Service Operations Manual –
Section 2: Rig – Brief Drilling Personnel on Surveying Procedures
Review this document and answer the following questions:
1. Should MWD Directional surveys be taken while the bit is on, or off bottom?
Off
2. Why should the driller work the drillstring up and down with no rotation prior
to an MWD survey being taken?
To work out any torque in the pipe & ensure that no pipe movement due to
unreleased torque takes place while the survey is being taken
3. When can the pipe be moved once the survey has been taken and the pumps
are on again?
After the FID is transmitted
Page 8
Toolface Steering
In directional drilling, the term ‘toolface’ originated when
whipstocks were first used as a means to deflect the
wellpath in a desired direction. A whipstock is a steel
wedge that is placed in the hole at the point where a
change in direction is required; its face is oriented
towards the direction that the drilling assembly needs to
take. In a vertical hole where the intention is to build
inclination in an easterly direction, for example, the face
of the tool might be lined up at surface towards east
before running it in-hole. As long as the whipstock does
not turn as it is lowered downhole, once it is
located in place at the kick-off point, it has a
toolface of 90 degrees relative to north. A
drilling assembly would then be run in-hole to
kick-off the well in that direction.
As more advanced methods of steering a well
were introduced, the term ‘toolface’ remained to
describe the orientation of deflected tools. The
MWD Directional Service transmits a toolface
value which, after corrections at surface,
commonly refers to the orientation of the bend
in a steerable motor. At inclination close to
vertical, the magnetic toolface orientation is reported as an azimuth references to
north; at higher inclination, the highside toolface is reported as the angular offset from
the highside of the hole. Drilling a directional wellpath with a steerable motor usually
requires a combination of ‘sliding’ and ‘rotating’. Sliding refers to periods where the
bend is oriented in the desired direction before drilling ahead with no drillstring rotation.
The drillstring is then rotated to drill straight (tangent) sections, as the bend is no
longer oriented in any one direction.
Page 9
Magnetic Toolface
In the whipstock example above, the toolface reference was north, so the actual
toolface value was 90 degrees, relative to this. We can use north as a reference when
the hole is at or close to 0 degrees inclination (vertical). Any toolface value while using
north as a reference indicates the azimuth that we want the wellpath to follow. If we
drilled ahead from vertical with a magnetic toolface of 180 degrees, the hole would
build inclination towards the south. We describe this type of toolface reference as
magnetic, as the MWD directional sensors use magnetometers to measure azimuth
relative to Magnetic North. MWD magnetic toolface measurements can therefore be
referenced to True North or Grid (Map) North if the appropriate corrections are made at
surface. Other terms for this type of toolface are ‘North Referenced Toolface’, or ‘Gyro
Toolface’ when gyroscopic sensors are used instead of magnetometers.
Example: If the Directional Driller drilled ahead with a magnetic toolface of 45
degrees, the wellpath would build inclination in a North-Easterly direction.
Magnetic Toolface
Page 10
Highside Toolface
Magnetic toolface is typically not used to steer a wellpath at higher inclinations. ‘Up’ is a
more useful reference, once the hole has built sufficient inclination. For example, if we
wish to steer a well which is already horizontal, our choices are limited to combinations
of up or down, combined with left or right. If we look at the hole from the perspective
of the drilling assembly, up is the highside of the hole. If we want to steer the wellpath
to the right, the bend must be oriented 90 degrees from this reference before drilling
ahead. Instead of projecting our steering direction onto a horizontal plane as we do
with magnetic toolface, highside toolface is projected onto the plane perpendicular to
the wellbore. In the case of a horizontal interval this would be the vertical plane. Drilling
ahead with a highside toolface of 0 degrees would therefore cause the wellpath to
continue building inclination, whereas a highside toolface of 180 degrees would cause
the wellpath inclination to decrease. Because highside toolface uses ‘up’ as a reference,
it is often referred to a gravity toolface (‘up’ being the opposite direction from gravity).
The accelerometers in the MWD directional sensor package are used to measure
highside toolface orientation.
Example: If the Directional Driller drilled ahead with a highside orientation of 45
degrees right of highside, the wellpath would build and turn right.
Highside Toolface
Baker Hughes MWD tools are set up as standard to transmit Magnetic Toolface below 3
degrees inclination and Highside Toolface when the borehole inclination exceeds 3
degrees.
Page 11
Scribeline Correction
We know that the orientation of the bend in the steerable system needs to be identified
at surface for the Directional Driller to determine whether the wellpath is being steered
in the right direction. As there may be several drilling assembly components between
the MWD tool and the motor, each with torqued connections, the MWD in most cases
tool will not be lined up with the orientation of this bend. We must therefore apply a
correction to the tools transmitted toolface in order
to correct for the misalignment.
MWD Scribeline
The MWD tool’s directional sensor package is lined
up internally to a reference mark on the outside of
the tool. The orientation of this ‘scribeline’ Motor Highside
reference with respect to either Magnetic North, or highside, depending on
borehole inclination is transmitted to surface. We therefore need to know
what the offset is between the scribeline reference and the motor bend
reference to correct this value.
Once the bottom hole assembly (BHA) is assembled, and each component is
fully torqued, there will usually be a misalignment between the MWD
scribeline and the motor bend ‘highside’ mark. The angular offset between
these marks can be measured on the rig floor and entered into the MWD
surface acquisition system. This offset is then added by the acquisition
system to the values transmitted by the MWD tool to give a toolface
corrected for scribeline offset. This correction is applied to both highside and
magnetic toolface values, in addition to the declination and convergence
which may also apply to magnetic toolface.
Page 12
Survey Corrections
Drillstring Interference
Magnetic interference from a number of sources can influence our magnetometer
measurements and subsequently affect the measured azimuth or any other calculated
value based on those measurements. The most common source is axial magnetic
interference caused by the magnetization of the drillstring. The significance of this
interference is dependent on factors such as directional sensor type, BHA, date,
location, and the expected inclination and azimuth.
Drillstring interference can be reduced by housing the survey sensors in drill collars
made from non-magnetic steel, known as monel or non-mag. Typically, adding monel
to the BHA to increase the distance of the sensors from any drillstring magnetic field
will minimize the effect of interference.
Magnetic Collars
Magnetic Collars
Non-Mag Collars
The amount of interference and its effect on the azimuth measurement can be
estimated for a given BHA configuration by using the Survey Utilities program, available
as part of the Advantage package.
Axial drillstring interference has the greatest effect where the horizontal component of
the earth’s magnetic field is small relative to the drillstring magnetization. The
horizontal component of the earth’s magnetic field gets smaller as we move from the
equator towards the poles. For a given location, the greatest effect from drillstring
interference occurs when drilling at an inclination of 90 degrees and an azimuth of
magnetic east or west. There is no effect on the horizontal component when drilling
vertically.
Page 13
If non-magnetic spacing is not sufficient to eliminate the effects of drillstring
interference, a correction algorithm may be applied. This MagCorr algorithm assumes
that only the Z-axis magnetometer is affected and cannot correct for any interference
from an external source which might affect the X and Y magnetometers. It calculates
an accurate azimuth by using the known local magnetic dip angle and total magnetic
field strength to correct the Z-axis magnetometer.
SAG
In an inclined borehole, gravity causes the BHA to lie to the low side of the hole.
Depending on the sensor position,
the reading may not read the
correct wellbore inclination:
A sensor located in these
positions would read
higher than the borehole
inclination
A sensor located in these
positions would read
lower than the borehole
inclination
A sensor located in these
positions would read the
correct borehole
inclination
Page 14
Stabilizers are used to support and center the BHA in the borehole. The tools or collars
(referred to as tubulars), will tend to flex and sag between stabilizers toward the low
side of the hole. This effect increases as inclination approaches 90 degrees (horizontal),
with no sag effect in a vertical well. If the inclination sensor is located in a tubular
affected by sag, it may no longer be measuring the true inclination of the borehole.
The SAG correction program software is used to model the BHA and determine the
correction to be applied to the inclination.
The degree of sag, and therefore the correction applied to the inclination is a function
of the stiffness of the tubulars, the placement and size of the stabilizers, buoyancy of
the drilling fluid, the inclination and curvature of the borehole at the survey point and
the position of the inclination sensor.
Page 15
Summary
Although the Directional Service provided by the MWD tool may be the most basic level
offered, in many ways it is also the most important. A range of advanced Formation
Evaluation and Drilling Optimization services are available to add on to this base-level
service. But, without an accurate survey listing or precise wellpath control, the
deliverables produced from these services will be of little use.
The role of MWD field personnel is to deliver directional data which meets defined levels
of accuracy. The consequences of providing data not within specification could, at
worst, result in collision with a producing well, possibly leading to catastrophic loss of
life and environmental damage. At the least, it would lead to a misplaced well and a
loss of revenue, with potential liability for the cost of a re-drill.
A range of data inputs to the surface acquisition system is required in preparation for
taking accurate MWD surveys. Although some of these are already covered within the
Wellbore Positioning suite of online modules, they will be discussed again during the
classroom instruction.
In addition to comprehensive training, consistent quality in our MWD Directional Service
deliverables requires strict adherence to defined procedures, and precise verification of
all inputs and a thorough QC of output data.
Page 16
Review Questions
Survey Procedures
1. What are the three measured parameters that make up a directional survey?
inclination, azimuth, measured depth
2. What does the directional sensor package consist of?
3 accelerometers, 3 magnetometers
3. Name the three characteristics of the earth’s magnetic and gravitational fields
that are used for directional survey Quality Control.
GT, BT, DIP
4. What would you expect to happen to the wellpath if drilling continued while the
bend in a steerable motor was oriented to the following values?
0 degrees Highside Build Inclination
90 degrees left of Highside Turn left
135 degrees right of Highside Drop Inclination & turn right
270 degrees from True North Build inclination to the West
225 degrees from True North Build inclination to South/West
90 degrees from True North Build inclination to the East
5. The scribeline offset is the angle in degrees measured from the MWD collar
scribeline to the _____________ scribeline, in the _____________ direction
looking downhole.
6. What could be the result of failure to correctly measure and apply the scribeline
offset?
steering the well in the wrong direction
Page 17
7. When drilling a well with an inclination of 1 degree, what would be the magnetic
toolface orientation used to steer the well and build angle in the East direction?
90 degrees
8. What does MagCorr correct for and which survey measurement is affected?
axial (drillstring) interference, azimuth
9. What parameters must be input to the MagCorr software to calculate the
correction?
total magnetic field strength (BTN) and dip angle ( DIPN)
10. What survey measurement is the SAG correction applied to?
inclination
Page 18
Pre-Read Summary
During part 1 of the MWD Academy, you will be trained on the operational procedures
for providing the NaviTrak Directional Gamma service at the rigsite. The content of this
document, along with the foundational training you have received so far, will help
prepare you for this course by providing you with the background knowledge required
to understand the applications of this service, and the theory of how it is delivered.
With most of the fundamental knowledge already covered here and in the appropriate
introductory training, the classroom instruction received during the MWD Academy will
focus on the practical aspects of service delivery at the rigsite. It is your responsibility
to ensure that you have a good understanding of all the pre-requisite material prior to
attending the MWD Academy.
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