Drillhole Survey
Deviflex Training - Trondheim Norway (2013)

Directional surveys obtain the measurements needed to calculate and plot the 3D well path.
Instruments for conducting directional surveys can be set up in several different variations,
depending on the intended use of the instrument and the methods used to store or transmit
survey information.

Types of survey instruments

Basically, there are two types of survey instruments:
  1. Magnetic
  2. Gyroscopic

Depending on the method used to store the data, there are film and electronic systems. Survey
systems can also be categorized by the methods used to transmit the data to the surface, such
as wireline or measurement while drilling (MWD).

Magnetic sensors

Magnetic sensors must be run within a nonmagnetic environment [i.e., in uncased hole either
in a nonmagnetic drill collar(s) or on a wireline]. In any case, there must not be any magnetic
interference from adjacent wells. Magnetic sensors can be classified into two categories:
  1. Mechanical compass
  2. Electronic compass

Mechanical compass
A mechanical compass uses a compass card that orients itself to magnetic north, similar to a
hiking-compass needle. Inclination is measured by means of a pendulum or a float device. In
the pendulum device, the pendulum is either suspended over a fixed grid or along a vernier
scale and is allowed to move as the inclination changes. The float device suspends a float in
fluid that allows the instrument tube to move around it independently as the inclination changes.

The only advantage of mechanical compasses is the low cost, while several disadvantages
have limited them from being used widely in directional surveys. The drawbacks are:
  1. High maintenance costs
  2. A need to choose inclination range
  3. Limited temperature capability
  4. The possibility of human error in reading film
  5. The inability to use them in MWD tools

Electronic compass
The electronic compass system is a solid-state, self-contained, directional-surveying
instrument that measures the Earth’s magnetic and gravitational forces. Inclination is
measured by gravity accelerometers, which measure the Earth’s gravitational field in the x, y,
and z planes. The z plane is along the tool axis, x is perpendicular to z and in line with the tool’s
reference slot, and y is perpendicular to both x and z. From this measurement, the vector
components can be summed to determine inclination. Hole direction is measured by gravity
accelerometers and fluxgate magnetometers. Fluxgate magnetometers measure components
of the Earth’s magnetic field orthogonally (i.e., in the same three axes as the accelerometers).
From this measurement, the vector components can be summed to determine hole direction.

Depending on the packaging of the electronic sensors, the electronic-compass system can be
employed in different modes, such as single-shot, multishots, and MWD, in which data are
sent to surface in real time through the mud-pulse telemetry system.

The electronic magnetic single-shot records a single survey record while drilling the well. The
sensors measure the Earth’s magnetic and gravitational forces with fluxgate magnetometers
and gravity accelerometers, respectively. The components of this survey system include the
probe and a battery stack that supplies power to the probe. The raw data are stored downhole
in the memory and retrieved at the surface to calculate the hole direction, inclination, and tool
face. The electronic magnetic multishot uses the same components as the electronic single-
shot; the only difference is that electronic multishots record multiple survey records. The MWD
acquires downhole information during drilling operations that can be used to make timely
decisions about the drilling process. The magnetic survey information is obtained with an
electronic compass, but, unlike previous systems that stored the information, the MWD
encodes the survey data in mud pulses that are sent up and decoded at the surface. The real-
time survey information enables the drillers to make directional-drilling decisions while drilling.
The sensors used in MWD tools are the same design as those used in electronic magnetic
single-shot and multishots (i.e., gravity accelerometers and fluxgate magnetometers).

The geomagnetic field
Both types of magnetic sensors rely upon detecting the Earth’s magnetic field to determine
hole direction. The Earth can be imagined as having a large bar magnet at its center, laying
(almost) along the north/south spin axis (see Fig. 1). The normal lines of the magnetic field will
emanate from the bar magnet in a pattern such that at the magnetic north and south poles, the
lines of force (flux lines) will lay vertically, or at 90° to the Earth’s surface, while at the magnetic
equator, the lines of force will be horizontal, or at 0° to the Earth’s surface. At any point on the
Earth, a magnetic field can be observed having a strength and a direction (vector). The strength
is called magnitude and is measured in units of tesla. Usual measurements are approximately
60 microtesla at the magnetic north pole and 30 microtesla at the magnetic equator. The
direction is always called magnetic north. However, although the direction is magnetic north,
the magnitude will be brllel to the surface of the Earth at the equator and point steeply into the
Earth closer to the north pole. The angle that the vector makes with the Earth’s surface is called
the dip.

The prevailing models used to estimate the local magnetic field are provided by the British
Geological Survey (BGS) or, alternatively, by the U.S. Geological Survey (USGS). These models
carry out a high-order spherical harmonic expansion of the Earth’s magnetic field and provide a
very accurate global calculation of the magnetic field rising from the Earth’s core and mantle.
The models are based on:

Measurements from hundreds of magnetic stations on the surface of the earth
Airborne magnetic surveys
Magnetic-field data gathered by satellites
Because even the field of the Earth’s core and mantle varies with time, these models are
updated on an approximately annual basis. Note that these models include neither effects from
materials near the surface of the earth (termed “crustal anomalies”), which can be quite
significant, nor separate effects from various electrojets in the Earth’s atmosphere,* the effects
of solar storms, or the diurnal variation in the earth’s magnetic field. At high latitudes,** these
effects can be quite significant. A way of getting around this problem is to make magnetic-
observatory-quality measurements directly at the wellsite; however, this is rarely possible. A
very useful alternative is to interpolate the field at a given location and time, as measured by at
least three nearby magnetic observatories, the triangle of which preferably includes the wellsite
being surveyed. This is referred to as interpolated in-field referencing. Scientists who use this
technique on surveys taken at high latitudes and with axial magnetic interference report
achieving an accuracy approaching that otherwise attainable only with gyros.

Gyroscopic sensors

Gyroscopic surveying instruments are used when the accuracy of a magnetic survey system
may be corrupted by extraneous influences, such as cased holes, production tubing,
geographic location, or nearby existing wells. A rotor gyroscope is composed of a spinning
wheel mounted on a shaft, is powered by an electric motor, and is capable of reaching speeds
of greater than 40,000 rev/min. The spinning wheel (rotor) can be oriented, or pointed, in a
known direction. The direction in which the gyro spins is maintained by its own inertia;
therefore, it can be used as a reference for measuring azimuth. An outer and inner gimbal
arrangement allows the gyroscope to maintain its predetermined direction, regardless of how
the instrument is positioned in the wellbore.

Gyroscopic systems (gyros) can be classified into three categories:
  1. Free gyros
  2. Rate gyros
  3. Inertial navigation systems

Free gyros
There are three types of free gyros: tilt scale, level rotor, and stable platform. The tilt scale and
level rotor are film systems, while the stable platform uses the electronic system, which has
shorter run time, faster data processing, and monitors continuously. Thus, most free gyros are
the stable-platform type, which uses a two-gimbal gyro system like the level-rotor gyro, but the
gimbals remain perpendicular to each other, even when the instrument is tilted during use. The
inner gimbal remains perpendicular to the tool axis (platform) instead of perpendicular to the

Rate gyros (north-seeking gyros)
These use the horizontal component of the Earth’s rotational rate to determine north. The Earth
rotates 360° in 24 hours, or 15° in 1 hour. The horizontal component of the Earth’s rate
decreases with the cosine of latitude; however, a true-north reference will always be resolved at
a latitude of less than 80° north or south. Therefore, the rate gyro does not have to rely on a
known reference direction for orientation. Inclination is measured by a triaxial gravity-
accelerometer package. Rate gyros have a very precise drift rate that is small compared to the
Earth’s spin rate. The Earth’s spin rate becomes less at higher latitudes, affecting the gyro’s
ability to seek north. This effect also increases the time required to seek north accurately and
decreases the accuracy of the north reference.

Inertial navigation systems
This is the most accurate surveying method. Inertial navigation systems use groups of gyros to
orient the system to north. It can measure movement in the x, y, and z axes of the wellbore with
gyros and gravity accelerometers. Because of the sensor design, this instrument can survey in
all latitudes without sacrificing accuracy.
Our Project
2013-2015 Tembapura
Drillhole Survey
PT. Duaem Gada Bayuagus