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Directional survey instruments
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:
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 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:
- Mechanical compass
- Electronic 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:
- High maintenance costs
- A need to choose inclination range
- Limited temperature capability
- The possibility of human error in reading film
- The inability to use them in MWD tools
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 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:
- Free gyros
- Rate gyros
- Inertial navigation systems
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 horizon.
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.
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.
Noteworthy papers in OnePetro
Ekseth, R., Torkildsen, T., Brooks, A.G., Weston, J.L., Nyrnes, E., Wilson, H.F. and Kovalenko, K. 2010. High-Integrity Wellbore Surveying. SPE Drilling & Completion 25 (4): 438-447. SPE-133417-PA. https://doi.org/10.2118/133417-PA.