Gyro Survey Tool Types In Oil & Gas Wells

Conventional Gyro

The conventional gyro or free gyro has been around since the 1930s. It obtains the azimuth of the wellbore from a spinning gyro. It only determines the direction of the wellbore and does not determine the inclination. The inclination angle is usually obtained with accelerometers. The film-based, single-shot gyro uses a pendulum suspended above a compass card (attached to the outer gimbal axis) to get the inclination. A conventional gyro has a spinning mass usually turns at 20,000 to 40,000 rpm (some turn even faster). The gyro will stay fixed if no external forces act on it and the mass is supported at its exact center of gravity. Unfortunately, it is not possible to keep the mass at its precise center of gravity, and external forces do act on the gyro. Therefore, the gyro will drift with time.

Theoretically, if a gyro starts spinning and is pointed in a specific direction, it should not substantially change direction over time. Therefore, it is run in the hole, and even though the case turns around, the gyro is free to move, and it stays pointing in the same direction. Since the direction in which the gyro is pointing is known, the direction of the wellbore can be determined by the difference between the orientation of the gyro and the orientation of the case containing the gyro. The orientation of the spin axis must be known before the gyro is run in the hole. This is called referencing the gyro. If the gyro is not referenced correctly, the entire survey is off, so the tool must be appropriately referenced before it is run in the hole for oil and gas wells.

Disadvantages

Another disadvantage of a conventional gyro is that it will drift with time, causing errors in the measured azimuth. The gyro will drift due to system shocks, bearing wear, and the Earth’s rotation. The gyro can also drift due to imperfections in the gyro. The defects can develop during the manufacturing or machining of the gyro, as the exact center of the mass is not in the center of the spin axis. The drift is less at the Earth’s equator and higher at higher latitudes near the poles. Generally, conventional gyros are not used at latitudes or inclinations above 70°. A typical drift rate for a traditional gyro is 0.5° per minute. The apparent drift caused by the Earth’s rotation is corrected by applying a special force to the inner gimbal ring. The applied force depends upon the latitude where the gyro will be used.

Because of these reasons, all conventional gyros will drift by specific amounts. The drift is monitored whenever a traditional gyro is run, and the survey is adjusted for that drift. If the reference or the drift is not adequately compensated, the gathered survey data will be incorrect.

Rate Integrating Or North-Seeking Gyro

A rate or north-seeking gyro was developed to prevent the shortcomings of the conventional gyro. A rate gyro and a north-seeking gyro are essentially the same things. It is a gyro with only one degree of freedom. The rate integrating gyro is used to determine true North. The gyro resolves the Earth’s spin vector into horizontal and vertical components. The horizontal component always points to the true North. The need to reference the gyro is eliminated, which increases the accuracy. The latitude of the wellbore must be known because the Earth’s spin vector will be different as the latitude varies.

During setup, the rate gyro automatically measures the Earth’s spin to eliminate the drift caused by the Earth’s rotation. This design feature makes it less likely to produce errors compared to a conventional gyro. Unlike a traditional gyro, the rate gyro doesn’t require a reference point to be sighted in, thereby eliminating one potential source of error. The forces acting on the gyro are measured by it, while the force of gravity is measured by the accelerometers. The combined readings of the accelerometers and the gyro allow the calculation of the inclination and azimuth of the wellbore.

A rate gyro will measure the angular velocity through an angular displacement. The rate integrating gyro calculates the integral of the angular velocity (angular displacement) through an output angular displacement.

Newer versions of the gyro can be surveyed while moving, but limitations exist. They do not have to remain stationary to get a survey. Total survey time can be decreased, making the tool more cost-effective.

Ring Laser Gyro

The ring laser gyro (RLG) uses a different type of gyro to determine the direction of the well. The sensor comprises three-ring laser gyros and three inertial-grade accelerometers mounted to measure the X, Y, and Z axes. It is more accurate than a rate or north-seeking gyro. The survey tool does not have to be stopped to take a survey, so surveys are quicker. However, the outside diameter of the ring laser gyro is 5 1/4 inches, which means this gyro can only run in a 7″ and larger casing (check our casing design guide). It can’t be run through a drill string, whereas a rate or north-seeking gyro can be run through a drill string or smaller diameter tubing strings.

Components

In its simplest form, the ring laser gyro consists of a triangular block of glass drilled out for three helium-neon laser bores with mirrors at the 120-degree points – the corners3. Counter-rotating laser beams – one clockwise and the other counter-clockwise coexist in this resonator. At some point, a photosensor monitors the beams where they intersect. They will constructively or destructively interfere with one another, depending on the precise phase of each beam.

If the RLG is stationary (not rotating) concerning its central axis, the relative phase of the two beams is constant, and the detector output is consistent. If the RLG is rotated about its central axis, the clockwise and counter-clockwise beams will experience opposing Doppler shifts; one will increase in frequency, and the other will decrease in frequency. The detector will sense the difference frequency from which precise angular position and velocity can be determined. This is known as the Sagnac effect.

What is being measured is the integral of angular velocity or angle turned since the counting began. The angular velocity will be the derivative of the beat frequency. A dual (quadrature) detector can be used to derive the direction of rotation.

Inertial Grade Gyro

The most accurate survey instrument in the oil and gas field is the inertial grade gyro, often called the Ferranti tool. It is the entire navigation system as adapted from aerospace technology. Because of the highest accuracy of this gyro, most survey tools are compared with it to determine their respective accuracies. The device uses three rate gyros and three accelerometers mounted on a stabilized platform.

The system measures the change in direction of the platform (platform rigs) and the distance it moves. It not only measures the inclination and direction of the well but also determines the depth. It does not use the wireline depth. However, it has an even bigger dimension of 10⅝ inch OD. As a result, it can only be run in casing sizes of 13 3/8″ and larger.

The gyroscope inclinometer from Vigor is tested in the simplest and easy-to-use form, and the customer only needs to install and debug it according to the video of Vigor after receiving the goods. If you need our help, Vigor's after-sales department will also reply 24 hours to help you deal with the problem urgently, if you are interested in Vigor's gyroscope inclinometer, please do not hesitate to get in touch with Vigor's engineer team to get the most professional technology and the best quality worry-free high-quality service.

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