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North Seeking gyro - Common tools description

Desription of typical North Seeking gyro:  Seeker (Baker Hughes Inteq), Wellbore surveyor (gyro/data), Finder (SDI), GCT (Schlumberger ), G2 (Sperry-Sun)

Seeker (Baker Hughes Inteq)

1 directional drilling survey tool description

The Seeker from Baker Hughes Inteq is a single axis gyro tool, i.e. the gyro has one sensing axis in the cross-borehole plane. As stated before, it applies gyrocompassing (low inclination) to determine azimuth. For that purpose four measurements at 90° toolface differences are performed. With two pairs of measurements at 180° apart toolface angle, gyro bias effects are eliminated.

2 QC parameters

The gyro temperature should not exceed the maximum calibrated value of 82 °C.

3 Additional running procedures

  • Initialisation and wellsite calibration check: The directional drilling survey tool is made up on the catwalk. Under normal survey conditions the extended 2" OD half flask should be used. The directional drilling survey tool should be warmed up to at least 46°C. Prior to each job a wellsite calibration check is performed by the Survey Engineer. Onshore the calibration check lasts about 30 minutes and includes a minimum of three 180° end to end rotations. Offshore, the calibration check lasts about a hour to eliminate low level vibrations. A minimum of 5 end to end rotations are made. Minimum lead time for this step is 2-3 hrs.
  • Inrun/outrun survey frequency: During the inrun, surveys should be taken at depth increments of 25 m (100 ft). Surveys are taken on the outrun typically every 100 m (300 ft). A survey takes about 1 minute.

Wellbore surveyor (gyro/data)

1 directional drilling survey tool description

The Well Bore Surveyor consists of a two axes gyro with its axes in the cross-borehole plane. Two measurements 180° apart toolface angle are made to eliminate gyro bias. The directional drilling survey tool is designed for gyrocompassing in low inclination mode. The Well Bore Surveyor can be run on wireline or as a multi-shot tool in casing or drillpipe in battery mode. The tool can not be pumped.

2 QC parameters

The mass unbalance spin axis offset is measured downhole and compared with the pre-calibration value. If required the actual value can be applied downhole.
The gyro temperature should not exceed the maximum calibrated value of 95°C.

3 Additional running procedures

  • Initialisation and wellsite calibration check: Verify that the shock watch on the gyro cup has not been set off. The directional drilling survey tool is made up on the catwalk. Warm the tool up to 60°C. Position the tool in a pre-selected toolface angle. A rotation along the tool should be applied to check for tool misalignment. The Survey Engineer should select a specific depth (inclination at least 30°) to perform a downhole check of the calibration values. At this depth 5 stations are recorded to calculate the mass unbalance offset (MBO). This value will affect the survey results and should be measured properly. Ensure that the measured toolface angle agrees with the pre-selected toolface angle to within 30°.
  • Inrun/outrun survey frequency: Survey the inrun or outrun in pre-selected depth increments, typically 25m (100 ft).

Finder (SDI)

1 directional drilling survey tool description

The Finder of Scientific Drilling International is a single axis gyro directional drilling survey tool, i.e. the gyro has one sensing axis in the cross-borehole plane. The tool applies gyrocompassing to determine azimuth. Due to its accuracy this can only be performed up to 15° inclination. For this purpose, four measurements at 90° toolface differences are performed. With two pairs of measurements at 180° apart toolface angle, gyro bias effects are eliminated. Above 15° inclination the tool applies the continuous mode. At the start of the continuous mode an initialisation is performed consisting of multiple gyrocompassing measurements. The Finder directional drilling survey tool can be run on wireline. For highly inclined wells the directional drilling survey tool can be pumped downhole.

2 QC parameters

North seek initialisation error is the major error source for the continuous mode. Mass unbalance spin axis offset and gyro bias both contribute to the gyro drift. Regular drift checks are performed to measure the actual gyro bias.
The gyro temperature should not exceed the maximum calibrated value of 82°C.

3 Additional running procedures

  • ·Initialisation and wellsite calibration check: The directional drilling survey tool is made up on the catwalk. Under normal survey conditions the 2.125" OD flask is used. Tool is warmed up to the relevant instrument log book temperature. Prior to each job a wellsite calibration check is performed by the Survey Engineer. Onshore, the calibration check lasts about 30 minutes and includes a 180° end to end rotation. Offshore, the calibration check lasts about a hour to eliminate low frequency vibrations.
  • ·Gyrocompassing mode (inclination < 15°): Surveys should be taken at pre-selected depth increments, typically every 25 m (100 ft). This mode is used until an inclination of approximately 15° is reached during an inrun or outrun.
  • ·Continuous mode (inclination > 15°): At an inclination of 15° the tool is switched over to continuous mode. At the start of this mode a series of gyrocompassing readings are taken. This is called North seek initialisation. The azimuth standard deviation of the North seek initialisation should be less than 0.25°. During the continuous mode drift checks/tunes are needed. A drift check is made if one of the following changes has occurred since the last drift check:
    • -15° of inclination change;
    • -15° of azimuth change;
    • -15° F (8.3°C) of temperature increase;
    • -15 minutes have elapsed since the last drift check.
  • Surveys should be taken at pre-selected depth increments, typically every 10 m (30 ft).

GCT (Schlumberger )

1 directional drilling survey tool description

The Schlumberger GCT is run in continuous mode for low and high inclinations. At the start of a survey a North seek initialisation consisting of multiple gyrocompassing measurements is performed. Table 154 gives a summary of the GCT specifications.

2 QC parameters

North seek initialisation is the major source of uncertainty. The drift rate is determined by taking drift measurements at the start/end of a survey and at bottom.

3 Additional running procedures

  • Initialisation and alignment: The tool is made up on the catwalk. Two knuckle joints should be included for holes with inclinations over 5°. At the start of a survey a North seek initialisation should be performed. This will take 1 1/4hrs. At the end of a survey another North seek and checking of the drift rates is performed. The latter will take 30 minutes and is used for quality control. When performing a North seek alignment offshore this should be done below seabed to eliminate tool movement. The directional drilling survey tool should be anchored and the wireline tension removed.
  • Drift checks: Drift checks should be taken in deviated wells (inclination > 10°) at the inrun at the start or tie-in point, at the bottom and at the outrun at the end or tie-in point. If the well temperature exceeds 110° C, drift checks at bottom should not be taken but during the outrun at a depth where the temperature has fallen below 110°C. No drift checks are required for vertical wells (inclination < 10°).
  • Inrun/outrun survey frequency: Surveys should be taken at pre-selected depth increments, typically every 10 m (30 ft).
  • The temperature should not exceed the maximum allowable operating temperature of 120°C for GCT-A , 150° C for GCT-B and 175° for GCT-BB.

G2 (Sperry-Sun)

1 directional drilling survey tool description

G2 of Sperry-Sun operates in low inclination gyrocompassing and low inclination continuous mode.

2 QC parameters

3 Additional running procedures

  • Initialisation and wellsite calibration check: On the wellsite a full onsite calibration check can be performed. However, in unstable environments it may be difficult to perform a full calibration and may be replaced by a downhole gyro calibration check.
  • At the start of a survey, two North seek initialisations are performed. The first initialisation is used to check the tool's repeatability. An initialisation consists of a number of alignments and lasts 15 minutes. Above 15° inclination a tie-in point can be used for referencing. In gyrocompassing mode the tool stops at pre-selected depth and a survey is taken. A survey takes 10 seconds. In continuous mode the survey data is transmitted every 10 m (25 ft). For both modes the drift is monitored and compared with theoretical drift, the latter based on calibration data. At the end of a survey another alignment is performed and the difference known as the closure rate between the two alignments is derived.
  • Drift checks: During the survey drift checks are taken. A drift check is made if one of the following changes has occurred since the last drift check:
    • -10° of inclination change;
    • -10° of azimuth change;
    • -10°C of temperature change.