North Seeking gyro directional drilling survey tools provide substantially more accurate survey data than conventional gyro tools. In the conventional gyroscopic survey tool, the gyro is oriented to a known reference direction or foresight.
A North Seeking gyro directional drilling survey tool uses a gyro to reference itself to True North, eliminating the need for a foresight. The foresight reference uncertainty and possible reading uncertainties are replaced by an operator independent North seek uncertainty. This referencing to True North is called gyrocompassing. It is based on the measurement of the horizontal component of the Earth rotation vector which becomes smaller for increasing latitudes and hence reduces the ability of these directional drilling survey tools to accurately seek North. In general their application limit of maximum latitude is 80 degrees North or South. North Seeking gyro tools are run on wireline transmitting survey data real time to surface.
North Seeking directional drilling survey tools have two basic modes of operation:
- Gyrocompassing mode, with the tool held stationary at each survey station whereby the azimuth is calculated independently at each survey station.
- Continuous mode. At the start of the survey interval the tool is referenced to True North by gyrocompassing. After that, the tool is run continuously with integration of the azimuthal tool rotation.
There are a number of North Seeking gyro directional drilling survey tools currently available: ·Seeker (Baker Hughes Inteq); ·Well Bore Surveyor (Gyro/Data); ·GCT (Schlumberger); ·Finder (SDI); ·G2 (Sperry-Sun).
In addition to the gyro, all these tools have a set of accelerometers for measuring Earth's gravity field, from which toolface and inclination are determined. In vertical wells a gyro toolface similar to the magnetic toolface of a magnetic tool is measured.
The directional drilling survey tools are run by Survey Engineers on wireline. North Seeking gyro surveys have a high level of quality control with respect to conventional gyro surveys and can be used for definitive surveys.
Furthermore, Scientific Drilling International's Finder Gyro While Drilling (GWD) can also be used as gyroscopic wireline steering tool and can be pumped down to survey highly inclined wells. Gyro/Data's Wellbore Surveyor can operate in battery mode, eliminating the need for a wireline.
Gyrocompassing mode
Gyrocompassing is a technique whereby the gyro directional drilling survey tool is held stationary and the component of the Earth's rate of rotation vector in horizontal plane is accurately measured by gyros. Knowing that the horizontal component of the Earth's rate of rotation is directed to True North, the ratio of the horizontal rotational field components measured is equal to tangent azimuth. All currently available North Seeking gyro directional drilling survey tools have the sensing axes in the cross-borehole plane. To allow determination of the measured horizontal components, the cross-borehole measurements are projected into the horizontal plane. Projection of the cross-borehole plane into the horizontal plane becomes inaccurate above 70° inclination.
As a consequence the presently available tools can not survey accurately in the gyrocompassing mode for hole inclinations above 70°.
Another disadvantage of the gyrocompassing mode is that the tools must be held stationary for some time at a survey station, lengthening the total survey time required.
Continuous mode
To reduce survey time some companies have implemented a continuous mode. In this mode the azimuthal change of the directional drilling survey tool is determined and its integral results in the actual azimuth. The measured rotation is corrected for the Earth's rate of rotation. At the start of a continuous survey an gyrocompassing initialisation is performed to allow for a tie-in. A tie-in can also be performed using a previous survey.
Typical running procedures
The following running procedures are to be carried out by the Survey Engineer. Additional procedures for initialisation and survey frequency are described for each tool separately.
1.Make-up the tool on the catwalk or another appropriate area. Select the proper running gear (e.g. for drillpipe, with heat shield etc.), warm up the gyro and perform a wellsite calibration or calibration check. Check QC parameters such as mass unbalances and scale factors with the pre-job office values.
2.Zero the wireline counter with the directional drilling survey tool sensors at the rotary table elevation.
3.For gyrocompassing tools: start surveying in pre-selected depth increments. Stop at the pre-selected depth and take a survey. Take an inrun and an outrun (except for Well Bore Surveyor where a single inrun or outrun is sufficient). Pull out of hole. Note the wireline depth counter.
4.For continuous tools: take a gyrocompassing alignment initialisation; if offshore this initialisation should be done below seabed to eliminate vibrations. Select the required depth interval and start surveying at a specified running speed. Take an inrun and outrun. Take drift checks during the survey accordingly. Pull out of hole. Note the wireline counter. Perform a gyrocompassing alignment and a drift check.
The Wellsite Drilling Engineer should give the Survey Engineer approximately 3 hours notice before running in the hole in particular to allow the directional drilling survey tool to be warmed up. The running procedures given above are to be used as a guideline.
Tie-in surveys
Where North Seeking gyro directional drilling survey tools are not used from surface, the co-ordinates of the last survey should be used as the tie-in point. An overlay of at least 300 m (1000 ft) with the previous survey should be made as an additional check. For the final survey the tie-in point co-ordinates of the last survey should be used. For tools operating in the continuous mode where incremental azimuth is determined (Finder, GCT and G2) the azimuth of the tie-in point can also be used for referencing. This is only recommended if the azimuth accuracy is better than the North seek initialisation performance of the tool. Where the previous survey is taken with the same tool, no North seek initialisation is required as this will not improve the survey accuracy.
The azimuth of the tie-in point shall not be used to replace a North seek initialisation if the azimuth accuracy of the tie-in point is less than the North seek initialisation performance.
Quality assurance and quality control
Quality assurance of North Seeking gyro directional drilling survey tools comprises a pre-job, post-job office and wellsite or downhole calibration check. The responsibility of the wellsite and office Drilling Engineer is to perform the quality control, i.e. to check that the calibration parameters specified on the quality sheet are within the acceptable values. The responsibility of the Survey Engineer is to check its equipment at the wellsite, to perform a survey according to the running procedures, to apply quality checks and to provide the final survey data.
The quality assurance and quality control parameters for every North Seeking gyro tool are different. The corresponding QC parameters are specified separately.
It is the responsibility of the Wellsite/Office Drilling Engineer to check the actual quality control parameter values against the acceptable limits specified. The actual surveys and limits should be specified by the Survey Engineer on the survey quality sheet as a vital element of the survey. Surveys should not be accepted without the corresponding quality sheet.
Uncertainties of north seeking gyro directional drilling survey tools
Uncertainties of North Seeking gyro directional drilling survey tools comprise:
- Ttool uncertainties and
- System uncertainties.
In the gyrocompassing mode tool, uncertainties are gyro mass unbalances, accelerometer and gyro scale factor. These uncertainties have to be checked by the Drilling Engineer against the acceptable values. The contribution of the various directional drilling survey tool uncertainty terms depends on the implementation of the gyro, i.e. number and orientation of sensing axis. In the continuous mode tool, uncertainties are gyro mass unbalances and gyro bias both resulting in a gyro drift. Regular gyro drift checks are performed to monitor drift.
System uncertainties are toolface dependent tool misalignment and depth uncertainties.