The tools measure Earth's gravity and magnetic field with solid state accelerometers and magnetometers (raw sensor data), respectively. Survey tools can be used as single and multi-shot tools dropped and retrieved on wireline, wireline steering tools and Measurement While Drilling (MWD) tools.
Solid state magnetic single and multi-shot directional survey tools are more accurate than conventional magnetic survey tools, and can be used to confirm MWD surveys and for definitive surveys.
The directional survey tools can be used in a full NMDC length or in combination with "correction technique" correcting raw sensor data for magnetic interference.
The survey data are stored in a solid state electronic memory. The downhole probe is battery-powered and self-contained. A surface computer is used to initially set up and process the data after the survey.
Principle of solid state magnetic survey tools
Solid state magnetic survey tools do not measure azimuth using a gimballed compass card (conventional directional survey tools) but derive azimuth from the horizontal components of the Earth's magnetic field measured with a set of magnetometers. This eliminates a gimballed system and allows magnetic interference corrections to be applied on the raw data measured. In addition inclination and toolface are derived from accelerometer readings. If required magnetic toolface can be obtained from the horizontal magnetic field components.
Solid state magnetic survey tool specifications
Sperry-Sun ESS and ESI
The main difference between the ESI and ESS is that the survey data within the ESI is stored in RAM and is maintained by a separate battery within the probe. Survey data in the ESS is stored in EEPROM (Electronically Erasable and Programmable Read Only Memory) and hence is not subject to loss due to battery failure. The ESS is considered more reliable and therefore is the preferred directional survey tool.
Sharewell CHAMP
A number of survey companies offer the Sharewell CHAMP tool as part of their survey services. They have purchased the basic probes and developed their own running equipment and surface computers. Although the basic probe is the same, the different running gear from the different survey companies results in slightly different specifications:
- Baker Hughes Inteq (BHI) - EMS (Electronic Magnetic Surveyor);
- Gyro/Data - GEMS (Gyro/Data Electronic Magnetic Surveyor);
- Scientific Drilling International (SDI) - EMM (Electronic Magnetic Multi-shot).
- Scientific drilling international super EYE. The Super EYE Electronic Multi-shot tool probe has a 1" diameter which allows it to be run in a 1.375" pressure barrel and makes it suitable for slim hole drilling and surveying operations.
Quality assurance
To assure the quality of solid state magnetic survey tools for all modes of operation (single and multi-shot tool, wireline steering tool, MWD tool) the following aspect should be given due consideration:
·calibration;
·magnetic interference correction or reduction;
·running procedures.
Calibration
Solid state magnetic instruments are calibrated by the survey company. The check calibration consists of measuring the Earth's magnetic field strength, dip angle, and gravity field strength and comparing the results against known local values. Maximum allowable differences are:
1.magnetic field strength ± 0.175 mT;
2.dip angle ± 0.3°;
3.gravity field strength ± 0.003 g.
If local values are unknown then the maximum allowable spread between measurements is twice the maximum allowable difference.
Magnetic interference
Solid state magnetic single and multi-shot directional survey tools can either be run in a full length of NMDC or in a reduced length of NMDC in combination with a magnetic correction technique. However, for highly inclined (>70°) East/West ± 20° (hence the intervals 70-110° and 250-290°) wells a full length of NMDC should be used.
Pre-survey checklist
The pre-survey checklist for solid state magnetic survey tools is similar to the checks required for conventional magnetic single and multi-shot tools.
In addition the Survey Focal Point should supply magnetic field data (magnetic field strength, dip angle and declination) as input for "correction technique".
Running procedures
The multi-shot directional survey tools are normally run by a Survey Engineer. It is recommended to run solid state magnetic multi-shot tools in tandem.
running procedure
Check shot
A check shot for comparison with MWD data can most economically be taken at the end of a bit run while pulling out of the hole. A check shot should not be confused with a rotational shot. Azimuth difference should be less than 1° for inclinations greater than 10°. The inclination difference should be less than 0.25°.
Rotational shot
A rotational shot should be made when either reduced or full length NMDCs are used. A rotational shot allows correction for cross-axial magnetic interference and toolface dependent misalignment of accelerometers and magnetometers.
Quality control
1.Check that the selected shots are assigned the correct depth from the time/depth record and measurements of the probe position
2.If tandem probes have been run, compare the inclinations and azimuths between the stacks. The azimuth differences should be less than 1° for inclinations greater than 10°. If no severe BHA deflection is present the inclination differences should be less than 0.25°. Where differences exceed these tolerances one of the data sets should be rejected.
3.Select the definite survey from either the top or bottom probe.
Uncertainties of solid state directional survey tools
The uncertainties associated with solid state tools are divided into tool uncertainties, running procedure errors and geomagnetic data uncertainties.
Sensor uncertainties are due to accelerometer and magnetometer imperfections: scale factor, bias and misalignment. Excessive sensor uncertainties are eliminated by proper calibration and calibration checks.
Running procedure uncertainties are mainly BHA deflection uncertainties. They may cause significant true vertical depth uncertainties specially in highly inclined wells. BHA deflection uncertainties may be corrected for.
Geomagnetic data uncertainties are input data uncertainties for local magnetic field strength, dip angle and declination. The declination is used in all calculations and so the consequent uncertainty is always present. Local magnetic field strength uncertainty and dip angle uncertainty only cause an azimuth uncertainty if raw data is processed by an azimuth correction technique.