Conventional magnetic survey tools consist of a mechanical inclinometer, i.e. a pendulum and a compass to determine azimuth. They are used as magnetic single-shot (MSS) and magnetic multi-shot (MMS).
Conventional magnetic survey tools are cheap to run but less accurate than magnetic solid state tools. They should be run in a full length of NMDC. Surveys should not be used as definitive surveys, except for isolated and vertical wells (incl<5°).
Principle of conventional magnetic survey tools
Conventional magnetic survey tools measure inclination, azimuth and toolface. They consist of a pendulum and a compass. Inclination is measured with a pendulum . As the pendulum is aligning itself along the gravity vector, it will always sit on the low-side. This results in the projected high-side following azimuthal tool rotations. On the other hand, the compass card which is gimballed will allow its 0° line to stay aligned to magnetic North irrespective of azimuthal tool rotations. This results in azimuth being the angle between the compass card and projected high-side. In the same way, toolface is determined. A pointer fixed to the tool housing will sense tool rotation while high-side is invariant to tool rotations. The angle between the toolface pointer and high-side is toolface.
Single-shot tools
Film-type magnetic single-shot (MSS) tools are manufactured by different companies. The main components are:
·an activation device (timer, motion sensor or monel sensor) which activates the single-shot;
·an angle unit (magnetic compass and inclinometer) which measures azimuth and inclination;
·a camera section (camera, photographic disc, lens and bulbs), pre-focused and loaded with film;
·a battery pack to provide power for the timer and the camera.
All angle units are oil filled to dampen shock loads. However, the instruments must be handled carefully as extreme shock loads will damage the internals and so cause measurement uncertainties of inclination and azimuth. Note that the pendulum acts opposite to the actual tool direction, hence a mirror image photographic disc is used.
Baker Hughes Inteq low angle units
The angle unit consists of a compass card mounted on a pivot and balanced so that the card is horizontal. The actual compass comprises one or two needle magnets mounted under the card. The inclination unit consists of a pendulum, with cross hairs at the end, suspended above the compass card. Concentric circles on the compass card provide the inclination scale. Inclination and azimuth uncertainties will occur when the point of suspension of the pendulum is not on the tool axis passing through the centre of the compass card.
Baker Hughes Inteq high angle units
The compass card in the high angle (drift arc) units is mounted on gimbals which maintains it in the horizontal plane. The inclination scale is a circular arc mounted on the inner gimbal. The inner gimbal is counterbalanced and suspended so that the base remains horizontal. The outer gimbal is aligned perpendicular to the hole direction by the low-side weight. The inclination is shown by a cross hair mounted on the outer gimbal; the cross hair lies over the graduated inclination scale. These units are the only ones that can be used in the absence of NMDCs in the drillstring when carrying out inclination-only surveys. This is because in the Baker Hughes Inteq high angle units, the movement of the inclination scale is independent of the compass card. All other units exhibit inclination uncertainties when magnetic fields are present which tilt the compass card.
Sperry-Sun low and high angle units
The Sperry Sun low angle units contain an inverted floating pendulum with the compass card on top of it. Magnetic needles are mounted under the compass card. Cross hairs on the axis of the tool indicate the value of the inclination. In the Sperry-Sun 0-90° unit, the floating pendulum is replaced by a floating hemispherical ball. This device requires a longer settling time.
Quality assurance
An uncertainty will occur when the needle magnets are not exactly aligned with the North as shown by the compass card. Regular calibration and correct handling of the compass can maintain the corresponding uncertainty to within ±0.5°.
Each kit box should contain two of each compass unit. The units in each pair should be used alternately; this gives an additional check on uncertainties. The duration of 50 surveys per set ensures that all compass units are checked prior to any one unit exceeding 25 surveys. All single-shot kit boxes should therefore be returned to the supplier for calibration checks after 50 surveys have been made with the kit, or the well is completed, or when the field test stand calibration is outside specifications, whichever occurs first.
Reading magnetic single-shot discs
·Pendulum type (low angle) units:
For low angle units, the hairline on the magnifying reader is positioned over the centre of the plumb bob cross hairs and on the zero inclination point (i.e. the centre dot of the photographic disc). The inclination is read directly from the position of the centre of the cross hairs on the calibrated photographic disc (i.e. by counting the number of concentric circles from the centre). The magnetic azimuth is read directly from the value shown by the hairline on the circumferential azimuth scale. The high-side, as shown, is defined as the upper direction of a line perpendicular to the borehole axis and which lies in a vertical plane. The toolface pointer is aligned with a T slot on the angle unit and the instrument barrel. When used in conjunction with a mule shoe, this slot is aligned with the mule shoe direction.
·High angle (drift arc) units:
For high angle units a magnifying reader is used to enlarge the photograph. The magnetic azimuth is read directly from the value shown by the vertical instrument hairline on the circumferential azimuth scale. The inclination is read directly from the value shown by the horizontal instrument hairline on the inclination scale.
·Floating-type (low and high angle) units
To read the disc, the hairline of the magnifying reader is positioned from the centre of the inclination scale through the intersection of the cross hairs. Note that in high angle holes, the direction to the centre of the inclination scale has to be estimated. The azimuth is read off on the outer scale, the inclination from the concentric circles. The toolface pointer is the line running from the cross hairs to the circumference of the disc. The toolface angle cannot be read directly from the disc. It has to be read from the special scale on the Sperry-Sun single-shot magnifying reader.
·Unreadable discs:
In the event that the single-shot disc cannot be read, the cause should be determined. Common faults are:
-film is transparent: a picture was not taken. Check for camera, timer, battery and bulk faults;
-film disc is black: film is overexposed or burnt. Check for timer failures and excessive temperatures;
-film is blurred: tool was not landed, or the instrument had landed but the drillstring was moving. Check timer and surface watch.
Multi-shot tools
The film-type magnetic multi-shot (MMS) tools differ from single-shot instruments in that a cine camera and associated activation device is used instead of a single-shot camera. A time control device enables the instrument to record photographs of azimuth and inclination at pre-set times.
Quality assurance
MMS tools should be properly calibrated and checked by the survey company. No additional quality checks can be performed at the wellsite other than those discussed in the pre-survey checklist.
Reading magnetic multi-shot films
Magnetic multi-shot pictures are read in the same way as single-shot discs. Before the final survey results are issued all pictures must be read independently by a second Survey Engineer.
Uncertainties of conventional magnetic tools
Any inclination and azimuth value read from a disc will include an uncertainty which is a compilation of tool, reading and magnetic interference uncertainties. In addition, uncertainties will be present due to running of the tool, i.e. BHA deflection, declination uncertainties etc.
Inclination uncertainties
Inclination uncertainties are due to tool uncertainties and reading uncertainties. For various inclination ranges the tool inclination uncertainties are typically ± 0.25° and reading uncertainties are typically between ± 0.25° and ± 0.5°.
Azimuth uncertainties
Azimuth uncertainties are due to tool uncertainties, reading uncertainties, magnetic interference uncertainties.
The instrument and reading uncertainties across the inclination range are typically as follows:
·instrument uncertainties ± 0.50°;
·reading uncertainties ± 0.50°;
·magnetic interference uncertainties of 0.25° if NMDC selection charts are used with clean NMDCs, the correct sensor spacing is applied and an acceptable azimuth uncertainty of 0.25° is used, up to 10° in extreme cases.