Magnetic Guidance Tool

The Magnetic Guidance Tool (MGT) is a downhole electromagnetic source for deployment in a reference well to enable drilling a second well at a controlled separation. WellServ Australia currently run the ‘Vector Magnetics’ Magnetic Guidance Tool (MGT) for all of its well intersection work.

The casing or drillpipe of the target well concentrates the current and generates an electromagnetic field. Sensors in the tool detect the magnitude, direction, and radial gradients of the electromagnetic and magnetostatic fields. A microcomputer on the surface collects these measurements and uses them to compute the distance and direction from hole high side and magnetic North to target. MGT service can be run in open hole, through drillpipe, or inside non-magnetic assemblies to measure distance and direction to a target casing or fish.



Wellserv currently has three MGT’s. Two of these are Type III and the third is a shorter prototype designed by Vector for the CSM industry. It has a shorter overall length to make it more user friendly for the operator, while still retaining the same accuracies that the Type III displays.

MGT COMPONENT SPECIFICATIONS

The specifications for the four types of MGTs currently available are shown in the table below. The basic difference between the four types is the length. Longer tools produce stronger magnetic fields and can be used out to a greater range. For close ranges, smaller tools must be used to avoid saturating the magnetic field sensors in the Steering Tool. Smaller tools are easier to transport and handle.

Tool Type Length Diameter Weight Min. Range a Max. Range b Max. Pressure Max. Pressure
I 1.5 m (5 ft) 3.8 cm (1.5 in) 7 kg (15 lbs.) 0.8 m (2.5 ft) 3 m (10 ft) 2800 kPa (400 psi) 50 C (120 F)
II 2.5m (8 ft) 5.1 cm (2 in) 25 kg (55 lbs.) 2m (6.5 ft) 9m (30 ft) 69,000 kPa (10,000 psi) 175 C (350 F)
III 4.9 m (16 ft) 5.1 cm (2 in) 48 kg (105 lbs.) 2.5m (8 ft) 12m (40 ft) 69,000 kPa (10,000 psi) 175 C (350 F)
IV 6.4 m (21 ft) 5.1 cm (2 in) 68 kg (150 lbs.) 3m (10 ft) 15m (50 ft) 69,000 kPa (10,000 psi) 175 C (350 F)



Notes:

For all tools the cable connection is a standard 1 7/16 inch single conductor GO-type monocable female cable head.


Tool Accuracy

The typical accuracy of MGT measured distances is given below. In all cases, it is assumed that the steering tool (MWD) and MGT inclinations and azimuths are acurately known and that the correct MGT calibration for MGT Pole Strength (Q) and Pole Separation (L) is used. The accuracy refers to either the High Side distance or the Right Side distance measurement. The accuracy figures given are for a steering tool with 10nT/ADC bit resolution for the magnetic sensors. Accuracy is better at longer ranges than the numbers given here when using steering tools with higher resolution.

MGT Type Typical Distance Accuracy (m)
Type I at 1m Range ±0.03 m
Type I at 3m Range ±0.05 m
Type II at 5m Range ±0.05 m
Type II at 9m Range ±0.2 m
Type III at 7m Range ±0.05 m
Type III at 12m Range ±0.4 m
Type IV at 7m Range ±0.05 m
Type IV at 15m Range ±0.5 m



Other applications of MGT’s tools:


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