Overview:
- Don’t drill near live wells
- Casing should be degaussed before running to minimize interference
with nearby wells. This can be done at the factory or onsite. A low frequency degaussing method is recommended
- Drillstring interference (in the Bz direction) must be subtracted. This
should be a constant value. This can be minimized by proper degaussing of motors and subs after magnetic inspection
- Interference from rig and/or conductor can be expected within 2-300
feet from end of conductor. This is usually before KOP
- Don’t use single station Z-axis correction methods when there is external interference
- Even if gyro surveys are used, monitor magnetics for unexpected close approaches. Measured interference > 2000 nT is a warning sign
MWD Surveying in Top-Hole Section
• Common practice is to use gyro surveys (gyroMWD or single shot wireline) when close to near by steel casing
• Distance varies with operator and/or contractor. Typically gyros are specified to 20-50 feet, or when MWD fails QC limits
• Magnetic interference from nearby wells changes the apparent azimuth measured by magnetic tools
• The interference cannot be measured with a single survey shot but it can be estimated using several survey shots at different depths
• The azimuth error can be estimated
• The effect is to make the Ellipsey of Uncertainty (EoU) larger
• Magnetic Interference does not effect the measured inclination
• In near-vertical wells azimuth errors have small effect on position
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View the entire Presentation:
Pad Drilling Using Magnetic MWD
Neil Bergstrom, P.E.IS – Devon Energy