Effect of Drilling Fluid Content on Directional Drilling: Shielding of Directional Magnetic Sensor in MWD Tools
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The topic of this report is "Effect of Drilling Fluid Content on Directional Drilling - Shielding of Directional Magnetic Sensor in MWD Tools" and it has been prepared as a part of the course TPG4910 Petroleum Engineering - Drilling Engineering, Master Thesis at the Norwegian University of Science and Technology (NTNU). The magnetic property of the drilling fluid is one of the substantial error sources for the determination of azimuth while drilling deviated wells. These errors may be in the range of 1-2  causing errors of 1-200 meter when drilling long, deviated intermediate sections. Therefore, these effects represent a significant cost to be mitigated. The error becomes even more pronounced if drilling occurs in arctic regions close to the magnetic North pole. Added clays, weight materials and tubular wear are anticipated to distort the geomagnetic field at the location of the magnetometers. The effect on the magnetometer readings is obviously linked to the amount of magnetic material in the drilling fluid. The problem has been studied both by laboratory experiments and analyses of down hole survey data. However, there are several inconsistencies in the results, and the phenomenon is not fully understood. In the present report it is shown how the magnetic distortion relates to some drilling fluid additives. A series of experiments have been conducted to increase the understanding of the effects. First a series of fresh water based bentonite drilling fluids were investigated to see the effect of bentonite on magnetic shielding. Thereafter, a series of fresh oil based drilling uidswere evaluated to observe the magnetic shielding effect of organophilic clays.Eroded steel (swarf) collected from the ditch-magnet of an offshore drilling location was added to the oil based drilling fluid to investigate how swarf and steel fines content of drilling fluids affects magnetic shielding. Finally, used oil and water based drilling fluids were investigated. The measurements done on the water based bentonite drilling fluid showed a shielding effect for the whole series of fluids. The highest shielding effect was 2.2%. This is high enough to cause significant errors on the magnetic survey azimuth calculations . Ferreous components in the bentonite clay are the main suspects in causing shielding of the earth's magnetic field in the fresh water based drilling fluid. The measurements done with the oil based drilling fluid showed little or no shielding effect for the series of fluids tested. The shielding effect peaked at 0.22%, considerably less than for the water based bentonite drilling u-ids. The synthetically made organophilic hectorite clay in the oil based fluidis believed to contain little or no ferreous components capable of magnetic shielding. However, when swarf from an offshore drilling location was added to the oil based drilling fluid the shielding effect was 25 %. This is a consider-able shielding effect of the earth's magnetic field, likely to cause substantial errors on the directional magnetic sensor in MWD tools. For the used oil based drilling fluid, the measurements showed a small, close to negligible, shielding effect on the earth's magnetic field. The shielding was 0.26%, a result similar to the fresh oil based drilling fluid. None of the used water based drilling fluids showed any signs of affecting the earth's magnetic field.