fax 01-972-952-9435. AbstractThe drilling fluid may degrade the accuracy of magnetic azimuth and borehole position significantly so that it complicates the drilling operation and reduces the possibility to hit the planned targets. As an example: Azimuth is distorted up to 5° because of the magnetic properties (susceptibility) of the drilling fluid; and the wellbore is displaced 50 meter in one of the analyzed survey sections.Survey data from 30 well sections in the North Sea and the Norwegian Ocean are analyzed in order to see whether the weight material in the drilling fluid may affect the magnetic characteristics for the drilling fluid. The cross-axial magnetic field intensity is attenuated more when applying ilmenite instead of barite as weight material. Another significant finding is that the attenuation is stronger in 12 ¼" sections than in 17 ½" and 8 ½" sections.Laboratory experiments also indicate that drilling fluids with barite are less susceptible than those with ilmenite, and that fresh fluids are more favourable than used fluids. Attenuation factors for cross-axial magnetic components, which are derived from laboratory measurements and a simplified model of the downhole conditions, become much lower than those derived directly from the survey data. This contradiction is explained by some physical and operational conditions.Guidelines have been established for how to control, reduce and eliminate the negative effect on the wellbore positions. The precautions and actions, which are presented, are dependent on the composition of the drilling fluid, the drilling operation, the wellbore directions, the magnetic interference sources, the accuracy of the geomagnetic reference field and the requirements for wellbore position accuracy and reliability.The ultimate method for getting around this problem is to measure azimuth with a gyroscopic tool. However, multistation analysis and corrections of magnetic survey data is also a satisfactory solution when certain operational and geometrical requirements are met.
The years ahead will see increased petroleum-related activity in the Barents Sea, with operations far off the coast of Norway.The region is at high geomagnetic latitude in the auroral zone, and therefore, directional drilling by use of magnetic reference will experience enlarged azimuth uncertainty compared with operations in the Norwegian and North Seas. Two main contributors to azimuth uncertainty are magnetic disturbances from electric currents in the ionosphere and axial magnetic interference from the drillstring. The former is more frequent in the Barents Sea than farther south, and the effect of the latter is increased because of diminished value of the magnetic horizontal component. Wellbore directional surveying for operations on the continental shelf in the North Sea and the Norwegian Sea rely on wellestablished procedures for near-real-time magnetic monitoring by use of onshore magnetic-reference stations. The different land and sea configuration, distant offshore oil and gas fields, higher geomagnetic latitude, and different behavior of the magnetic field require the procedures to be reassessed before being applied to the Barents Sea. To reduce drilling delays, procedures must be implemented to enable efficient management of magnetic disturbances. In some areas of the Barents Sea, the management requires new equipment to be developed and tested before drilling, such as seabed magnetometer stations. One simple way to reduce drillstring interference is increasing the amount of nonmagnetic steel in the bottomhole assembly (BHA). To maintain azimuth uncertainty at an acceptable level in northern areas, it is crucial that wellbore-directional-surveying requirements are given high priority and considered early during well planning. During the development phase of an oil and gas field, the planned wells must be assigned adequate positional-uncertainty models and, if possible, be designed in a direction that minimizes the wellbore directional uncertainty.
Time-dependent current fluctuations in the Earth's ionosphere cause inaccuracies in wellbore directional surveying. These inaccuracies increase at higher latitudes, and although monitoring and correction are possible, they become less valid as the distance between the monitoring site and the rigsite increases, which is a particular problem for offshore drillsites. The characteristics of the ionosphere currents indicate that the most favorable location for monitoring stations is on the same geomagnetic latitude as the drillsite. Such an arrangement has been used to monitor and correct directional surveys at the Haltenbanken area of the Norwegian Sea over a period of approximately 2 years. Haltenbanken is approximately 200 km west of the Norwegian coast at latitude 65 N, where magnetic-storm activity can have a significant effect on directional surveying. A monitoring station was set up on the coast at the same geomagnetic latitude as Haltenbanken. To test the idea that magnetic disturbances are similar along constant magnetic latitude, an additional monitoring station was established 200 km east of the main station. The data broadly confirmed the hypothesis, although isolated events were observed when this was not the case. The challenges of surveying at offshore sites north of 62 N latitude are probably greater than the oil and gas industry is accustomed to-but such challenges will become more significant if the Arctic Ocean is opened to drilling operations. The technique described in this paper may contribute to safer and more-productive offshore operations at high latitudes.
Time-dependent current fluctuations in the Earth's ionosphere cause inaccuracies in wellbore directional surveying. These inaccuracies increase at higher latitudes and although monitoring and correction is possible, it becomes less valid as the distance between the monitoring site and the rigsite increases, a particular problem for offshore drill sites. The characteristics of the ionosphere currents indicate that the most favourable location for monitoring stations is on the same geomagnetic latitude as the drill site. Such an arrangement has been used to monitor and correct directional survey at the Haltenbanken area of the Norwegian Sea over a period of approximately two years. Haltenbanken is located about 200km west of the Norwegian coast at latitude 65° north, where magnetic storm activity can have a significant effect on directional surveying. A monitoring station was set up on the coast at the same geomagnetic latitude as Haltenbanken. To test the idea that magnetic disturbances are similar along constant magnetic latitude, an additional monitoring station was established 200km east of the main station. The data broadly confirmed the hypothesis, although isolated events were observed when this is not the case. The challenges of surveying at offshore sites north of 62° N latitude are probably greater than the oil and gas industry is accustomed to; challenges that will become more significant if the Arctic Ocean is opened to drilling operations. The technique described in this paper may contribute to safer and more productive offshore operations at high latitudes.
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