TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIn field operations from a semi-submersible rig offshore northern Norway, ilmenite has been used as weight material in the drilling fluid. From environmental considerations, it has been desirable to exchange barite with ilmenite to minimise the discharges of heavy metals while drilling with water based drilling fluids. Field trials during the eighties and early nineties have shown that use of ilmenite as weight material can cause severe erosion problems. By adjusting the particle size distribution of the ilmenite to a narrow cut with the particle mean size around 10 micron, the erosion was brought down to a level generally underneath the erosion level experienced when drilling with barite.The paper describes in detail the effect of ilmenite and barite onto erosion of surface lines and manifolds connected to the bulk system, mud mix-, transfer-and supercharge pumps, and mud pump. The erosion of these pipes was reduced when using ilmenite in water based drilling fluids. In oil based drilling fluid there were not observed any significant erosion when using either weight materials. The change in wear on the mud pump pistons and liners will be outlined. Furthermore, the impact the different weight materials have on the erosion of Measurement While Drilling equipment (MWD) will also be outlined. It is also shown that a reduction in rig air pressure for transporting ilmenite in the bulk handling system has reduced the erosion in the exposed bulk pipelines. Finally, the consequences for running the drilling fluid operation are presented.

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.

Barite has to date been the most common weighting material for drillingfluids. Several heavy metal components are associated with the barite. During discharge of drilled cuttings from the use of water based drilling fluid some parts of these heavy metals can be dissolved into the sea. If oil based drilling fluid is used no discharge of drilled cuttings is allowed into the seaon the Norwegian Continental shelf. This cuttings can be re-injected into the formation or can be brought onshore for treatment. If barite is heated to more than 850°C during this treatment, the toxic component barium oxide may be released. The negative impacts from the use of barite in drilling fluids can be avoided if an alternative weight material is used. On the Norne field in Norway a field trial was performed using ilmenite as the weighting material. The paper describes the improved heavy metal content of the ilmenite compared to the barite. Furthermore, it is shown that the heavymetals present are not as biologically available in ilmenite as in barite. The paper describe how the ilmenite improved the ability for the fluid to be recycled and reused. Ilmenite is more resistant to grinding during the drillingprocess, resulting in more stable particle size distribution. This implies a reduced need for dilution of the drilling fluid. This leads to reduced discharges to the sea if water based drilling fluid is used, and less volume tohandle and treat in the case of oil based drilling. The dark color of the ilmenite made it easier to detect and repair any leaks in the transport and bulk handling systems offshore. This will lead to reduced dust problems and an improved occupational hygiene. No significant dust problems were observed during the test at Norne. Introduction Current drilling practice is to use water based drilling fluids whenever feasible, since permanent depositions on the sea bed of cuttings produced when drilled with these water based drilling fluids are allowed. Still, there is a major emphasis onto minimizing the negative impacts from drilling operations on the environment. This emphasis covers the change to less and less harmful chemicals in the drilling operations. Even some of the minerals added to the drilling fluid may contribute to the environmental impact. The leaching of heavy metals from weighting agents to the sea may give a negative impact. The heavy metal content of ilmenite is lower on most components than that of barite. In addition is the bio-availability of the heavy metals lower. Therefore, it is believed to be more desirable to use materials like ilmenitethan barite as weighting agents in drilling fluids. History Ilmenite has been applied earlier in drilling operations in the North Sea. Two tests have been conducted. In the first test two wells were drilled in 1979and 1980 with ilmenite weighted drilling fluids. The ilmenite was a fairly coarse ground material compared to the presently used ilmenite. According to Blomberg et al (1), the drilling fluid properties were easier to control compared to drilling with barite. This is because ilmenite has a lower tendency of being ground down to finer materials. Consequently there is a lesser need offluid dilution. During the drilling operations excessive wear on different types of equipment was observed. This abrasion was anticipated to be a result of usage of a rather coarse ilmenite material. Although the abrasion was anticipated to be merely a result of the particle size distribution than the material itself, no further field evaluations were conducted until the earlynineties.

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 1-200m if 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 the 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 downhole survey data. However, there are several inconsistencies in the results, and the phenomenon is not fully understood.In the following 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 experiments showed that presence of free iron ions do not contribute to magnetic distortion. Next, a series of experiments with bentonite-based fluids were conducted, showing the effect of bentonite on magnetic shielding. Measurements on a series of clean, laboratory made oil-based drilling fluids showed that the magnetic shielding did not increase by the addition of organophilic hectorite clays. Finally, eroded steel from an offshore drilling location was added into the oil-based drilling fluid. These swarf and steel fines significantly increased the magnetic shielding of the drilling fluid. This paper also outlines how much the drilling direction may be distorted by the presence of these additives and contaminants.

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