Multistation Analysis and Geomagnetic Referencing Significantly Improve Magnetic Survey Results

Abstract

Improvements in the quality of wellbore directional surveys using multistation analysis and geomagnetic referencing are demonstrated, with case studies to illustrate the benefits to drilling projects. Multistation analysis is a technique that provides compensation for drillstring magnetic interference. The benefits of multistation analysis can be further improved when used in conjunction with geomagnetic referencing, which takes account of localized crustal effects in the earth's magnetic field. In recent years these improved magnetic surveying techniques have gained increased industry acceptance. Their practical application is delivering the capability to reach tighter drilling targets, and in some circumstances to dispense with the need to run gyro surveys. These techniques are providing tangible and cost saving benefits to help meet the demanding technical and economic needs of these projects. A series of well profiles for wells drilled on projects that use these techniques are presented, each of which use individually mapped local crustal magnetic field models and different drilling assembly configurations. Treatment of each of the major magnetic surveying error sources is discussed, and the effect on surveyed well position when compared with independent overlapping gyro surveys is presented. The benefits from this real-time drilling surveying process also demonstrate the value of the "drilling correction" as a means of drilling more accurately to plan, with a reduced need for correction runs, or post-drilling changes to the planned well trajectory. Introduction Modern directional drilling operations depend on magnetic surveying instruments to provide confirmation that the positional objectives of the well, and hence the drilling target, have been met. These Measurement While Drilling (MWD) instruments are deployed in the bottom hole [drilling] assembly (BHA) in order to provide real-time surveyed position feedback to the driller, and minimize time taken for surveying overall. Magnetic survey tools are robust, relatively easy to calibrate, maintain and run, and their performance is well known and modeled to a common standard in the industry1. Today, the difficulty in making accurate magnetic survey measurements is largely driven by uncertainty caused by two major error sources, declination reference error and interference caused by drillstring magnetism. A growing body of work in the area of geomagnetic referencing2,3, has provided a direct improvement to the reference error problem. Concurrent advances in the treatment of drillstring magnetic interference compensation, particularly with the use of multistation analysis4 have also brought further improvements to magnetic survey accuracy. Experience has now shown that when geomagnetic referencing and multi-station analysis are combined in a robust manner, well positioning accuracy approaching that previously only provided by gyroscopes, can be delivered. A series of case studies based on survey management services provided to BG Group, ChevronTexaco and TotalFinaElf Exploration UK from several geomagnetically disparate North Sea fields are presented to demonstrate the real and practical cost savings achieved using these techniques.

Improving models of the Earth’s magnetic field for directional drilling applications

“…Lowdon and Chia, 2003;Chia and Lima, 2004), ultimately MWD relies upon 47 measuring the direction of the well-bore relative to the direction of the local geomagnetic field 48 (Russell et al, 1995). Hence accuracy is inherently limited by knowledge of the direction of the 49 Earth's magnetic field at the MWD survey position.…”

Space weather effects on drilling accuracy in the North Sea

“…MS analysis provides a unique magnetic signature for each MWD tool and its drilling assembly. Provided the wellbore geometry and environmental conditions remain constant the correction derived for each assembly can be applied to all the data from the same survey run (Lowdon and Chia, 2003). The form of the magnetic signature makes it possible to determine if the errors are likely to be sensor-related or the result of external environmental factors, which may be an early indication of the onset of a drilling-related issue.…”