This section discusses the primary azimuth reference systems currently used in directional drilling. This will include True North and Magnetic references with particular detail given to Grid Coordinate systems (i.e. UTM, Lambert, Geographic, and Local). A simple field-proven method is also presented to help avoid confusion when converting from one system to another. More than one multi-million dollar directional drilling project has missed its intended target(s) due to errors and/or misunderstandings surrounding the azimuth reference system in use.
The confusion arises primarily from the necessity to change from one system to another between the well planning phase, where most maps are drawn with respect to a local Grid North, and the drilling phase where surveying is performed with respect to a Magnetic or True North reference. The field company representative is faced with a confusing array of possible conversions, including declination corrections from Magnetic North to True North, True North to Grid North, Magnetic to Grid North, or Grid to Magnetic North. Is the correction to be added or subtracted from the survey measurement? Is the convergence magnitude and sign correct for the grid system used?
With all these questions, it is easy to see why this seemingly simple task is often performed improperly and the mistake not realized until the target is missed. The rig foreman often passes on the responsibility for field convergence application to the service company supplying the surveys or to the directional driller. While this practice may appear sound in theory, it usually creates additional confusion as basic information is often poorly communicated or misconstrued. It is not uncommon that on projects where several service companies perform different surveys (i.e. MWD, single shots, multi-shots, and gyros) that each supplier comes up with a different convergence value.
A case in point involved a recent high visibility multimillion dollar directional drilling project. In this incident, a well known well planning company drew the well maps with respect to the local grid coordinate system, with a footnote stipulating that the directional contractor would be responsible for grid and magnetic declination convergence. When the operation began, the rig site was manned by a company representative, two consulting drilling engineers, and a directional driller all responsible for deviation control.
The directional company was not accustomed to deriving grid corrections and solicited help from the
company representative. He assumed the local grid was UTM (later learned to be state plane) and the appropriate UTM convergence was applied. He then had the directional company's office redraw the well maps rotated by that UTM correction. The office complied and added in the magnetic declination as well. The directional driller missed this fact, however, and continued to apply a declination correction at the rig site as drilling continued. It was not until the project was completed and the target missed that the errors were realized.
This project was more closely supervised than a normal directional well, yet it serves as a classic example of how easily the relative relationships between coordinate systems can be poorly communicated and inappropriately applied. The remainder of this paper will examine methods to reduce these azimuth convergence errors by utilizing field experience and suggested communication procedures between all involved parties.
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