MWD makes use of the Earth’s magnetic field as a reference in its azimuth calculations. The sensor errors present in modern MWD instruments are small in comparison to typical variations in the Earth’s magnetic field, which constitutes a rather unstable reference. The MWD positional uncertainty is therefore, to a large extent, a result of environmental errors originating from natural fluctuations in the magnetic field and distortions caused by adjacent magnetic material, and not variable sensor quality. Knowledge and techniques on how to minimize the effects of the unstable magnetic reference have been known for many years, and the practical implementation does not vary significantly between the major MWD suppliers, who have focused on factors other than survey accuracy in their marketing. It was therefore possible to create an MWD error model1 which has been accepted as an industry standard.
For gyro surveying, in which direction is determined with respect to the Earth’s rotation vector, the situation is quite the opposite. State of the art gyro instruments are far less accurate than the stability of the Earth’s rotation rate. Tool design and operational procedures vary significantly between the different suppliers, and survey accuracy is regarded as a competitive advantage. For these reasons, the SPE WPTS decided to provide the drilling industry with a framework for mathematical error modeling of gyroscopic survey tools2, and not to supply numerical error parameter inputs. It was left to the gyro service providers themselves to supply the rest of the industry with the necessary gyro model inputs, mainly without any external industry accepted guidelines or review/audit processes in place.
In this situation, operators and directional drillers might believe they are operating within acceptable safety margins while, in reality, they may operate with critically low safety margins. In the following, validation methods based either on direct measurements of the error terms involved, or on the comparison of multiple surveys in the same well based on the statistical analyses of real downhole data are outlined. Specific examples are given of how realistic uncertainty estimates for some existing gyroscopic tools are obtained.
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