Uncertainty analysis in determining the position of a drill bit with the covariance method

Abstract

During oil drilling, the position of wells is indirectly determined by measurements near the drilling bit. However, the measurements are disturbed by noise causing errors in the measurements. It is important for large oil companies like Shell to determine the position of wells with a certain degree of uncertainty. Therefore, the main objective of this thesis is to assess the uncertainty in the nal well position of MSA corrected survey data. Two methods are investigated in this thesis to determine the positional uncertainty; Multi-Station Analysis in combination with the Minimum Curvature Method and Multi-Station Analysis in combination with the Covariance analysis method. In addition to Multi-Station Analysis, a second method has been investigated to determine sensor errors estimates and their uncertainty, namely the probabilistic estimation from Weighted Least Squares method. The methods with their assumptions are discussed, whereby the Covariance analysis method is discussed in more detail. Thereafter, the methods are tested with various test trajectories. The test results are compared to subsequently draw conclusions. It is clear that Multi-Station Analysis is the most reliable method to estimate sensor errors with their uncertainty. However, it was striking when the azimuth angle is not xed but varies, that certain errors are very poor. In addition, it could be concluded that the choice to systematically or randomly consider positional uncertainty has a major in uence on the positional uncertainty that arises. The thesis is completed with recommendations for further research.