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In the graphs below, the y axis is used to plot the terms shown for each observation and if a smooth curve is drawn through the observed points, the TVD, Northing and Easting (from the start of the well to any MD) are simply the areas under the graphs.
Figure 107: Using a smooth curve drawn through the observed points to show North, East and TVD.
The area of the cos(I) graph is the TVD, the area of the sin(I)cos(A) graph is the Northing and the area of the sin(I)sin(A) graph is the Easting. By fitting smooth curves through the points, the curvature of the wellpath effectively varies smoothly through the survey rather than being fixed to a given radius between observations. Now consider the effect of a poor survey on the above graphs.
h ncons s n su y w ll ‘pull’ h ow ds s lf nd h dff nc makes can be determined by the difference in the area (The blue region in figure 107). In other words, varying curvature can compare the area under any of the three graphs, with and without a given survey observation and determine the effect each observation has on the wellpath position. Surveys consistent with those on either side will have very little effect if they are removed. This produces 3 co ‘shf’ n V , o hng ndE s ngc d by h ogu obs on. This vector can be split into a high/low side component and a left/right component ndplo d g ns m su ddphfo y us ful cons s ncy ‘s gn u ’ known as a varying curvature analysis. This provides the surveyor with a digital spellchecker on his data since any typing errors will show up as highly inconsistent with the surrounding surveys.
Figure 108: Spotting inconsistencies in the survey.
CONTENTS
Points to Ponder
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