The Inclination Difference Test. As indicated in the georeference QC section, the inclination cannot be controlled against gross sag errors, and often also not against gross tool misalignments, by the survey itself. External data, usually in the form of an independent overlapping survey, will be needed to fulfill this goal. The inclination difference test, which is based on two independent overlapping surveys, has been developed to obtain control over the sag and misalignments. It does not test the sag and misalignment errors directly but looks at the resultant effectof all error sources affecting the inclination accuracy. The test is only capable of indicating that something is wrong with the inclination estimate in at least one of the two surveys being compared. It cannot be used to detect the specific error source that may be affecting the outcome of the test.
The inclination difference test is a Chi-squared test2 designed to detect systematic inclination differences present throughout the survey. It is stressed that the test relies totally on independence between the two inclination measurements used to form each inclination difference, which means that the two overlapping surveys must be performed with different surveying instruments and running gear/bottomhole assemblies (BHAs).
Figure 2 shows the inclination profile and the inclination differences between a magnetic MWD survey and a drop gyro survey. The jumps in the inclination differences may either be caused by depth differences
or BHA dependent inclination errors like the sag. The depth difference option was ruled out in this case, and it became important to determine whether or not the inclination errors giving rise to these inclination
differences were in accordance with the error models. A 15 station inclination difference test was run to address this question. The resulting Chi-square test value of 102 is far greater than the associated
tolerance level of 34, and it is concluded that gross inclination errors are present in at least one of the two surveys.
Another example of inclination differences is shown in Figure 3. A 15 station inclination difference test was conducted resulting in a Chi-square test value of 16, approximately half of the tolerance level of 34. Because the error model estimates for the performance of both systems are similar in size in this case, it can be concluded that both surveys are in accordance with their respective error models.
The Azimuth Difference Test.The azimuth difference test is based on the same mathematical principles as the inclination difference test. A three station azimuth difference test run immediately after a bit run is an excellent check against severe BHA magnetization errors and is generally recommended in connection with any bit run that involves a significant BHA change.
An error in the magnetic declination is an example of a potentially significant azimuth error that only can be verified through an independent overlapping non-magnetic survey. The azimuth difference test has been developed for this task. It does not estimate the declination error directly, but it is capable of indicating that something is wrong with the azimuth in at least one of the two surveys involved. Other QC tests are needed to identify the real cause of a failed test.
Figure 4 shows the azimuth profile and the azimuth differences between the same two surveys used in the second inclination difference test example. The jumps in azimuth differences are not consistent with the behavior that would arise through depth differences, and they must, therefore, have their origin in real azimuth errors in one or both of the surveys. The question is whether these errors are too large to satisfy the error model.
A 15 station azimuth difference test was run resulting in a Chi-square test value of 112, substantially greater than the tolerance value of 34. The test fails, indicating that gross errors affecting azimuth estimation must be present in either one or both of the surveys. This result also serves as a reminder that a pass of the inclination difference test cannot be taken as evidence that the azimuth estimates also lie within acceptable bounds. To isolate the cause of this failed test, the MWD survey was run through a magnetic multi-station estimation process that removed most of the azimuth differences, as can be seen from a comparison of Figures 4 and 5.
A repeat of the 15 station azimuth difference test with the multi-station corrected MWD data resulted in a Chi-square test value of 9.7, well within the tolerance level. The test is now passed, and it can be concluded that the multi-station corrected MWD data did perform within the tolerances defined by the expected MWD error model. The errors predicted by the gyro error model are much smaller than those predicted by the MWD error model. In view of this fact, it is not possible to conclude for certain that the continuous gyro survey is performing within its error model.
The Coordinate Difference Test.This is another useful test based on the same type of Chi-square statistics used in the inclination and azimuth difference tests, but it deals with lateral, along-hole, and vertical positional differences instead of inclination and azimuth differences. For optimal performance, the independent survey QC process should include inclination, azimuth, and coordinate difference tests, and constitutes the most powerful survey QC tool available.
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