The shortcomings described in the previous sections demonstrate that a QC system based purely on georeference comparison and/or multi-station QC tests is not adequate to prove that a given survey is performing within the assumed error model. Significant error sources, including errors in depth, magnetic declination, continuous gyro drifts, drill string sag, and often survey tool misalignment, are uncontrolled with these tests. Additional QC tests based on repeated or duplicate measurements at the same survey station, in-run/out-run comparisons and comparisons with independent overlapping surveys are therefore vital to QC these additional terms. Repeated measurement tests are discussed here and the application of independent overlapping surveys is described in the section which follows this.
The rotation-shot misalignment test. It is necessary to control the misalignment and the gravity driven sag errors, in addition to controlling the accelerometer sensor errors, in order to document the quality of an inclination measurement. Since accelerometer tests are not sensitive to misalignment and sag, further tests are required. The rotation-shot-misalignment test has been developed to control toolface dependent misalignments; it does not QC the survey against sag errors.
The test is built around repeated inclination measurements taken at different tool faces at the same depth. Different tool faces are usually achieved by rotating the drill-pipe between the measurements. Therefore, the test is only suited for MWD and for gyros run at a locked orientation inside drill-pipes. It is noted that the accelerometer package should always be tested, using the methods described earlier in relation to accelerometer testing, and accepted prior to running this test.
Rotational Check-Shot Test. A series of measurements taken at different tool faces at a common depth forms a basis for additional tests that investigate the stability and tool face dependence of measured magnetic field strength, dip angle, and azimuth. Tests of this type have been employed for many years to detect and in some cases to correct for transverse magnetometer errors that might arise from drill collar hot spots.
Dual Depth Difference Test. The depth measurement is regarded as the most difficult directional surveying parameter to QC fully against gross errors. There exists no reference measurement in this case. The only possible downhole QC measurement is, therefore, an independent depth measurement, and this is usually not available for single surveys, for example for MWD, gyro while drilling and drop gyros. The only exception is for wireline gyro surveys run with a casing collar locator (CCL) inside casing or drill pipe. Two depth measurements, a wireline and a pipe depth measurement, are then available at every pipe connection. The depth difference at pipe connections between the pipe depth measurement and the associated wireline depth measurement can be used for QC.
The In-Run/Out-Run Misalignment Test. This test can be used to detect and secure against severe misalignment errors in continuous surveys, where the internal rotation-shot misalignment QC test is usually inapplicable2. Because tool face rotation is necessary to estimate the misalignment components, this test is suited only for surveys with gradually changing tool face, such as wireline surveys run on roller centralizers. Other misalignment tests have to be used for surveys with constant or near to constant tool face.
The in-run/out-run misalignment test can only be used if both in-run and out-run survey data exist for the entire survey section. Both the in-run and the out-run data should be proven free from gross accelerometer errors by applying internal QC testing, such as the gravity error test or the multi-station accelerometer test before running this test.
The in-run/out-run misalignment test makes use of the in-run/out-run inclination differences taken at the same depths as the fundamental QC measurements. It is, therefore, essential that the in-run and out-run depths at which the measurements are taken correspond to the same location in the well.
The Continuous Azimuth Drift Test. Continuous gyro and inertial navigation system (INS) surveys have an accumulative azimuth error behavior9-10, and it is not practical to QC each individual azimuth output. It is more convenient to examine the propagation of the azimuth error over the entire survey, and the continuous azimuth drift test has been developed precisely for this purpose2. It makes use of in-run/out-run azimuth differences to estimate the two recommended error model parameters, the linear gyro drift and the gyro random walk. To achieve this, the out-run azimuths ideally should be depth correlated relative to the in-run before running the test.
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