The quality of a survey is determined by comparing the actual measured borehole uncertainties against the theoretical uncertainties of the survey tool. Survey uncertainties can be divided into tool, technique and running environment uncertainties.
The magnitude of tool uncertainties can be derived from quality assurance and quality control parameters. The magnitude of technique and, in particular, running environment uncertainties can only be determined if more than one survey is taken. For example, declination or gyro foresight are running environment uncertainties which can not be determined from a single survey but only if an additional independent survey is taken.
1 Tool quality assessment
Tool quality assessment consists of reporting and checking actual values of quality check parameters with specified tolerances.
2 Survey data comparison
Survey data comparison comprises comparison between magnetic survey data and gyroscopic survey data. Survey data comparison shall be performed by the Drilling Engineer. The main objective of survey data comparison is to see whether systematic uncertainties are present. Systematic uncertainties are: depth, azimuth and inclination uncertainties.
Depth difference: Depth differences arise from wireline depth uncertainties, tie-in depth uncertainties and depth reference uncertainties. Depth uncertainties are due to wireline stretch and will therefore show the trend to increase linearly with depth.
Azimuth difference: Azimuth differences arise from tool azimuth uncertainties due to magnetic interference and azimuth reference uncertainties. A constant systematic azimuth difference between a magnetic and gyroscopic survey is due to an uncertainty in azimuth reference such as declination or foresight. Such a systematic difference independent of inclination can not be caused by incorrect magnetic interference correction, magnetic interference reduction, gyro drift as the effect of these sources of uncertainty is inclination dependent. In a tangent section where inclination is constant the azimuth difference still can vary if different BHAs are used hence different grades of magnetic interference.
Inclination difference: Inclination differences arise from tool inclination uncertainties, tool misalignments and BHA deflection. Tool inclination uncertainties and tool misalignment are toolface dependent. These uncertainties can be determined by inrun/outrun comparison by taking multiple surveys at the same depth or along a tangent section. In a tangent section where inclination is constant the inclination difference still can vary if different BHAs are used.
Based on the survey evaluation a survey is accepted or rejected. A survey is rejected on basis of tool quality or running and environment conditions.