Multistation analysis (MSA) is a technique widely used in the industry to correct systematic errors of magnetic MWD surveys associated with drillstring interference (DSI). However, to ensure an acceptable uncertainty of the correction we must understand the geomagnetic reference accuracy.
One of the main problems for magnetic MWD surveys is that the inaccuracy of the BGGM (BGS Global Geomagnetic Model) from the British Geological Survey (BGS), does not allow for correction of axial DSI for wells with inclination more than 55° in the west/east direction by the MSA method. If there is a local magnetic reference error, the BGGM does not have the required resolution to map it. This may mean that the azimuth error from the MSA correction will be significantly greater than the error caused by DSI. Thus, both a compensation for DSI and a good magnetic reference value are required to accurately position wells with MWD tools.
A modification of the MSA algorithm uses a model of distortion caused by DSI when a trajectory deviates. Within residual uncertainty, the results thus calculated are combined with the conventional MSA. That allows the possibility of avoiding the current limitation of DSI correction uncertainty.
The described algorithm avoids errors caused by geomagnetic reference inaccuracy in the case of a trajectory with changing direction and inclination. For cases with enough variation of the trajectory, we can correct axial DSI for horizontal wells in the west/east direction with high accuracy. Also, the method can be used for calculation of actual reference offsets from values derived from a geomagnetic model like the BGGM.
The developed method is an evolution of MSA that keeps all the advantages of the conventional method and avoids the limitation caused by geomagnetic reference inaccuracy. Application of this algorithm is most beneficial in regions where BGGM accuracy is low, as it is in Russia. In addition, the method allows greatly improving the correction of surveys for horizontal wells near an east/west direction in the case of a deviated trajectory.