Research Ofactive Magnetic Field Calibration System Based On The Triaxial Square Helmholtz Coils

The increasing depletion of oil resources is an important problem in thedeveloping process of the human society. In order to improve the oil recovery rate ofexisting oil fields, the number of highly difficult wells is increasing, such as clusterstructure well, around the barrier wells, multi-lateral horizontal well, communicatingwell. The magnetic measurement parameters of the directional probe directly affectsthe reliability of wellbore trajectory and decide whether the construction accuratefollowing the design trajectory in the MWD(Measurement-While-Drilling). Therefore,how to improve the magnetic measurement accuracy of the directional probe andestablish of high-precision probe calibration system (within10nT) is one of theimportant research topics of major oil companies in the world.Through cooperating with Shengli Drilling Technology Research Institute ofSinopec, the target of directional probe calibration system was located by researchingthe actual working conditions in the MWD: the active magnetic field calibrationsystem based on a triaxial Helmholtz coils, at the same time the system can counteractthe Earth’s magnetic field and the laboratory vicinity environmental magnetic fieldand can produce the desired uniform and stable magnetic field in which we cancalibrate the directional probe. The main works include designing thethree-dimensional Helmholtz coils system, designing the three-channels currentsource, researching the measurement and traceability methods of magnetic field underthis system, counteracting the geomagnetic field and generating vector artificialmagnetic field, and the design of calibration system’s software.Firstly, the dissertation analyzed the research status of the directional probecalibration system at home and abroad, then introduced the earth’s magneticfield.Established the technology line of calibration system’s research according to theresearch objectives and built a model of the active magnetic field calibration system,the model consists of three parts: triaxial Helmholtz coils magnetic field generatorsystem, the magnetic field measurement and traceability system,and the calibrationsystem software.In the paper, how to produce a uniform magnetic field, themeasurement and traceability methods of high-precision weak magnetic field wereintroduced, the parameters of round and square Helmholtz coils were derived andcalculated in theory. Ansoft magnetic field simulation was used to verify the reliabilityof the theoretical calculation based on the parameters of Helmholtz coils.Then thepaper explained why the triaxial square Helmholtz coils was better than the circularcoil on the practical application and uniform magnetic field region. On this basis, theparameters of triaxial square Helmholtz coils was designed in order to suit forpractical engineering. Secondly, the method of magnetic field measurement matching the triaxialHelmholtz coil magnetic field generator was analyzed, how to use the three-waycurrent source and a triaxial non-magnetic turntable was introduced in brief. Then thematching miniature triaxial fluxgate and the means of data acquisition were illustrated.The functional relationship between the target magnetic field and the load current wasimportantly derived, the relationship provides a theoretical guidance for the entiretriaxial Helmholtz coils how to accurately cancel out the geomagnetic field andgenerate different artificial magnetic field vector. The method of control the three-waycurrent source, the magnetic field measurement system, data processing, feedbackmagnetic field and the way to deal with the magnetic field deviation were explained inthe calibration system control software.Finally, the requirements of environmental in the laboratory where placedcalibration system was proposed, and the magnetic environment of the institute wasmeasured. By analyzing and processing of the measured data, the best place wherecan set up the three-axis Helmholtz coil. After the completion of the experimental testplatform, the coefficient between the target of the magnetic field and load current wassolved by using the measured data, and the deviation of a magnetic field wascorrected by the software. The experimental results show that the triaxial squareHelmholtz coil active magnetic field calibration system have been achieved thetechnical requirements of the project, the precision of the artificial magnetic field canbe controlled in10nT, the deviation of magnetic declination and inclination of is lessthan0.01°.