Drill bit positioning method based on connected well guidance for coalbed methane

On the basis of the features of low-frequency alternating magnetic field, the mathematical model of low-frequency alternating magnetic field near the magnetic source in drill bit has been set up. According to the measurement principle of measurement-while-drilling (MWD) tool, the measurement coordinate system has been set up, and the method for conversion from magnetic source coordinate system to measurement coordinate system has been derived, so as to form the drill bit positioning method under measurement coordinate system. On this basis, the accurate connection guiding instruments and tools have been designed and made, the drilling process for magnetic guiding connected well has been developed, and the field application has been carried out. The results indicate that: by adding the near-drill-bit magnetic tool to conventional combination of drilling tools, optimizing the parameters, carrying out real-time measurement of trajectory and controlling the trajectory posture, the connection error less than 1m has been achieved in the 730m horizontal section, so that the horizontal well and vertical well can be connected accurately, and the technical support has been provided for the development of coalbed methane.