Research on Fast Measurement Method of Borehole Trajectory Based on Fiber Optic Gyro MWD System

The measurement-while-drilling (MWD) system usually uses the fluxgate, which can be sensitive to the earth magnetic field, to measure the borehole trajectory, but it is easily disturbed by the external magnetic field and has low measurement accuracy. The fiber-optic-gyroscope (FOG)-based inertial navigation system (INS) has the advantages of anti-electromagnetic interference. It can improve the measurement accuracy of MWD. Using the analytic coarse alignment method of INS to measure the borehole trajectory requires the downhole instruments to be completely stationary and takes more than three minutes. But in the process of drilling, it is difficult for the instruments to keep completely static. If the static time is too long, it is very easy to cause downhole accidents. Therefore, in order to realize the project of FOG-Based MWD system, the fast measurement method of borehole trajectory must be researched. This paper presents a method of inertial frame alignment based on gravity information, which can reduce the effects of disturbances by using the integral of gravity information. The swing and sliding downhole motions are simulated to verify the algorithm. The results show that in the low frequency swing environment, the azimuth of the analytic coarse alignment oscillates in the range of 0° to 180° and the azimuth accuracy of inertial frame alignment is better than 2° at 100s. In the sliding experiment, the azimuth convergence time of inertial frame alignment is 60s and the precision is 0.6° at 120s. The new method can meet the requirement of the fast measurement of borehole trajectory.