Simulation of QPSK-based acoustic telemetry through drillstring

Abstract

Faster data communication speed is required for better real-time drilling control. The fastest communication method currently in use is acoustic telemetry, with a maximum level of 20 bps. The communication speed is limited because the decay time of the chirp signal is about 1/20 second. In order to overcome the communication limitation due to the chirp signal interval, the QPSK modulation method was applied. This method of transferring data with a phase difference of the carrier frequency is a robust communication method commonly used in digital wireless communication. In this study, the FDTD (finite-difference time-domain) method was applied and the drilling noise taken at drilling sites was added to simulate the actual drilling site environment. According to the study results, the BER (bit error rate) should be less than 0.1 in order to recover data from signals received at the surface. The center frequency of the chirp communication method is about 650 Hz, and at this frequency, the attenuation of the sound wave energy according to the drilling depth is relatively high. To overcome the signal attenuation, it is necessary to install a signal repeater approximately every 1 km. In this study, by lowering the QPSK carrier frequency to 83.5 Hz, it was possible to communicate without a repeater up to a drilling depth of 3 km. In this case, the maximum communication speed possible is 167.5 bps, which is favored for faster and accurate real-time drilling control. In order to enable this communication, measures are required to overcome high level of drilling noise in the low frequency region. A design plan is needed to install acoustic attenuators in the ground receiver and increase the capacity of the signal exciter amplifier and battery in the transmitter section near the drill bit.