Near-bit apparent resistivity measurement method based on SPWM excitation

Abstract

In order to improve the measurement accuracy of near-bit apparent resistivity and its measurement range, this paper excites the underground transmitting antenna by a sinusoidal pulse-width modulated (SPWM) current excitation method, which can improve the response of the receiving antenna and circuit in the resistivity measurement system under the usual square wave excitation, thus improving the measurement accuracy of near-bit apparent resistivity. In this paper, the effect of square wave, SPWM wave and sine wave excitation on resistivity measurement was studied, and the modulation algorithm and hardware control are programmed by DSP (Digital Signal Processor), and the SPWM signal is output by full-bridge inverter circuit and tuned antenna. The indoor experimental results show that the near-bit apparent resistivity measurement method based on the SPWM wave pulse excitation can achieve the measurement in the range of formation equivalent resistor value of less than 600kΩ. There is a high trend matching between near-bit apparent resistivity LWD curve and RT resistivity curve of well completion electrical logging in field tests of formation resistivity less than 2000Ω·m, which proves that the method can improve the accuracy of near-bit apparent resistivity measurement.