Study On Method And Several Issues Of Multi-Frequency Nuclear Magnetic Resonance Logging

Nuclear Magnetic Resonance (NMR) logging has some advantages in oil exploration that other traditional logging approaches could not compare with. Because of the blockade from developed country in this technique, the development in NMR logging kept empty in our country. In this paper, based on the description of NMR principle and its application in oil logging, the systerm and several crucial technology issues of our country’s first generation NMR logging instrument have been discussed and analyzed. The main research work could be concluded in the following several aspects.(1) Based on the’inside-out’pattern and’Gradient field+Radio-Frequency pulses’ sensor architecture, combine the relationship between NMR signal and formation parameter, the mathematic physic model of NMR logging was established. At the same time, the contributions of magnetic, antenna, electronic cuicirts and pulse sequences on the apparatus’Signal-to-Noise (SNR) were expressed to provide theorectical foundation for tool’s development.(2) The mechanism for CPMG pulse sequence to measure transverse relxation time was depicted through Bloch equations. And the influence of finite pulse width on spin echo was analyzed systematically. Based on these, the best value of the initial pulse interval had been proven theoretically, that is 2t shorter than the normal value Experiment showed that SNR had been improved by 0.75dB when initial pulse interval’s best value was adopted.(3) The physical mechanism for stimulated echo to cause the fisrt few echoes deviating from expected value was discussed. And it was proposed to establish mathematic revelence between echo and RF field through power calibration. So that, the error caused by stimulated echoes could be corrected. Some expecitments had been taken to examine the performance of this approach. If the effects of stimulated echoes were ignored completely, results deviated too much. If the first few echoes were discarded, the error would be 24.5% when processing low SNR logging data. But the error was only 4.9% when adopting the power calibration.(4) The instrument’s overall performance was examined and evaluated systematically. In laboratory, during the 8 hours experiment, porosity error is less than 0.96%, standard deviation is 0.93; the T, error is no more than 1.8% and the standard deviation is 4.6ms. Besides, the SNR kept about 39.8dB in the lab. Through the experiments in oil field, the instruments showed good stability, repeatability. The porosity deviation is 1.4 and accuracy is 90% of the most advanced NMR logging tool. In addition, measurement accuracy is below 5 porosity unit which is 3 in the world’s most leading apparatus.