Investigation of high frequency 1D NMR to characterize reservoir rocks

Abstract

Typical laboratory Nuclear Magnetic Resonance (NMR) measurements for core analysis are made at a frequency of 2 MHz, principally to verify downhole logging measurements with frequency ranging from 0.7 to 2 MHz. However, higher frequencies have been introduced in measuring shale properties. In this study, we investigate the differences between low frequency (2 MHz) NMR and higher frequency (12 MHz) NMR responses with different TE (Echo Time) ranging from 114 to 300μs in various rock types including sandstones and shales. We found that 12 MHz instrument attains a greater SNR (Signal to Noise Ratio) ranging from 7.3 to 9.8 times higher with fewer scans than the 2 MHz instrument. While this is an insignificant factor for highly porous rocks, it is very important when measuring shales, which requires numerous scans to achieve an acceptable SNR levels. This is also significant while tracking time dependent fluid changes in rocks. We demonstrate novel technique of the dynamic monitoring for a CO2 displacement of oil experiment with 12 MHz NMR instrument. T2 spectra were obtained every 20 min to document the oil saturation profile. From this saturation profile, breakthrough time and residual oil saturation were determined to be 340 min and 54%, respectively. Total porosity measurements were almost identical between 2 MHz and 12 MHz with an average difference of ±0.38 p.u. For shale samples only, the difference in porosity measurement was much less, i.e., ±0.16 p.u. Our results suggest that higher frequency NMR produces similar total porosity estimates, T2 geometric means, and T2 spectra for some conventional sandstones with low paramagnetic mineral content and shale lithologies.