A Practical Method to Compensate for the Effect of Echo Spacing on the Shale NMR T2 Spectrum

Abstract

The low field nuclear magnetic resonance (NMR) technique is considered as one of the most effective methods to characterize the pore size and fluid distribution in geophysical prospecting. The signal of NMR is noninvasive and lithology independent, while the effect of NMR responses influenced by the echo spacing (TE) is significant when applied to unconventional reservoirs which are featured with ultralow porosity and pore radius. TE is expected to be as low as possible to ensure most of the hydrogen signals are accounted for, but it is hard to achieve in downhole NMR measurement. We develop a practical method to compensate for the influence of TE on the transversal relaxation time (T2) spectrum of shale. Based on the experiments with different TE, the relationship between the NMR calibrated porosity and the TE is established to recover the signal amplitude. The effect of TE on the peak position is investigated, and then an empirical equation is constructed to move the peak position to its original state. By the above two-step correction algorithm, the T2 spectrums of shale at large TE are compensated precisely. The proposed method provides an effective way to obtain the real T2 spectrum and achieves good results in the downhole NMR data processing of shale reservoir.