A Practical Method to Compensate for the Effect of Echo Spacing on the Shale NMRT₂Spectrum

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 (T E ) is significant when applied to unconventional reservoirs which are featured with ultralow porosity and pore radius. T E 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 T E on the transversal relaxation time (T 2 ) spectrum of shale. Based on the experiments with different T E , the relationship between the NMR calibrated porosity and the T E is established to recover the signal amplitude. The effect of T E 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 T 2 spectrums of shale at large T E are compensated precisely. The proposed method provides an effective way to obtain the real T 2 spectrum and achieves good results in the downhole NMR data processing of shale reservoir.

Applicability of Nuclear Magnetic Resonance Experiment in Analyzing Pore and Fluid Distribution Characteristics of Tight Sandstone: A Case Study in the Julu Area, Bohai Bay Basin, China

“…However, in fact, the T 2 spectra can be influenced by both the sample and experimental parameters. For example, TE is a key parameter in NMR experiments, and its improper setting results in the T 2 spectrum not reflecting the complete information on the pores of the sample . Too small a TE can lead to overcharacterization of the basal signal (kerogen and structural water) and this can result in an overestimate of the porosity, while too large a TE can lead to signal deficiency and a small T 2 spectrum range. , At the same time, when the sample contains more paramagnetic minerals and the TE is excessive, there is a non-negligible lack of T 2 signal due to the internal magnetic field. , …”

“…And the settings of experimental parameters largely affect the variation of T 2 spectrum distribution. Predictably, when the NMR porosity is smaller than the gas-measured porosity, decreasing the TE can obtain a wider T 2 spectrum, more short relaxation signals, and a reduction in the difference between the NMR porosity and the gas-measured porosity. , For samples S2 and S7, whose true surface relaxivity may be relatively small, the experimental parameter TE is relatively suitable; thus, there is little movable fluid in the short relaxation time ranges and the error in T 2l‑ρ k is relatively small (Figure b,g). In general, there is a large amount of movable fluid in the wide pore throats and large pore spaces corresponding to relatively long T 2 relaxation times, and there is almost no movable fluid in the small pore throats and rough pore walls corresponding to relatively short T 2 relaxation times.…”

“…33 The effects of the diffusive pore coupling cannot be neglected for samples with complex pore structures and fast relaxation components. 34−36 Furthermore, too large an echo time (TE) setting can lead to a partial miss of the short relaxation signal corresponding to the small pore, 37,38 which can affect the calculation of surface relaxivity. The true surface relaxivities of the different samples vary, indicating that the appropriate experimental parameters for the sample are also different.…”

“…For example, when the sample contains many paramagnetic minerals, it may lead to an inhomogeneous internal magnetic field, which in turn triggers an off-resonance frequency and a reduction in echo amplitude and relaxation time . The effects of the diffusive pore coupling cannot be neglected for samples with complex pore structures and fast relaxation components. − Furthermore, too large an echo time (TE) setting can lead to a partial miss of the short relaxation signal corresponding to the small pore, , which can affect the calculation of surface relaxivity. The true surface relaxivities of the different samples vary, indicating that the appropriate experimental parameters for the sample are also different.…”