NMR transverse relaxation of the clay-rich shale in inhomogeneous magnetic field: A numerical study

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

“…10,38 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. 33,62 As shown in the results of the NMR experiments, some samples showed large changes in signal in the short relaxation range before and after centrifugation (Figure 6), and the ϕ NMR was generally smaller than the ϕ helium (Tables 1 and 4). Those results suggest that there may be incomplete testing of the short relaxation signal of the small pores in the experiments, which would lead to a large geometric mean value of T 2 and a small calculated surface relaxivity.…”

“…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. , …”

“…More importantly, the T 2 spectra of saturated samples were often assumed to reflect the complete pore information in past studies, but in fact, the accuracy of the T 2 spectra is affected by the matching of sample and experimental parameters. 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.…”

Application Limits and Influencing Factors in Characterization of Rock Cores Using the Phase-Encoded T2-y Method in Low-Field NMR

“…The application of the SE-SPI method in oil recovery has been reported in several publications. , Nonetheless, it is evident that the observed signals and their processed results are subject to various influencing factors, including machine performance, acquisition model, magnetic field properties, and the experimental environment. For instance, the homogeneity of the magnetic field is frequently compromised by several factors. − A primary concern is the nonuniformity of static magnetic field and the internal magnetic field resulting from magnetic susceptibility differences or variations in mineral composition within the rock matrix . Another issue stems from the eddy currents induced by the switching of magnetic field gradients, which persist beyond the cessation of the magnetic gradients .…”