Contributed Review: Nuclear magnetic resonance core analysis at 0.3 T

Abstract

Nuclear magnetic resonance (NMR) provides a powerful toolbox for petrophysical characterization of reservoir core plugs and fluids in the laboratory. Previously, there has been considerable focus on low field magnet technology for well log calibration. Now there is renewed interest in the study of reservoir samples using stronger magnets to complement these standard NMR measurements. Here, the capabilities of an imaging magnet with a field strength of 0.3 T (corresponding to 12.9 MHz for proton) are reviewed in the context of reservoir core analysis. Quantitative estimates of porosity (saturation) and pore size distributions are obtained under favorable conditions (e.g., in carbonates), with the added advantage of multidimensional imaging, detection of lower gyromagnetic ratio nuclei, and short probe recovery times that make the system suitable for shale studies. Intermediate field instruments provide quantitative porosity maps of rock plugs that cannot be obtained using high field medical scanners due to the field-dependent susceptibility contrast in the porous medium. Example data are presented that highlight the potential applications of an intermediate field imaging instrument as a complement to low field instruments in core analysis and for materials science studies in general.

Obtaining T 1 - T 2 distribution functions from 1-dimensional T 1 and T 2 measurements: The pseudo 2-D relaxation model

“…It is this relationship that is exploited in the P2DRM. NMR experiments on brine-saturated core samples of many different types of sandstones and carbonates have shown consistency with the equation pair[4,20,41,42]. These experiments empirically show that for large pores, , , and the ratio , while for small pores, the experiments are consistent with the relation , hereafter…”

Characterization and Analysis of Porosity and Pore Structures

“…Consequently, the industry standard for laboratory NMR core analysis has been set at an 1 H resonance frequency of  0 = 2 MHz, corresponding to a magnetic fi eld strength of B 0 = 0.05 T. The use of low fi eld also limits the infl uence of internal gradients, enabling quantitative analysis (Mitchell et al 2010). More recently researchers have been exploring the use of other fi eld strengths to assess core such as 0.3 T as reviewed by Mitchell and Fordham (2014).…”

“…The inherent signal-to-noise ratio (SNR) of the measurement is improved by increasing the fi eld strength, enabling better spectral (chemical) and spatial (image) resolution. Furthermore, nuclei with lower gyromagnetic ratios than 1 H are more readily accessed (e.g., 23 Na, 19 F, 31 P, 13 C, 2 H) for studies of molecular structure and chemical reaction monitoring (Mitchell and Fordham 2014). High-fi eld NMR also offers the advantage of shorter radio frequency (rf) probe recovery times, allowing the detection of short relaxation time components in solids.…”

“…A wide range of NMR spectroscopic methods are available for non-destructive characterization of porous materials. According to Mitchell and Fordham (2014) the majority of NMR spectrometers are high fi eld instruments (magnetic fi eld strength B 0 > 1.5 T) used for chemical analysis and biomedical studies. In these applications, the use of a high magnetic fi eld offers several advantages.…”

In Situ Chemically-Selective Monitoring of Multiphase Displacement Processes in a Carbonate Rock Using 3D Magnetic Resonance Imaging

“…It has been demonstrated that 23 Na MRI can be used to provide direct images of the location of brine solution using 23 Na observation (Washburn and Madelin 2010;Mitchell and Fordham 2014). Of relevance to the present study, Washburn and Madelin (2010) have demonstrated a 23 Na MRI method to monitor the movement of brine into an oil-saturated Bentheimer sandstone during a spontaneous imbibition process; however, no direct information on the hydrocarbon phase can be obtained.…”