Multi Field Evaluation of T2 Pore-Size Distributions and T1-T2 2D Maps
Authors:   Green Imaging Technologies, Dick Michael , Kenney Taylor ,   Green Imaging Technologies, Veselinovic Dragan ,   Green Imaging Technologies, Marica Florin ,   University of New Brunswick MRI Research Centre, Balcom Bruce J. ,   University of New Brunswick MRI Research Centre, Green Derrick ,   Green Imaging Technologies
Published: November 2024

Low-field NMR instruments (~2 MHz) are popular for use in petrophysics laboratories as they compare favorably and reliably to NMR logs done downhole in the field. The lower field also reduces the issue of high magnetic susceptibility of core samples as compared to higher field instruments. However, higher field instruments present several distinct advantages, including faster scan times for a given SNR and superior detection of short relaxation elements due to their shift to longer relaxation times. For these reasons, higher field instruments have become more popular in recent years for use in core analysis. Previously, we investigated the validity of comparing T2 distributions and T1-T2 maps for various samples recorded at 2, 12, and 20 MHz. In this paper, we continue this work by increasing the suite of samples studied and expanding the fields investigated to even higher frequencies. This paper will include T2 measurements as well as T1-T2 maps of bulk fluid (doped H2O), sandstone plugs, carbonate plugs, and shale plugs at three new higher field strengths (33, 65, and 126 MHz). This new higher field data, when coupled with the previous data for these samples (2, 12, and 20 MHz), will create a data set with almost a two-order of magnitude increase in magnetic field strength. This new data set will lead to an excellent understanding of the effect of field strength on the NMR measurement of T2 distributions and T1-T2 maps. Beyond the samples listed above, a set of measurements has also been completed at all six magnetic fields on common clay and kerogen samples. Clay and kerogen can be common rock components in many petrophysical samples (especially unconventionals). As a result, it is important to understand how their T2 distributions and T1-T2 maps are also affected by field strength. For example, does the T2 distribution of clay-bound water change with a field in the same manner as water within the pores of a sample? For all samples tested, the higher magnetic field decreased the scan time for the same SNR for both T2 and T1-T2 measurements. For example, for the shale sample, a T1-T2 map with an SNR of 165 took 2,895 minutes at 2 MHz but only 4.5 minutes at 20 MHz. This corresponds to a 645 times decrease in scan time. The higher field also increases the separation between water and light hydrocarbons from heavier components in T1-T2 maps. It was found that T2 distributions can shift to both shorter and longer values with an increasing magnetic field. This makes interpreting T2 distributions recorded at different fields difficult. For the clay samples, a significant difference in T2 distributions or total signal amplitude as a function of frequency was not observed. With the increase in the prevalence of higher field NMR instruments, there is now plenty of NMR core analysis data taken at various magnetic fields being recorded in labs throughout the petrophysics industry. This paper has shown how T2 measurements and T1-T2 maps on various samples compare at six magnetic field strengths. We hope this work can be used throughout the industry as calibration when comparing measurements at various fields.