Objectives/Scope
Low field NMR instruments ( ̴2MHz) are popular for use in petrophysics labs as they compare favorably, and reliably, to NMR logs done downhole. 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 signal to noise ratio (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. However, very little investigation has been done on the validity of comparing NMR data recorded with low field instruments and to data recorded with higher field NMR. This paper looks to fill this void.
Methods
This paper will include T2 measurements and T1-T2 maps of bulk fluid (doped H2O), as well as sandstone , carbonate and shale plugs at three different field strengths (2 MHz, 12 MHz and 23 MHz). T2 NMR is a multiexponential decay that is dependent on pore size. T1-T2 maps are a two-dimensional measurement where fluids within a core sample are discriminated based on their relative viscosities. For these reasons, it is important to understand how T2 distributions and T1-T2 maps will vary with field strength. Without this understanding T2 distributions and T1-T2 maps recorded on the same sample could be interpreted differently when recorded at various fields.
Results
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 a SNR of 165 took 2895 mins at 2 MHz but only 4.5 mins at 23 MHz. This corresponds to 645X decrease of 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 for all samples T2 distributions can shift to both shorter and longer values with increasing magnetic field. This makes interpreting T2 distributions recorded at different fields difficult.
