Practical Limitations Of Low Field NMR Measurements of 13C and 23Na

Low magnetic field 1H NMR has been employed for decades as a tool in core analysislaboratories. Measurement of other NMR active nuclei have not been as common because thesemeasurements typically require very expensive strong magnetic field instruments with cryogenic cooling.However, in recent years there has been new data showing that NMR measurements of other active nucleican be beneficial for core analysis. For example, it has been shown that 13C NMR measurements (inconjunction with 1H measurements) can be employed to determine the relative saturation of water and oil ina core sample, help determine wettability and examine CO2-Brine-Rock interactions for evaluation of carboncapture utilization and storage. Our goal has been to develop a new NMR probe capable of observing both13C and 23Na at lower field. This new 13C/23Na probe was designed to work in our existing 0.3 Tesla magnet.This meant that one magnet can be employed to measure both hydrogen, sodium and carbon NMR data bysimply swapping out the 1H probe (Larmor Freq. – 12MHz) for the 13C/23Na probe (13C-Larmor Freq. – 3.25MHz and 23Na-Larmor Freq. – 3.41 MHz). While 1H and 23Na have a natural abundance of 100%, 13C hasa natural abundance of only 1%. This leads to a lower signal-to-noise ratio (SNR) for the 13C data leadingto longer scan times as compared to 1H. In addition, employing lower magnetic fields for 13C and 23Nafurther reduces the NMR signal leading to even lower SNR further lengthening scan times. Anotherexperimental challenge that had to be overcome in development of the 13C/23Na probe was the fact thatworking at a lower field made the probe more susceptible to acoustic ringing. A lot of effort was spentexploring how different copper shielding configurations and different materials employed for the probe bodycould reduce the acoustic ringing. Despite all these challenges, the new 13C/23Na probe was successfullycompleted and commissioned. Eventually, T2 and T1 distributions, one dimensional saturation profiles andT1-T2 maps were acquired for 13C of different bulk samples including glycerol, decane and crude oil. For23Na, brines with differing sodium concentrations (2-25%) were measured both in bulk and in saturated coresamples. Finally, both the 23Na and 13C T2 measurements were employed in conjunction with and 1H T2measurements to derive the relative saturation of water and oil in different samples.