Whole-core analysis by ¹³C NMR

A whole-core nuclear magnetic resonance (NMR) system was used to obtain natural abundance ¹³C spectra. The system enables rapid, nondestructive measurements of bulk volume of movable oil, aliphatic/aromatic ratio, oil viscosity, and organic vs. carbonate carbon. ¹³C NMR can be used in cores where the ¹H NMR spectrum is too broad to resolve oil and water resonances separately. A 5 1/4 -in. ¹³C/¹H NMR coil was installed on a General Electric (GE) CSI-2T NMR imager/spectrometer. With a 4-in.-OD whole core, good ¹³C signal/noise ratio (SNR) is obtained within minutes, while ¹H spectra are obtained in seconds. NMR measurements have been made of the ¹³C and ¹H density of crude oils with a wide range of API gravities. For light- and medium-gravity oils, the ¹³C and ¹H signal per unit volume is constant within about 3.5%. For heavy crudes, the ¹³C and ¹H density measured by NMR is reduced by the shortening of spin-spin relaxation times. ¹³C and ¹H NMR spin-lattice relaxation times were measured on a suite of Cannon viscosity standards, crude oils (4 to 60° API), and alkanes (C₅ through C₁₆) with viscosities of 77°F ranging from 0.5 cp to 2.5 × 10⁷ cp. The ¹³C and ¹H relaxation times show a similar correlation with viscosity from which oil viscosity can be estimated accurately for viscosities up to 100 cp. The ¹³C surface relaxation rate for oils on water-wet rocks is very low.