Whole-Core Analysis by 13C NMR

Abstract

Summary 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. car bonate carbon. 13C NMR can be used in cores where the ‘HNMR spectrum is too broad to resolve oil and water resonances separately. A 51/4-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. 13 C 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 at 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 13 C surface relaxation rate for oils on water-wet rocks is very low. Nonproton decoupled 13C NMR is shown to be insensitive to kerogen; thus, 13 C NMR measures only the movable hydrocarbon content of the cores. In carbonates, the 13C spectrum also contains a carbonate powder pattern useful in quantifying inorganic carbon and distinguishing organic from carbonate carbon.