Whole-Core Analysis by 13C NMR

Abstract

Summary. A whole-core nuclear magnetic resonance (NMR) system was used to obtain natural abundance (13)C spectra. The system enables rapid, nondestructive measurements of bulk volume of movable oil, aliphatic/ aromatic ratio, oil viscosity, and organic vs. carbonate carbon. (13)C NMR can be used in cores where the (1)H NMR spectrum is too broad to resolve oil and water resonances separately. A 5 1/4-in. (13)C/(1)H NMR coil was installed on a General Electric (GE) CSI-2T NMR imager/spectrometer. With a 4-in.-OD whole core, good (13)C signal/noise ratio (SNR) is obtained within minutes, while (1)H spectra are obtained in seconds. NMR measurements have been made of the (13)C and (1)H density of crude oils with a wide range of API gravities. For light-and medium-gravity oils, the (13)C and (1)H signal per unit volume is constant within about 3.5%. For heavy crudes, the (13)C and (1)H density measured by NMR is reduced by the shortening of spin-spin relaxation times. (13)C and (1)H NMR spin-lattice relaxation times were measured on a suite of Cannon viscosity standards, crude oils (4 to 60 degrees API), and alkanes (C5 through C16) with viscosities at 77 degrees ranging from 0.5 cp to 2.5 X 10(7) cp. The (13)C and (1)H 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 (13)C NMR is shown to be insensitive to kerogen; thus, (13)C NMR measures only the movable hydrocarbon content of the cores. In carbonates, the (13)C spectrum also contains a carbonate powder pattern useful in quantifying inorganic carbon and distinguishing organic from carbonate carbon. Introduction We recently reported the use of (1)H NMR spectroscopy and imaging in rapid, nondestructive core analysis. The advantage of (1)H NMR is its high inherent sensitivity, which allows good SNR to be obtained, even on small cores in a short period of time. Proton NMR spectroscopy generally applies to obtaining oil and water saturations in carbonates and clean siliclastic rocks. In shalier materials, however, the proton resonances of oil and water broaden and finally overlap so that the two fluids are not spectroscopically resolved. In these cases, unless one of the phases is doped with paramagnetic agents, only the total fluid volume, not the individual saturations, can be measured.(13)C NMR offers a complementary approach to (1)H NMR for quantifying and characterizing hydrocarbons in cores. (13)C NMR has the advantage of unambiguous quantification of hydrocarbons in the presence of water, particularly in cases where oil and water (1)H NMR signals are not spectroscopically resolved. Just as in (1)H NMR, (13)C NMR also yields information about oil viscosity and such average structural parameters as aliphatic/aromatic ratios. The only nucleus of carbon with a magnetic moment, (13)C has low sensitivity because of both low natural abundance (1.108%) and small gyromagnetic ratio (1.071 MHz/kilogauss). Its absolute sensitivity relative to the proton is 1.76 × 10(-4), which makes (13)C difficult to measure in small core samples. In 4-in. -OD whole core, however, the large quantity of material enables one to obtain good SNR, with averaging times of 5 to 15 minutes. To analyze whole core, a (13)C/1 H double-resonance 5 1/4 -in. -ID NMR coil was built for a GE CSI-2 Tesla NMR imager/spectrometer. The (13)C/1 H coil allows measurements of both water and oil with (1)H NMR and oil and carbonate content with (13)C NMR. The coil is sufficiently large that 4-in.-OD whole core can be analyzed while inside a 4 3/4-in.-OD fiberglass core barrel. Because (13)C NMR can now be acquired routinely on whole core, there is a need for fundamental data on the (13)C NMR properties of hydrocarbons in cores. Thus, we analyzed carbon and properties of hydrocarbons in cores. Thus, we analyzed carbon and proton aliphatic and aromatic densities for a wide variety of crude proton aliphatic and aromatic densities for a wide variety of crude oils and measured the relationship between (13)C and (1)H spin-lattice relaxation times and oil viscosities. Using these data, we applied (13)C NMR to measure oil volumes in cores and to quantify organic vs. inorganic carbon in carbonates. Apparatus As described elsewhere, two NMR imager/spectrometers with 12-in. bores were adapted for rock studies. The field homogeneity of the superconducting magnets can be shimmed to better than 1 ppm over a 5.5-in.-diameter spherical volume, so that the width ppm over a 5.5-in.-diameter spherical volume, so that the width of an NMR line in 4-in.-OD clean whole core is less than 2 ppm. The modifications to the standard spectrometer for core analysis consist of self-shielded gradient windings, which switch magnetic field gradients up to 18 gauss/cm in a 6-in.-ID region in less than a few hundred microseconds, and large-bore multiple-nuclide radio-frequency (RF) coils with high spatial uniformity. The whole-core, (13)C/(1)H RF coil consists of a (13)C birdcage coil, 5 1/4-in. ID by 8 in. long, concentric with a larger (1)H bird-cage coil, 5 3/4-in.. ID by 6 in. long. Because of the concentric configuration, the (13)C coil partially short-circuits the (1)H coil, and loss of sensitivity of about a factor of two is observed for protons. This is not a problem, however, because of high proton protons. This is not a problem, however, because of high proton sensitivity and large sample size. For the 5 1/4-in. coil, the 90 RF pulse lengths for (13)C and (1)H are 90 and 100 microseconds, respectively. The RF field homogeneity in the (13)C coil, mapped with a small vial of (13)C-enriched methanol, Was found to vary by less than 5% in a cylindrical region 4 in. in OD by 6 in. long. A programmable table, controlled by output pulses from the CSI console, conveys the core through the magnet. The automatic table and RF coaxial switching enable any interleaved sequence of (13)C and (1)H data acquisition. (13)C NMR Spectra of Oils High-resolution (13)C NMR spectra of neat crude oils were recorded on a Bruker AM-360 NMR spectrometer under a pulse-and-acquire sequence with a 45 degrees RF pulse, pulse-and-acquire sequence with a 45 degrees RF pulse, 20-second recycle delay, and (1)H decoupling with composite pulses. Fig. 1 shows typical aliphatic and aromatic portions pulses. Fig. 1 shows typical aliphatic and aromatic portions of the (13)C NMR spectra of neat NM0104 and WT1175A crude oils. The spectral differences result from structural differences in the crudes and can be used to "fingerprint" the oils. Both aliphatic and aromatic spectral regions show complex patterns of resonances resulting from the effects of aliphatic branching and aromatic ring substitution and condensation. In oil-bearing siliclastic rocks, the (13)C NMR spectra are broadened by inhomogeneities in the local magnetic fields and consist of wide aliphatic and aromatic bands. The aliphatic band is between 10 and 40 ppm downfield, relative to tetramethylsilane, whereas the aromatic band is from 120 to 140 ppm. Figs. 2a and 2b show the (13)C NMR spectra of two 2-in.-OD x2-in.-long sandstones saturated with Wasson crude, showing the aliphatic and weaker aromatic bands. Figs. 2c and 2d show that in carbonates, in addition to the aliphatic and aromatic bands, the (13)C NMR spectra also contain a carbonate powder pattern. The powder pattern arises from all the orientations of carbonate crystallites relative to the applied magnetic field and extends from 120 to 195 ppm, with an average chemical shift of 170 ppm. SPEFE P. 183