NMR Imaging for Core Analysis

Abstract

Abstract 2 Tesla, 12" horizontal bore NMR systems have been adapted for imaging/spectroscopy of oil cores. Strong and fast shielded gradients (up to 18 G/cm with a settling time of less than a few hundred microseconds) have been developed to produce good quality images of cores with very short relaxation produce good quality images of cores with very short relaxation times. Accurate and rapid spectroscopic porosity measurements have been made on carbonate and sandstone whole cores, including some shaly samples. Porosity images have been made of carbonates and clean sandstones. Oil/water saturation has been determined spectroscopically in carbonates and clean sandstones, and selective water and oil images have been produced. Other NMR nuclei, such as H2 (deuterium), C13, produced. Other NMR nuclei, such as H2 (deuterium), C13, F19, and Na23 can be used for a variety of oil core applications. In particular, we have produced C13 spectra of oil in many different rocks; these spectra can potentially determine oil saturation in shaly materials where the proton NMR lines of oil and water overlap. Introduction Nuclear Magnetic Resonance (NMR) has long been used in the oil industry in the form of Nuclear Magnetism logging as a research tool for the measurement of rock properties such as porosity, permeability and residual oil saturation. NMR porosity, permeability and residual oil saturation. NMR imaging adds the extra feature of three dimensional visualization of fluids in porous media on the millimeter and submillimeter scale, and therefore can be used to study the spatial distribution of core porosity and fluid saturation as well as fractures, pore size distribution and drilling mud invasion. Unlike X-ray CT, which images both rock matrix and pore fluids, NMR images only mobile fluids and the interaction of those fluids with the confining surfaces of the pores. We have adapted 12" horizontal bore 2 Tesla NMR imaging systems for whole core analysis and fluid flow studies. In certain rock formations, such as clean sandstones and carbonates, the NMR linewidth is narrow and the oil and water saturations can be easily determined by chemical shift spectroscopy or imaging. In other cases, such as shaly (clay containing) sandstones, the water and oil linewidths are significantly broadened. NMR imaging under these circumstances requires fast and strong magnetic field gradients. To this end we have designed and built self-shielded gradient windings which switch gradients of up to 18 Gauss/cm in a 6" ID region in less than a few hundred microseconds. In addition, multiple nuclide volume imaging/spectroscopy coils (i.e. H1 and F19, H1 and C13) with high spatial uniformity have been developed for measurement/imaging of multiple fluid phases inside cores. phases inside cores. We have found that part of the NMR signals in some of the shaly sandstones can be refocussed and should therefore be imageable with very fast data acquisition. NMR capabilities are presently being extended to shaly sandstones with short presently being extended to shaly sandstones with short relaxation times and broad NMR linewidths. NMR AND NMR IMAGING NMR is performed by placing a sample in a strong, uniform static magnetic field Bo. Radiofrequency (RF) coils are used to apply a radiofrequency magnetic field to the sample and to receive signals back from precessing nuclei in the sample. P. 101