Porosity and permeability from mobile NMR core-scanning

Two types of mobile Nuclear Magnetic Resonance (NMR) core-scanners were used to measure porosity and estimate the pore-size distribution of water-saturated drill cores. Both devices are experimental prototypes and differ in their magnetic field strength and homogeneity. Whereas the magnetic field of the unilateral NMR-MOUSE(R) is applied to the sample from one side and is inhomogeneous within the object, the other device is constructed from a Halbach magnet and encloses the sample in a large cylindrical volume resulting in a nearly homogeneous magnetic field. Non-destructive NMR measurements were made on full cylindrical and split semi-cylindrical cores, including limestone, sandstone, basalt, shale, and unconsolidated clay-rich sediments with varying values of porosity, pore size and magnetic susceptibility. Porosity calculated from amplitudes of transverse relaxation measurements with both instruments agree with porosity values determined by independent measurements. The homogeneous magnetic field of the Halbach core-scanner generates distributions of transverse relaxation that are hardly influenced by diffusion. Permeability therefore could be determined for limestone samples with low magnetic susceptibility. In addition to their use in the laboratory, both tools because of their small size and low weight are conveniently shipped, e.g. to drilling platforms. They allow rapid wellsite analysis of large-size cores in a fresh state without prior preparation.