Mobile NMR for rock porosity and permeability

Three different mobile Nuclear Magnetic Resonance (NMR) core-scanners were used to measure porosity and permeability of water-saturated drill cores and core plugs in a non-destructive way. In addition to their use in the laboratory, the small and light-weight devices are conveniently shipped, e.g. to drilling platforms. They allow rapid wellsite analysis of large-size cores in a fresh state without prior preparation. The sensors are experimental prototypes and differ in their magnetic field strength and homogeneity. The magnetic field of the NMR-MOUSE® is applied to the sample from one side and is inhomogeneous within the object. The two Halbach core-scanners enclose the samples in a large cylindrical volume with a nearly homogeneous magnetic field. Besides one-dimensional (1D) relaxation measurements which can be performed with all sensors, the second version of the Halbach core-scanner is also suitable for two-dimensional (2D) relaxation measurements on core plugs. 1D transverse relaxation measurements were made on fully cylindrical and split, semi-cylindrical cores of limestone, sandstone, basalt, peridotite, 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 all instruments agrees well with porosity determined by independent methods. Transverse relaxation measurements within the homogeneous magnetic field of the Halbach core-scanners can be used for permeability prediction. In the case of sandstone and limestone samples with high porosity, a standard calculation scheme from NMR logging in the oil industry yields good results. However, standard methods cannot be applied for an accurate permeability prediction for samples with low porosity and small pore sizes associated with high internal magnetic field gradients. Therefore, a new model theory was developed, which describes the pore radius dependence of the surface relaxivity as both an analytical and a more practicable empirical equation. Regarding corrected surface relaxivity values, permeability can be predicted accurately from the logarithmic mean of the T2 distribution from the physically based Kozeny-Carman equation