High-spatial-resolution nuclear-magnetic-resonance method for investigation of fluid distribution in whole cores

Abstract

Low field Nuclear Magnetic Resonance (NMR) has been widely used in laboratories to investigate hydrocarbon and water in small core plugs to provide accurate petrophysical measurements and for calibration of downhole NMR logs. One fundamental limiting factor of current NMR technologies on large whole core is that the spatial resolution is limited by the length of the radio-frequency (r.f.) detection coil. In addition, the end effects of finite coil length can compromise the accuracy of NMR measurement when the core is longer than the coil. Herein a method is developed to overcome these problems to acquire high-spatial-resolution (HSR-) NMR data of the fluid in the long rock samples. The method is implemented by first conducting a series of NMR measurements in synchronization with a stepwise movement of the long sample through the NMR r.f. coil with the step increment equal to the desired spatial resolution. It then inverts the acquired data to obtain HSR-NMR results using a robust inversion algorithm. This new method provides a non-destructive measurement of fluid content with any desired spatial resolution and is especially useful for tight rock samples with short NMR relaxation times. The method is tested on 23 preserved whole cores from a source rock reservoir. The measured fluid distribution from the HSR-NMR method matches well with the fluid content from GRI method on the same sections of the preserved cores. In addition, the measured fluid distribution correlates well with the kerogen content along the cores with obvious lamination in the samples.