Separation of solid and liquid components in organic-rich chalks using NMR relaxation

Abstract

Currently there is great interest in interpreting the 1H NMR T2 relaxation (i.e., transverse relaxation) of porous geological media containing both liquid-like and solid-like signals. This has an impact on the interpretation of commercial NMR core and log analysis of organic-rich shales, such as shale oil and shale gas, where T1-T2 relaxation maps are routinely used to identify sweet spots and producibility of the hydrocarbon reservoir. We report a novel method to separate liquid-like components with an exponential decay (T2e) in transverse magnetization from solid-like components with a Gaussian decay (T2G). The method uses novel pulse sequences together with a 20 MHz 1H NMR relaxometer optimized for reservoir core plugs. The method is applied to obtain 2D T1-T2 maps in organic-rich chalks saturated with water or heptane, as well as bitumen-extracted samples. The T1-T2 maps clearly distinguish liquid-like signals (including micro/meso-macro pore fluids, heptane dissolved in bitumen, and clay-bound water) from solid-like signals (including kerogen, bitumen, and clay hydroxyls) in the organic-rich chalks. The liquid-like (T2e) components in the T1-T2 maps show a clear contrast between water and heptane in the micro/meso-macro pores, which shows potential for improved fluid typing and saturation in organic-rich chalks and gives insights into diffusive coupling and bitumen blockage of the pore network. The solid-like (T2G) components in the T1-T2 maps are used for clay mineral identification, determination of kerogen content, and quantification of solvent-extracted bitumen versus bitumen expelled from kerogen due to swelling from dissolved heptane.