(337q) Low-Field Nuclear Magnetic Resonance Signals and Applications in Unconventional Formations.
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2023
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Research Interests
- Currently there is great interest in interpreting the 1H NMR 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 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 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 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.
- As kerogen matures over geological times, bitumen is generated inside kerogen. With further maturation, the bitumen may be expelled out of the kerogen into the inorganic meso-macro pores, which affects important petrophysical properties of the rock such as wettability, permeability, acoustics, and electrical resistivity. We use a novel 2D NMR relaxation method at ``high-field'' to report the first-ever quantification of of bitumen in meso-macro pores versus bitumen in kerogen nano-pores, and we show how bitumen evolves with depth (i.e., natural maturation) in organic-rich chalk. Specifically, we use 1H NMR T1-T2 relaxation with a solid-echo to quantify the bitumen extracted by solvents in the meso-macro pores versus the bitumen expelled from the kerogen nano-pores after heptane saturation, all as a function of depth and overburden stress in the reservoir. We correlate the NMR derived quantities against RockEval pyrolysis, we show the evolution of the kerogen nano-pore volume and size versus depth, and we show evidence for universal T1 cross-relaxation in organic matter.
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