Abstract
The formation process, structure, and distribution of gas hydrate in sediments have become focal points in exploring and exploiting natural gas hydrate. To better understand the dynamic behavior of gas hydrate formation in sediments, transverse relaxation time (T2) of nuclear magnetic resonance (NMR) is widely used to quantitatively characterize the formation process of gas hydrate and the change in pore characteristics of sediments. NMR T2 has been considered as a rapid and non-destructive method to distinguish the phase states of water, gas, and gas hydrate, estimate the saturations of water and gas hydrate, and analyze the kinetics of gas hydrate formation in sediments. NMR T2 is also widely employed to specify the pore structure in sediments in terms of pore size distribution, porosity, and permeability. For the recognition of the advantages and shortage of NMR T2 method, comparisons with other methods as X-ray CT, cryo-SEM, etc., are made regarding the application characteristics including resolution, phase recognition, and scanning time. As a future perspective, combining NMR T2 with other techniques can more effectively characterize the dynamic behavior of gas hydrate formation and pore structure in sediments.
Highlights
Gas hydrate is a cage-type crystalline compound formed by water and gas molecules at low temperature and high pressure, in which water molecules interact with each other through hydrogen bonds to form cavities and encase gas molecules in a cage-like structure [1,2]
The measurement of nuclear magnetic resonance (NMR) T2 is mainly realized by low-field NMR (LF-NMR) equipment which mainly consists of a powerful magnet (
NMR T2 test provides a fast, easy, and non-destructive way of detecting hydrogenNMR T2 test provides a fast, easy, and non-destructive way of detecting hydrogenbearing fluids, which is widely used to monitor the dynamic behavior of gas hydrate bearing fluids, which is widely used to monitor the dynamic behavior of gas hydrate forformation and pore structure in sediments [30,45,46,47,48,49,50,51,52,53,56,66,68,69,70]
Summary
Gas hydrate is a cage-type crystalline compound formed by water and gas molecules (methane, carbon dioxide, and hydrogen) at low temperature and high pressure, in which water molecules interact with each other through hydrogen bonds to form cavities and encase gas molecules in a cage-like structure [1,2]. Previous literature mainly focuses on two major types of hydrate-bearing sediments: drilled hydrate sediment samples and synthesis of gas hydrate in the pore space of sediments such as quartz sand or clay minerals [22,23,30,31] Most scholars use the latter sediment due to the difficulty of obtaining natural samples [32]. This review is not exhaustive, it summarizes recent advances in the literature relating to the application of NMR T2 in in situ quantitative characterization of gas hydrate formation in sediments, evaluates the current developments, and considers future advances in this promising experimental technique. (1) overview of the principles of NMR T2 and LF-NMR equipment; (2) brief review of quantitative characterization of gas hydrate formation in sediments; (3) analysis of the differences and similarities between NMR T2 method and other technologies; (4) discussion on the research trends and potential future development of the application of NMR T2 in quantitative characterization of gas hydrate formation in sediments
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