Gas hydrates are non-stoichiometric crystalline compounds comprising of guest molecules of suitable size and shape and host cages formed by water molecules. Cages by hydrogen bond of water molecules can accommodate guest molecules inside. Their crystal structures are classified into three kinds; structure I, II and H (sI, sII and sH). Natural gas hydrates have been a particular concern of the oil and gas industry as the operating conditions of offshore flowlines may be favorable for the formation of gas hydrate that results the blockage of offshore flowlines. Therefore, offshore flowlines transporting oil and gas have to be operated very carefully to avoid the formation of gas hydrates. As means to cope with this problem, suitable chemical injection is generally conducted and kinetic hydrate inhibitor (KHI) is a representative example. KHIs bind to the surface of the hydrate crystal and retard the nucleation and growth of gas hydrate, thus the hydrate inhibition is accomplished during specific period and the system is secured.
In this study we investigated the structural characteristics of gas hydrate formation in the presence of KHIs. Polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap) were selected as KHIs that are water-soluble polymeric materials. Solid-state NMR has become a brilliant method to analyze its structure and cage occupancies. Hydrate samples for 13C solid-state NMR were prepared according to our previous work1. The 13C NMR analysis for each samples was repeated twice to confirm the hydrate structure and cage occupancies. Results show that KHIs well retard natural hydrate formation and there are preferential specific structures. Two kinds of hydrate structures (sI and sII) were obviously detected and their ratio of sI/sII according to the included KHIs showed an evidence to the inhibition capability of KHIs.