Development and performance evaluation of chitosan-graft-poly (N-vinyl pyrrolidone) as a dual-function inhibitor for hydrate decomposition and reformation

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In the process of drilling and producing natural gas hydrates, the hydrate is prone to decomposition, leading to wellbore instability and well control problems, and the natural gas produced by the decomposition is prone to form hydrate again in the low-temperature and high-pressure wellbore, creating flow obstacles. The existing inhibitors, however, fail to effectively inhibit the decomposition and reformation of hydrates. In this work, N-vinylpyrrolidone was used to modify chitosan, and an inhibitor namely DFHI with a dual function of inhibiting hydrate decomposition and reformation was developed. The inhibitory effects of DFHI were analyzed using the optimized experimental method for evaluating hydrate decomposition and reformation, and the mechanism was analyzed. The results showed that the memory effect of hydrate reformation gradually weakened or even disappeared with the rising ambient temperature during hydrate decomposition. The inhibitory performance of DFHI exhibited a positive correlation with its concentration. 0.5% DFHI resulted in a 74.7% delay in the decomposition time of methane hydrate, along with an extension of the induction time for both hydrate formation and reformation by 137.2% and 262.1%, respectively, significantly weakening the memory effect of hydrate reformation. The mechanism of DFHI inhibiting hydrate decomposition primarily involves surface adsorption on hydrates and thickening effects that retard mass and heat transfer processes. During the process of hydrate formation and reformation, DFHI molecules tend to aggregate at the surface of methane bubbles, hindering gas–water contact and diffusion, particularly when numerous micro-nano bubbles generated by hydrate dissolution are present; furthermore, the adsorption of DFHI molecules on the surface of hydrates delays their nucleation and growth.