Experimental investigation on hydrate anti-agglomerant for oil-free systems in the production pipe of marine natural gas hydrates

During the production of marine natural gas hydrates, as gas hydrates are easily formed, they tend to agglomerate, deposit on the pipe walls, and eventually block the pipe. In this study, high-pressure autoclave and micromechanical force apparatus were used to investigate the effects of anti-agglomerants and kinetic inhibitors on the hydrate agglomeration in the oil-free gas–water system, and the anti-agglomeration mechanism was analyzed. The results indicate that when the hydrate conversion rate reached about 27%, a complete blockage occurred. In the oil-free system, conventional anti-agglomerants did not exhibit any anti-agglomeration performance; while kinetic inhibitors slowed down the hydrate formation, but could not inhibit their agglomeration. An amphiphilic amide compound, namely DCA, effectively prevented hydrate agglomeration and blockage under the simulated hydrate production conditions of 8-h stirring and subsequent 8-h shut-in. From the perspective of interparticle interaction, 1.0 wt% DCA reduced the cohesion force between cyclopentane hydrate particles by 62% and 70% in liquid cyclopentane and air, respectively. These effects were attributed to the adsorption of DCA on the surface of hydrate particles, which converted the surface characteristic from hydrophilic to hydrophobic; thus, no liquid bridge could be formed between particles. Moreover, DCA formed an isolating layer and offered steric hindrance, thus preventing the adhesion and agglomeration of hydrates.