Effects of Diameter Transitions on Cyclopentane Hydrate Blockage and Dissociation in Flow Loops

Hydrate blockage in pipelines poses a significant risk during oil and gas production as well as natural gas hydrate exploitation. Existing studies on hydrate blockage have primarily focused on pipelines with constant diameters, while the formation and evolution of hydrate blockage in variable-diameter pipelines remain insufficiently understood. However, diameter transitions such as reducers are ubiquitous in practical oil and gas production systems. In this study, the formation, blockage, and dissociation behaviors of cyclopentane hydrates in two types of reducing pipelines were systematically investigated using a self-designed flow loop equipped with reducers, under varying cyclopentane-water volume ratios and flow conditions. The results show that at low flow velocities of 0.4 and 0.8 m/s, cyclopentane preferentially accumulates in the larger-diameter section and cannot be uniformly dispersed in water, resulting in incomplete hydrate formation. At a higher flow velocity of 1.6 m/s, diameter reduction significantly increases the susceptibility of the pipeline to hydrate blockage, and the blockage time decreases progressively with increasing cyclopentane-to-water ratio. For the thin pipe to the thick pipe (pipeline I), hydrate deposition initiates at the upper shoulder of the reducer and subsequently propagates downward to form a complete blockage. The unplugging process occurs in two stages: first, the hydrate plug in the larger pipe section migrates downstream and gradually dissociates; subsequently, hydrates in the smaller pipe are transported into the larger pipe under the action of the inverter pump, where they further dissociate. For the thick pipe to the thin pipe (pipeline II), hydrate deposition follows a similar initial pattern; however, during dissociation, secondary blockage occurs due to the reduced pipe diameter, leading to a longer dissociation time under identical heating conditions compared with reducing pipeline I. These findings provide important insights into hydrate blockage mechanisms in reducing pipelines and offer practical guidance for hydrate blockage prevention and removal at diameter-transition sections.