Investigation of flow and viscosity characteristics of hydrate slurries within a visual-loop system

Abstract

With the gradual advancement of oil and gas exploration into deep offshore, the hydrate blockage has emerged as a critical concern for the flow assurance. We conducted constant-velocity hydrate formation and variable-velocity rheology experiments with a novel visual-loop to analyze slurry flow and viscosity change in pipelines. Results showed staged pressure variations during hydrate formation-aggregation-deposition process, and it could be analyzed judiciously with a developed viscosity model. Initially, hydrates dispersed as small flocculent particles with minor aggregation, gradually raising differential pressure, and the critical viscosity model parameter, hydrate aggregation rate (m) was <1. Subsequently, particle aggregation and wall adhesion dominated, resulting in reduced hydrate flow volume and possible blockage of special pipelines (e.g., dead-leg), with m-values >1. Finally, as hydrate growth continued, substantial adhesion to the pipeline reduced flow diameter, significantly increasing blockage risk. However, the addition of sufficient surface-active ingredients improved hydrate dispersibility and enabled the slurry to maintain the first stage, exhibiting long-term stability with an m-value <1. Additionally, the apparent viscosity of the hydrate slurry within the pipeline was accurately determined utilizing a novel approach, accounting for its yield-pseudoplastic behavior. The calculated viscosities closely matched post-sampling rheometer measurements, and were effectively predicted by the developed viscosity model.