Rheological study of low wax content hydrate slurries considering phase interactions

Abstract Regarded as two of the most challenging flow assurance issues during the production of oil and gas in offshore fields or ultra-deep seas, wax and hydrates may coexist in multiphase pipelines, greatly jeopardizing the safety of operation and transportation. Understanding the coupling rheological properties of the coexistence system is vital to guarantee the transportability of pipelines. In this work, the viscosity of the fluid with different wax contents during hydrate formation process were obtained, and rheological tests of waxy hydrate slurries were carried out. It was found that the relative viscosity of wax-containing flow systems was much higher than that of the wax-free flow system, and increased with the hydrate volume fraction monotonously. Aside from the independent contribution of hydrates and wax to the overall viscosity of waxy hydrate slurry, an “additional” viscosity increment was observed in wax-containing systems both in flow systems and rheometer systems, which was evidenced to be caused by the phase interactions among wax, hydrates and water droplets. Results of the rheological tests in the flow loop indicated that the thixotropy of waxy hydrate slurries in flow systems weakened as the wax content increased. A viscosity model for waxy hydrate slurries considering the phase interactions was developed, which was capable of calculating the cohesive force between hydrate particles in the presence of wax. The model could predict the viscosity of waxy hydrate slurries that formed under another formation and shearing condition with relative deviation smaller than 20%.