Dynamics of methane hydrate particles in water-dominant systems during transient flow

[Display omitted] •Quantitative investigation of hydrate particle size distributions under shear.•Visual observations with an in-line video camera to capture changes in behavior.•Rapid changes in particle size due to agglomeration when flow rate was changed.•New conceptual picture of hydrate slurries with microscale behaviour of particles.•Low circularities of hydrate particles suggesting a high intrinsic slurry viscosity. Natural gas hydrates are expected to be a new energy resource due to their diverse geographic distribution with huge potential for energy recovery. Test production systems for these resources typically consist of separate gas and water production lines. While this should limit the possibility of re-association during transit, the water lines may suffer from gas breakthrough. Hydrate formation and agglomeration in these water-dominant conditions is not well understood, but may lead to bedding, and, ultimately, plug formation. In this work, we interrogate the dynamics of agglomerating methane hydrate particles experiencing a change in flow conditions using a flowloop instrumented with an in-line video camera under two sets of conditions: i) a sudden stop; and ii) a stepwise decrease in the flow velocity. Particle size distributions were observed using the camera, where the size, number, andrateof particle transition were all explored. The particle size was sensitive to flow velocity, where changing shear conditions led to a new size distribution being established within seconds. This indicates that hydrate agglomeration takes place rapidly, which is an important insight for transient simulations. An associated change in slurry rheological properties was observed, where large, agglomerated particles were typically rough and irregular, suggesting a high intrinsic viscosity. Finally, a conceptual model for hydrate blockage formation in water-dominant systems was developed, offering a new microscopic view of hydrate particles and their agglomeration that incorporates the roles of particles spanning different size categories. This will be of particular use not only in evaluating the risks of re-association in production lines for hydrate reservoirs, but in understanding the dynamics associated with potential hydrate formation in CO2sequestration applications.