(85g) Avoiding Flowline Plugging: Hydrates Pressure Drop Signature, Plugging Onset Prediction and Deposition Mechanism

This paper presents work done towards understanding indicators of hydrate plugging onset and the governing plugging mechanisms. Extensive multiphase flowloop studies were conducted to establish the pressure drop signature, temperature response, and visual observation of plugging behavior of the hydrate transporting systems. It was observed that for low viscosity carrier fluids, i.e., low viscosity oils, hydrate transportability, plugging onset and plugging mechanisms were strongly dependent on multiphase flow effects. Specifically, the main factors included liquid loading, watercut, mixture velocity and presence or absence of additive. General observation for these low viscosity systems included plugging risk increasing with: increasing watercut, decreasing liquid loading, absence of additives, low mixture velocity and increased sub-cool. Other factors that increased the risk included salinity, pressure and rate of cooling. Onset of plugging was deduced from pressure drop increasing or plateauing or increasing with decreasing mixture velocity. On the other hand, onset of bedding led to scattered temperature readings. Overall, hydrate deposition and plugging result from severe agglomeration leading to particle-wall interaction.

The second part of the paper presents results for the high viscosity carrier fluids, i.e., high viscosity oils. These systems were characterized by very high pressure losses. These systems are very sensitive to temperature change, i.e., cooling into hydrate zone was accompanied by huge pressure drop increase. Visual observations through viewport showed very little activity close to the pipe wall. It was hypothesized that flow in these systems was governed by significant boundary layer effects, i.e., as viscosity further increased due cooling, the laminar boundary transformed into creeping flow and finally stalled. In these systems, hydrates transportability and bedding strongly depended on emulsion properties. These systems were characterized by significant shear heating and difficult restarts.

Overall, hydrate deposition and plugging result from formation of strong emulsions with increasing yield stresses that lead stalled flow. Results showed that use of anti-agglomerants improves transportability of hydrates.

Keywords: rheology, particulate flow, slurry, flow regime, flow assurance, emulsions

Solids Flow, Handling, and Processing