Consideration of Various Parameters and Scenarios in the Simulation of Hydrate Formation

Abstract

This study investigates the effect of various parameters on hydrate formation under two scenarios of regular and cold start-up operations in a flowline and a subsea network. Parameters including hydrate adhesion forces, required subcooling for hydrate nucleation, and a weighting parameter for hydrate-oil slip (COIL) are evaluated. The effect of methanol injection and the mentioned parameters on hydrate formation are compared to 0.2 hydrate fraction threshold from literature. Hydrate formation from oil having 184 m3/m3 GOR and 35% WC is studied using two scenarios of regular and cold start-up operations in a 6-km flowline and a subsea tieback network (consisting of two branches and a main flowline). The overall heat transfer coefficient is 22.7 W/(m2.K), and the simulation is conducted by OLGA-CSMHyK-MUTIFLASH. Slurry relative viscosity, hydrate fraction, and hydrate propensity in terms of temperature difference known as DTHYD are used as indicators, and a hydrate fraction threshold of 0.2 is considered. Adhesion forces (0.5 - 0.005 N/m), required subcooling (3.61 - 10 °C), COIL (0.2-1), and methanol injection are investigated. During regular operation, the flow pattern remains stratified in a single flowline having 35% WC and 1 COIL. By increasing the required subcooling for hydrate nucleation from 3.6 °C to 10 °C, the hydrate fraction was reduced from approximately 1.7% to zero. COIL has the greatest effect on hydrate fraction. The reduction of adhesion forces had a noticeable effect on oil viscosity compared to the other indicators. Plug formation is not expected in the studied single flowline and subsea network under normal operation. On the other hand, a potential plug based on higher hydrate formation occurs in a cold start-up operation even under the effect of the studied parameters. However, the chance of plug formation is considerably reduced by injecting 20 wt% methanol. Overall, assessing the three indicators of hydrate formation (slurry relative viscosity, hydrate fraction, and DTHYD) are critical and provide more realistic insight about hydrate formation compared to using only one of the indicators for the evaluations. This work investigates the three aforementioned indicators of hydrate formation rather than relying on only one indicator (e.g., hydrate fraction) under regular and cold restart operations. The study evaluates hydrate formation based on a hydrate fraction threshold of 0.2 for a potential plug, compared to thermodynamically preventing hydrate formation.