(154a) Understanding and Modeling Hydrate Formation in High Water Producing Oil Pipelines

As oil/gas subsea fields mature, the amount of water produced increases, which results in an increased risk of hydrate plug formation in the flowlines. It is important to understand the mechanism of hydrate plug formation in high water cut systems in order to manage hydrate plug prevention. In this work, we will present an extensive series of hydrate formation and dissociation experiments in a four-inch diameter flowloop at the ExxonMobil research facility at Friendswood, TX. The effect of mixture velocity (1 to 2.5 m/s) and liquid loading (50 to 90%) on hydrate plug formation was studied for 100% water cut (no oil present) systems and 85% water cut systems, with methane as the gas hydrate former. The pressure drop across the pump due to flow did not increase until a certain concentration of hydrates, defined as F_transition, was reached. The 5-50 times rise in pressure drop was explained by a combination of hydrate deposit at the pipe wall (reduction in the flow cross-section) and high frictional losses due to formation of hydrate bed (high concentration hydrate-water mixture). F_transition was found to be unaffected by liquid loading and the presence of salt in water while it increased with increasing flow velocity. A conceptual picture for hydrate growth and plugging mechanism for 100% water cut systems was proposed, which involved a transition from homogeneous to heterogeneous suspension flow of hydrate particles in water which leads to increased interaction between continuously growing hydrate particles leading to the formation of a hydrate bed/deposit in the pipe.