Modeling the effects of hydrate wall deposition on slug flow hydrodynamics and heat transfer

Offshore oil and gas exploitation is usually accompanied of water (brine) and sand. High water cuts at high pressure scenarios (deep waters) and low temperature (cold waters) favor the formation of gas hydrates. Pipe blockage due to gas hydrates formation is a main concern in the oil and gas industry due to the profit losses caused by either production impairments or interruptions, and to the high costs associated to the elimination of such blockages. Hydrates form mainly at: (i) the water-gas interface, forming a suspension, or (ii) on the pipe inner wall, forming a deposit that constricts the flow. Both cases will affect flow hydrodynamics and heat transfer, and predicting how the flow will be affected is essential for improving exploitation efficiency and safety. Due to the gas and liquid volumetric flow rates that are typical of this kind of operation, the spatial distribution of the phases is often assumed to fall within the slug flow pattern region. Slug flows are characterized by the intermittent passage of elongated bubbles flowing over a liquid film, followed by liquid slug bodies that may contain dispersed bubbles in their interior. The present work introduces modifications on an existing mechanistic slug flow model to consider the effects brought by the deposition of hydrates on the pipe inner wall. The main differences herein considered are: (i) a cross sectional area reduction, which is related to higher pressure drops, and (ii) a conductive thermal resistance at the pipe wall, since the hydrate is an insulating material. Assuming a hydrate layer of constant thickness already deposited on the wall - that is, both the mass transfer and the heat generation associated to hydrate formation have already ceased - the slug flow model is used to verify the consequences of hydrate deposition to the slug flow hydrodynamics and heat transfer. Simulations for methane-water mixtures flowing along a 1.5-km length, 26-mm ID horizontal pipeline are presented so that the consequences to the pressure and temperature distributions, to the mixture heat transfer coefficient, to the unit cell geometry and to the slug flow frequency can be explained.