Hydrate Blockage Formation And Development In Multiphase Flow In Deepwater Wellbore

Field experience indicates that hydrates formed in wellbore may result in severe conduit blockage and other safety problems in deepwater oil and gas development.This thesis is targeted at deepwater well testing operations and focused on annular flow.In this paper,the hydrate formation,transpotation,deposition and plugging behavior in deepwater wellbore are studied.A set of wellbore multiphase model is developed to characterize the flow behavior in deepwater wellbore with hydrate formation and deposition.With the model,a method is developed to quantitatively predict when and where hydrate blockages form in testing tubing during deepwater gas well testing.The formation and evolution behavior is obtained.Based on the gas-liquid contact feature and the inter-phase heat and mass transfer characteristics,hydrate formation dynamics in droplets,liquid film and condensate are obtained.Hydrates in the wellbore are mainly generated from droplets in the gas core.These hydrates are carried by the high-speed gas and migrate a rather long distance before depositing onto the tubing wall.On the contrary,hydrates formed at the surface of liquid film are stuck by strong adhesion forces exerted by the tubing wall.These hydrates tend to directly deposit on the wall rather than fly downstream with the gas stream.The deposited hydrates form a continuously growing hydrate layer,resulting in reduced effective tubing diameter.This is the main cause of tubing blockage.A model is developed to quantitatively predict hydrate blockage.Considering the interaction between hydrate behavior and multiphase flow and heat transfer,a wellbore multiphase flow model is established.By introducing a source/sink term,the continuity equations of gas and liquid phases take into account the gas and water consumption during hydrate formation.The continuity equations of hydrate take into account the opposing effects of hydrate formation and deposition.In the momentum equation,the influence of reduced tubing diameter and changes in surface roughness and gas/liquid/hydrate flow rates(Reynolds numbers)on the pressure drop are considered.In the energy equation,the effect of the Joule-Thomson effect caused by non-uniform hydrate deposition is taken into consideration.In addition,the thermal resistance of the hydrate layer is also incorporated.A numerical algorithm scheme is developed to solve the model.Published flowloop experiment data and field data are used to test the reliability of the proposed model.The concept,Hydrate Blockage Free Window(HBFW),is introduced to describe the occurrence of hydrate blockage during deepwater gas well testing.Based on the HBFW,a method is developed to quantitatively predict when and where hydrate blockages form in testing tubing during deepwater gas well testing.The formation and evolution behavior of hydrate blockage is obtained.The hydrate layer thickness is non-uniformly distributed in the axial direction.In some regions,the hydrate deposition rate is high and the hydrate layer grows faster.Hence hydrate blockages form more quickly in these regions and they are the high blockage risk region(HBRR).A new method,the HBFW-based method is proposed to prevent tubing blockage during well testing operations.This method can significantly reduce the consumption of hydrate inhibitors compared with the traditional total formation prevention method.This paper further discusses the application of this model in the optimization of hydrate control scheme and the early monitoring of hydrate plugging.This study can provide theoretical basis for hydrate plugging prediction and prevention.