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Multiphase flow modeling and deposition of hydrates in oil and gas pipelines
Rao, Ishan
Rao, Ishan
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2013
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2014-03-01
Abstract
Multiphase flow is a ubiquitous feature of oil and gas production and hence, all flow assurance issues involve multiphase flow. The risk of plugging due to gas hydrate formation remains one of the most prevailing flow-assurance problems in deepwater subsea oil and gas operations. Due to the potentially severe economic impact of forming a gas hydrate plug, it is critical to develop gas hydrate formation models, which predict temporal and spatial gas hydrate plug formation in flowlines. Gas hydrate formation and accumulation mechanisms depend on the flow regimes of the system; in turn, gas hydrate formation can affect the flow regime of fluid flow. Currently, there are no multiphase tools that account for this coupling and interdependence of gas hydrate formation and flow regime. The main purpose of this thesis was to understand investigate the inter-coupling of the two most important pieces of flow assurance: hydrates and multiphase flow. A simple hydrodynamic slug flow model, based on fundamental multiphase flow concepts, coupled with a transient hydrate formation model, was used to study the effect of hydrates in a gas/water system undergoing fluid flow. The model includes a hydrate kinetic model, mass and energy balances, and pressure drop components. The validity of the model was tested against data measured in two high-pressure flowloop facilities at ExxonMobil, Friendswood, TX and at Tulsa University, OK. Flowline geometry was incorporated into the models to predict the slugging and accumulation of hydrates for different geometries. This hydrodynamic model predicts flow regime transitions among stratified, stratified-wavy, slug, and bubble flow with and without hydrates. A first pass model for the slip between gas and liquid phases, using dimensionless numbers, was developed for a general fluid system. The effect of inclination/gravity was studied and incorporated in the gas-liquid slip. Using this model to simulate hydrate formation in subsea pipelines, it was shown that higher hydrate accumulations are formed with increasing water-hydrate slip. The role of slip in assessing the location of a hydrate plug was demonstrated by simulations of the Werner Bolley field tests. This two-phase flow model was extended to incorporate hydrates as a separate third phase and a comprehensive multiphase flow framework, CSMFlow, for gas-liquid-hydrate was developed. Using a specific model for the slip relations between the phases, the model can predict the classical gas-liquid flow regime map and the impact of hydrates as a third, solid phase has on such flow regime maps. Slug flow in CSMFlow was quantified by capturing parameters like number of slugs, slug length distribution and slug frequency. Two-phase flow simulation results for the Caratinga field using CSMFlow were compared with OLGA®, where the prediction of the number of slugs was similar for both models, but the Log-Normal distribution for the slug length was broader in the case of CSMFlow than OLGA, and liquid holdup values were lower for OLGA®,. The mechanism of hydrate deposition on cold surfaces in water-saturated gas systems was investigated to demonstrate the hydrate film-growth phenomenon, which is dependent on the heat transfer in the system. Calculated porosities of the hydrate deposit dropped from ~90% to ~30-40% in the first 20 hours and remained constant, before decreasing to ~5% during the annealing stage. A combined heat and mass transfer approach was used to predict hydrate film growth on a pipe surface in saturated single-phase fluid systems. In gas-dominated systems, the hydrate deposit thickness increases very slowly, and the deposit thickness/growth rate increases with increasing water saturation of the gas phase and porosity of the deposit. This thesis shows that a relatively simple model can be useful in the predictions of multiphase flow and in particular how hydrates affect the flow behavior, and therefore hydrates must be explicitly accounted for as a separate phase. Slug length in the system increases with increasing superficial gas velocity at constant initial liquid loading and increasing inclination angle. Due to the presence of the hydrate phase, there is a transition from two phase to three phase flow although number of slugs changed slightly when the hydrate fraction increased from 1 to 5 vol.%. The single-phase flow deposition module was used to predict hydrate deposition in the Caratinga field for different fluids. The gas and condensate phase show similar deposition trends, while the Caratinga crude oil shows the lowest deposition. Flow assurance engineers will be able to apply the predictions and new information from the model developed in this thesis to facilitate informed decisions for risk assessment, design and optimization of oil/gas transport facilities.
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