--> A Model for Gas Hydrates Formation in Water Dominant Flow Established employing a Flow loop Investigation

AAPG Asia Pacific Region Geosciences Technology Workshop:
Gas Hydrates – From Potential Geohazard to Carbon-Efficient Fuel?

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A Model for Gas Hydrates Formation in Water Dominant Flow Established employing a Flow loop Investigation

Abstract

With the expected growth in the global energy demand in the future, natural gas hydrate resources coming into prominence due to their diverse geographic distribution with huge potential for energy recovery. In the direction of hydrate exploitation, the first offshore production test in the Eastern Nankai Trough area in 2013 was accomplished employing a depressurization technique which stimulated the continuous production of gas and water from the reservoirs. Technically at this point, there is a huge risk of pipeline blockages due to the favorable conditions for hydrate formation. Hydrate formation has already been considered one of severe flow assurance problems in oil and gas industries, however it is not explored in the hydrate exploitation domain (especially plugging issues in water-dominant system). Thus, the focus of the current work, conducted as a part of “Japan’s Methane Hydrate R&D Program (MH-21)”, is to develop a model to predict the hydrate formation rate in water-dominant turbulent bubbly flow with a consideration of hydrate slurry viscosity based on the result obtained from flow loop experiments. This proposed model will be able to provide the salient information to predict and access the hydrate risk in future gas hydrate productions. The flow loop experiments were conducted to obtain the data for hydrate formation rate, an interfacial area between gas and water, and the viscosity of hydrate slurry. The flow loop is about 16.7 m long with 10 mm diameter, consists of a circulation pump, a flow meter, temperature and pressure gauges, differential pressure (dP) gauges, an in-line camera and ports for gas or water injection. The in-line video camera can shoot a multiphase flow of gas, water and hydrate in the flow loop. All the experiments were conducted using methane gas and water at 3oC temperature with a void ratio of ≤2 vol%. The hydrate volume fraction was increased step-by-step (0-20 vol%) by slowly injecting methane into the flow loop. The flowloop pressure ranged 35-50 bar during the gas injection. The hydrate formation rate was calculated from temperature, pressure and the gas injection rate. The interfacial area was estimated from the video images shot by the in-line camera. Minor and major axes of each bubble can be detected by an image processing, and the total surface area of bubbles is calculable from them. The viscosity of hydrate slurry was estimated from flow velocity and dP. The model was developed based on the mass transfer limited model, which assumes that hydrate growth depends on transfer of guest gases from vapor phase to water phase. Key challenges of the modeling work were how to determine the mass transfer coefficient, the surface area of gas bubbles, and the influence of the hydrate volume on the viscosity of hydrate slurry. A rise in the viscosity leads to a decrease in the mass transfer coefficient. To estimate these properties, sub models were also required. An eddy cell model proposed by Lamont and Scott (1970) was employed for the mass transfer coefficient. Both intrinsic viscosity and differential effective medium theories were considered for the estimation of viscosity. The developed model and sub models were validated by comparing the model predictions with the experimental data, and it was confirmed that the predicted values basically fell within same order of experimental values.