Hydrates on suspended bubbles: development of a high pressure counter-flow system and initial measurements

Abstract

>In the wake of the Deepwater Horizon oil well blowout, significant research was undertaken to study this disaster. In particular, studies into the role of gas hydrates in deep sea blowouts, whether this is in the open ocean or within containment vessels. However, the buoyancy of gas of oil and hydrates in deep ocean causes the phases to rise too quickly for easy study. As a result of this issue, the Center for Hydrate Research designed and constructed CSM High-Pressure Water Tunnel (HPWT) to study hydrates at deep sea conditions, 4°C and above 1000 psi. The HPWT utilizes counter-flowing water to suspend the gas bubbles in a viewport, referred to as the Bulls-eye. The entire system is designed to operate at high pressures, with the minimum pressure rating value at 3200 psi (Bulls-eye). Pressure control and gas injection are controlled through the use of Isco syringe pumps. Temperature control is done through a chiller circulating a water-ethylene glycol mixture in copper tubing running countercurrently to the stainless steel flow lines. Gas injection is recorded by the Isco volume difference over the course of the experiment, which is then used in unison with Peng-Robinson equation of state to calculate the number of moles of methane. The system is capable of recording temperatures, pressures, and flow rates in a LabView program, which also controls the pumps and chiller. A high-speed/high-resolution camera is used to record videos of the bubbles at a rate up to 1000 frame per second and a resolution of 2560 x 1680 pixels. The HPWT is able to produce hydrates at the interface between the bulk water and the suspended methane gas bubbles. Initial studies in the system looked for the volume of methane added for hydrate formation and also approximations of hydrate growth rates. The amount of methane for hydrate formation was compared to predictions from CSMGem and Multiflash. The point of initial hydrate plate formation was found to occur near the predicted Multiflash concentrations of saturated water and sI hydrate equilibrium, even though the system exists at three-phase conditions. The average value of the ratio between experimental and predicted concentrations for hydrate plates was found to be 1.02. Complete hydrate shells were also seen in the system and formed when the experimental to predicted mole fraction ratio was 1.95. These results indicate that hydrates may form in accordance with two-phase equilibrium, but a complete shell will not form until there is almost twice the amount of gas in the system. The hydrate shell area growth rates were compared to literature values and found to both be on the order of 0.4 mm 2/s. These results are only an estimate as the video recording techniques do not capture the entire hydrate formation process, but instead are taken from image estimates at different times. These results are encouraging as most of the literature values were done at static conditions, as opposed to the flow conditions in the HPWT. In general, the results from these initial studies show that hydrate formation may prove difficult following deep sea well blowouts. Initial experiments show hydrate formation occurs at the two-phase equilibrium line predicted by Multiflash, but a complete shell will not form until twice as much gas as added. This amount of gas may not be present following a deep sea blowout. Most importantly, the HPWT is capable of forming hydrates to further study their impact in the open ocean and in deep sea blowouts. The system was designed to incorporate other studies that may prove helpful, such as the insertion of model cofferdams to study hydrate formation in containment units. The system also can be used to simulate oil and water flows, such as those in oil pipelines, to study flow assurance issues involving hydrates.