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Phase equilibria of gas hydrates containing hydrogen sulfide and carbon dioxide
Ward, Zachary Thomas
Ward, Zachary Thomas
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2015
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2016-10-08
Abstract
Production of gas fields containing carbon dioxide and hydrogen sulfide is becoming more prevalent in the global oil and gas industry. As conventional wells are depleted and the economics of unconventional gas production become favorable, the industry has responded by producing gas fields containing acid gas (CO2 and/or H2S). CO2 and H2S are the cause of a wide range of flow assurance issues that the industry are currently experiencing, including their strong tendency to form gas hydrate crystals that can plug flowlines. However, due to the challenging experimental conditions (long time scale, toxicity, high pressure, specialty cell materials required due to corrosion in these systems) for phase equilibria measurements of acid gas systems, there is a paucity of available data in the literature. The purpose of this thesis is to provide advanced understanding and new data critical to the development and validation of model predictions of gas hydrate phase equilibria for systems containing CO2 and H2S. New hydrate phase equilibria measurements were performed for pure H2S hydrates, binary CH4 + H2S hydrates, ternary CH4 + C3H8 + H2S hydrates, and ternary CH4 + C3H8 + CO2 hydrates using the isochoric pressure search method in stainless steel and Hastelloy C autoclaves, as well as using a step-scan technique in a high pressure differential scanning calorimeter. These measurements were compared with literature and commercial software used in academia and industry for predicting hydrate formation temperatures and pressures. These new data were then incorporated into the non-ideal hydrate solid solution Gibbs energy minimization multi-phase equilibria program, CSMGem, to better predict hydrate formation for hydrates containing CO2 and H2S. The CSM Gibbs Energy Minimization model is based on a statistical thermodynamics model for predicting the hydrate phase fugacity. Two fitted parameters, the soft-core radius, sigma, and the potential well depth, epsilon are key in this thermodynamic model. These parameters, when optimized to the new experimental data, showed significant improvement in the model to predict hydrate phase equilibria for systems containing mixtures of light hydrocarbons and H2S. To accomplish this parameter optimization, a new interface for CSMGem was developed and written, and the Fortran files that provide the core of the program were updated to operate with modern development software. This new interface provides an important new platform for future expansion and revision of CSMGem, facilitating a better understanding of hydrate thermodynamics and model improvements in future work. New measurements for a CH4 + C3H8 + CO2 hydrate were also performed and compared with the literature. However, inconsistencies were found between the data collected in this work and the sparce data reported in the literature. Metastability, when not considered carefully, can cause significant errors in hydrate phase equilibria measurements. An analysis was performed on all measurements performed in this work to determine the impact of metastability. While pure H2S, pure CH4, and binary CH4 + H2S gas systems did not show signs of significant metastability, systems containing C3H8 showed significant metastability. A method was devised to directly quantify how a system responds to changes, and if the hydrate phase is metastable during dissociation. This analysis will allow for higher quality data collection and confidence that results are as accurate as possible. During phase equilibria measurements for a ternary CH4 + C3H8 + CO2 hydrate using a high pressure visual autoclave, visual evidence of the growth phenomena showed a wall-growth dominated hydrate formation to plugging mechanism. This shows a significant departure from the classical discrete particle formation into a hydrate slurry that is normally found in CH4 hydrate. These studies can provide new insight into the hydrate formation to plugging mechanism for the complex hydrate systems found in pipelines. These new initial studies suggest that hydrate deposition and wall/film-dominated hydrate formation and growth can be key steps in hydrate plugging onset of such systems; a topic currently underexplored and poorly understood in literature, but critical to the flow assurance engineering community.
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