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Modeling and experiments of gas hydrates and other solids management
Navaneetha Kannan, Seetharaman
Navaneetha Kannan, Seetharaman
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2024
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2026-04-04
Abstract
Gas hydrate plug formation and blockage in oil and gas flowlines pose major flow assurance issues, leading to significant economic and safety concerns. The hydrocarbon production flowlines provide favorable conditions for hydrate formation, including low temperature, high pressure, guest molecules (e.g., methane), and water. Under these conditions, deposited hydrates, if not remediated, can eventually plug the pipe, and impede the entire production system. This thesis presents new findings on hydrate formation and plug dissociation processes through experiments and modeling.
The thesis introduces a novel two-step mechanistic model in which one-sided depressurization (1SD) followed by the injection of Thermodynamic Hydrate Inhibitor (THI) accelerates the plug dissociation process. This new model integrates simultaneous heat and mass transfer changes due to the injection of THI, along with its flow rate and concentration effects. This work addresses safe operating conditions for hydrate plug depressurization and the influence of major variables contributing to hydrate plug dissociation.
For the first time, a unique field case with multiple plug scenarios is presented, supported by field data, to assist engineers facing similar challenges. This study highlights the major knowledge gap in the remediation of multiple hydrate plugs in the presence of THI from a safety standpoint.
A comprehensive literature review of gas hydrate issues in subsea deadlegs is conducted to provide recommended guidelines to assess and mitigate their safety risks for the first time. A case study examines hydrate formation risks in a spool section (deadleg) of a Norwegian subsea gas condensate asset using thermodynamic predictions and OLGA simulations. The study confirms the effectiveness of monoethylene glycol (MEG) in mitigating hydrate risks in a deadleg under field conditions.
The experimental studies assess gas hydrate agglomeration in gas condensate systems using rocking cells, both with and without inhibitors. The findings reveal that underinhibited systems (with methanol) show increased hydrate volume percent due to smaller water droplet in condensate emulsions, accelerating the rate of hydrate formation. The research categorizes hydrate transportability under different MeOH concentrations and subcooling conditions into three distinct zones: non-flowable gas hydrates, flowable underinhibited hydrate slurries, and full inhibition. It also demonstrates that MeOH can delay nucleation and reduce hydrate plugging risks, particularly at lower subcooling temperatures, highlighting the importance of temperature control in effective hydrate management.
Further rocking cell experiments investigate the effectiveness of a Kinetic Hydrate Inhibitor (KHI) exposed to high temperatures (50 ℃) in preventing structure I gas hydrate formation. Tests were performed in natural gas containing CO2 systems with no inhibitor, unheated KHI, heated KHI, and a combination of MEG with heated KHI. Results show that heating does not impair KHI performance, and the addition of MEG boosts the KHIs effectiveness in delaying nucleation and countering the dissociation temperature hysteresis observed in KHI-only systems. This highlights the viability of using KHI and MEG together as a cost-effective solution for managing gas hydrates.
Parallel to the study of gas hydrates, the thesis also investigates briefly other solid deposits, such as asphaltenes and wax. An in-house model has been developed based on the Asphaltene Instability Trend (ASIST) method for predicting asphaltene precipitation and onset pressure under reservoir conditions, enabling early management of potential asphaltene-related issues in oil fields. As field composition varies over time, integrating thermodynamic modeling with experimental evaluation of fluid properties at ambient conditions is crucial.
The effect of pipe surface treatment on wax deposition has been investigated using a temperature-controlled mini-loop setup by circulating an oil-wax mixture. The mechanical shear force tests demonstrated that surface treatments can significantly reduce the forces needed to remove wax, suggesting improved pigging processes.
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