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Studies of hydrate cohesion, adhesion and interfacial properties using micromechanical force measurements
Brown, Erika P.
Brown, Erika P.
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2016
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2017-05-17
Abstract
The oil and gas production industry continues to innovate in new exploration and production techniques that allow the extraction of energy resources from increasingly extreme conditions. One consequence of this advancement is the increasing threat of hydrate plugs forming in the oil and gas production lines due to favorable thermodynamic conditions for hydrate formation. Complete inhibition of hydrates, which is traditionally the preferred method in hydrate treatment, can become prohibitively expensive or challenging due to environmental regulations. As such, the industry has observed a shift in focus from hydrate avoidance to hydrate management. In this strategy, hydrates are allowed to form and flow as a slurry. The goal in hydrate management is to prevent the aggregation and deposition of hydrates so that the flowline can produce without impediment. In order to accomplish this, a sound understanding of the cohesive (particle-particle) and adhesive (particle-surface) forces of hydrates particles is needed. This thesis seeks to advance the knowledge available on hydrate cohesion and adhesion, especially in the presence of surfactant additives. Several models involve the cohesive/adhesive force in their calculations; the Capillary Bridge Theory uses interfacial variables to predict the inter-particle force, while the Camargo and Palermo Model balances the cohesion force with shear forces to predict the extent of aggregation in a hydrate-bearing system. Each of these systems was analyzed for the sensitivity of each variable and how the inter-particle force affects each system. A method for measuring the contact angle of water on the hydrate surface was developed to study changes in hydrate wettability in systems both without and with surfactants present. This method was verified using two apparatuses and three operators, and it was shown that the contact angle measurement was repeatable. Using this method, the contact angle of a water droplet on a cyclopentane surface was found to be 94◦±5◦. This contact angle is less hydrophilic than previous estimates, and it represents an important update to prediction efforts using Capillary Bridge Theory. Based on the updated estimate of the contact angle, the embracing angle for a pure system was estimated as α = 4.9◦. Studies were also conducted to determine whether the Micromechanical Force (MMF) apparatus could be used to rank anti-agglomerants (AAs) by their performance. Four AAs were used in a blind test and were ranked based on the reduction in cohesion force measured. The final ranking determined agreed closely with the results provided by an industrial lab us- ing a typical macro-scale method. The MMF, which focuses on interfacial-scale interactions, is attractive for ranking measurements due to the speed, precision, visualization capabilities and small sample size needed. The visual nature of the MMF measurements also provided insight into the mechanisms of the AAs and morphological changes that resulted from AA addition. Changes in the wettability of the particles were proposed as a mechanism due to a strong correlation between contact angle and force measured in the presence of the AAs. In addition to particle-particle interactions, particle surface interactions were studied in the presence of AAs. It was found that AAs decreased the adhesion force between a stainless steel surface and a hydrate particle, but that the forces may increase if the surface was coated with a petroleum wax. Forces also increased with the addition of dissolved wax for a system with no AA as well as an oil-soluble AA. A water-soluble AA exhibited no changes with the addition of wax to the system. Changes in the hydrate shell micro-structure were studied by measuring the force necessary to puncture a hydrate particle using a glass cantilever. It was found that the addition of model surfactants caused the force needed to puncture the shell to decrease. The reduced shell strength was compared to other phenomena in the system such as interfacial tension, growth rate and cohesion force. Model surfactant studies were continued by comparing the force reduction for several chemicals that were added simultaneously to the system. Three classes of interactions were identified based on these measurements: synergistic, antagonistic and those showing no interaction. The effect of subcooling on cohesion was investigated based on improved measurement and error calculation techniques. It was found that the cohesion force increases significantly near the equilibrium temperature, but tapers to a near-constant value at high subcoolings. This trend agrees with measurements made on ice systems, as well as trends observed in the water layer on ice particles. The temperature dependence was found to persist for annealing times up to two hours. Finally, design of a high pressure Micromechanical Force apparatus was performed, including the identification of nano-manipulators which make the cell design possible. This system was tested using 3D printing for low pressure experiments before the final design was produced. Initial tests indicated reproducible experiments using gas hydrates and showed that CO2 was not an appropriate hydrate former for this system.
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