Roar Larsen

Clathrate hydrate growth and inhibition

Single crystals of structure II (sII) and structure I (sI) clathrate hydrates were grown in aqueous tetrahydrofuran (THF) and ethylene oxide (EO) solutions. Normal growth habits from the melt are predominantly {111} crystallographic planes for sII, and {110} for sI. Addition of polymeric so-called kinetic inhibitors in very small amounts changed the growth habit of sII to thin, 2-dimensional hexagonal {111} plates. Growing crystals in THF-rich or THF-poor solutions produced similar-looking flat plates. Adding the same inhibitors to the sI system caused rapid small-scale branching of the crystals. The highly branched sI specimens were found still to be single crystals. Higher concentrations of inhibitor were found to stop the growth of the crystals completely, for both sI and sII. These concentrations were as low as 0.1 wt.% at low supercooling. Experiments showed the polymers to be practically irreversibly adsorbed. A physical hypothesis was developed to explain the observed growth inhibition. This hypothesis appeals to a structural fit of the inhibitor polymers on the hydrate surface and strong bonding in specific configurations.

Melt growth of tetrahydrofuran clathrate hydrate and its inhibition: method and first results

Abstract Tetrahydrofuran clathrate hydrate single crystals grow from the supercooled melt as octahedra, showing that {1 1 1} is the slowest-growing face. The addition of poly(vinylpyrrolidone), poly(vinylcaprolactam) or a random terpolymer of the previous two with dimethylaminoethylmethacrylate causes the crystals to grow as hexagonal plates also bounded by {1 1 1} faces. Plate thickness decreases with increasing concentration of the impurity, until growth is altogether inhibited at a concentration of polymer that depends on supercooling.

Melt growth and inhibition of ethylene oxide clathrate hydrate

Growth and inhibition of single crystals of ethylene oxide clathrate hydrate provide experimental accessibility to similar phenomena in methane clathrate hydrates. Ethylene oxide hydrate single crystals were grown from the supercooled melt as rhombic dodecahedra, exhibiting {1 1 0} as the slowest growing face. The addition of poly(N-vinylcaprolactam) (PVCap) or a random terpolymer of PVCap, poly(N-vinylpyrrolidone), and dimethylaminoethylmethacrylate causes small scale branching of crystals, but with a uniform crystal orientation. A few tenths of a weight percent of these additives cause complete crystal growth inhibition up to several degrees of supercooling.

NMR Imaging Study of Hydrates in Sediments

Abstract: In this study, hydrates were generated in synthetic sediments in a laboratory cell. After hydrate formation took place and the sediment solidified, the samples were investigated both visually and by the use of nuclear magnetic resonance (NMR) imaging. The hydrates in this initial study were formed from model systems at low pressure. The results show hydrates distribution effects. Scans in the NMR apparatus were also made of the unfrozen samples to serve as a basis for comparison. NMR here maps the mobility of hydrogen atoms and their distribution in a sample. The relevant factor is the density of mobile H‐atoms, and this is shown to be about five times smaller for a volume of (for example) tetrahydrofuran (THF, C4H8O) hydrate than for the fluids of water and/or THF. This correlates very well with an observed signal decrease by a factor of six in an NMR‐studied sample after hydrate formation had taken place.

Growth and Inhibition of Ethylene Oxide Clathrate Hydrate

Abstract: Growth and inhibition of single crystals of ethylene oxide clathrate hydrate provide experimental accessibility to similar phenomena in methane clathrate hydrates. Ethylene oxide hydrate single crystals were grown in a simple experimental setup. The crystals grew from the subcooled melt as rhombic dodecahedra, macroscopically exhibiting {110} as the slowest growing faces. The addition of minute amounts of poly(N‐vinylcaprolactam) (PVCap) or a random terpolymer of PVCap, poly(N‐vinylpyrrolidone), and dimethylaminoethylmethacrylate (VC‐713) caused small‐scale branching of crystals, but with a seemingly uniform crystal orientation. A few tenths of a weight percent of these additives caused complete crystal growth inhibition at several degrees of subcooling.

CRITICAL DESCRIPTORS FOR HYDRATE PROPERTIES OF OILS: COMPOSITIONAL FEATURES

In petroleum production systems, hydrate morphology is observed to be influenced by the crude oil composition. This work is aimed at identifying which crude oil compositional parameters that need to be determined in order to evaluate natural anti-agglomerating properties of crude oils, i.e. the critical compositional descriptors. The compositional features of 22 crude oils have been studied, and multivariate data analysis has been used to investigate the possibility for correlations between several crude oil properties. The results show that biodegradation together with a relatively large amount of acids are characteristic for non-plugging crude oils, while excess of basic compounds is characteristic for plugging crude oils. The multivariate data analysis shows a division of the nonbiodegraded oils, which are all plugging, and the biodegraded oils. In addition, the biodegraded oils seem to be divided into two groups, one with plugging oils and one with mostly non-plugging oils. The results show that the wettability can be predicted from the variables biodegradation level, density, asphaltene content and TAN.

PETROLEUM HYDRATE DEPOSITION MECHANISMS: THE INFLUENCE OF PIPELINE WETTABILITY

The mechanisms by which hydrates deposit in a petroleum production-line are likely to be related to pipeline surface properties, e.g. pipeline material, surface energy and roughness. In this work, the wettability alteration of pipeline surfaces from contact with oil, as well as the adhesion energy between water and solid in the presence of oil is investigated. Contact angles are determined as a function of solid material and oil composition, for both model oils and crude oils. Although contact angles in oil/brine/solid systems have been extensively reported in the literature, the variety of solids that may mimic a pipeline is limited. In this study, we include various metal surfaces in addition to glass and a coating. Initial results from using near infrared imaging for collecting contact angle data in non-translucent systems are also presented.

A Proposed Thermodynamic Model for Gas Hydrate Equilibrium in Electrolyte Solutions

In this work, the thermodynamic stability of gas hydrates is investigated in the presence of electrolyte solutions. The proposed model is based on the Van der Waals–Platteeuw model for gas hydrate equilibrium, and the Pitzer and Mayorga model is employed to calculate the water activity in the electrolyte solutions. Available values for the Pitzer model parameters are usually adjusted using experimental data at 25°C, which is usually higher than the gas hydrate formation temperature. In order to eliminate this problem, those adjustable parameters are re-optimized using experimental data from the literature at the lowest temperature. In the case of mixed electrolyte solutions and without using any adjustable parameters, a mixing rule is proposed to estimate the water activity. The new mixing rule is based on the ionic strength of the mixture and estimates the mixture water activity by using properties of the single electrolytes that constitute the mixture. The results show the proposed model can calculate hydrate equilibrium conditions with good accuracy, especially at low concentrations, which is the case for most industrial applications.