Catalysts for Enhanced CO2-CH4 Exchange in Natural Gas Hydrates. An experimental feasibility study of exchange enhancement by use of chemical additives
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Gas hydrate is a solid state of gas and water at high pressure and low temperature conditions. Vast energy potential is associated with gas hydrates and extensive on-going research aims at addressing the technical viability of production from hydrate deposits. Two different approaches to produce natural gas from hydrate reservoir have been proposed. Either decompose the hydrate by altering thermodynamic conditions or expose the hydrate to a thermodynamically more stable hydrate former inducing a replacement process of the encaged CH4 molecule in the lattice structure with the introduced new hydrate former. The latter has gained recent attention both in research and industrial communities. CO2 is an attractive candidate for such process due to both offering a better hydrate stability and possibilities for sequestrating a climate gas as gas hydrates in the earth. The work presented in this thesis is a series of experiments which studied processes involved during hydrate formation and hydrate dissociation within porous rocks. Methane hydrate was successfully and repeatedly formed within Bentheim sandstone core samples. The generated PVT-data were used to estimate the amount of methane stored in hydrate, the amount of free methane in the pores as well as the post hydrate formation water saturation. A comparison of data acquired in this study with in-house data demonstrated a trend towards higher post hydrate formation water saturation for increased initial water saturation and higher brine salinity. A number of experiments were conducted to study hydrate dissociation based production methods. Depressurization as a production method was investigated and production data acquired were compared with numerical simulation results acquired using TOUGH + HYDRATE. Thermal stimulation was investigated where temperature of the hydrate system was increased stepwise in order to find the dissociation threshold temperature at the experimental conditions. These data were later used to test the hydrate stability calculator CSMGem. Production by in situ exchange with liquid CO2 was studied during six experiments. These experiments were categorized by temperature during the exchange and presence of chemical additives during the exchange process. Two baseline exchange experiment was conducted at 83bar and 9.6 ºC using pure CO2. Another exchange experiment was conducted at 83bar and 4ºC to study the impact of temperature on the exchange rate. Enhancement of the exchange rate would potentially benefit from both increased methane production as well as the larger amount of CO2 stored in hydrate. Initial experiments of using Monoethanolamine (MEA) and Methyldiethanolamine (MDEA) to enhance the exchange rate were performed at 83bar and 4ºC. MEA and MDEA are respectively primary and tertiary alkanolamines that react with CO2 in an exothermic reaction. The generated heat from the reaction has the potential of triggering hydrate dissociation. Two experiments were conducted where slugs of MDEA and MEA were added to the injected CO2. Heat loss along the injection line resulted in low or no effect on the production. In order to minimize the heat loss, the chemical additive and CO2 had to be injected separately and react within or at the inlet of the core. The experimental setup had to be modified in order to allow for the latter. The amount of heat generated from the reaction between the injected chemical additive and CO2 resulted in dissociation of methane hydrate and high methane recovery. As a part of this master thesis, a mass flow meter was implemented, tested, and used in the production line enabling more accurate production measurements. Data acquired by mass flow meter in conjunction with data from a gas chromatograph were used to quantify the production as a function of time. In addition, a new confinement system using confinement buffers were implemented offering better confinement stability during the experiments. A new experimental setup was designed and built during spring 2012 as a part of the work presented here.