Hydrate Phase Transition-Risk, Energy Potential and CO2 Storage Possibilities

Abstract

Natural gas hydrate (NGH) can cause crucial flow assurance problems to the oil and gas industry. It is being considered as a potential vast energy resource for the world in the future. It could also potentially provide a long-term offshore storage possibility for carbon dioxide. Therefore, the risk of hydrate formation during processing and pipeline transport of natural gas and CO2, thermodynamics and kinetics of hydrate formation, and simultaneous CH4 production from in-situ hydrate and CO2 long-term offshore storage in form of CO2 hydrate are important research concerns. The main scientific method used in this project is classical thermodynamics based on thermodynamic properties calculated using methods in Quantum Mechanics and Classical Mechanics. Classical thermodynamics was used together with residual thermodynamics description for every phase; this includes the hydrate phase, to analyse different routes to hydrate formation between hydrate formers (or guest molecules) and water. NGHs are formed from water and natural gas at high pressures and low temperatures conditions under the constraints of mass and heat transport. The problem is that natural gas is usually produced together with water and operations are usually at elevated pressures and low temperatures. Current industrial approach for evaluating the risk of hydrate formation is based on liquid water condensing out of the bulk gas at dewpoint and at a specific pressure-temperature (P-T) condition. In this method, the maximum allowable water content will be kept below the projected dew-point mole-fractions during transport, considering the operational P-T conditions. However, a previous study in our research group suggested that solid surface, particularly rust (Hematite) is another precursor to hydrate formation; rust provides another route for liquid water to drop out through the mechanism of adsorption. And pipelines are generally rusty before they are mounted in place for operations. The two approaches have been applied to study the risk of water dropping out from natural gas from different real gas fields. The approach of adsorption of water onto Hematite (rusty surfaces) completely dominates. The dew-point method over-estimates the safe limit (maximum mole-fraction) of water that should be permitted to flow with bulk gas about 18 – 20 times greater than when the effect of hematite is considered, depending on the specific gas composition. That suggests that hydrate may still form when we base our hydrate risk analysis on dew-point technique. The presence of higher hydrocarbon (C2+) hydrate formers causes a decrease in allowable water content with increasing concentration of ethane, propane and isobutane for the temperature range of 273 – 280 K. As their concentrations increase in the bulk gas, these C2+ act to draw down the water tolerance of the gas mixture to a point where they completely dominate or dictate the trends. For the inorganic components, CO2 has little or no significant impact on the allowable upper-limit of water when its concentration increases. While the presence of H2S causes a consideration reduction in water tolerance of the system as its concentration in the mixture increases. The presence of 1 mol% of H2S in the bulk gas may cause about 1 % reduction in water tolerance. The reduction in maximum content of water could be up to about 2 – 3 % and up to about 4 – 5 % if the concentration increases to 5 mol% and 10 mol% respectively. It is not appropriate to interpret hydrate stability entirely based on equilibrium P–T curves as often done in literature. The hydrate stability curve of CO2 hydrate has lower pressures (thus more stable) compared to that of CH4 hydrate but only to a certain temperature. That is the quadruple-point were phase-split occurs causing the pressures of CO2 hydrate going above that of CH4 hydrate due to the increase in density caused by the CO2 liquid phase. A free energy analysis revealed that CO2 hydrate has lower free energy across the entire temperature range, thus more stable at all the temperatures. Therefore, hydrate stability should rather be based on free energy analysis since in real situations hydrate cannot reach equilibrium. Consequently, the most stable hydrate is the hydrate with the minimum free energy. The hydrate with the least or most negative free energy will first form under constraints of mass and heat transport, then followed by the subsequent most stable hydrate. Among the hydrocarbon guest molecules studied, the most stable hydrate is hydrate of isobutane, followed by that of propane, and then by ethane. Induction times are sometimes mistaken as hydrate nucleation times, which is why some works report nucleation times of hours. Hydrate formation is a nano-scale process, and the hydrate nucleation times computed for both heterogeneous and homogeneous hydrate formation in this project are in nano-seconds. The long times experienced before hydrates are detected are caused by mass transport limitations due to the initial thin hydrate film formed at the interface between water and the hydrate former interface. Another misunderstanding about hydrate nucleation is that only one uniform-phase hydrate is formed from either a single guest or a multicomponent mixtures of hydrate formers. Based on the combined first and second laws of thermodynamics, nucleation will commence with the most stable hydrates, under the constraints of heat and mass transport. Nucleation can happen via different routes: hydrate formation will originate at the interface between the guest molecule phase and water. A range of hydrates with different compositions of the original hydrate former(s), different densities and different free energies will form from aqueous solution (dissolve hydrate formers). Theoretically, hydrate can also nucleate from water dissolved in the guest molecules phase. Such hydrate cannot be stable because of the little mass of water that will dissolved in the guest molecule phase as well as limitation of heat transport, especially in the case of hydrocarbon guests like methane which is a poor heat conductor. The thermodynamics of simultaneous natural gas production from in-situ CH4 hydrate and CO2 long-term offshore storage was studied. Two processes where studied: mixing of nitrogen with the CO2 and injecting the mixture into the hydrate reservoir and the implication of the enthalpies of hydrate phase transitions. The study indicated that the proportion of CO2 needed in the CO2/N2 mixture is only about 5 – 12 % without H2S in the gas stream. While it is about 4 – 5 % and 2 – 3 % with the presence of 0.5 % and 1 % of H2S respectively. Virtually, direct solid-state CO2–CH4 swap will be extremely kinetically restricted, and it is not significant. Enthalpy changes of hydrate phase transition in literature obtained from experiment, Clausius-Clapeyron and Clapeyron models are limited and often lack some vital information needed for proper understanding and interpretation. Information on thermodynamic properties such as pressure, temperature (or both), hydrate composition, and hydration number are often missing. The equation of state utilised is also not stated in certain literature. Several experimental data also lack any measured filling fractions, and frequently, they apply a constant value which suggests that the values may be merely guessed. In addition, older data based on Clapeyron equation lack appropriate volume corrections. The calculations of both Clausius-Clapeyron and Clapeyron equations are based on hydrate equilibrium data of pressure and temperature from experiments or calculated data. But hydrate formation is a non-equilibrium process. Information about superheating above the hydrate equilibrium conditions to totally dissociate the gas hydrate to liquid water and gas is normally lacking. The values vary considerably in such a way that some of them decided to base their results on average values over a range of temperatures. For example, Gupta et al. (2008) conducted a study with experiment, Clausius-Clapeyron and Clapeyron equations but all the results varied substantially. We therefore propose a method based on residual thermodynamics which does not have the limitations of the current methods. We do not expect much agreement of our results with a lot of the literature, firstly, because of the limitations of the other methods, especially, the simplicity of both the Clapeyron and Clausius-Clapeyron equations. Secondly, the remarkable disagreement among current data reported in literature. The residual thermodynamics scheme used in this project is based on the unique and straight forward thermodynamic relationship between change in free energy and enthalpy change, with thermodynamic properties evaluated from residual thermodynamics. Such properties are change in free energy as the thermodynamic driving force in kinetic theories, equilibrium curves, and enthalpy changes of hydrate phase transition. With residual thermodynamics, real gas behaviour taking into account thermodynamic deviations from ideal gas behaviour can be evaluated. The results of enthalpy changes of carbon dioxide hydrate phase transitions using residual thermodynamics in this project are around 10 – 11 kJ/mol guest molecule greater than the ones of methane hydrate phase transition for 273 – 280 K range of temperatures. Calculations based on kJ/mol hydrate within the same temperature range gave 0.5 – 0.6 kJ/mol hydrate. Anderson’s results using Clapeyron equation are a little close to the results obtained in this work, precisely 10 kJ/mol and 7 kJ/mol guest molecule at 274 K and at 278 K respectively. While Kang et al. (2001) in their experiment put this difference at 8.4 kJ/mol guest molecule at 273.65 K. However, in replacement of in-situ CH4 hydrate with CO2, it is not the temperature-pressure curve that is most essential, but what is most important is the difference in free energies of both hydrates, CH4 hydrate and CO2 hydrate, and the enthalpies of CO2 hydrate formation relative to the enthalpies of CH4 hydrate dissociation. The free energy of CO2 hydrate is around 1.8 – 2.0 kJ/mol more negative or lower than the free energy of CH4 hydrate within a temperature range of 273.15 – 283.15 K (0 – 10 °C). That confirms that hydrate of CO2 is more stable thermodynamically than hydrate of CH4. It is pertinent to state that this proposition is still under investigation, and it is still under development. In addition, there are constraints that are also under study. Hydrate formation at the interface between CO2 gas and liquid water is very rapid, forming a hydrate film which will quickly block the pore spaces thereby limiting further CO2 supply. Studies also need to be done on finding the most efficient and effective way to reduce the thermodynamic driving force, either by using any thermodynamic inhibitor or other substances.