Analytical study of CO2–CH4 exchange in hydrate at high rates of carbon dioxide injection into a reservoir saturated with methane hydrate and gaseous methane

Methane hydrate exists in huge amounts in certain locations, in sea sediments and the geological structures below them, at low temperature and high pressure. Production methods are in development to produce the methane to a floating platform. There it can be reformed to produce hydrogen and carbon dioxide, in an endothermic process. Some of the methane can be burned to provide heat energy to develop all needed power on the platform and to support the reforming process. After separation, the hydrogen is the valuable and transportable product. All carbon dioxide produced on the platform can be separated from other gases and then sequestered in the sea as carbon dioxide hydrate. In this way, hydrogen is made available without the release of carbon dioxide to the atmosphere, and the hydrogen could be an enabling step toward a world hydrogen economy.

A mathematical model of formation of carbon dioxide gas hydrate upon injection of warm carbon dioxide into a natural stratum saturated with methane and methane hydrate has been presented. The case when methane hydrate decomposes into gas and water on two frontal boundaries and the subsequent formation of carbon dioxide hydrate from carbon dioxide and water has been discussed. The regions where this mode is implemented depending on stratum permeability have been studied based on the pressure–temperature plane of the gas being injected into the stratum.

The mechanism of replacement of methane by carbon dioxide in the hydrate in the process of CO2 injection into a reservoir with formation of fronts of methane hydrate dissociation and carbon dioxide hydrate generation is investigated. It is found that such a replacement regime can be implemented in both low- and high-permeability reservoirs. It is shown that in the highintensity injection regime the heat flux from the well does not affect propagation of the fronts of methane hydrate dissociation and carbon dioxide hydrate generation. In this case the replacement regime is maintained by only the heat released at formation of carbon dioxide hydrate. An increase in the injection pressure may lead to suppression of methane hydrate dissociation and termination of the replacement reaction. The critical diagrams of existence of the regime of conversion of methane hydrate to carbon dioxide hydrate are constructed.

Kinetic mechanisms of methane hydrate replacement and carbon dioxide hydrate reorganization

Hydrate phase transition kinetics from Phase Field Theory with implicit hydrodynamics and heat transport

A mathematical model of the replacement of methane with carbon dioxide in hydrate at high rates of carbon dioxide injection into a reservoir is proposed. The replacement-reaction zone is modeled by the front of the methane hydrate conversion to carbon dioxide hydrate. A comparison is made with a regime that takes into account the formation of two surfaces of phase transitions, i.e., the front of dissociation of CH4 hydrate and the front of formation of CO2 hydrate. It is shown that a high injection pressure suppresses the dissociation of methane hydrate, and no replacement reaction is implemented. With significant volumes of hydrate in the reservoir, only a partial formation of carbon dioxide hydrate occurs. The critical diagram of the replacement-reaction modes is constructed.

In the presented work, the influence of carbon dioxide parameters on the features of CO2 – CH4 replacement in methane hydrate is investigated for the case of a negative reservoir temperature (less than 0 °C). The mathematical model is built on the assumption that when carbon dioxide is injected into a reservoir saturated at the initial time with methane and its hydrate, three regions are formed: near, far and intermediate. The near region, adjacent to the outer boundary of the reservoir and through which carbon dioxide is injected, is saturated with carbon dioxide and its hydrate; the far region of the reservoir is saturated with methane and its hydrate, and the intermediate one, separating the near and far regions, is saturated with methane and ice. Computational experiments have shown that the presented scheme with the formation of three regions, where the intermediate region is saturated with methane and ice, is valid only for certain values of the parameters of injected carbon dioxide. It was found that with an increase in the intensity of phase transitions, the formation of carbon dioxide hydrate occurs from carbon dioxide and water; in this case, three or four regions can be formed in the reservoir. In the case of the formation of three regions, the intermediate region contains only water and methane instead of ice and methane. When four regions are formed in the reservoir, in addition to the three regions adopted in the problem statement, an additional region is formed, saturated with methane and water. For the case of low values of the intensity of injection of carbon dioxide, the mode of replacement of methane in the hydrate with carbon dioxide is possible. In this case, only two regions are formed in the reservoir, without the formation of an intermediate region saturated with methane and ice.

Sequestration of carbon dioxide (CO2) in gas hydrate and its disposal in shallow sediments under hydrate stability conditions is an efficient way of reducing carbon dioxide emission to the atmosphere. Laboratory modelling of carbon dioxide hydrate formation in freezing and frozen sand demonstrates that sediments within and below permafrost are potentially suitable reservoirs in this respect. The experiments are conducted in high-pressure cells with automatic pressure and temperature monitoring during hydrate formation in ice-bearing sand samples saturated with carbon dioxide at constant negative temperatures of −1°C to −8°С. The work includes testing the sensitivity of pore carbon dioxide formation to temperature, cyclic freezing/thawing, and initial ice saturation. According to experimental evidence, pore carbon dioxide hydrate can form in a large range of negative temperatures, while pore moisture can be liquid or solid. Hydrate formation accelerates when the samples are exposed to cyclic freezing and thawing. The formation of carbon dioxide hydrate is most active in the 45%–65% range of initial ice saturation. The results have implications for possible patterns of carbon dioxide hydrate formation in permafrost by injection of carbon dioxide into the zone of hydrate stability and for respective permafrost responses.

The process of injection of carbon dioxide into a porous reservoir, which initially contains methane and its hydrate, is considered. The initial thermodynamic conditions in the reservoir correspond to the stable existence of methane hydrate. During the entire injection process, the reservoir temperature is negative, that is, water can be present either in the form of ice, or in the composition of methane or carbon dioxide hydrates, and the pressure is such that carbon dioxide does not condense. For the considered process, a mathematical model of non-isothermal filtration of a two-component gas is presented, taking into account the formation and decomposition of hydrates. Numerical experiments have been carried out on the basis of this mathematical model. The graphs of the distributions of the main parameters in the reservoir (partial pressures of the gas mixture components, temperature, saturations of the porous medium with various substances), as well as graphs of the time variation of the total mass of carbon dioxide buried in the reservoir and the mass of carbon dioxide in the hydrate are given. It is shown that the injected carbon dioxide displaces methane from the injection boundary and replaces the methane in the hydrate and the greater the pressure of the injected carbon dioxide, the greater the mass of the hydrate in which gas is replaced.

Most of the natural gas hydrates are found in deep marine sediments and permafrost regions, where the presence of salts and porous media are quite evident. With this, we develop a computationally efficient mathematical model that can expound the clathrate hydrate dynamics in a reservoir-mimicking environment. Along with proposing the chemical potential difference as the driving force to take care of the thermodynamic aspect, a nonstoichiometric reaction with nth-order kinetics is for the first time introduced in the line of adsorption kinetics. To make this thermokinetic model more rigorous, the diffusion part is further formulated with the kinetic factor, along with incorporating various practical aspects, including reaction surface renewal and hydrate formation in nanometer-sized pores of irregular and distributed particles. Finally, to examine the validity of this rigorous model, experimental case studies of methane (CH4) and carbon dioxide (CO2) hydrate formation in various porous media with pure and saline water are used. In addition, we compare the developed model with the existing formulations of gas hydrate dynamics, and it is perceived that the proposed model outperforms the existing models with reference to the experimental data of methane and carbon dioxide hydrate formation at diverse geological conditions.

Based on the equations of continuous medium mechanics, a mathematical model is constructed for the injection of liquid carbon dioxide into the porous medium saturated with methane and water for the case when the injection is accompanied by the formation of gas hydrates of carbon dioxide and methane. Self-similar solutions to the one-dimensional problem describing the evolution of hydrodynamic and temperature fields are constructed. The dynamics of movement of frontal boundaries of phase transitions with respect to injection pressure and temperature, as well as with respect to permeability and initial pressure of the porous medium is investigated. It is shown that intensity of the formation of methane and carbon dioxide gas hydrates increases with an increase in injection pressure and reservoir permeability, and at rather high values of permeability, pressure drop in the reservoir and injection temperature, the frontal formation boundaries of methane and carbon dioxide gas hydrates may merge. The dependence of the limit values of injection pressure and temperature corresponding to the merger of the phase transition boundaries on the permeability and initial pressure is studied in this work.

The carbon dioxide injection into a reservoir which contains methane and water in the free state is investigated. A mathematical model for forming carbon dioxide hydrate at the phase transition front that separates the methane and carbon dioxide regions is proposed. Conditions at the interface are derived and an asymptotic solution of the problem is found. It is shown that the CO2 hydrate formation may occur at pressures and temperatures that do not lie on the dissociation curve. Critical diagrams of the process, which determinine the range of parameters at which the complete transition of the injected gas into the hydrate state takes place are obtained.

During the formation of gas hydrate crystals, stable isotope fractionation of the guest molecules occurs. For methane and ethane, hydrogen stable isotope fractionation has been reported in the range of a few per mil, but for carbon stable isotope fractionation, there is no difference in 13C, or the difference is below the detection limit. As for carbon dioxide in natural gas hydrates, little is known about stable isotope fractionation of carbon dioxide. In this study, we report the stable isotope fractionation during formation of synthetic carbon dioxide hydrates under various temperature conditions. Fine ice powder and pure carbon dioxide gas were introduced into a pressure cell and adjusted to be above the equilibrium pressure of the hydrate and below the liquefaction pressure of the carbon dioxide at each temperature. After the formation of carbon dioxide hydrate, the residual gas was collected and the hydrate was recovered under the liquid nitrogen temperature to obtain the hydrate-bound gas. The carbon stable isotope ratios of carbon dioxide in the hydrate and residual gases were determined using a stable isotope mass spectrometer. Over a wide range from 243 K to 278 K, the 13C of residual gas was always larger than the hydrate phase, indicating that the carbon dioxide hydrate preferentially enclathrated lighter molecules. This trend is consistent with the results of a previous study. These results suggest that the equilibrium pressure of 13CO2 hydrate is slightly higher than that of 12CO2 hydrate.

A new mathematical model of gaseous carbon dioxide injection into formation saturated with free methane and methane hydrate is proposed. It is assumed that the methane hydrate is converted to carbon dioxide hydrate under suitable thermodynamic conditions. At a high injection rate the replacement zone is narrow and modelled by the reaction front. The full system of boundary conditions at the front is derived. The obtained similarity solution describes three various regimes of the injection. At a low reservoir temperature and hydrate saturation the simple regime of methane displacement is realized. In contrast, at a high injection rate and large values of the initial temperature and hydrate saturation the CO2 hydrate becomes superheated due to the excess of heat. At moderate values of the reservoir temperature, hydrate saturation and the injection pressure the self-sustained replacement reaction is developed.

The scheme and the corresponding theoretical model of process of methane replacement by liquid carbon dioxide at his injection in a gas hydrate layer are offered. The flat one-dimensional automodel solution corresponding to injection of carbon dioxide in semi-infinite layer through flat border is constructed. It is fixed that such solution can contain three characteristic zones in general case: the far zone, where there is a methane filtration in the porous medium which is partially saturated by methane hydrate in the absence of phase transitions; the melted zone of products of decomposition of methane hydrate (water and methane) can be formed in the intermediate zone (this zone is formed because of heating owing to injection of warm carbon dioxide); the near zone where there is a filtration of liquid carbon dioxide in the layer which is partially saturated by hydrate of carbon dioxide. The analysis of influence of parameters of layer, his initial state (temperature, pressure and hydrate saturation) and the parameters of the injected dioxide of carbon on the various modes of filtration is conducted.