Hironori Haneda

VARIABLE-COMPLIANCE-TYPE CONSTITUTIVE MODEL FOR METHANE HYDRATE BEARING SEDIMENT

In order to evaluate a methane gas productivity of methane hydrate reservoirs, it is necessary to develop a numeric simulator predicting gas production behavior. For precise assessment of longterm gas productivity, it is important to develop a mathematical model which describes mechanical behaviors of methane hydrate reservoirs in consideration of their time-dependent properties and to introduce it into the numeric simulator. In this study, based on previous experimental results of triaxial compression tests of Toyoura sand containing synthetic methane hydrate, stress-strain relationships were formulated by variable-compliance-type constitutive model. The suggested model takes into account the time-dependent property obtained from laboratory investigation that time dependency of methane hydrate bearing sediment is influenced by methane hydrate saturation and effective confining pressure. Validity of the suggested model should be verified by other laboratory experiments on time-dependent behaviors of methane hydrate bearing sediment.

EXPERIMENTAL STUDY OF ENHANCED GAS RECOVERY FROM GAS HYDRATE BEARING SEDIMENTS BY INHIBITOR AND STEAM INJECTION METHODS

The inhibitor and steam injection methods have been examined using a laboratory-prepared methane hydrate bearing sediment. New experimental apparatuses have been designed and constructed. In the case of inhibitor injection, the measurement of gas production vs. time suggested that the inhibitor increased dissociation rate. Core temperature decreased upon the inhibitor injection, in contrast to that in the case of pure water injection. The observed pressure differentials between the inlet and outlet of the core sample suggest that the inhibitor effectively prevented the hydrate reformation within the dissociating core sample. In the case of steam injection coupled with depressurization, it can be seen that the effect of steam (or hot water) injection was clear in the later stage of dissociation, compared with that in the case of depressurization alone. The inner (core) temperature change indicates that the coupling of depressurization and steam injection induces MH dissociation from upstream and downstream to the center of the sample. However, it starts from an upstream region and continues downstream steadily in the case of steam (hot water) injection alone.

Experimental Study on Improvement of Permeability and Gas Production Behavior by the Simultaneous Injection of Hot Water and Nitrogen-Estimation of Permeability in Methane Hydrate Reservoir, Part3-

Methane hydrate is one of the potential resources of natural gas in the near future, because it exists in marine sediments or in permafrost regions worldwide. Some extraction methods of methane hydrates from the reservoir has been proposed, such as depressurization, thermal stimulation and inhibitor injection. These are all based on the in-situ dissociation process of methane hydrates that is transformed into methane methane and water. Therefore, it is very important to clarify the physical phenomena of gas-water multiphase flow in porous media and the properties of formation and dissociation of methane hydrates.We have carried out the experimental study on hot water injection as one of the thermal stimulation methods. From the results, it was found that (1) when temperature in downstream zone of sand column was lower than equilibrium condition, additional hydrate growth was promoted at downstream zone due to migration of cooled water and dissociated gas, (2) as a result, differential pressure increased exponentially, and water permeability of sand column decreased drastically.For inhibition of hydrate growth and improvement of permeability in hydrate reservoir, we conducted further experimental work on the simultaneous injection process of nitrogen and hot water. Nitrogen has the effect as an inhibitor as well as methanol and salts. In this experiment, firstly, nitrogen was injected into sand column to displace free methane gas, and then hot water injection was started. In the progress of dissociation, temperatures in the sand column and differential pressures, production rate of dissociated gas were measured. Additionally, based on measuring data, water permeability in dissociation process was estimated. Due to the inhibitor effect of nitrogen, it was possible to continue water injection without permeability reduction. Thus dissociated gas production was completed earlier in comparison with normal hot water injection process.

The Effect of CO2-Air Mixture Compositions on the Formation and Dissociation of CO2 Hydrate

Abstract: The disposal of carbon dioxide to the marine and sea bed sediments as CO2 gas hydrate is an innovative technique for solving the global environment issue. Experiments on the formation and dissociation of gas hydrate have been carried out using a pressure vessel to investigate the effect of carbon dioxide concentration in the gas phase. From the experiment results, the following are clarified: (1) There is a strong relationship between the partial pressure of carbon dioxide, concentration, and the temperature of formation and dissociation of gas hydrate. Therefore, the use of this relation enables the estimation of equilibrium conditions of the gas mixture. (2) The initial formation rate varies from 0.1 to 0. 5 ml/(min·g). In terms of average values, the initial formation rate increases as the carbon dioxide concentration of the initial gas mixture increases. (3) From the analysis of component gas of gas hydrate and space gas, it can be assumed that nitrogen and oxygen are also incorporated into the hydrate structure cage as guest molecules. Moreover, it can be seen that the carbon dioxide concentration in the initial space gas is higher than that in the space gas at the time of gas hydrate formation. Therefore, this hydrate technology applies to the concentration of carbon dioxide. In future, we will attempt to carry out tests on the formation and dissociation of CO2 hydrate under a low concentration of CO2. Furthermore, we will analyze the structure of gas hydrate using Raman spectroscopy to clarify that nitrogen and oxygen are incorporated into the gas hydrate cage as guest molecules.

Study on CO2 Hydrate Formation as Stockpiling in Marine Sediments

Publisher Summary Methane hydrate reservoirs are found in marine sediments, and various proposals have been reported to produce natural gas from such reservoirs. The original concept of a methane hydrate production system has been proposed, in which carbon dioxide hydrate is used. In this technique, the CO2 hydrate layer develops in the upper part of the methane hydrate reservoirs, and the artificial roof is constructed to prevent a landslide; it may also prevent the emission of decomposed methane gas into the marine environment. In addition, the system features the sequestration of CO2 by the formation of CO2 gas hydrate after the mining of methane gas hydrate. This makes it possible to maintain the stability of sea floor, and to prevent geo-hazards such as sudden landslides. If this technique is put into practice, the energy problem and global environmental problems will be solved at the same time. This chapter presents the result of experiments on the formation and the growth of CO2 hydrate using an apparatus that simulates marine sediments. Both of the landslides at the sea floor and the emission of methane into the sea can be prevented by using the technique. In addition, the geological structure can be reinforced by the formation of a solid CO2 hydrate layer where the methane hydrate was extracted. This also features the sequestration of CO2 gas into the sediment. The experimental data of gas consumption was compared with theoretical data.

Behavior of CO2 Hydrate Growth below Melting Point of Ice-Research on formation and dissociation of gas hydrates.

The mining systems of methane hydrate reservoirs using CO2 hydrate are proposed, i.e. the artificial block system and the CH4-CO2 replacement system. In order to accomplish the concepts, it is necessary to understand the mechanism of formation kinetics of gas hydrates. There are few experimental data on the growth rate of CO2 hydrate, particularly in the temperature range below melting point of ice, and the mechanism has not been well understood.In this study, two types of experiments were carried out on formation kinetics of CO2 hydrate below melting point of ice, using Raman spectrometry and gas consumption method.Spectra of CO2 trapped in the hydrate cages were directly observed by Raman spectrometry. The result showed the amount of CO2 molecules increased with time, and increasing rates depend on the temperature and pressure conditions. From the experiment of gas consumption method, it was found that the growth rate of CO2 hydrate has two stages. It is indicated that the first stage of growth is rapid growth mode caused by quasi-liquid-layer or locally melted water by the latent heat, and then the latter stage is slower growth mode between gas and ice. In the temperature range of-5 ∼-1 °C, more than 40 % of total reaction gas was consumed in the first stage of growth. Considering that the reaction at the first stage is similar to that of H-LW-V system, an empirical equation was applied to the experimental results. Reaction coefficients in the first stage were estimated to determine the growth of CO2 gas hydrate. From these results, we address that proper use of the first stage growth of CO2 gas hydrate is important to develop the advanced mining systems for methane hydrate.