多孔質体におけるメタンハイドレートの生成・分解挙動―メタンハイドレート貯留層の浸透率評価に関する研究 Ｉ―

Natural gas hydrates are mainly stored in the pores of the sedimentary layer in permafrost regions, and the formation characters in porous media were particularly important for the exploitation and utilization of natural gas hydrates. So, it is essential to understand the formation process of methane hydrate in porous media below freezing point. In this study, the formation process of methane hydrate was studied in a closed system in porous media below freezing point. The results indicated that the initial pressure played an important role in the formation characteristics of methane hydrate in porous media below freezing point. The higher the initial pressure was, the larger the formation rate of methane hydrate. And the maximum formation rate attained 6.46 × 10−4 mol/h when the initial pressure was 9.0 MPa under the same temperature and particle size conditions. Furthermore, the final conversion rate was larger at higher initial pressure and the final conversion rate attained 56.5% at an initial pressure of 9.0 MPa. Furthermore, the pressure disturbance could improve the formation process of methane hydrate and the final conversion rate was larger to some extent. The relevant results will provide a theoretical reference for natural gas hydrates exploitation.

Experimental study on gas production from methane hydrate in porous media by SAGD method

Formation and storage characteristics of CO2 hydrate in porous media: Effect of liquefaction amount on the formation rate, accumulation amount

The behaviors of methane hydrate formation in porous media are investigated in three-dimensional vessels. The effects of the fugacity difference, the water–gas ratio, and the volume of the vessel on the formation rate of methane hydrate are studied. The results show that the formation rates are disproportionate to the fugacity differences but in proportion to the volume of the vessel and the change of the initial gas–water ratio has little effect on the rate of hydrate formation. Meanwhile, according to the discussion about the temperatures and resistances in hydrate reservoir, it is confirmed that methane hydrate forms from the boundaries to the center of the vessel. Furthermore, a new method is designed to form methane hydrate samples in porous sediments with high hydrate/water saturation and low gas saturation.

Natural gas hydrate, as an unconventional resource, has been attracting increasing attention. Understanding the characteristics of methane hydrate formation and dissociation in porous media is important for developing gas hydrate-bearing reservoirs. This work discusses the use of low-field nuclear magnetic resonance (LF-NMR) technology to investigate the formation and dissociation of methane hydrate in the sandstone. In this work, an experimental assembly wherein methane hydrate can form and dissociate, is used to conduct LF-NMR measurements. LF-NMR, as a noninvasive measurement technology, combines the transverse relaxation time (T2) measurement with the magnetic resonance imaging (MRI). T2 measurements can explore the characteristics of methane hydrate formation and dissociation in core samples from a pore-scale perspective. MRI can display the spatial distribution of water from a core-scale perspective. The excess-gas method and the excess-water method are successively applied to form methane hydrate, and depressurization is applied to dissociate methane hydrate in the laboratory. The characteristics of methane hydrate formation and dissociation is studied in the sandstone. Experimental results show that the signal intensity of short T2 and long T2 decreases simultaneously in the process of the methane hydrate formation using the excess-gas method, indicating that methane hydrate is formed in both small and large pores. When using the excess-water method, the signal intensity of long T2 decreases, and the signal intensity of short T2 increases in the process of the methane hydrate formation, indicating that methane hydrate is mainly formed in large pores. Methane hydrate is dissociated simultaneously in both small and large pores when using the depressurization method. Water content in small pores gradually increases. Capillary pressure causes some water to remain in the core samples following dissociation. Water content in large pores decreases initially and then increases during depressurization. In the early stages of depressurization, more water leaves large pores than is generated by hydrate dissociation. In the later stages of depressurization, less water leaves the large pores than is generated by hydrate dissociation. This study may inspire the new understanding on distribution of fluid in sediments during the process of accumulation and exploitation of natural gas hydrates.

Effects of water-gas two-phase flow on methane hydrate dissociation in porous media

The Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium), which was organized to attain the exploration and exploitation of methane hydrate (MH) resources, has been evaluating MH reservoirs located in the Eastern Nankai Trough from the viewpoints of geology, geophysics, petrophysics and reservoir/production engineering. As one of these studies, we have been predicting gas/water production performances from these MH reservoirs showing diverse characteristics. This paper presents the results of our examinations on the producibility of gas from MH reservoirs located in the Eastern Nankai Trough by a variety of MH dissociation/production methods and on the feasibility of the future development of MH in terms of gas production and economics.Eastern Nankai Trough MH reservoirs, which are composed of alternating beds of sand, silt and clay in turbidite sediments, have various conditions of clay distribution as well as of initial pressure, temperature, permeability and MH saturation. Some of these reservoirs contain MH of high saturation at a certain interval (MH concentrated reservoirs), while in the others MH is deposited sparsely (MH non-concentrated reservoirs). First, the numerical near well model for one of the typical MH reservoirs was constructed mimicking this alteration of sand, silt and clay layers by simplified grid layer system, which was used for the simulation to clarify the mechanisms of MH dissociation and production by the application of the depressurization method.The detailed numerical reservoir models were then constructed for the vicinity of the wells located both in MH concentrated and in non-concentrated reservoirs, consulting the well log and core interpretation results. MH dissociation/production performances were predicted through numerical simulation assuming the application of various MH dissociation methods such as depressurization, wellbore heating, hot water huff'n'puff and hot water flooding. The simulation studies revealed the difference in the gas production between MH concentrated and non-concentrated reservoirs. These studies also suggested that the methane recovery from MH concentrated reservoirs by depressurization methods ranged from 30 to 60% and that the energy efficiency by thermal methods was very small despite high methane recovery. Furthermore, simple economic analyses on the basis of these simulation results exhibited the promise that some MH reservoirs in the Eastern Nankai Trough could be economically developed if the well spacing and MH dissociation/production methods were appropriately designed.

Investigation of the methane hydrate surface area during depressurization-induced dissociation in hydrate-bearing porous media

Methane hydrate formation and dissociation behaviors in montmorillonite

The replacement technique of CO2–CH4 hydrate in naturally occurring gas hydrates is a promising method for achieving CO2 storage and CH4 recovery in permafrost regions. Revealing the formation characteristics of CH4 hydrate becomes an urgent need in porous medium below the freezing point. For this purpose, the formation experiment was carried out under different conditions and the formation characteristics were systematically investigated. The result indicated that the initial pressure was the key factor affecting the formation process of CH4 hydrate. And the final pressure drop was greater at higher initial pressure conditions. The final pressure drop reached to 0.92 MPa when the initial pressure was 9 MPa under the same temperature conditions. The results also illustrated that, the formation rate of hydrate was the fastest when the temperatures were closely to the freezing point. The average formation rate of CH4 hydrate reached to 0.646 mmol/h when the temperature was 273.15 K. Moreover, the smaller the particle size was, the higher the final conversion rate of ice. The final conversion rate reached to 56.5% when the initial pressure was 9 MPa. The results are of significance for exploiting natural gas hydrate in the permafrost regions. Highlights The formation characteristics of methane were studied in porous media below the freezing point. The higher initial pressure could enhance the final conversion rates of ice. The formation rate of methane was the fastest when the temperature was closely to the freezing point. The pressure disturbance could improve the formed amount of methane hydrate in porous media.

Effects of pressure and sea water flow on natural gas hydrate production characteristics in marine sediment

The numerical simulator TOUGH+HYDRATE (T+H) was used to predict the transient pure methane hydrate (no sediment) dissociation data. X-ray computed tomography (CT) was used to visualize the methane hydrate formation and dissociation processes. A methane hydrate sample was formed from granular ice in a cylindrical vessel, and slow depressurization combined with thermal stimulation was applied to dissociate the hydrate sample. CT images showed that the water produced from the hydrate dissociation accumulated at the bottom of the vessel and increased the hydrate dissociation rate there. CT images were obtained during hydrate dissociation to confirm the radial dissociation of the hydrate sample. This radial dissociation process has implications for dissociation of hydrates in pipelines, suggesting lower dissociation times than for longitudinal dissociation. These observations were also confirmed by the numerical simulator predictions, which were in good agreement with the measured thermal data during hydrate d...

Effect of capillary term parameters on the thermodynamic modeling of methane hydrate formation in porous media

Natural gas hydrate is a promising energy resource, but it is challenging to recover methane from the clay pores rich with sediment organic matter due to the inadequate information on hydrate evolution in the organoclay complex. Molecular dynamics simulations were conducted to investigate the methane hydrate formation and dissociation in the sodium montmorillonite interlayer (Na-MMT) with fatty acids by characterizing the four-body structural order parameter, radial distribution functions, and cage types. Results demonstrated the slight inhibition of fatty acid types (butyric and isovaleric acid) at molar concentrations (0.6% and 1.5%) on the methane hydrate formation in the context of this study. This finding was a result of several main processes including the formation of "quasihydrate" water structures around the hydrophobic carbon chains of fatty acids, the disruption of hydrogen bond networks by the hydrophilic carboxyl of fatty acids, and the restriction on the thermal motion of the sodium ions by coordination with the carboxyl of fatty acids. Results also demonstrated the promotion of fatty acids on the methane hydrate dissociation in the Na-MMT, which highlighted the role of fatty acid accumulation in the accelerated breakdown of hydrates at relatively low decomposing temperature. Overall results provided theoretical support for better understanding the formation mechanisms and decomposition strategies of methane hydrates in the heterogeneous sediment environments since fatty acids are microbial intermediates of special importance in the hydrate-bearing sediment.

Investigation on the induction time of methane hydrate formation in porous media under quiescent conditions