窒素圧入によるメタンハイドレート貯留層からの新規ガス回収法に関する実験的研究

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.

Chapter 1 - Methane production techniques from methane hydrate reservoirs

In order to obtain fundamental data for the evaluation of permeability in methane hydrate reservoir, we have conducted experimental and analytical studies on the behavior of formation and dissociation of methane hydrate in porous media and the change in permeability in the process of formation and dissociation. The experimental apparatus for permeability measurement was originally designed and manufactured. Very uniform methane hydrate reservoir was achieved by the method of gas injection into sand column. The artificial sample of sand column with methane hydrate was satisfactory for the experiment of absolute permeability measurement. As a result of the experiment, it was found that the formation rate of methane hydrate in porous media depends on the particle size distribution of sands composing porous media. The interfacial condition between methane gas and pore water was an essential parameter of formation. In an experiment of dissociation of hydrate by the injection of hot water, the increase of permeability in methane hydrate reservoir was observed as a function of time. The large decrease in permeability was also obtained as a result of reformation due to flow of dissociated gas in porous media. The apparent absolute permeability drastically decreased as the hydrate saturation in sand column becomes lower. The values of the permeability in the case of hydrate saturation 40 % were 1/30 to 1/50 compared with the original values without any hydrate formation in porous media.

An experimental study on the effect of nitrogen injection on the deformation of coal during methane desorption

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

In recent years, the use of nitrogen has increased as gas injection to recover oil fluids near the critical point. The behavior of hydrocarbon mixture phases in the critical region shows very interesting complex phenomena when facing a recovery project with nitrogen. Therefore, it is important to have experimental information of the PVTx thermodynamic variable, often scarce, for this type of critical phenomena. This paper reports the experimental measures of the volumetric behavior and phases of synthetic fluid in a nitrogen injection process. The experiment was performed at laboratory scale, and it obtained variations on the saturation pressure, gas oil ratio, density and composition of the hydrocarbon phase when nitrogen was injected at molars of 10, 20, 30 and 40% on different volumetric portions of the mother sample. In addition, the data obtained experimentally was used to demonstrate the capacity of tune to compositional models. The data provided represents a valuable contribution to the understanding of phenomena associated with retrograde and near critical regions, as well as their use in tuning and developing more elaborate models such as Cubic Equations of State (EOS). It is worth highlighting the importance of this data in the potential processes of nitrogen, CO2, and lean gas injection, which require knowledge of the gas-oil ratio, saturation pressures, density and composition of the fluid in current production. The identification of the phenomena shown, represent a potential application to the modeling of displacements and maintaining the pressure in the improved recovery when scaling up the laboratory data to the field / reservoir conditions.

Geological setting and characterization of a methane hydrate reservoir distributed at the first offshore production test site on the Daini-Atsumi Knoll in the eastern Nankai Trough, Japan

Numerical investigation on the long-term gas production behavior at the 2017 Shenhu methane hydrate production site

Study on CO2 Hydrate Formation as Stockpiling in Marine Sediments

Production strategy for oceanic methane hydrate extraction and power generation with Carbon Capture and Storage (CCS)

In recent years, the exploration and utilization of methane hydrate from permafrost and off-coast regions has become a very hot topic. Aiming at its potential as a vast energy resources in future use, the current study is focused on the low emission extraction and utilization system for oceanic methane hydrate and its economic and technological feasibility. In this study, recent real extraction tests in both China and Japan in 2017 are firstly discussed and compared, where the challenges in production period and production rate are explained. In those tests, depressurization method is used to dissociate the methane hydrate for gas production form the unconsolidated reservoir layers below the seabed. Based on the conditions of Japan, a systematic utilization system is proposed for oceanic methane hydratewhich is consisted of extraction system, power generation system, Carbon Capture and Sequestration (CCS) system and hot fluid injection system. Vertical well systems are adopted in this design and the strategic warming up process is used before depressurization extraction, so as to obtain optimal production rate of methane gas. The inclusion of CCS makes it possible to utilize methane hydrate with very low carbon emission. It is estimated that the electricity price obtained by this system can be comparable with the current civil electric price in Japan if the production rate is around 1.0×105 m3/d. Also in this study, the real extraction scenarios and possible strategies of Nankai Trough in Japan and Shenhu Area in China are re-visited and discussed by numerical models. It is found that the only depressurization or thermal stimulation method have the problems of wellbore deformation, pressure problem and energy efficiency problems, while combined strategic modes show the possibility of long-term continuous production. For the basic dissociation parameter analysis, the conditions near the equilibrium curve show higher production rate, which is due to higher initial reservoir temperature. Such results indicate the importance of heat supply inside the reservoir layer as the methane hydrate dissociation is endothermic. It is shown that with the increase of reservoir pressure, such temperature effects become more critical as the optimal initial temperature increases according to equilibrium curve. This results support the system design with strategic warming-up process in a sister well during methane hydrate extraction. In addition, the effects of reservoir layer thermal management, hot reservoir fluid flow effects on the dissociation behaviors are analyzed in this study. For vertical well analysis, the production rates under different permeability parameters show the same trend with real production, which in turn help confirm the in-lab permeability tests. Horizontal well simulation show the importance of overburden and under burden. If the confining layers are less permeable, the production rate becomes higher due to the limited intrusive water flow from the confining layers. In sum, this study summarized the advantages and disadvantages of oceanic methane hydrate extraction/utilization methods and put forward a low-carbon emission system. It is hoped that this study will contribute to the future development of both production prediction and real extraction tests.

Depleted methane hydrate (MH) reservoirs are potential sites for CO2 storage. Hydrate formation during the CO2 flow process in a dissociated MH sample was simulated to clarify the formation characteristics of CO2 hydrates and their effect on CO2 storage. Experiments included MH formation and dissociation, CO2 injection, water‐injection and CO2 hydrate dissociation, and liquid‐water distribution as monitored by magnetic resonance imaging (MRI). It was observed that the initial water saturation determined the hydrate saturation in the artificial sediment, and the depressurization range was the main factor influencing MH dissociation for the excess gas sample. Pressure is the key factor influencing hydrate formation during CO2 flow. An increase of the CO2 flow rate led to a decrease of both hydrate saturation and conversion of the injected CO2. The cumulative amount of injected water is not the key factor controlling CO2 hydrate formation, but it does determine the residual water saturation.

Liquid nitrogen is a vital medium during the extinguishing and chilldown process of coal spontaneous combustion in coal mine goafs. In this paper, the heat and mass transfer of liquid nitrogen in a loose medium was investigated. A laboratory system including a temperature sensing system was designed and built to explore the effects of different nitrogen injection positions and angles on the chilldown effect. The results indicate that after liquid nitrogen injection, the temperatures in the liquid nitrogen flow area and the bottom of the model can be quickly reduced to −196 °C, which was the best chilldown effect zone. With the vaporization of the liquid nitrogen, the cryogenic nitrogen gradually diffused. At 20,000 s, the bottom temperature was about –63 °C, the middle was –30 °C, and the upper was 0 °C. When the model angle was 0°, the effective chilldown zone was the largest. As the angle increased, the effective chilldown volume and holding time decreased significantly. The model angle had a greater impact on the chilldown. For the position of nitrogen injection, the inlet was set slightly farther from the gas outlet’s position, leading to a larger coverage for the liquid nitrogen and cryogenic nitrogen chilldown. This study can provide a theoretical basis for the flow and chilldown of nitrogen injection in goafs.

Numerical Study on the Application of In situ Low-temperature Oxidation for Enhanced Recovery from Methane Hydrate Reservoir

A three-dimensional, compositional, multiphase flow simulator for methane-hydrate reservoirs is developed in this study. It is used to study the production characteristics of class 1 methane-hydrate reservoirs. The effects of well-completion location, well spacing, and production schedule on gas production efficiency are also examined. All simulation studies in this work implement a constant bottom-hole pressure (at 14.7 psia) as a production scheme for exploring maximum production capacity from the reservoir. The simulation study shows that the presence of gas hydrate on top of a conventional gas reservoir can dramatically improve gas productivity. Unlike conventional gas reservoirs, the water production rate of gas-hydrate reservoirs increases with time (when a constant bottom-hole pressure is implemented as a production scheme). Moreover, it shows that moving well-completion location in free-gas zone (in relation to the movement of the interface between free-gas and hydrate zones) provides better production performance and the best completion location is in the middle of free gas zone. As expected, the results also show that smaller well spacing yields higher gas production. However, for a particular system used in this work, it does not show substantial improvement of production efficiency. For a multiple-well system, the simulation results indicate that production efficiency can be improved by putting the wells on production at different times.