The first phase of Japan's Methane Hydrate R&D Program expired in March 2009, and the second phase started in April 2009. Pre-history of the program is summarized. Major topics of the program are picked up briefly.
Methane hydrate, often called “fiery ice”, has been examined as a possible future energy source. MH21 (the Research Consortium for Methane Hydrate Resources in Japan) has advanced methane hydrate exploitation study covering the resource assessment around the Japan arc and the development of methane hydrate production technology since 2002. The existence of more than 10 methane hydrate concentrated zones were delineated in the Eastern Nankai Trough area and it was calculated that methane hydrate resources (in-place) in those methane hydrate concentrated zones corresponded to 20 tcf of methane gas. To verify the production techniques, twice onshore production tests have been conducted in the Canadian Arctic under international collaborations. Following the first production test in 2002 in which the thermal stimulation technique lead to the world's first gas production from methane hydrate-bearing sediment, the second test was performed to realize the depressurization for in-situ methane hydrate dissociation in 2007 and 2008. The discovery of the methane hydrate concentrated zones and the applicability of the energy efficient depressurization technique are important milestones to bring methane hydrate from an exotic material to real resource.
The role of the Research Group for Production Method and Modeling is to develop practical natural gas production methods from methane hydrate resources off-shore Japan. While natural gas produces from methane hydrate resources, unique behavior occurs in the methane hydrate sediment due to the physical properties of methane hydrate, and it does not observe in the conventional gas production. Four research subjects have been settled and carried out in Phase 1. In the subject of “Characterization of the sediment”, in-situ analysis system of physical properties of methane hydrate sediment has been developed, and various properties of fundamental properties, frame structures and mechanical properties of natural core sample were analyzed. Finally, methane hydrate reservoir parameters have been clarified. In the subject of “Clarification of the dissociation behavior of the sediment”, to establish the models for behavior over the entire sediment from the pore scale at the methane hydrate sediment dissociation, the analysis of the mass and the heat transfer phenomenon at the dissociation were executed. And the specific behavior during production process such as ice formation, methane hydrate re-generation, changes in mechanical properties are modeled. These models were introduced to the production simulator. In the subject of “Development of the production simulator”, various calculation modules such as a dissociation rate, a consolidation property and a permeability property, were developed. Exclusive production simulator (MH21-HYDRES) and geomechanical simulator (COTHMA) have been developed. Also, the performance of MH21-HYDRES was confirmed through the international bench mark test. In the subject of “Development of the dissociation and the recovery method”, various dissociation methods were tested to establish an efficient, economical production method. Finally, depressurization method was proposed as an efficient method, and it was verified in the on-shore production test at the Mackenzie Delta/Canada. In this review, the research achievements of the Research Group for Production Method and Modeling were introduced briefly.
In order to clarify physical properties of methane hydrate-bearing sediments, seismic wave velocities and electric resistivities of natural and artificial samples were measured in relation to methane hydrate saturation. First, a laboratory experiment system was developed so that P- and S-wave velocities and electric resistivities were measured simultaneously under the condition of in situ pressure and temperature. Natural methane hydrate samples were prepared from the stored cores kept in liquid nitrogen at a low temperature. Artificial methane hydrate samples were produced in the tri-axial cell of the experiment device. All core samples were saturated by brine just before the measurements. P-wave velocities of the artificial methane hydrate samples increased by 133 m/s at every 10% increase of MH saturation. S-wave velocities of those samples increased by 118 m/s at every 10% increase of MH saturation. Poisson's ratios calculated from the P- and S-wave velocities were in the range from 0.32 to 0.47. These measurement results agreed well with those obtained by the previous study on the relationship between MH saturation and seismic wave velocities of the wireline logging around the Eastern Nankai Trough area offshore Japan. In addition, it was found that the relationship was explained by one of the rock physics models for the MH bearing sediments as Matrix-support type. Electric resistivities of the same samples as those used to measure the seismic velocities increased by 0.54Ωm at every 10% increase of MH saturation, though the resistivities run up as the saturation was more than 50%. These also agreed well with those obtained from the wireline logging.
Methane hydrate (MH) is being highlighted as next-generation hydrocarbon resources mainly because of its huge in place and cleanness. The Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium), which was organized to attain the exploration and exploitation of MH resources, has been implementing a variety of research projects. As part of such research projects, we have been developing a state-of-the-art numerical simulator (called ‘MH21-HYDRES’) for rigorously predicting MH dissociation and production performances. The main functions of this simulator and the efforts toward improving and verifying this simulator are introduced in this paper. The gas production from MH reservoirs is significantly different from that from conventional oil and gas reservoirs in terms of the mechanism and the phenomena, since (1) MH is a solid, (2) reservoir behaviors are associated with the chemical reactions such as MH dissociation/formation, and (3) reservoir properties, especially permeability, change drastically by MH dissociation. Therefore, it is impossible to predict MH reservoir performances by conventional oil and gas reservoir simulators, which lead to the development of the own numerical simulator specialized for MH reservoirs. Currently MH21-HYDRES can be applied to three-dimensional Cartesian and two-dimensional radial coordinate systems. This simulator can also deal with six components of methane, water, nitrogen, carbon dioxide, methanol and salt, and with five phases of gas, water, ice, MH and (precipitated) salt. The main features of this simulator are to calculate the kinetics of endothermic dissociation and exothermic formation reactions of MH as well as multi-phase flow behaviors resulting from these reactions. The simulator divides a target reservoir into multiple grid blocks, for which the pressure, temperature, water saturation, methanol and salt mass fractions, etc. are calculated solving the system of discretized non-linear equations for the component mass conservation and the overall energy conservation. To shorten the computational time, it is preferable to reduce the total number of grid blocks and hence to increase the size of grid blocks. On the other hand, larger grid blocks result in more significant numerical errors, which is more serious in a MH simulator than in a conventional oil and gas simulator. To resolve these problems inconsistent with each other, the Dynamic Local Grid Refinement (DLGR) function was incorporated into the simulator. DLGR enables a shorter computational time without reducing the accuracy of prediction, by allocating fine grid blocks only to the regions of importance where MH is being dissociated/formed at every time step. In addition, we are attempting to parallelize MH21-HYDRES utilizing the published MPI (message passing interface) and the parallelized solver for a linear equation system. Although the parallelized MH21-HYDRES is still in the prototype stage, using four processors, we could successfully achieve the computational speed that is about three times faster in comparison with serial processing. The international code comparison for MH simulators is being held, led by National Energy Technology Laboratory and U.S. Geological Survey. We are participating in this comparison to verify the performances of MH21-HYDRES. Although other simulators in this project show the deficiency for complex and large scale problems, MH21-HYDRES could provide reasonable solutions from the view point of the stability and robustness as well as the accuracy of calculated results, which may manifest the successful coding of this simulator.
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.
Methane hydrate (MH) 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 MH from the reservoir have been proposed, such as depressurization, thermal stimulation and inhibitor injection. These are all based on the in-situ dissociation process of MH that is transformed into methane gas and water. However, there are some technical and economical problems for operation of these methods. Therefore, we have proposed a new enhanced gas recovery method by nitrogen injection. Nitrogen has the effect as an inhibitor as well as methanol and salts to shift an equilibrium condition of hydrate to high-temperature and low-pressure. In this study, we constructed the numerical model for MH dissociation process in porous media by nitrogen injection on the basis of experimental observations. The component of gas phase was treated as a two components system consisting of methane and nitrogen, and an equilibrium condition of methane-nitrogen system was introduced into a numerical model. Through the history-matching of temperature change and gas production behavior in laboratory-scale experiments, we confirmed the validity of the constructed numerical model.
A gas production system from methane hydrate layers by hot water injection using dual-horizontal wells has been proposed. Experiments with physical and numerical reservoir models have been carried out in order to simulate gas production characteristics with the system. In the experiments, the reservoir models consisting with ice of NaHCO3 aqueous solution formed in glass-bees porous medium were used to express the dissociation heat of methane hydrate by melting one of ice. Gas production at dissociation front of methane hydrate was simulated by a gas generation with the chemical reaction of NaHCO3 included in the ice and HCl mixed in hot water injected at ice melting front. In the system, a dissociated region including dual horizontal wells filled with hot water, named as hot water chamber, was generated to produce gas continuously. The gas production rate has the maximum peak just after breakthrough of injected water between two horizontal wells, then it declined and gas was produced by almost constant rate. We have successfully developed the numerical model, and matched the history of physical gas production. Moreover, numerical simulations of gas production by the hot water injection into a field of methane hydrate sediment using dual horizontal wells 500 m in length were carried out for a methane hydrate reservoir of 15 m in layer thickness, 60% of hydrate saturation, 100 md and 25 md in horizontal and vertical absolute permeabilities, respectively. The cumulative gas production is simulated as 1.5 to 1.9×106 m3 std for initial five years, and the total thermal energy injected is 2.3 to 3.5 times of net dissociation energy of methane hydrate. Furthermore, a new gas prodution scheme, which uses four pairs of dual horizontal drilled wells into a methane hydrate sediment with area of 1 km×1 km, has been presented and evaluated with the numerical simulation as the cumulative gas production for 15 years is 1.3×108 m3 std.
Gas production technology from natural gas hydrate is under investigation. Recently, the efficiency of depressurization method has been clarified by laboratory experiments and numerical analyses. However, the decrease of gas-production rate could occur during depressurization due to the temperature drop of the reservoir in the vicinity of production well. The solution to this problem is important for efficient and stable gas production from methane hydrate reservoirs. In order to investigate the effect of ultrasonic irradiation on dissociation enhancement, experiments of methane-hydrate dissociation with ultrasonic irradiation are conducted under depressurized condition. Pressure vessel is originally designed for synthesizing methane hydrate sediments and for irradiating ultrasonic wave toward the synthesized sediments. Gas production rates are examined in the experiments of depressurization method and combined method of ultrasonic wave and depressurization. The efficiency of the combined method is evaluated by measured gas production rates. Results of the heat-balance calculations on the dissociation experiments are also reported.
Japan relies on imports for the greater part of its energy needs, so that securing reliable future energy resources have become even more significant due to the recent violent fluctuation in the price of oil. In addition, as global environmental problems, such as global warming, become more serious, the need to shift to cleaner energy sources, such as natural gas, becomes more pressing. In this social context, methane hydrate, a new natural gas resource that has been confirmed to exist in the sea surrounding Japan, is expected to become a cleaner source of domestic energy in the future. “Methane Hydrate Exploitation Program in Japan” was published in July 2001 and the Research Consortium for Methane Hydrate Resources in Japan (official abbreviated title: MH21 Research Consortium) was established to oversee the completion of phase-1 of this plan. In this consortium, Engineering Advancement Association of Japan (official abbreviated title : ENAA), which is the group responsible for conducting Environmental Impact Assessment, EIA, has conducted a number of research and developed activities to establish basic technologies associated with EIA and completed almost all of aims during phase-1. In this paper, we introduce overview of our R&D about EIA which should be necessary to implement on methane hydrate production.