Methane Hydrate Saturation Research Articles

Discrete element modelling of geomechanical behaviour of methane hydrate soils with pore-filling hydrate distribution

Methane hydrate soil is a natural soil deposit that contains methane hydrate in its pores. The micro-scale processes of the geomechanical behaviour of methane hydrate-bearing soils are investigated by the Discrete Element method (DEM). A series of DEM simulations of triaxial compression tests were performed to study the influence of methane hydrate saturation (S h ) on the stress–strain relationship, the volumetric response and on the macroscopic geomechanical properties such as friction and dilation angle. Results of the numerical simulations are compared with laboratory triaxial test data performed on sandy methane hydrate samples. The simulations showed that for the pore-filling case, the hydrate contribution to the strength of the sediment is of a frictional nature, rather than of a cohesive nature.

Laboratory-Scale Experiments of the Methane Hydrate Dissociation Process in a Porous Media and Numerical Study for the Estimation of Permeability in Methane Hydrate Reservoir

An experimental study of the dissociation of methane hydrate (MH) by hot-water injection and depressurization was carried out at the National Institute of Advanced Industrial Science and Technology (AIST). These experiments helped us understand some important aspects of MH behavior such as how temperature, pressure, and permeability change during dissociation and gas production. In order to understand the experimental results, a model of MH dissociation in a porous media was designed and implemented in a numerical simulator. In the model, we treated the MH phase as a two-component system by representing the pore space occupied by MH as a separate component. Absolute permeability and relative permeability were formulated as a function of MH saturation, porosity, and sand grain diameter and introduced into the numerical model. Using the developed numerical simulator, we attempted history matching of laboratory-scale experiments of the MH dissociation process. It was found that numerical simulator was able to reproduce temperature change, permeability characteristics, and gas production behavior associated with both MH formation and dissociation.

Key Factors for Depressurization-Induced Gas Production from Oceanic Methane Hydrates

Oceanic methane hydrate (MH) deposits have been found at high saturations within reservoir-quality sands in the Eastern Nankai Trough and the Gulf of Mexico. This study investigates the key factors for the success of depressurization-induced gas production from such oceanic MH deposits. A numerical simulator (MH21-HYDRES: MH21 Hydrate Reservoir Simulator) was used to study the performance of gas production from MH deposits. We calculated the hydrate dissociation behavior and gas/water production performance during depressurization for a hypothetical MH well. Simulation runs were conducted under various initial reservoir conditions of MH saturation, temperature, and absolute permeability. A productivity function (PF) was introduced as an indicator of gas productivity, which is a function of gas production rate, water production rate, and discount rate. The simulations showed that recovery factors over 36% and maximum gas production rates over 450 000 Sm3/d were expected for the most suitable conditions of a class 3 deposit (i.e., an isolated MH deposit that is not in contact with any hydrate-free zone of mobile fluids). However, gas productivity was affected by formation temperature and initial effective permeability. The values of PF increased with increasing formation temperature when the initial permeability of the deposit was higher than a threshold value (the threshold permeability); however, it decreased for the deposit below the threshold permeability. The threshold permeability was estimated to be between 1 and 10 mD in the class 3 deposit. These results suggest that key factors for the success of depressurization-induced gas production from oceanic MH are as follows: (1) The initial effective permeability of the MH deposit is higher than the threshold value, and (2) the temperature of the MH deposit is as high as possible.

未固結堆積物のメタンハイドレート胚胎層の岩石物理モデル

It is well known that the methane hydrate exists below the sea floor over 500 m or the polar region under the frozen ground in the Earth. We made the cross-plot of the methane hydrate and Vp, Vs, or Vp/Vs from the well log data at the eastern Nankai Trough in Japan and the Mackenzie Delta in Canada. We found that as the methane hydrate saturation increases, Vp and Vs increase, and Vp/Vs decreases from these plots. We discussed the rock physics model of methane hydrate bearing layers. We calculated Vp, Vs, and Vp/Vs as the function of the methane hydrate saturation, based on four imaginary rock physics model of methane hydrate bearing layers. From the comparison between the real well sonic data and the calculated data from four models, we inferred that the rock physics model of methane hydrate bearing layers was matrix-supporting model at both the eastern Nankai Trough and the Mackenzie Delta areas. However, there were some error and difference with the Vp, Vs and Vp/Vs from the matrix-supporting model and real well sonic data between the eastern Nankai Trough and the Mackenzie Delta. We estimated the effect of Vp, Vs, and Vp/Vs for the change of clay content of matrix-supporting model. As the clay content decreases, the Vp and Vs increase, and Vp/Vs decreases. The clay content from well core data corresponds with the calculated Vp, Vs, and Vp/Vs as the function of the methane hydrate saturation and the clay content from the matrix supporting model. In case of the estimation of methane hydrate saturation from Vp, Vs, and Vp/Vs, we need to consider the geology, especially sand/clay ratio of the methane hydrate bearing layers.

モデル粒子堆積物を用いた孔隙径分布と浸透率に関する研究

Natural gas hydrates in sediment are expected to be developed as a resource of natural gas and have been studied as a possible future energy resource. Gas permeability and water permeability in the methane-hydrate (MH)-bearing sediments are important factors for estimating the efficiency of producing methane gas from the natural gas hydrate sediment. Permeability of MH-bearing sediment is considerably affected by several properties of sediment, i.e., pore-size distribution, porosity, cementing, MH saturation, and MH-bearing features. There is no “universal” equation of permeability related with these properties that is applicable to all porous media including MH-bearing sediment. In order to clarify the relation between permeability and these properties, permeability, grain-size distribution, porosity, pore-size distribution, and specific surface area of glass beads and sandy sediments have been measured. The grain-size distribution has been measured by a dynamic laser light scattering-diffraction system. Mercury porosimetry has been used to measure porosity, pore-size distribution, and specific surface area. Proton nuclear magnetic resonance measurement combined with a permeability measurement system was used to characterize the porosity, pore-size distribution and the permeability. The permeability was measured by water.As a reference, four empirical equations of permeability, i.e., the Krumbeim and Monk equation, the Kozeny-Carman equation, an approximated equation using mercury porosimetry data, and the Schlumberger Doll Research equation, which incorporate factors such as grain size, porosity, pore size, and specific surface area, were scrutinized for calculating permeability.A semi-empirical (SE) equation of the permeability of glass beads and sandy sediment has been derived based on the Kozeny-Carman equation by pore-size distribution and porosity that had been measured by mercury porosimetry. This SE equation has been approximated by the relation of porosity, pore-size distribution, tortuosity, and specific surface area of the sediment.The permeabilities calculated by the SE equation correspond well to the permeability measured by water flow.

メタンハイドレート探査手法とその基本コンセプト

Methane hydrates, which are ice like materials and constructed with methane and water molecules, are spread widely under permafrost and deep water areas. It is thought that methane hydrates are ones of occurrence type of methane and can be future energy resources.BSR (bottom simulating reflector) occurs at the basement of GHSZ (gas hydrate stability zone) and therefore it is used to detect occurrences of methane hydrates in deep waters using reflection seismic data. At the same time, BSR is only the first step for methane hydrate exploration because BSR does not provide information about thickness, saturation and type of methane hydrate bearing zone. From the viewpoint of resource exploration, practical methods to analyze details of methane hydrate resource fields have been required.We constructed an effective strategy to delineate methane hydrate concentrated zones, of which reservoir are turbidite sand bodies, utilizing four following indicators in 3D seismic data: (A) BSR, (B) turbidite sequence (above BSR), (C) strong seismic reflectors and (D) relatively higher interval velocity, suggesting methane hydrate concentrated sand layers. It enabled us to delineate methane hydrate concentrated zones and evaluate their rock volume successfully in the eastern Nankai Trough as similar as prospects and leads in conventional oil and gas fields are delineated by geological and geophysical approaches.

メタンハイドレートを含む模擬堆積物の三軸圧縮特性におよぼす拘束圧の影響

In this study, effect of confining pressure on mechanical properties of artificial methane hydrate sediment was experimentally investigated and discussed. Two kinds of drained triaxial compression tests were conducted for artificial sediment specimens containing Toyoura sand and synthetic methane hydrate under a pore water pressure of 8 MPa and a temperature of 278 K; “constant confining pressure test” and “two-stage confining pressure test”. In the former test, confining pressure was kept constant at 8.5 MPa, 9 MPa, 10 MPa or 11 MPa during axial loading process. In the latter test, confining pressure was altered from 8.5 MPa to 11 MPa during axial loading process. The findings can be summarized as follows.(1) A specimen was restrained from deforming laterally by confining pressure.(2) Strength and stiffness of a specimen were enhanced by confining pressure.(3) Two strengths under different confining pressures could be obtained from a single specimen by two-stage confining pressure test. (4) Strength was formulated as a function of confining pressure and methane hydrate saturation based upon earlier findings.These findings concerning the effect of confining pressure seem to be of considerable help for a proposal of constitutive equation and numerical simulation in the future.

Prediction of Gas Productivity From Eastern Nankai Trough Methane-Hydrate Reservoirs

Summary The Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium) has been evaluating methane-hydrate (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 applicability of a variety of MH dissociation/production methods to these MH reservoirs and on the feasibility of future development 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 and of initial pressure, temperature, permeability, and MH saturation. Some of these reservoirs contain MH of high saturation at a certain interval (MH concentrated reservoir), while in the others, MH is deposited sparsely (MH-nonconcentrated reservoir). Detailed numerical reservoir models were constructed for both MH-concentrated and -nonconcentrated reservoirs, consulting the well-log and seismic-interpretation results. MH-dissociation/-production performances were then predicted through numerical simulation, assuming the application of various MH-dissociation methods (such as depressurization, well-bore heating, hot-water huff'n'puff, and hot waterflooding. The simulation studies clarified the difference in the gas production between MH-concentrated and -nonconcentrated reservoirs. These studies also revealed that the permeability not only of sand layers but also of clay layers has a significant effect on the gas productivity from MH-concentrated reservoirs. Furthermore, it was suggested that the hot-water injection was effective when it was applied as a secondary-recovery method after depressurization. Simple economic analyses on the basis of these simulation results exhibited the promise that some MH reservoirs in the Eastern Nankai trough could be developed economically if the well spacing and MH-dissociation/-production methods were designed appropriately.

メタンハイドレートを胚胎する砂質堆積物の特徴とメタンハイドレート胚胎メカニズムへの影響

Methane hydrate-bearing sediments taken by a Pressure Temperature Core Sampler (PTCS), were recovered from the NE-Nankai Trough to clarify the accumulating mechanisms whereby methane hydrate accumulates in pore spaces. The sediments were of the sand and mud alternation layer, which were turbidities and hemi-pelagic mud, respectively. Their sediment features such as grain size, porosity, and methane hydrate saturation of pore spaces were analyzed systematically. Relationships among grain size distribution, Bouma's sequence and hydrocarbon gas volume produced by methane hydrate dissociation were confirmed, thus we understood the correspondence of the features of turbidities features and methane hydrate saturation; i.e., we understood that the part of high methane hydrate saturation had a porosity of 50-55%, median of 1.9-3.3φ, and grain sorting of from -1.4 to -0.4. These features indicate the lower section of turbidite called as Ta and/or Tb. Other sections of turbidite had small amounts of methane hydrate. From these results, we estimated that only permeable sections could maintain the accumulation of methane hydrate from first to last stage. If their pores had been filled with gas-saturated water and the pressure/temperature condition was sufficient for methane hydrate, the methane hydrate would precipitate homogeneously in sediments from its initial accumulation. However, methane hydrate precipitation reduces permeability because methane hydrate growth in pores causes a reduction of pore volume. During accumulation, it is inevitable that gas-rich fluid could not reach into low permeable layers, although it could still be supplied into high permeable layers. This mechanism does not need any cap rock or some trap system in sediments, and it is quite a different mechanism from conventional oil and gas accumulation.

東部南海トラフメタンハイドレート資源の生産性評価

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.

メタンハイドレート堆積物の浸透率特性―東部南海トラフ基礎試錐メタンハイドレート堆積物の浸透率評価を目的として―

Natural gas hydrates in sediment are expected to be developed as a resource of natural gas and have been studied as a possible future energy resource. Gas permeability and water permeability in the methane-hydrate (MH)-bearing sediments are important factors for estimating the efficiency of producing methane gas from the natural gas hydrate sediment. Permeability of MH-bearing sediment is considerably affected by several properties of sediment, i.e., pore-size distribution, porosity, cementing, MH saturation, and MH-bearing features.Glass beads sediment and sandy sediment with different grain sizes have been measured as fundamental research and for application to natural MH-bearing sediment. Sediment pore-size distribution and porosity have been measured by NMR and mercury porosimeter. In order to obtain pore-size distribution with higher spatial resolution, a technique of numerical conversion of NMR-T2 distribution have been developed.A semi-empirical (SE) equation of the permeability of glass beads and sandy sediment has been derived based on the Kozeny-Carman equation by pore-size distribution and porosity that had been measured by mercury porosimetry. This SE equation has been approximated by the relation of porosity, pore-size distribution, tortuosity, and specific surface area of the sediment.The SE equation has been applied for converted NMR-T2 distribution of MH-bearing sediment. The permeabilities of methane hydrate bearing sediment calculated by the SE equation correspond well to the permeability measured by water flow.

Productivity from Eastern Nankai Trough Methane-Hydrate Reservoirs

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 19382, "Prediction of Gas Productivity From Eastern Nankai Trough Methane-Hydrate Reservoirs," by Masanori Kurihara, SPE, Akihiko Sato, and Hisanao Ouchi, Japan Oil Engineering Company; Hideo Narita, National Institute of Advanced Industrial Science and Technology; Yoshihiro Masuda, SPE, the University of Tokyo; Tatsuo Saeki and Tetsuya Fujii, Japan Oil, Gas, and Metals National Corporation, prepared for the 2008 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed. The MH21 Research Consortium of Japan evaluated methane-hydrate (MH) reservoirs in the eastern Nankai trough from the viewpoints of geology, geophysics, petrophysics, and reservoir/production engineering. The applicability of several MH-dissociation/-production methods to these MH reservoirs and the feasibility of future development in terms of gas production and economics were examined. Simple economic analysis on the basis of those simulation results indicated promise that some MH reservoirs in the eastern Nankai trough could be developed economically if well spacing and MH-dissociation/-production methods were designed and applied appropriately. Introduction A numerical simulator was developed for rigorous prediction of MH-dissociation and -production behaviors, at both core and field scales. Use of this numerical simulator enabled studies to predict MH-reservoir performance to study the effectiveness of several production methods in terms of the gas producibility from MH reservoirs with diverse characteristics. These studies, however, were conducted targeting imaginary reservoirs as well as those roughly mimicking eastern Nankai trough reservoirs by simplifying reservoir properties. Exploratory drilling was carried out in 1999 and 2004, and a 3D-seismic survey was conducted in 2002, which clarified the characteristics of the reservoirs. In this study, the performance of MH dissociation and production were forecast by use of more-realistic reservoir models that rigorously reflect the geological/petrophysical properties analyzed through those exploration activities. MH Reservoirs in the Eastern Nankai Trough. Six exploration wells were drilled in which logging-while-drilling (LWD) data, wireline-logging data, and core samples were acquired offshore Tokai in 1999. These wells confirmed the presence of an interval with relatively high MH saturations. Additional exploratory drilling targeted a broader area in 2004. A total of 32 exploration wells were drilled to obtain well-log data and core samples at 16 locations in the Kumano basin, Second Atsumi Knoll, and offshore Tokai, as shown in Fig. 1. Japan Oil, Gas, and Metals National Corporation has been analyzing these data, aiming at estimating the initial volume of methane in place in MH reservoirs in the eastern Nankai trough. During these analyses, it was revealed that the MH reservoirs are a sequence of thin layers (most thinner than 0.4 m) of sand, silt, and clay within turbidite beds. The MH saturation is negligibly small in clay layers, even if MH is contained in these layers.

Sulfate‐methane transition as a proxy for average methane hydrate saturation in marine sediments

We develop a relationship between the sulfate‐methane transition (SMT) and average gas hydrate saturation (AGHS) for systems dominated by methane migration from deeper sources. The relationship is explained by a one‐dimensional numerical model that simulates gas hydrate accumulation in marine sediments. Higher methane fluxes result in shallow SMT depths and high AGHS, while lower methane fluxes result in deep SMTs and low AGHS. We also generalize the variation between AGHS and scaled SMT depth, a procedure that aids prediction of AGHS at different sites from observations of the SMT, such as along Cascadia Margin.

円盤状堆積層試料を用いた圧密・浸透連成試験によるメタンハイドレート堆積層における絶対浸透率の定式化―メタンハイドレート貯留層の浸透率評価に関する研究（第５報）―

Methane hydrate (MH) is one of the potential natural gas resources in near future, because it exists in marine sediments or in permafrost regions worldwide. To evaluate the productivity of methane gas from MH reservoirs, it is necessary to develop a gas production simulator for doing case studies.In this study, for the purpose of numerical simulation on MH dissociation process, we carried out an experimental study for estimating permeability of a MH reservoir with compaction of the sediments due to decrease of effective stress in pore space. In actual hydrate fields, it is supposed that vertical consolidation of sediments occurs whereas gas and water flow in a horizontal direction. To reproduce such a situation, we attempted to measure the permeability of artificial MH sediment under the horizontal radial flow condition by using a specially designed apparatus.First, we investigated the relationship between absolute permeability and MH saturation in this radial flow system. When MH saturation increased from 0 to 0.30, permeability decreased from 12.0μm2 to 1.19μm2 that was one 12th of the value of sand matrix without any MH formation. The same tendency of permeability changes was obtained in liner flow system as previously reported, so the measuring technique of permeability by using radial flow system has been established.Second, by using the similar apparatus, compaction-permeability tests were conducted to formulate absolute permeability as a function of porosity and MH saturation. From experimental results, it was found that permeability decrease due to consolidation was remarkable in the cases of small sand grain diameter and large initial porosity. We determined the parameters of permeability reduction factor for porosity through a numerical analysis, and derived the final form of absolute permeability equation for MH reservoirs.

メタンハイドレートを含む模擬堆積物の強度および残留強度のひずみ速度依存性

In this study, triaxial compression tests were conducted for Toyoura sand containing synthetic methane hydrate at conditions of confining pressure 9 MPa, pore water pressure 8 MPa and temperature 278 K. In the test, strain rate was switched between C1 and C2 (C1 C2) at every predetermined strain interval, and two stress-strain curves at strain rates of C1 and C2 were estimated from a single specimen.As a result, the peak strength at the faster strain rate C2 was higher than that at the slower strain rate C1. It was also found that strain-rate dependency of peak strength increased with methane hydrate saturation. It can be said that methane hydrate saturation significantly affected to the strain-rate dependency and then failure mechanism of the methane hydrate sediments.Strain-rate dependency of residual strength (stress in the post failure region) was discussed. The ratio, (stress at the strain rate C2 / stress at the strain rate C1), was almost constant in the region. This finding may suggest that the mechanism of strain-rate dependency or time-dependent behavior of methane hydrate sediments does not change in the post-failure region.

減圧法におけるメタンハイドレート分解時の圧密挙動ならびに浸透率特性に関する室内実験研究―メタンハイドレート貯留層の浸透率評価に関する研究（第６報）―

Methane hydrate (MH) is one of the potential resources of natural gas in the near future, because large amount of MH exists in marine sediments or in permafrost regions worldwide.Depressurization process is regarded as the most effective process for gas recovery from the viewpoints of gas productivity and economical efficiency, compared with the other in-situ dissociation processes of MH. However, increase of effective stress during depressurization causes consolidation of MH sediments and permeability reduction. As a result, decrease of gas productivity is also supposed. Therefore, it is very important to understand the behavior in MH reservoir, especially in developing the extraction system for MH, and when considering the environmental impacts due to the development. In this study, we conducted an experimental study on consolidation and gas production behavior during MH dissociation by depressurization, using the special type apparatus. To reproduce the real flow condition of gas and water, we used disc shape samples as simulated MH sediment. Horizontal radial flow in porous media during MH dissociation was constructed whereas vertical load system was used to simulate rock pressure conditions in real MH sediment.From an experimental observation, it was found that dissociation and gas production during depressurization consisted of the following three stages depending on temperature and pressure conditions, such as 1) expansion of methane gas in pore space by depressurization, 2) dissociation due to the latent heat of sand grain and each phases and 3) dissociation due to thermal conduction from outer temperature boundary. In addition, we confirmed that increase of effective stress at the initial stage of depressurization was dominant factor on compaction behavior, and deformation after constant effective stress condition was primarily dependent on the creep effect rather than MH dissociation.Then, dissociation pressure, sand grain size, MH saturation and initial temperature was changed as experimental parameters, we discussed the effect of these parameters on MH dissociation, consolidation, and dissociated gas production.

In-situ Observations by NMR for Formation and Dissociation of MH in Porous Media

In-situ observation of methane-hydrate (MH) formation and dissociation in sandy sediments, and water-permeability measurements for the sediments have been performed by the apparatus combined proton nuclear magnetic resonance (1H NMR) and permeability measurement system, which we had originally designed. For three kinds of sand, Toyoura, No. 7 and No. 8 silica sand, the changes of pore-size distributions under MH formation and dissociation processes were discussed from the changes of transverse relaxation time, T2, of liquid water in pore space. With forming MH, it was found that intensity and peak-top T2 of T2 distribution decreased. The intensity of T2 distribution is proportional to the number of protons of water and methane in pore space. Decreasing intensity means the consumption of water and methane to form MH, and the shorter peak-top T2 indicates smaller apparent pore size, in which MH was assumed to be solid material as well as sand grain. These changes are thought to indicate that MH growth make apparent pore size gradually smaller. On the other hand, MH dissociation is supposed to advance from water-hydrate interfaces, where apparent pore size becomes larger gradually. All the sediments used here have the same tendency of decreasing pore-size with changing water saturation due to MH formation and growth. Measured water permeability ratio, which is defined as the ratio of effective water permeability to absolute one, during MH dissociation increased rapidly in the case of higher MH saturation, Sh, condition and went up slowly at lower Sh condition, while that estimated from SDR model increased gradually for all Sh condition. But it was found that the expanded SDR model which was introduced an exponent factor to the logarithmic mean of T2 (T2LM) of original SDR model could express the change of measured permeability. Here, the expansion of T2LMmeans extremely larger specific surface area (SSA) of pore space at higher S SUB>h. We interpret that higher SSA bring stronger resistance to water mobility, which leads extremely smaller water permeability at higher S SUB>h.

Methane hydrate accumulation model for the Central Nankai accretionary prism

A two-dimensional transient model of methane hydrate accumulation is designed and applied in the Nankai accretionary prism. Model results show that the formation and distribution of methane hydrate and free gas is related to value of the thermal flux, the fluid flux, the methane flux supplied from below, and methane source within the sediment. The duration of the methane flux is of significant importance. The older the sediments, implying a longer period of methane flux supply from below and within the sediments of the model, the larger the volume percentage of free gas will be. This can explain the occurrence of BSR in a area with lower methane flux observed, but absence of the BSR in the other area with higher methane flux observed. Modeling also predicts that the maximum saturation of methane hydrate is about 1% of the pore space in the area of ODP Site 1178, and the methane hydrate occurrence zone is probably 90–200 m below the sea floor (mbsf), which is supported by both direct and indirect evidence for hydrate. In the area near ODP Site 808, the fault zones having high fluid flow and methane flux may possibly result in methane hydrate formation without BSR occurrence.

メタンハイドレート存在下での浸透率モデルの構築と室内分解実験のシミュレーション―メタンハイドレート貯留層の浸透率評価に関する研究（第４報）―

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. To evaluate the productivity of methane gas from MH reservoirs, it is necessary to develop a gas production simulator for doing case studies.We have carried out an experimental research on MH dissociation process by hot-water injection as one of the thermal recovery methods, for clarifying the physical phenomena of gas-water multiphase flow in porous media and the properties of formation and dissociation of MH. In this study, we developed a new numerical theory for MH dissociation process in porous media, based on the phenomena experimentally observed. In the developed theory, MH phase is treated as two components system derived from irreducible and free water. As MH is not mobile due to its solid characteristics, we consider gas-water two-phase flow with MH formation. We also consider the change in reservoir temperature due to hear transfer, and formation and dissociation of MH. Absolute permeability and relative permeability of sand packs were measured under the presence of MH and models for the permeabilities were formulated as functions of MH saturation, etc. These models can estimate dramatic decrease in absolute permeability at high MH saturations, whereas relative permeability curves are shifted to lower water saturation at high MH saturations.Using the developed theory, we carried out numerical studies for a laboratory core experiment of MH dissociation by hot-water injection. Simulation results on temperature distribution and gas production behavior during MH dissociation sufficiently agreed with the experimental data. Furthermore, it was found that the simulation was able to reproduce experimental pressure behavior associated with permeability change due to MH formation and dissociation.

Methane Hydrate and Free Gas on the Blake Ridge from Vertical Seismic Profiling

Seismic velocities measured in three drill holes through a gas hydrate deposit on the Blake Ridge, offshore South Carolina, indicate that substantial free gas exists to at least 250 meters beneath the bottom-simulating reflection (BSR). Both methane hydrate and free gas exist even where a clear BSR is absent. The low reflectance, or blanking, above the BSR is caused by lithologic homogeneity of the sediments rather than by hydrate cementation. The average methane hydrate saturation above the BSR is relatively low (5 to 7 percent of porosity), which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3.