Occurrence Of Natural Gas Hydrate Research Articles

Mechanisms for upward migration of methane in marine sediments

Methane, a non-negligible component of the global carbon budget, could be discharged upward through marine sediments to ocean floor by certain migration mechanisms. Although quite some studies have been conducted, the mechanisms for methane migration have not been well reviewed yet, especially in hydrate-bearing sediments. In this study, methane migration mechanisms are classified into diffusion and advection processes which include water movement, free gas flow, sediment failures, and recently developed gas migration through hydrate channels. The occurrence of natural gas hydrate might affect methane migration in three ways: (1) reducing the permeability of marine sediments and consequently hindering the upward movement of methane either in gas or liquid phase, (2) enhancing the geomechanical strength of marine sediments, which prevents the creation of new pathways for methane escape by sediment failures, and (3) benefiting upward methane migration by constructing hydrate channels at the interface of continuous gas columns. Generally, dissolved methane could hardly break through the gas hydrate stability zone and sulfate-methane transition zone because of the high consumption rate for methane in these two zones. For free methane gas, the capillary force is a strong resistance to free gas flow in porous sediments. However, whether for dissolved methane or free methane gas, discharge along pre-existing fractures or failure surfaces might be considerable. In addition, methane discharge by gas flow through hydrate channels is still hard to constrain. Finally, based on current research uncertainties in constraining the methane flux to the ocean, the research outlook is also addressed. It is suggested that more investigations should be conducted in three aspects: the flow characteristic of high-permeability conduits, the quantitative correlations of geomechanical properties and hydrate distribution, and the occurrence conditions of hydrate channels.

Seismic characterization and modelling of the gas hydrate system in the northern Bay of Bengal, offshore Bangladesh

The offshore Bangladesh includes the northern Bengal fan, where sediment supply from the Ganges and Brahmaputra rivers has resulted in the accumulation of up to 20 km of shallow-marine, fluvio-deltaic and slope sediments that have accumulated during rapid tectonic subsidence since the late Miocene. The high sedimentation rates, along with high organic matter content, make this area favorable for the formation of natural gas from both microbial and thermogenic sources. Here we use multichannel seismic reflection profiles and modelling of the gas hydrate stability zone (GHSZ) to present the first evidence for the occurrence of natural gas hydrate in the offshore Bangladesh. First, we analyze the sediments of the shelf and slope areas, which are characterized by downslope sediment transport features and by the presence, in places, of faults/fractures as well as widely distributed amplitude anomalies and seismic facies that we relate to the presence of gas. A high-amplitude reversed polarity reflection of variable continuity that mimics the seafloor and cross-cut stratigraphy is interpreted as a Bottom Simulating Reflector (BSR). The BSR is observed in several areas that are predominantly located in the E-SE of the study area, in water depths of 1300–1900 m and at depths below seafloor of 250–440 m. Sediments above BSR locations generally show higher seismic interval velocities reaching values of ∼1920–1940 m/s, which are consistent with the presence of gas hydrate in shallow marine sediments. Furthermore, the BSR lies at approximately the same depth as the theoretical base of the gas (methane) hydrate stability zone (BGHSZ), calculated assuming a 3.5 % wt pore water salinity and using existing geothermal gradient and seafloor temperature data from the study region. However, in places, the BSR lies deeper or shallower than the base of the modelled BGHSZ. These discrepancies include areas where faults/fractures and seismic evidence linked to fluid flow from deeper reservoirs reach the GHSZ disrupting its stratigraphic continuity. At these locations, we suggest that faults/fractures act as fluid migration pathways causing localized heat-flow perturbations and/or changes in the hydrate-forming gas composition both likely affecting the depth of the GHSZ. Our results provide the first evidence of the gas hydrate potential in the offshore Bangladesh and should drive future research and data acquisition aiming to understand the composition, saturation and thickness of the gas hydrate-bearing sediments in this region.

Numerical prediction of phase equilibrium conditions of gas hydrates up to 200 MPa in the presence of sodium and calcium chloride solutions

The occurrence of natural gas hydrates has both beneficial and negative practical implications. Nowadays, the most numerical studies in gas hydrates formation field were limited to a pressure of 74 MPa. However, these components are abundantly formed at permafrost and under the seafloor in which the pressure exceeds 100 MPa. In this study, the prediction of equilibrium conditions of gas hydrates was investigated numerically in pure gas and gas mixture up to 200 MPa. The inhibition of hydrates formation was examined by using NaCl and CaCl2 at different molar fractions. The Lee-Kesler-Plocker equation of state was selected as the most adequate, to predict gas hydrate stability limits, compared to other numerical approaches. The modeling results indicate that our model is in good agreement with experimental findings up to 200 MPa, but due to the lack of data in literature the maximum pressure compared was 163.25 MPa for gas mixture in pure water. The salt inhibition investigation also shows a good accuracy of our model at molar fractions less than 0.05 in both pure gas and gas mixture. It can be concluded that this model permits to study the stability limits of hydrates in wide range of gas composition and chloride salt solutions.

Hydrate formation and dissociation characteristics in clayey silt sediment

The geological occurrence of natural gas hydrates in clayey silt sediments has been extensively confirmed. Compared with sandy sediments, clayey silt is characterized by a large specific surface area, high capillary pressure, and high bound water content, which leads to distinct characteristics of hydrate formation and dissociation. The formation and dissociation of methane hydrates in clayey silt sediments with illite or montmorillonite were experimentally investigated. It was determined that an increase in the water content of silt and illite-silt sediments from ~9.47% to ~28.41% caused a decrease in the water-to-hydrate conversion ratio owing to the formation of a hydrate film between water and CH4 gas. However, in montmorillonite-silt sediments, the water-to-hydrate conversion rate increased between 9.47% and 18.94% of water saturation and decreased between 18.94% and 28.41% of water saturation under the combined effects of weakly bound water content and capillary pressure. In the hydrate dissociation experiment, the temperature of sediments with ~9.47% water content decreased more, which was caused by the smaller specific heat capacity of sediments with lower water content. Meanwhile, the temperature drops below the freezing point during the hydrate dissociation process, which is caused by the strong inhibition of hydrate phase equilibrium by montmorillonite. This study demonstrates the effects of clay type and water content on hydrate formation and dissociation, which is helpful for understanding the production behavior of clayey silt hydrate reservoirs.

Influence of Gas Supply Changes on the Formation Process of Complex Mixed Gas Hydrates.

Natural gas hydrate occurrences contain predominantly methane; however, there are increasing reports of complex mixed gas hydrates and coexisting hydrate phases. Changes in the feed gas composition due to the preferred incorporation of certain components into the hydrate phase and an inadequate gas supply is often assumed to be the cause of coexisting hydrate phases. This could also be the case for the gas hydrate system in Qilian Mountain permafrost (QMP), which is mainly controlled by pores and fractures with complex gas compositions. This study is dedicated to the experimental investigations on the formation process of mixed gas hydrates based on the reservoir conditions in QMP. Hydrates were synthesized from water and a gas mixture under different gas supply conditions to study the effects on the hydrate formation process. In situ Raman spectroscopic measurements and microscopic observations were applied to record changes in both gas and hydrate phase over the whole formation process. The results demonstrated the effects of gas flow on the composition of the resulting hydrate phase, indicating a competitive enclathration of guest molecules into the hydrate lattice depending on their properties. Another observation was that despite significant changes in the gas composition, no coexisting hydrate phases were formed.

Effects of protein macromolecules and metabolic small molecules on kinetics of methane hydrate formation in marine clay

Numerous efforts have been made to mimic the natural marine environments to understand the behavior of hydrate formation and accumulation in the sediments under seafloor. Yet it is found that the role of organic matters in the kinetics of hydrate formation is largely veiled, which should be much enriched as a result of the activities of the marine ecosystem. Therefore in this study, the kinetic effects of protein macromolecules and metabolic small molecules on CH4 hydrate formation were examined; the organic compounds were extracted from the samples gathered from the hydrate reservoir located in the South China Sea. A total of 2058 species classified into 10 types of protein macromolecules and metabolic molecules were identified. Low-field nuclear magnetic resonance technique was used to in-situ monitor the hydrate formation process. The results showed that the metabolic molecules could remarkably promote hydrate formation, with more water converted into hydrate within a shorter time. Proteins have been widely studied to inhibit hydrate formation; here it is for the first time confirmed that the proteins extracted from the natural samples drilled in the hydrate reservoir could act as kinetic inhibitor, lowering the possibility of hydrate nucleation. It was further indicated that hydrate formation majorly consumed the water in the macro pores; this was followed by a diffusion-limitation process with the hydrate shell acting as mass transfer barrier slowing down the water conversion. The water in the micropores was yet found to be very difficult to participate in the reaction, potentially resulting from the unavailable gas-water contact. The findings could provide insights into the kinetics of hydrate formation in the presence of organic matters abundant in natural sediments and expand the current understandings on the occurrence of natural gas hydrate under marine environments.

Multibeam water column data research in the Taixinan Basin: Implications for the potential occurrence of natural gas hydrate

A multi beam sonar survey is carried out in the continental slope of the Taixinan Basin to obtain submarine topographic and water column data. The data are processed to obtain water column images. Anomalous water column images, displaying plume characteristics, are found in gas hydrate enriched areas in the Taixinan Basin. This indicates the presence of natural gas resources in the Taixinan Basin. The multibeam sonar system is shown to provide an accurate and effective approach for detecting sub-sea gas hydrate.

Application of X‐Ray Computed Tomography Technology in Gas Hydrate

Clathrate hydrates are formed by water molecules and natural gas molecules under a certain temperature and pressure. Natural gas hydrates are mainly distributed in the marine sediments and the permafrost in nature. As a kind of potential clean energy, determining the structural characteristics of hydrates in sediments is the prerequisite for the safe and economic exploitation of natural gas hydrate. X‐ray computed tomography (CT) is a potential method to nondestructively detect the occurrence of natural gas hydrate. The basic physical properties such as porosity, hydrate saturation, and sediment permeability of the hydrate‐bearing sediments are analyzed by calculating the CT numbers or combining with the network model. Therefore, the X‐ray CT is used to study the hydrates produced in the laboratory and the hydrates formed in the nature by scientists all over the world. By summarizing the previous research, this article focuses on the important role of X‐ray CT in the hydrate research from the following three aspects: analyzing the basic physical properties of hydrate sediment, identifying the occurrence mode of hydrate in sediments, and measuring the hydrate formation and decomposition process. In response to the research progress and future research directions, relevant suggestions had been put forward.

Evidence for pore-filling gas hydrates in the sediments through morphology observation

To provide an evidence of natural gas hydrate occurrence state, a series of experiments on multiple growth and dissociation of 90.0% methane/10.0% propane hydrates at 1.3 MPa and 270.15 K were carried out in two sediments for morphology observation via a visible jacketed-reactor. The gas hydrate crystals were observed to form and grow on the surface of sediments at the initial growth. During the thermal decomposition, gas and liquid products had an unceasingly impact on the sediments, then gas/liquid–solid migration occurred, and a large number of cavitation appeared. In the later growth and dissociation experiments, the gas hydrate particles were in suspension or supporting states in the interstitial pore space between the sediment particles, indicating that the gas hydrate displayed a pore-filling characteristics. Through analyzing the distribution of gas hydrates and bubbles, it was found that the amount of gas hydrates distributed in the sediments was improved with multiple growth-dissociation cycle proceedings. Gas migration enhanced the sediment movement, which led to the appearance of the increasing quantity of gas bubbles in the sediments during cycles. Salts affected the growth of the gas hydrates and the migration of sediment grains, which also restricted the accumulation of gas bubbles in the sediments. According to the Raman analysis, the results showed that sII hydrates were formed for CH4 and C3H8 gas mixtures in different sediments and solutions with hydration number of 5.84–6.53. The Salt restricted the access of gas into the hydrate cages.

Reservoir volume of gas hydrate stability zones in permafrost regions of China

Gas hydrate is a promising energy resource. Permafrost regions in China are natural gas hydrate prospects. In this study, we summarized the conditions for gas hydrate formation, and analyzed the probable thickness and area of gas hydrate stability zones in the permafrost regions in China. Then the probability of porosity within the gas hydrate stability zones was estimated from lithology, burial history, and compaction curves. Finally, the total reservoir volume for gas hydrate stability zones in permafrost regions was estimated from the joint probability distribution of the effective thickness, area, and porosity. The probable total reservoir volume of the gas hydrate stability zones in the permafrosts in China is in the range of 2.995 × 1012 m3 to 5.081 × 1012 m3, with the best estimate at 4.002 × 1012 m3. The Qinghai-Tibetan Plateau including Qilian (where the occurrence of natural gas hydrate has been proven by coring) has about 55% of the total reservoir volume of the gas hydrate stability zones in China, indicating it has a great resource potential. In the Mohe Basin the reservoir volume of the gas hydrate stability zone ranges from 0.49 × 1012 m3 to 0.79 × 1012 m3, suggesting it is a hopeful future resource exploration area. The best estimate of the total reservoir volume of the gas hydrate stability zones in Northwest China is 1.2 × 1012 m3 which is considerable. However, occurrence of gas hydrate is yet to be confirmed by future exploration. These estimates provide much needed data for further gas hydrate resource exploration and evaluation, as well as environmental protection.

Sensitivity analysis of P-waves and S-waves to gas hydrate in the Shenhu area using OBS

Compared to towed streamers, ocean-bottom seismometers (OBS) obtain both S-wave data and richer wavefield information. In this paper, the induced polarization method is used to conduct wavefield separation on OBS data obtained from the Shenhu area in the South China Sea. A comparison of the changes in P- and S-waves, and a comprehensive analysis of geological factors within the area, enable analysis and description of the occurrence of natural gas hydrate in the study area. Results show an increase in P-wave velocity when natural gas hydrate exists in the formation, whereas the S-wave velocity remains almost constant, as S-waves can only propagate through the rock skeleton. Therefore, the bottom-simulating reflection (BSR) response of the P-wave is better than that of the S-wave in the frequency analysis profile. In a wide-angle section, the refractive wave of the hydrate layer is evident when using P-wave components but identification is difficult with S-wave components. This velocity model illustrates the sensitivity of P- and S-wave components to gas hydrate. The use of this polarization method and results of analysis provide technical and theoretical support for research on hydrate deposits and other geological features in the Shenhu area.

Microstructure Observations of Natural Gas Hydrate Occurrence in Porous Media Using Microfocus X-ray Computed Tomography

Natural gas hydrates are globally considered a potential alternative form of energy suitable for sustainable development. The microstructure of natural gas hydrates in sediments governs their seismic and acoustic exploration, stability of seafloors, and gas production from hydrate deposits. To investigate the microstructure and occurrence of natural gas hydrates in pores, natural gas hydrate-bearing porous media were directly observed using microfocus X-ray computed tomography (CT). The spatial distributions of free gas, natural gas hydrates, water, and grains were identified. The results indicated the preference of natural gas hydrates to form primarily within pore spaces and not to cement the adjacent grains, which was described by the floating model. Moreover, the migration of gas and water within pore spaces during hydrate formation appeared random, and natural gas hydrates were found to nucleate preferentially at the gas–water interface. The values of porosity and hydrate saturation obtained via reco...

Evidence of natural gas hydrate in Kunlun Pass basin, Qinghai-Tibet Plateau, China

The possible occurrence of natural gas hydrate in permafrost of the Qinghai-Tibet Plateau has significant potential impact on energy resources, climate change and the environment. New data from a scientific drilling project in the Kunlun Pass basin indicates the presence of natural gas hydrate in permafrost on the plateau at Kunlun Pass, Qinghai Province, China. A full suite of petrophysical logs was run in the borehole, and gas geochemistry was logged during drilling. Gas recovered from multi-layer strata below the depth of 250 m was characterized by intermittent gas peaks and methane concentration ranges from 22% to 32% during drilling. Petrophysical analysis indicates that these multi-layer strata are dominated by typical gas hydrate features including low density, high lateral resistivity, and high acoustic velocity. Corroborating petrographic evidence includes authigenic carbonate minerals and pyrite found in these strata, closely related with methane and expected as a product of gas hydrate dissociation.

Geophysical signatures for low porosity can mimic natural gas hydrate: An example from Alaminos Canyon, Gulf of Mexico

AbstractNatural gas hydrate in sand sediments can increase both the measured compressional velocity and resistivity. The same geophysical signatures occur, however, in low‐porosity sand. We investigate the possible occurrence of natural gas hydrate in a sand interval in Alaminos Canyon Block 21 (AC 21) in the Gulf of Mexico, drilled by the U.S. Gas Hydrate Joint Industry Project. The sand interval has an increase in resistivity (~2.2 Ω m) and a strong peak and trough at the top and bottom of the sand on exploration seismic, which has been interpreted as natural gas hydrate. We reexamine the logging data and construct a new reservoir model that matches the measured resistivity, the high‐density sublayers in the sand, and the surface seismic trace. Our model shows that the sand interval in AC 21 is most likely water saturated; and the slight increase in resistivity, higher‐measured density, and the seismic amplitudes are caused by a reduction in porosity to ~30% in the sand interval relative to a porosity of ~42% in the surrounding marine muds. More broadly, we show that the average depth where the porosity of marine muds becomes lower than sand sediment is 900 mbsf, though it could be as shallow as 600 mbsf for high‐porosity sands. In any case, the similar geophysical signatures for water‐saturated sand and low saturations of natural gas hydrate in sand probably occur throughout the gas hydrate stability zone at most sites worldwide.

Seismic facies analyses as aid in regional gas hydrate assessments. Part-I: Classification analyses

The identification of areas of natural gas hydrate occurrence, defining its concentration and regional distribution, as well as understanding the processes that control gas hydrate formation is a crucial component of regional gas hydrate assessments. In this study, we incorporate an additional element into a regional assessment strategy by including the depositional environment defined through seismic facies classes. The seismic facies classification is attempted using regional 2D seismic data and a 3D seismic volume, as well as core and log-data from two gas hydrate drilling expeditions carried out in the Ulleung Basin, East Sea, to conduct a fully integrated gas hydrate assessment. The facies classification described in this study is part of a procedure to conduct an assessment including: (1) Lithologic description of the recovered core and describe the deposition character of the sediment facies identified; (2) seismic facies classification using 2D and 3D seismic data and their derived various seismic attributes; (3) Linking seismic facies classes to core-derived sedimentological descriptions to define the host-strata most favourable for gas hydrate occurrences; (4) Defining reservoir physical properties and linking these to seismic facies classes for reservoir property description; (5) Calculating regional elements of the gas hydrate petroleum system (top of gas hydrate occurrence and base of gas hydrate stability); (6) Merging seismic facies and reservoir properties, definition of the top and base of the gas hydrate stability zone, to calculate gas hydrate saturations in each seismic facies and the total volume of gas hydrate present in the study area; (7) Defining probabilistic elements of the predicted volume of gas hydrate by combining statistical information of the individual input parameters through exhaustive Monte Carlo simulations. The seismic facies classification algorithm used in this study is based on identifying clusters (classes) that show similar properties in the seismic data and derived attribute characteristics. Our classification scheme uses seven seismic attributes (amplitude, instantaneous frequency and amplitude (also referred to as envelope), sweetness, acoustic impedance, thin-bed- and parallel-bedding indicator) and incorporates the use of principle component analysis for data reduction. The achieved classifications are overall robust and yield identical facies classes for the co-located 2D and 3D data. Linking the seismic facies classes to the recovered core and sedimentological descriptions results in a sedimentological interpretation of the seismic facies. Cross-plots of individual components as well as analysing the Eigen-vectors and -values from the principle component analysis (PCA) helps understand the physical meaning of these seismic facies classes and their seismic character. The five facies classes assigned to the 3D volume and 2D seismic line after PCA represent the mass transport deposit (MTD) units (class #1), hemipelagic mud with some sandy turbidite (class #2), hemipelagic mud with many sandy turbidites (class #3), as well as water-bearing mud with few sand layers and occurring mostly at shallow depth near the seafloor or beneath the base of the gas hydrate stability zone (classes #4 and #5). Within the study area and seismic data sets used, gas hydrate occurrences are only linked to facies classes #2 and #3.

Accumulation and exploration of gas hydrate in deep-sea sediments of northern South China Sea

The large deep-sea area from the southwestern Qiongdongnan Basin to the eastern Dongsha Islands, within the continental margin of northern South China Sea, is a frontier of natural gas hydrate exploration in China. Multiform of deep-sea sedimentations have been occurred since late Miocene, and sediment waves as a potential quality reservoir of natural gas hydrate is an most important style of them. Based on abundant available data of seismic, gravity sampling and drilling core, we analyzed the characteristics of seismic reflection and sedimentation of sediment waves and the occurrence of natural gas hydrate hosted in it, and discussed the control factors on natural gas hydrate accumulation. The former findings revealed the deep sea of the northern South China Sea have superior geological conditions on natural gas hydrate accumulation. Therefore, it will be of great significance in deep-sea natural gas hydrate exploration with the study on the relationship between deep-sea sedimentation and natural gas hydrate accumulation.

Geochemical characteristics of pore water in shallow sediments from north continental slope of South China Sea and their significance for natural gas hydrate occurrence

Northern slope of the South China Sea is a potential area of gas hydrates and research focus areas. Analyzed geochemical parameters about pore water of the anions and cations of major components and δ 13 C DIC in the sediments from Shenhu sea area, Dongsha sea area and Southwest Taiwan Basin and collected by gravity piston. The results showed that Cl − concentration were no significant changes with depth at the three stations and their values were consistent with the normal sea water, SO 4 2- concentration showed significant changes in gradient descent from core top to bottom at three stations and Ca 2+ , Mg 2+ , Ca 2+ /Mg 2+ showed a similar downward trend with SO 4 2− concentration, but the amplitude is inconsistent. The SMI of Shenhu sea area, Dongsha sea area, Southwest Taiwan Basin were separately 11m, 8m, 6m and δ 13 C DIC of pore water in the sediments range from -10 ‰ to -27 ‰, which were similar with geochemistry characteristics of pore water in the sediments at other international regions of being found gas hydrate, such as Blake Ridge and Mexico Gulf. These geochemistry characteristics may suggest that natural gas hydrate reservoir released methane in deep sediments and methane was mixture gas as main causes of pyrolysis. Summary a series anomaly indicators and identification methods that use these geochemical anomalies characteristics about pore water of shallow sediments to trace gas hydrate in deep stratum.

Rock physics modeling for assessing gas hydrate and free gas: a case study in the Cascadia accretionary prism

We investigate the estimation of gas hydrate and free gas concentration using various rock physics models in the Cascadia accretionary prism, which is one of the most intensively studied regions of natural gas hydrate occurrences. Surface seismic reflection data is the most useful and cost-effective in deriving seismic velocity, and hence estimating gas hydrate and free gas across a BSR with depth, if a proper background (without gas hydrate and free gas) velocity is chosen. We have used effective medium theory of Helgerud et al. (EMTH) and, a combination of self-consistent approximation and differential effective medium (SCA-DEM) theory coupled with smoothing approximation for crystalline aggregate. Using the SCA-DEM (non-load-bearing) and EMTH (load-bearing) modeling, we calculate the average saturations of gas hydrate as 17 and 19%, respectively within ~100 m thick sedimentary column using velocity, derived from the surface seismic data. The saturations of gas hydrate are estimated as 15 and 18% using the SCA-DEM, and 20 and 25% using EMTH from the logging-while-drilling and wire-line sonic velocities, respectively. Estimations of gas hydrate from Poisson’s ratio are in average 50% for EMTH and 10% for SCA-DEM theory. We obtain the maximum saturation of free gas as 1–2% by employing the SCA-DEM theory either to seismic or sonic velocities, whereas the free-gas saturation varies between 0.1 and 0.4% for EMTH model. The gas hydrate saturation estimated from the sonic velocity and the free gas saturation derived from both the seismic and sonic velocities using the SCA-DEM modeling match quite well with those determined from the pressure core data in the study region.

Permafrost-associated natural gas hydrate occurrences on the Alaska North Slope

In the 1960s Russian scientists made what was then a bold assertion that gas hydrates should occur in abundance in nature. Since this early start, the scientific foundation has been built for the realization that gas hydrates are a global phenomenon, occurring in permafrost regions of the arctic and in deep water portions of most continental margins worldwide. In 1995, the U.S. Geological Survey made the first systematic assessment of the in-place natural gas hydrate resources of the United States. That study suggested that the amount of gas in the gas hydrate accumulations of northern Alaska probably exceeds the volume of known conventional gas resources on the North Slope. Researchers have long speculated that gas hydrates could eventually become a producible energy resource, yet technical and economic hurdles have historically made gas hydrate development a distant goal. This view began to change in recent years with the realization that this unconventional resource could be developed with existing conventional oil and gas production technology. One of the most significant developments was the completion of the BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well on the Alaska North Slope, which along with the Mallik project in Canada, have for the first time allowed the rational assessment of gas hydrate production technology and concepts. Almost 40 years of gas hydrate research in northern Alaska has confirmed the occurrence of at least two large gas hydrate accumulations on the North Slope. We have also seen in Alaska the first ever assessment of how much gas could be technically recovered from gas hydrates. However, significant technical concerns need to be further resolved in order to assess the ultimate impact of gas hydrate energy resource development in northern Alaska.

天然堆積物中でのメタンハイドレート形成における堆積物粒子径，粘土鉱物等の影響

Accurately estimating natural gas hydrate resources is one of the most important issues when assessing the energy potential of natural gas hydrate, which relies largely on the precision of data on the hydrate saturation level in sediments of a reservoir. The primary mode of occurrence of gas hydrate recognized in the eastern Nankai Trough area is in sediment pores. It is suggested that the distribution of coarse-grained sands is one of the most important factors controlling the occurrence of natural gas hydrates. This research aims to elucidate the particle size and clay mineral effects on hydrate saturation in sediments through an experimental approach. The specimens tested include sand, silty sand, and silt, representing the main sediment types recovered from the gas hydrate distribution region of the eastern Nankai Trough. The results obtained from the experiments clearly indicate a particle size and clay content-dependent trend of low saturation in fine sediment but high saturation in coarse sediment. These results are generally consistent with NMR logging results for high-saturation samples, but are somewhat different for samples with low or medium saturation levels. To obtain a better understanding of the mechanism of these two factors, studies were carried out to investigate the saturation level of methane hydrate in a series of silica powders and clay. The results indicate that particle size and clay contents are the two key factors determining the saturation level of gas hydrate in sediments—the finer the particle size and/or the higher the clay content, the lower the hydrate saturation.