Distribution Of Methane Hydrate Research Articles

Subsurface heat and salts cause exceptionally limited methane hydrate stability in the Mediterranean Basin

Abstract Knowledge of the global reservoir of submarine gas hydrates is of great relevance for understanding global climate dynamics, submarine geohazards, and unconventional hydrocarbon energy resources. Despite the expected presence of gas hydrates from modeling studies, the land-locked Mediterranean Basin displays a lack of evidence of extensive gas hydrate presence from samples and seismic data. We modeled the theoretical Mediterranean distribution of methane hydrate below the seafloor and in the water column using available geological information provided by 44 Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) boreholes, measured geothermal gradients, and thermohaline characteristics of the water masses. We find that the pervasive presence of high-salinity waters in sediments, coupled with the unique warm and salty water column, limit the thickness of the theoretical methane hydrate stability zone in the subsurface and deepen its top surface to 1163–1391 m water depth. The theoretical distribution of methane hydrates coincides well with the distribution of shallow, low-permeability Messinian salt deposits, further limiting the formation of pervasive gas hydrate fronts and controlling their or distribution due to the prevention of upward hydrocarbon gas migration. We conclude that the Mediterranean Basin, hosting the youngest salt giant on Earth, is not prone to the widespread formation and preservation of gas hydrates in the subsurface and that the gas hydrate potential of salt-bearing rifted continental margins may be considerably decreased by the presence of subsurface brines.

Experimental Study of the Formation Rate and Distribution of Methane Hydrate in Layered Sand.

The geological structure and gas hydrate occurrence are stratification-dependent in the vertical direction. It is necessary to explore the formation processes and distribution characteristics of methane hydrate in layered porous media. The sand sample consists of two equal parts in a testing cylinder. The upper part is 0.5–1 mm sand in particle diameter, and the lower parts are 0.075∼0.5, 0.5∼1, and 1∼2 mm. The experimental results show that the formation rate of methane hydrate gradually decreases as the reaction goes on, and it is higher in layered sand than in nonlayered sand in the beginning. With the increase of the sand size in the lower part, saturation of methane hydrate gradually decreases in the upper part and increases in the lower part. In the layered sand, saturation of methane hydrate is higher in the sand layer whose particle size is bigger. The abovementioned results can be used to predict the favorable area where methane hydrate may appear in different stratigraphic structures in nature.

Study on the Effects of Heterogeneous Distribution of Methane Hydrate on Permeability of Porous Media Using Low‐Field NMR Technique

AbstractThe permeability of sediments is an important parameter that effects flow characteristics of the gas and water in the process of natural gas hydrates development. The distribution of gas hydrate is generally heterogeneous in porous media. It poses a great challenge to the permeability measurement of hydrate‐bearing sediments. To study the effect of heterogeneous distribution of methane hydrate on the permeability of porous media, methane hydrate formation in the sandstone and permeability measurement was carried out using a low‐field nuclear magnetic resonance (NMR) measurement system in situ conditions. The spatial distribution of methane hydrate in the core samples was determined by combining the T2 distribution with magnetic resonance imaging. The results show that the distribution of methane hydrate is heterogeneous in the core samples. The temporal and spatial variation of permeability happens in the process of hydrate dissociation. The heterogeneity of methane hydrate distribution severely affects the analysis of permeability change. Considering the heterogeneity of methane hydrate distribution, a new method was proposed to obtain the quantitative relationship between hydrate saturation and relative permeability based on magnetic resonance imaging and the equivalent seepage capacity method. The results show that the relative permeability models obtained by this method agree better with experimental results. The optimum coefficients of relative permeability models obtained by equivalent seepage capacity method change significantly in contrast with the direct fitting method. The variation of the optimum coefficient is up to 300% in this study. The model coefficients are related to the pore characteristic of hydrate‐free porous media.

Mechanical Properties of Methane Hydrate-Bearing Interlayered Sediments

The complex distribution of gas hydrate in sediments makes understanding the mechanical properties of hydrate-bearing sediments a challenging task. The mechanical behaviors of hydrate-bearing interlayered sediments are still poorly known. A series of triaxial shearing tests were conducted to investigate the strength parameters and deformation properties of methane hydrate-bearing interlayered sediments at the effective pressure of 1 MPa. The results indicate that the stress-strain curves of hydrate-bearing interlayered sediments are significantly different from that of hydrate-bearing sediments. The peak strength, Young’s modulus, initial yielding modulus, and failure mode are deeply affected by the methane hydrate distribution. The failure behaviors and mechanism of strain softening and hardening patterns of the interlayered specimens are more complicated than those of the integrated specimens. This study compares the different mechanical behaviors between integrated and interlayered specimens containing gas hydrate, which can serve as a reference for the prediction and analysis of the deformation behaviors of natural gas hydrate reservoirs.

Pore habit of methane hydrate and its evolution in sediment matrix – Laboratory visualization with phase-contrast micro-CT

Abstract Describing the pore habit of methane hydrate in sediment matrices is essential for understanding natural distribution of methane hydrate, methane trace (transport and solidification) in the hydrate stability zone, physical properties of hydrate-bearing sediments, and the associated influence on potential gas production. Pore habit visualization in natural media at pore scale even with laboratory synthesized cores has been challenging due to the similar densities of methane hydrate from pore liquid. In this work, we used phase-contrast assisted micro-CT with potassium iodine-doped brine to visualize four phases: sand particles, pore fluid, methane hydrate and methane gas. This study visualizes the pore habit of methane hydrate at various stages including during hydrate formation in excess-gas systems, its evolution after brine injection to replace pore fluid, and hydrate formation in excess-water systems. Hydrate tends to adopt round and smooth surfaces when in contact with water while exhibits relatively angular interfaces when in contact with methane gas. Hydrate formation in excess-gas systems results in a partial cementing and partial mineral-coating pore habit, while hydrate in excess-water systems develops mainly as pore-filling, and locally cementing or mineral-coating where big gas pockets exist at the initial state. Pore liquid replacement from methane gas to brine triggers a shift of hydrate pore habit towards pore-filling. Methane hydrate evolution over time produces bigger hydrate particles but with less contact area with sand particles. The effects of hydrate pore habit become less important as hydrate particle size exceeds the pore size. Additionally, hydrate formation could trap residual methane gas and brine as inclusions.

On the Importance of Advective Versus Diffusive Transport in Controlling the Distribution of Methane Hydrate in Heterogeneous Marine Sediments

AbstractThe occurrence of methane hydrate in marine reservoirs often correlates with the physical properties of the host sediments. High hydrate saturations (>60% of the pore volume) found in association with coarser‐grained strata have been attributed to both enhanced advective transport through more permeable sediment layers and to perturbations in phase equilibrium related to pore space geometry that results in increased diffusive transport. To assess the relative importance of these mechanism in controlling hydrate occurrence, we develop a 1‐D model for hydrate growth along dipping, coarse‐grained layers embedded in a fine‐grained sediment package. We explicitly account for pore size effects on methane solubility and permeability‐driven variations in fluid flux. We show how the vertical distribution of hydrate varies in response to changes in grain size and rates of fluid advection, sedimentation, and in situ methane production. We then use our model to simulate centimeter‐scale variations in hydrate saturation observed at Walker Ridge Block 313, Hole H in the Gulf of Mexico. We find that the largest concentrations of hydrate are controlled by diffusion, while increased advective methane supply favors more distributed growth throughout high‐permeability regions. Our results hold promise for using well log‐derived estimates of hydrate saturation to infer sediment properties and the sources and rates of methane supply during reservoir emplacement.

Quantifying gas hydrate from microbial methane in the South China Sea

Methane hydrate has been confirmed to be present in the South China Sea (SCS) based on drilling expeditions and geophysical indications. It is necessary to access the methane hydrate inventory because of its role in the energy resources and climate change. The aim of this study is to constrain the inventory of methane hydrates in the SCS formed by the microbial degradation of organic matter within the gas hydrate stability zone (GHSZ). Bathymetry, seafloor temperature, and geothermal gradient datasets enabled us to estimate the GHSZ at a resolution of 30″×30″ for the SCS. We conclude that the GHSZ occurs at water depths of >500 m in most areas of the SCS. The methane hydrate distribution formed by microbial methane production in the SCS was computed using the parameterized transfer function recently proposed by Wallmann et al. (2012). The result indicates widespread accumulations of methane hydrate along the continental slopes in the SCS. The microbial methane hydrate is distributed over a 7.69 × 1011 m2 area and the total abundance of methane hydrate in the SCS is estimated to be 42.8 Gt C.

Numerical simulation of pore-scale formation of methane hydrate in the sand sediment using the phase-field model

Methane hydrate is considered as a promising energy resource for the near future. To predict the gas productivity from the methane hydrate in the subsea sand-sediment, it is important to know effective permeability accurately of the sediment bearing methane hydrate. Hence, the hydrate morphological distribution: namely, what is the shape and morphology of hydrate, in the sediment should be elucidated, because the permeability is strongly affected by the hydrate distribution. In this study, to know where hydrate is formed in the pore of porous media, we proposed a numerical model for estimating the microscopic distribution of methane hydrate in sand sediment, using the classical nucleation theory and the phase-field model. The former theory gave the probabilities of hydrate nucleation positions in the gas-water-sand three-phases and the latter method provided the mobility of the front of the hydrate formation. A necessary hydrate formation rate constant was determined by history-matching with an experiment in the literature. Using the obtained rate constant, we numerically simulated hydrate formations within the microscopic computational domains.

Methane Hydrate Distribution from Prolonged and Repeated Formation in Natural and Compacted Sand Samples: X-Ray CT Observations

To study physical properties of methane gas hydrate-bearing sediments, it is necessary to synthesize laboratory samples due to the limited availability of cores from natural deposits. X-ray computed tomography (CT) and other observations have shown gas hydrate to occur in a number of morphologies over a variety of sediment types. To aid in understanding formation and growth patterns of hydrate in sediments, methane hydrate was repeatedly formed in laboratory-packed sand samples and in a natural sediment core from the Mount Elbert Stratigraphic Test Well. CT scanning was performed during hydrate formation and decomposition steps, and periodically while the hydrate samples remained under stable conditions for up to 60 days. The investigation revealed the impact of water saturation on location and morphology of hydrate in both laboratory and natural sediments during repeated hydrate formations. Significant redistribution of hydrate and water in the samples was observed over both the short and long term.

Numerical modeling of gas hydrate emplacements in oceanic sediments

We have implemented a 2-dimensional numerical model for simulating gas hydrate and free gas accumulation in marine sediments. The starting equations are those of the conservation of the transport of momentum, energy, and mass, as well as those of the thermodynamics of methane hydrate stability and methane solubility in the pore-fluid. These constitutive equations are then integrated into a finite element in space, finite-difference in time scheme. We are then able to examine the formation and distribution of methane hydrate and free gas in a simple geologic framework, with respect to the geothermal heat flow, fluid flow, the methane in-situ production and basal flux. Three simulations are performed, leading to the build up of hydrate emplacements largely linear through time. Models act primarily as free gas accumulators and are relatively inefficient with respect to hydrate emplacements: 26–33% of formed methane are converted to hydrate. Seepage of methane across the sea-floor is negligible for fluid flow below 2. 10 −11 kg/m 2/s. At 5.625 10 −11 kg/m 2/s however, 9.7% of the formed methane seeps out of the model. Moreover, along strike variation arising in the 2-dimensional model are outlined. In the absence of focused flow, the thermodynamics of hydrate accumulation are primarily one-dimensional. However, changes in free methane compressibility (density) and methane solubility (the intrinsic dissolved methane flux) subtlety impact on the formation of a free gas zone and the distribution of the hydrate emplacements in our 2-dimensional simulations.

Capillary controls on methane hydrate distribution and fracturing in advective systems

[1] We simulate 1-D, steady, advective flow through a layered porous medium to investigate how capillary controls on solubility including the Gibbs-Thomson effect in fine-grained sediments affect methane hydrate distribution in marine sediments. We compute the increase in pore fluid pressure that results from hydrate occluding the pore space and allow fractures to form if the pore fluid pressure exceeds a fracture criterion. We apply this model to Hydrate Ridge and northern Cascadia, two field sites where hydrates have been observed preferentially filling cm-scale, coarse-grained layers. We find that at Hydrate Ridge, hydrate forms in the coarse-grained layers reaching saturation of 90%, creating fractures through intervening fine-grained layers after 2000 years. At northern Cascadia, hydrate forms preferentially in the coarse-grained layers but 2 × 105 years are required to develop the observed hydrate saturations (∼20%–60%), suggesting that hydrate formation rates may be enhanced by an additional source of methane such as in situ methanogenesis. We develop expressions to determine the combinations of sediment physical properties and methane supply rates that will result in hydrate-filled coarse-grained layers separated by hydrate-filled fine-grained layers, the conditions necessary to fracture the fine-grained layers, and the conditions that will lead to complete inhibition of hydrate formation as pore space is constricted. This work illustrates how sediment physical properties control hydrate distribution at the pore scale and how hydrate distribution affects fracturing behavior in marine sediments.

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.

地震波属性を用いたメタンハイドレート賦存層抽出の試み

The result of MITI Nankai Trough wells drilled in 1999-2000 proved the occurrence of methane hydrate in the Nankai Trough area from recovered core and well logging data. This result gave a big impact on both the Japanese strategy for energy and scientific interest. And it eventually motivated Japanese government to commence the national project named “Japan's Methane Hydrate Exploitation Program”, which started since 2001. From the results of the 2D and 3D seismic survey acquired in the Nankai Trough area by METI in 2001 and 2002, widely-distributed BSR was recognized in the area, which was expected the wide distribution of methane hydrate. For the volume quantification, we needed to evaluate reservoir parameters of methane hydrate reservoir such as thickness, saturation, and porosity of sediments. And we inferred methane hydrate bearing layers are inhomogeneous and discontinuous in space and depth. And we also found a good correspondence between methane hydrate layer and the zone which is characterized by seismic attribute as high P-wave velocity, high P-wave impedance, high S-wave impedance, low pseudo Poisson's ratio and high attenuation.Consequently, we conclude that the pre- and post-stack seismic attributes are so helpful tools to delineate the methane hydrate reservoir.

Global Distribution of Methane Hydrate in Ocean Sediment

In this paper, we present an equilibrium thermodynamic model to accurately predict the maximum depth of hydrate stability in the seafloor, including the effects of water salinity, hydrate confinement in pores, and the distribution of pore sizes in natural sediments. This model uses sediment type, geothermal gradient, and seafloor depth as input to predict the thickness of the hydrate zone. Using this hydrate model and a mass-transfer description for hydrate formation, we have also developed a predictive method for the occurrence of methane hydrates in the ocean. Based on this information, a prediction for the distribution of methane hydrate in ocean sediment is presented on a 1° latitude by 1° longitude (1° × 1°) global grid. From this detailed prediction, we estimate that there is a total volume of 1.2 × 1017 m3 of methane gas (expanded to atmospheric conditions), or, equivalently, 74 400 Gt of CH4 in ocean hydrates, which is 3 orders of magnitude larger than worldwide conventional natural gas reserves. ...

Study of the natural gas hydrate trap zone and the methane hydrate potential in the Sverdrup Basin, Canada

The methane hydrate stability zone beneath Sverdrup Basin has developed to a depth of 2 km underneath the Canadian Arctic Islands and 1 km below sea level under the deepest part of the inter-island sea channels. It is not, however, a continuous zone. Methane hydrates are detected in this zone, but the gas hydrate/free gas contact occurs rarely. Interpretation of well logs indicate that methane hydrate occurs within the methane stability zone in 57 of 150 analyzed wells. Fourteen wells show the methane hydrate/free gas contact. Analysis of the distribution of methane hydrate and hydrate/gas contact occurrences with respect to the present methane hydrate stability zone indicate that, in most instances, the detected methane hydrate occurs well above the base of methane hydrate stability. This relationship suggests that these methane hydrates were formed in shallower strata than expected with respect to the present hydrate stability zone from methane gases which migrated upward into hydrate trap zones. Presently, only a small proportion of gas hydrate occurrences occur in close proximity to the base of predicted methane hydrate stability. The association of the majority of detected hydrates with deeply buried hydrocarbon discoveries, mostly conventional natural gas accumulations, or mapped seismic closures, some of which are dry, located in structures in western and central Sverdrup Basin, indicate the concurring relationship of hydrate occurrence with areas of high heat flow. Either present-day or paleo-high heat flows are relevant. Twenty-three hydrate occurrences coincide directly with underlying conventional hydrocarbon accumulations. Other gas hydrate occurrences are associated with structures filled with water with evidence of precursor hydrocarbons that were lost because of upward leakage.

Model Simulation for Generation and Migration of Methane Hydrate

A one-dimensional numerical simulation model has been developed to forecast the distribution of methane hydrate in the subsurface. The model includes the generation and migration of methane, and the process of methane-hydrate formation after sedimentation. The following processes are consideredfor each cell within the model; 1) Calculation of methane generated within the sediment 2) Process of migration of dissolved methane with compacting water 3) Process of migration of gaseous methane as a separate phase, by buoyancy 4) Evaluation of sealing capacity from the hydrostatic equilibrium equation 5) Calculation of the amount of methane hydrate formation from in-situ temperature and pressure The model will predict the distribution of methane hydrate after applying the above processes for all the sediments. We use this model to simulate the process of methane hydrate formation at ODP Leg 164 Site 997 at Blake Ridge, offshore Atlantic coast of the USA. At this location, the methane hydrate zone indicated by the bottom simulating reflector (BSR) is about 100 m shallower than the base of gas (methane) hydrate stability zone (BGHS) which is calculated from the model using measured temperatures and pressures. To explain this difference, the following hypotheses are proposed; 1) The BSR is a remnant of the past the BGHS that was created when temperature was higher or sea level was lower. 2) Methane hydrate is being generated between the BSR and the BGHS at present. The model simulation indicates that analysis, based not only on present physical conditions but also on the geohistrical time scale, is necessary to explain the present day distribution of methane hydrate.

Abstract: Characterizing volumes and distribution of methane hydrate and underlying free gas with wide angle seismic data on the Blake Ridge, offshore South Carolina

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Methane Hydrate Accumulation along the Western Nankai Trough

Abstract: A one‐dimensional gas hydrate accumulation model is developed to simulate the existence of hydrate in the Nankai Trough margin, where the total organic carbon content is extremely low. In order to study the accumulation of gas hydrate along a seismic profile, our model assumes an initial 1000 m thick turbidite sedimentation at the trench axis, and then the sediment experiences tectonic uplift to form the Nankai accretionary prism. Methane generated by microbial processes during the sedimentation is partly trapped in the pore space and partly migrated upwards due to compaction and subsequent porosity decrease at deeper levels. Upward methane gas flux from the deeper part (below 1000 m) is predicted by the model because the maximum thickness of the accreted sediment exceeds ten kilometers in the Nankai Trough accretionary prism. The basic geological parameters input to the model are those obtained from the ODP site 808, which was drilled at the lower inner trench slope. The methane hydrate stability zone is essentially controlled by bottom water temperature, regional heat flow and hydrostatic pressure. The simulation shows that methane hydrate will not accumulate in the Nankai Trough margin if only microbial methane generation is considered because the organic carbon content in the Nankai Trough area is as low as 0.75%. However, if we assume an upward flux of 5 kg of methane per square meters per 10,000 years, it will cause approximately 10 to 25% hydrate saturation under current physical conditions. The origin of the methane may be thermogenic and/or microbial. The resultant distribution of methane hydrate shows very good correlation with the distribution of the BSR.

Interest of in situ turbidimetry for the characterization of methane hydrate crystallization: Application to the study of kinetic inhibitors

In this paper we describe a new experimental set-up which makes possible the in situ determination of the particle size distribution of methane hydrate during its formation in a pressurized reactor. It has been installed on two research sites: one at the Institut Français du Pétrole (IFP) and the other at the Ecole des Mines de Saint-Etienne (EMSE). Thanks to this apparatus, new data can be obtained, in particular concerning the granular aspects of the crystallization processes and the reproducibility of the results. Particularly, we propose a new experimental procedure which allows us two important improvements: 1. a better reproducibility of the induction time, 2. the possibility of doing a large number of experiments in a day (around fifteen). Using this protocol, the effect of some kinetic inhibitors of the crystallization processes are shown: increase of the induction time and/or decrease of the quantity of hydrate formed.

Potential distribution of methane hydrate beneath the European continental margins

Gridded datasets have been constructed for the physical parameters controlling the formation of gas hydrate. Simultaneous solutions to the gas hydrate phase and pressure‐temperature equations were obtained for each node of these grids. The results show sub‐bottom depths of potential methane hydrate BSRs. These are presented as colour contoured grids for the thickness of the hydrate stability zone for the European margins. The chart proposes that if gas hydrates exist over these areas, then this is the potential depth to the BSR. This depth is generally greater along the continental margins and also increases with increasing age of the margin. Shallowing of the BSR can be seen over areas of thinned continental crust and plateaus. The geological factors controlling gas hydrate formation determine areas of likely occurrence so the apparent paucity of identified hydrate layer BSRs off the European margins, particularly in the Mediterranean, is most notable