Gas Hydrates In Sediments Research Articles

Multi-channel 2D seismic constraints on pore pressure- and vertical stress-related gas hydrate in the deep offshore of the Mahanadi Basin, India

Estimation of pore pressure and in-situ vertical stress magnitude is essential for understanding the geomechanical behavior of the gas hydrate sediments in deep offshore of the Mahanadi basin. The basin located at northern side of eastern continental margin of India (ECMI) contains gas hydrate in clay/silt sediments. The pore pressure and vertical stress are mapped on two 2D-multi channel seismic data of lines (MH-38A and MH-38B) with aid of information of three wells (namely NGHP-01-19, NGHP-01-09 and NGHP-01-08). Initially, the coefficients of best fit curve have been computed from velocity-effective stress plot for the individual wells and applied on the seismic velocity to transform into the effective stress. The vertical stress has been computed from both the seismic and log data. Therefore, the pore pressure is predicted by subtracting the effective stress from the vertical stress. The pore pressure have been mapped in gas hydrate stability zone (GHSZ) and sediments below bottom simulating reflector (BSR). The pore pressure and vertical stress gradient are 10.11 MPa/km and 10.67 MPa/km, respectively. The pore pressure and vertical stress from seismic data are closely matched at well location with excellent goodness of fit (R2) varying from 0.82 to 0.95. Normal pressure is observed in the gas hydrate bearing sediments but slightly high pressures are noticed below the BSR indicating presence of free gas. The pore pressure from seismic data will guide drillers for choosing the mud weight during well drilling and casing optimizing in other part of deep offshore in the Mahanadi basin.

Influence of heat conduction and heat convection on hydrate dissociation by depressurization in a pilot-scale hydrate simulator

Natural gas hydrate, as an unconventional energy resource, has generated considerable research interest. It is generally accepted that depressurization method is the most practical and economically promising way to produce gas from gas hydrate sediments. Rates of hydrate dissociation by depressurization depend on heat transfer rate, and the heat transfer during hydrate dissociation mainly includes heat conduction and heat convection. In this paper the Pilot-Scale Hydrate Simulator (PHS), with an inner volume of 117.8 L, was applied to investigate the influence of heat conduction and heat convection on hydrate dissociation. Different thermal boundary conditions and different flow directions during gas recovery from hydrate reservoir by depressurization were performed in the PHS. In addition, the method of studying the effect of different directions of heat convection by changing well locations was firstly proposed in this paper. It was obtained from experimental results that the hydrate dissociation rate with an isothermal boundary is faster than that with a semi-adiabatic boundary, and heat conduction is the dominant factor in hydrate dissociation by depressurization in the constant pressure stage. The influence of heat convection on hydrate dissociation in the constant pressure stage may not be obvious, but during the depressurizing stage, the opposite direction of fluid flow and heat transfer can promote hydrate reformation, and has effect on fluid flow characteristics inside the reservoir. These findings can provide theoretical references for field tests of exploiting natural gas hydrate.

Experimental Investigation on Deformation of Natural Gas Hydrate in the Process of Decompressing

The study of the mechanical properties of natural gas hydrate sediments is an important part in the research of gas hydrate extraction technology. However, in the past, most of the researches focused on triaxial compression experiments, and few people studied the deformation in the process of depressurization mining. In order to study this problem, we have simulated the relationship between the axial deformation of natural gas hydrate sediments and aerogenesis in the process of depressurization. In this experiment, the in-situ formation and decompression decomposition of hydrate are simulated, and the overlying pressure of 13.5MPa is loaded, and the axial deformation of hydrate sediment is recorded. The conclusion is that, in the process of hydrate decompression decomposing, the strain increased with the amount of gas produced, the relationship between strain value and gas production percentage is a proportional function, and the proportional coefficient increases with the increase of hydrate saturation.

A unified effective medium model for gas hydrates in sediments

A unified effective medium model is developed to incorporate the endpoints of perfectly smooth and infinitely rough sphere components and to allow partitioning between rough and smooth grains. We incorporate the unified model into the framework for gas hydrates in unconsolidated sediments using pore-fluid and rock-matrix configurations for grain placement, while reviewing other developments that have taken place in the past four decades. The unified rock-matrix model is validated with data available from the 2002 Mallik gas hydrates project well 5L-38. Gas-hydrate saturation and neutron-porosity logs from this well are used to generate synthetic P- and S-wave velocity models for several values of the friction coefficient. First, we overlaid crossplots of P- versus S-wave velocities for synthetic and measured velocities, and we compared the match until a good choice was found for the friction coefficient. Second, we plotted the synthetic velocities as separate logs of P- and S-wave velocities for each friction coefficient; the synthetic velocity logs were then overlaid on the measured velocities calculated from the sonic logs. Results of a direct comparison of the synthetic and measured velocity logs provide valuable insights into the validation of the unified effective medium model. Recognizing the significance of the Hertz-Mindlin-type effective medium models for gas hydrates in unconsolidated sediments, we incorporate the previous efforts into a single “unified” model and define a common nomenclature. Although we attempt to assign a single friction coefficient value to each hydrate window, it is not surprising that in a real and heterogeneous environment, the value might vary with depth, as it does here at the larger spatial scales. We determine and quantitatively estimate that gas hydrates in sediments are well-predicted with a friction coefficient closer to a smooth sphere model than a rough sphere model.

Keystone Arctic paleoceanographic proxy association with putative methanotrophic bacteria

Foraminifera in sediments exposed to gas-hydrate dissociation are not expected to have cellular adaptations that facilitate inhabitation of chemosynthesis-based ecosystems because, to date, there are no known endemic seep foraminifera. To establish if foraminifera inhabit sediments impacted by gas-hydrate dissociation, we examined the cellular ultrastructure of Melonis barleeanus (Williamson, 1858) from the Vestnesa gas hydrate province (Arctic Ocean, west of Svalbard at ~79 °N; ~1200-m depth; n = 4). From sediments with gas hydrate indicators, living M. barleeanus had unusual pore plugs composed of a thick, fibrous meshwork; mitochondria were concentrated at the cell periphery, under pore plugs. While there was no evidence of endosymbioses with prokaryotes, most M. barleeanus specimens were associated with what appear to be Type I methanotrophic bacteria. One foraminifer had a particularly large bolus of these microbes concentrated near its aperture. This is the first documented instance of bona fide living M. barleeanus in gas-hydrate sediments and first documentation of a foraminifer living in close association with putative methanotrophs. Our observations have implications to paleoclimate records utilizing this foundational foraminiferal species.

Identifying the morphologies of gas hydrate distribution using P-wave velocity and density: a test from the GMGS2 expedition in the South China Sea

Pore-filling and fracture-filling are two basic distribution morphologies of gas hydrates in nature. A clear knowledge of gas hydrate morphology is important for better resource evaluation and exploitation. Improper exploitation may cause seafloor instability and exacerbate the greenhouse effect. To identify the gas hydrate morphologies in sediments, we made a thorough analysis of the characteristics of gas hydrate bearing sediments (GHBS) based on rock physics modeling. With the accumulation of gas hydrate in sediments, both the velocities of two types of GHBS increase, and their densities decrease. Therefore, these two morphologies cannot be differentiated only by velocity or density. After a series of tests, we found the attribute as a function of hydrate concentration show opposite trends for these two morphologies due to their different formation mechanisms. The morphology of gas hydrate can thus be identified by comparing the measured with its background value, which means the of the hydrate-free sediments. In 2013, China's second gas hydrate expedition was conducted by Guangzhou Marine Geologic Survey to explore gas hydrate resources in the northern South China Sea, and both two hydrate morphologies were recovered. We applied this method to three sites, which include two pore-filling and three fracture-filling hydrate layers. The data points, that agree with the actual situations, account for 72% and 82% of the total for the two pore-filling hydrate layers, respectively, and 86%, 74%, and 69% for the three fracture-filling hydrate layers, respectively.

Seismic Attenuation for Characterization of Gas Hydrate Reservoir in Krishna-Godavari Basin, Eastern Indian Margin

ABSTRACT Gas hydrates have received global attention as a possible alternative non-conventional energy resource. Hence, the detection, characterization and quantification of gas hydrates are very important for evaluating the resource potential. Presence of gas hydrates in sediments above the bottom simulating reflector or BSR is associated with low attenuation or high quality factor (Q), whereas, free gas bearing sediments below the BSR exhibit high attenuation or low seismic Q. Here the logarithm spectral ratio (LSR) method is applied to marine seismic reflection data along two cross lines (18 and 46) in the Krishna-Godavari (KG) basin in eastern Indian margin, where gas hydrates have already been established by drilling/coring. The interval Qs is calculated for three sedimentary layers (A, B, and C) bounded by the seafloor, BSR, one reflector above and another reflector below the BSR at some common depth points (CDPs) to study the attenuation characteristics of sediments across the BSR. The estimated average interval Q (160) for the hydrate bearing sediments (layer B) is much higher than the average interval Q (80) for both the loose clayey sediments (Layer A) and underlying free gas saturated sediments (layer C). This demonstrates that estimation of seismic quality factor Q can be used for characterization of gas hydrate reservoir.

Mechanical experiments and constitutive model of natural gas hydrate reservoirs

Natural gas hydrate is a new type of green energy resources and has great development prospects, and it has attracted worldwide attentions. The exploitation of natural gas hydrate may result in a series of geological disasters. Therefore, the constitutive model of natural gas hydrate bearing sediments needs to be established to reveal deformation laws of the reservoir sediments and accurately evaluate mechanical properties of hydrate reservoirs. This is the basic guarantees for the effective exploitation of natural gas hydrate resources. The triaxial compressive tests were conducted on samples of natural gas hydrate sediment. Furthermore, the Duncan-Chang hyperbolic model was modified by considering the influences of hydrate saturation based on the test results to obtain the constitutive model according with the deformation characteristics of natural gas hydrate reservoirs. The results show that the stress-strain curves of natural gas hydrate reservoirs show unobvious compaction stage and peak strength, short elastic stage, long yield stage, and significant strain hardening characteristics. After applying loads on natural gas hydrate bearing sediments, the internal solid particles were dislocated and slid. When the loads were small, the sediments demonstrated elastic deformation. With the increase of loads, plastic flows appeared in the interior, and the hydrate crystals were re-orientated, thus the sediments showing plastic deformation. Initial tangent elastic modulus increased with the effective confining pressures, which had little correlations with hydrate saturation. Furthermore, the damage ratio increases with the increase of effective confining pressures, while slightly decreases with the increase of natural gas hydrate saturation. The predicted results of stress-strain curves of sediments with different hydrate saturations well coincide with the results of triaxial compressive tests, indicting the feasibility and rationality of this model.

An experiment investigation into the effect of particle properties on natural gas hydrate sediments#br#

An experiment investigation into the effect of particle properties on natural gas hydrate sediments#br#

TOC濃度変動に基づく日本海堆積物の「標準層序」とガスハイドレート胚胎層との対比

TOC濃度変動に基づく日本海堆積物の「標準層序」とガスハイドレート胚胎層との対比

Pore water content in equilibrium with ice or gas hydrate in sediments

A new method is proposed for estimating the contents of liquid water in equilibrium with bulk ice and bulk gas hydrate in hydrophilic porous media. In this paper water content equilibrium with bulk ice is represented as unfrozen water, and water equilibrium with gas and gas hydrate is nonclathrate water.The method consists in measuring moisture- and temperature-dependent pore water potential (and water activity) in natural sediments or synthetic porous samples at atmospheric pressure, with subsequent thermodynamic calculations of pressure- and temperature-dependent unfrozen and/or nonclathrate water contents using the measured data. The experiments are applied to synthetic samples with known pore sizes (controlled pore glasses, CPG) and natural kaolinite and polymineral clay samples. Water potential and thermodynamic activity are measured on a Decagon Devices WP4T meter. Thermodynamic calculations for water contents in hydrophilic porous media are performed using specially derived equations for temperature-dependent unfrozen water in equilibrium ‘bulk ice - confined water’ systems (at negative Celsius temperatures) and for nonclathrate water in equilibrium ‘gas - bulk gas hydrate - confined water’ systems depending on both temperature and pressure of gas hydrate former (methane, natural gas, etc.). The water contents in porous samples estimated with the new method show good agreement with the results of direct contact measurements for equilibrium water in contact with ice or gas hydrate at given temperatures and pressures.

The interaction of climate change and methane hydrates

AbstractGas hydrate, a frozen, naturally‐occurring, and highly‐concentrated form of methane, sequesters significant carbon in the global system and is stable only over a range of low‐temperature and moderate‐pressure conditions. Gas hydrate is widespread in the sediments of marine continental margins and permafrost areas, locations where ocean and atmospheric warming may perturb the hydrate stability field and lead to release of the sequestered methane into the overlying sediments and soils. Methane and methane‐derived carbon that escape from sediments and soils and reach the atmosphere could exacerbate greenhouse warming. The synergy between warming climate and gas hydrate dissociation feeds a popular perception that global warming could drive catastrophic methane releases from the contemporary gas hydrate reservoir. Appropriate evaluation of the two sides of the climate‐methane hydrate synergy requires assessing direct and indirect observational data related to gas hydrate dissociation phenomena and numerical models that track the interaction of gas hydrates/methane with the ocean and/or atmosphere. Methane hydrate is likely undergoing dissociation now on global upper continental slopes and on continental shelves that ring the Arctic Ocean. Many factors—the depth of the gas hydrates in sediments, strong sediment and water column sinks, and the inability of bubbles emitted at the seafloor to deliver methane to the sea‐air interface in most cases—mitigate the impact of gas hydrate dissociation on atmospheric greenhouse gas concentrations though. There is no conclusive proof that hydrate‐derived methane is reaching the atmosphere now, but more observational data and improved numerical models will better characterize the climate‐hydrate synergy in the future.

Natural Gas Evolution in a Gas Hydrate Melt: Effect of Thermodynamic Hydrate Inhibitors.

Natural gas extraction from gas hydrate sediments by injection of hydrate inhibitors involves the decomposition of hydrates. The evolution of dissolved gas from the hydrate melt is an important step in the extraction process. Using classical molecular dynamics simulations, we study the evolution of dissolved methane from its hydrate melt in the presence of two thermodynamic hydrate inhibitors, NaCl and CH3OH. An increase in the concentration of hydrate inhibitors is found to promote the nucleation of methane nanobubbles in the hydrate melt. Whereas NaCl promotes bubble formation by enhancing the hydrophobic interaction between aqueous CH4 molecules, CH3OH molecules assist bubble formation by stabilizing CH4 bubble nuclei formed in the solution. The CH3OH molecules accumulate around the nuclei leading to a decrease in the surface tension at their interface with water. The nanobubbles formed are found to be highly dynamic with frequent exchange of CH4 molecules between the bubble and the surrounding liquid. A quantitative analysis of the dynamic behavior of the bubble is performed by introducing a unit step function whose value depends on the location of CH4 molecules with respect to the bubble. It is observed that an increase in the concentration of thermodynamic hydrate inhibitors reduces the exchange process, making the bubble less dynamic. It is also found that for a given concentration of the inhibitor, larger bubbles are less dynamic compared to smaller ones. The dependence of the dynamic nature of nanobubbles on bubble size and inhibitor concentration is correlated with the solubility of CH4 and the Laplace pressure within the bubble. The effect of CO2 on the formation of nanobubble in the CH4-CO2 mixed gas hydrate melt in the presence of inhibitors is also examined. The simulations show that the presence of CO2 molecules significantly reduces the induction time for methane nanobubble nucleation. The role of CO2 in the early nucleation of bubble is explained based on the interaction between the bubble and the dissolved CO2 molecules.

Molecular Dynamics Simulation of the Crystal Nucleation and Growth Behavior of Methane Hydrate in the Presence of the Surface and Nanopores of Porous Sediment.

The behavior of hydrate formation in porous sediment has been widely studied because of its importance in the investigation of reservoirs and in the drilling of natural gas hydrate. However, it is difficult to understand the hydrate nucleation and growth mechanism on the surface and in the nanopores of porous media by experimental and numerical simulation methods. In this work, molecular dynamics simulations of the nucleation and growth of CH4 hydrate in the presence of the surface and nanopores of clay are carried out. The molecular configurations and microstructure properties are analyzed for systems containing one H2O hydrate layer (System A), three H2O hydrate layers (System B), and six H2O hydrate layers (System C) in both clay and the bulk solution. It is found that hydrate formation is more complex in porous media than in the pure bulk solution and that there is cooperativity between hydrate growth and molecular diffusion in clay nanopores. The hydroxylated edge sites of the clay surface could serve as a source of CH4 molecules to facilitate hydrate nucleation. The diffusion velocity of molecules is influenced by the growth of the hydrate that forms a block in the throats of the clay nanopore. Comparing hydrate growth in different clay pore sizes reveals that the pore size plays an important role in hydrate growth and molecular diffusion in clay. This simulation study provides the microscopic mechanism of hydrate nucleation and growth in porous media, which can be favorable for the investigation of the formation of natural gas hydrate in sediments.

Benthic fluxes of dissolved organic carbon from gas hydrate sediments in the northern South China Sea.

Hydrocarbon vents have recently been reported to contribute considerable amounts of dissolved organic carbon (DOC) to the oceans. Many such hydrocarbon vents widely exist in the northern South China Sea (NSCS). To investigate if these hydrocarbon vent sites release DOC, we used a real-time video multiple-corer to collect bottom seawater and surface sediments at vent sites. We analyzed concentrations of DOC in these samples and estimated DOC fluxes. Elevated DOC concentrations in the porewaters were found at some sites suggesting that DOC may come from these hydrocarbon vents. Benthic fluxes of DOC from these sediments were 28 to 1264 μmol m−2 d−1 (on average ~321 μmol m−2 d−1) which are several times higher than most DOC fluxes in coastal and continental margin sediments. The results demonstrate that the real-time video multiple-corer can precisely collect samples at vent sites. The estimated benthic DOC flux from the methane venting sites (8.6 × 106 mol y−1), is 24% of the DOC discharge from the Pearl River to the South China Sea, indicating that these sediments make an important contribution to the DOC in deep waters.

Thermophysical method for quantitative estimation of hydrate content in samples imitating bottom sediments

Gas hydrates in sediments can dissociate on heating, which causes a characteristic change in temperature over time. This can be used to detect hydrates and estimate their content. The geothermal method for gas hydrate prospecting consists in obtaining thermograms and defining regularities related to the presence of gas hydrates in the medium. The aim of this study was a quantitative estimation of hydrate content in sediments, based on data from repeated measurements of the temperature of a linear heat source (needle probe). The power of the source is chosen so that hydrate does not decompose in the first measurement but decomposes in the second. Afterward, we solve the optimization inverse problem of determination of the model parameters, one of which is a hydrate content. Experimental data on heating of the laboratory samples are processed, and hydrate contents are obtained with an accuracy of 30%. These values agree with independent estimates. The analytical solution of the axially symmetric problem of gas hydrate dissociation, based on the solution of the Stefan problem, is applied as a mathematical model for hydrate heating and decomposition in the sample.

Experimental study on a pressure-coring technology based on a freeze-core valve for marine hydrate-bearing sediment sampling

Recovery of hydrate-bearing sediments under near in situ conditions remains a topic worthy of investigation. To avoid the frequent sealing failures of mechanical valve-based samplers, a freeze-core-valve-based sampler is proposed for pressure-coring sampling in submarine sediment. A series of preliminary experiments were conducted to characterize the freeze-core valve, including the performance of retaining pressure and adaptation to different sediments. The results show that the freeze-core valve exhibited a strong performance in retaining pressure and can be to various sediments without limitations due to sediment conditions, including pore water salinity, clay mineral content, particle size and grain grading. The retained pressure for a 58-mm nominal diameter and 80-mm-long freeze-core valve formed at −40 °C can reach 43 MPa without leakage, which can meet the requirement of the pressure-coring sampling. The pressure retaining performance can be further improved by lowering the freezing temperature or increasing the length of the freezing-core valve. According to the preliminary tests, the sampling technique based on the freeze-core-valve has the potential to solve the problem of the low recovery success rate of gas hydrate sediment cores in the future.

Phase behavior of gas hydrates in nanoporous materials: Review

A precise understanding of phase behavior for a variety of both artificial and natural processes is essential to achieving scientific and technological goals. There has been growing research interest in gas hydrates confined in nanoporous media aiming to simulate and analyze the unique behavior of natural gas hydrates in sediments. Moreover, the appearance of peculiar properties due to the confinement effect stimulates research on gas hydrate technology for gas separation, such as CO2 capture from versatile pre/post combustion emissions. In spite of their importance, reliable phase equilibrium data on gas hydrates confined at a nanoscale are scattered throughout the literature, while those in bulk state are abundant. Accordingly, we surveyed the previous studies on the phase behavior of gas hydrates in various nanoporous materials to include and provide valuable information and knowledge for start-up researchers in various gas hydrate fields.

A methodology of porosity estimation from inversion of post-stack seismic data

Post-stack inversion of seismic data is routinely carried out to derive acoustic impedance (AI) and, hence petrophysical properties in an area. We have been introducing here an uncommon methodology of inverting post-stack seismic data into porosity from porosity log. The post-stack inversion for estimation of direct porosity is performed by utilizing an estimated porosity wavelet, low frequency model and model based inversion. This methodology is implemented on two types of 2D post-stack seismic dataset; (a) deep water Mahanadi (MN) offshore containing gas hydrate sediments and (b) clay rich, shaly sediments in Krishna–Godavari (K–G) shallow offshore. The total porosity (φ) estimated from density log for the depth interval of 1725–2032 m ranging from 49 to 75% has been used as input for porosity inversion from the seismic data of unconsolidated sediments at 1701 m bathymetry in MN basin. The total porosity for the depth interval of 410–1494 m ranging from 5 to 45% has been used as input for porosity inversion from the 2D post-stack seismic data of shallow offshore sediments at 31 m bathymetry in K–G basin. This prediction is applied to dataset having good (for MN) and poor correlation (for K–G) between AI and porosity. In MN basin, the porosity along 2D multichannel seismic ranges from 53 to 65% in the gas hydrated zone with maximum value of 70% at the free gas filled unconsolidated sediment below the bottom simulating reflector (BSR). The water bearing silt/clay sediments indicates porosity of about 60–65% below the seafloor. In K–G basin, the porosity in Raghavapuram shale varies from 13 to 30% with maximum value of 52% is observed in Paleocene sediments.

Detection of gas hydrate sediments using prestack seismic AVA inversion

Bottom-simulating reflectors (BSRs) in seismic profile always indicate the bottom of gas hydrate stability zone, but is difficult to determine the distribution and features of gas hydrate sediments (GHS). In this study, based on AVA forward modeling and angle-domain common-image gathers we use prestack AVA parameters consistency inversion in predicting gas hydrate sediments in the Shenhu area at northern slope of South China Sea, and obtain the vertical and lateral features and saturation of GHS.