Gas Hydrate-bearing Sediments Research Articles

Lithofacies characterization of gas hydrate prospects discovered during the National Gas Hydrate Program expedition 02, offshore Krishna-Godavari Basin, India

Abstract Lithofacies of host sediments associated with gas hydrate reservoirs may have a significant influence on the formation of hydrates. The type of sediments and the depositional processes responsible for the gas hydrate-bearing sediments along the eastern margin of the India have been studied by integrating core data and logging-while-drilling downhole logs. Borehole data correlated with seismic data were used for the interpretation of geological process responsible for gas hydrate formation. The acquired seismic profiles were correlated with the downhole logs by identifying the shape of the well logs curves that indicate finning up and blocky patterns. Turbidite sands and debris flows were also identified from the log and core sediment studies. This study identified two prominent types of depositional systems: (1) turbidity currents and (2) debris flows were mainly responsible for the deposition of gas hydrate-bearing reservoirs in the study area. Based on these studies two well-developed gas hydrate systems have been identified in the Krishna-Godavari Basin and hydrates were determined to be distributed mostly in sand dominated facies. Fine-grained sand in the identified turbidite deposits were shown to be a good hydrate reservoir. The hydrate-bearing sediments in the central part of the Krishna-Godavari Basin (Area C in D6 block) consist of mostly debris flows, which is consistent with scattered gravel- and sand/silt-sized grains in channel-levee systems.

A comprehensive review on the geomechanical properties of gas hydrate bearing sediments

Abstract Natural gas hydrates (NGH) that form in sediments beneath the ocean beds are a potential source to meet growing global energy demand. Gas hydrate bearing sediments (GHBS) exhibit improved geomechanical properties (viz., shear strength, stiffness, compressibility, and Poisson's ratio) that are responsible for their greater stability as compared to the native sediments. However, dissociation of gas hydrates during extraction of gas, instability in the GHBS could trigger submarine slope failures, seabed subsidence, and failure of the foundations of seafloor installations. In order to reduce the risk of occurrence of such geohazards, it is imperative to ascertain the stability of GHBS by understanding the variations in their geomechanical properties through field and laboratory investigations, which are time consuming and prohibitively expensive for many researchers. With this in view, the present study attempts to decipher the variation of geomechanical properties with respect to various parameters that affect them (viz., hydrate saturation (SH), hydrate morphology, and confining pressure-temperature conditions), by critically synthesizing the existing data in the literature. It has been noticed that the shear strength and stiffness are the major parameters influencing the stability of GHBS and both share power law relationship with SH. Further, it is noticed that the cohesion is the predominant component of the shear strength of GHBS and shows significant improvement for SH values greater than 15–50%. The variability of cohesion and stiffness with respect to SH have been analysed in tandem with the volumetric deformation, during the shearing process, to propose particle level mechanism hypothesis that provides an insight in to the stress-strain behavior of GHBS. The proposed relationships and mechanisms would aid in understanding the behavior and development of improved modeling of geomechanical properties of GHBS.

Numerical modeling for the mechanical behavior of marine gas hydrate-bearing sediments during hydrate production by depressurization

Abstract Natural gas hydrate, which is called inflammable ice, has drawn global interest. Many countries have carried out the trial production of natural gas from marine gas hydrate formations. However, due to the failure of the bonded structure and the disappearance of the hydrate in the pores of formations, improper production might induce geological hazards such as submarine landslide, non-uniform subsidence and local large deformation of the formations. In this study, a constitutive model based on super-loading and sub-loading surface theory is proposed to investigate the mechanical behavior of gas hydrate-bearing sediments (GHBSs). The mechanical behavior of both GHBSs and the structured clay are modeled by introducing rotational hardening rule, sub-loading surface and super-loading surface. A fully coupled thermo-hydro-chemo-mechanical (THCM) model is applied to study the mutiphysical responses of GHBSs during hydrate production by depressurization. The simulation results of the two-layer model show that the mutiphysical responses are dependent on not only the different material properties of the two layers but also the difference between the hydrate dissociated and undissociated zones. Although depressurization is the driving factor of hydrate dissociation, the heat transfer from the clay layer is also a key factor in the hydrate dissociation. Hydrate dissociation results in the variation in the stiffness and strength of the GHBSs leading to stress and strain and strain redistribution. The yielding and structural failure of sediments mainly occur in the hydrate dissociated zone.

Geomechanics involved in gas hydrate recovery

Gas hydrate is regarded as a promising energy owing to the large carbon reserve and high energy density. However, due to the particularity of the formation and the complexity of exploitation process, the commercial exploitation of gas hydrate has not been realized. This paper reviews the physical properties of gas hydrate-bearing sediments and focuses on the geomechanical response during the exploitation. The exploitation of gas hydrate is a strong thermal–hydrological–mechanical–chemical (THMC) coupling process: decomposition of hydrate into water and gas produces multi-physical processes including heat transfer, multi-fluid flow and deformation in the reservoir. These physical processes lead to a potential of geomechanical issues during the production process. Frequent occurrence of sand production is the major limitation of the commercial exploitation of gas hydrate. The potential landslide and subsidence will lead to the cessation of the production and even serious accidents. Preliminary researches have been conducted to investigate the geomechanical properties of gas hydrate-bearing sediments and to assess the wellbore integrity during the exploitation. The physical properties of hydrate have been fully studied, and some models have been established to describe the physical processes during the exploitation of gas hydrate. But the reproduction of actual conditions of hydrate reservoir in the laboratory is still a huge challenge, which will inevitably lead to a bias of experiment. In addition, because of the effect of microscopic mechanisms in porous media, the coupling mechanism of the existing models should be further investigated. Great efforts, however, are still required for a comprehensive understanding of this strong coupling process that is extremely different from the geomechanics involved in the conventional reservoirs.

Effect of reformation of gas hydrate on the gas phase permeability of montmorillonite

Permeability is one of the key parameters to determine the process of gas hydrate exploitation. The relationship between the gas hydrate reformation and permeability of gas hydrate-bearing sediments is the basic work of marine hydrate reservoir exploration and development. In this paper, the effect of reformation of natural gas hydrate on the gas phase permeability of montmorillonite was investigated. In the experiment, the marine gas hydrate-bearing clayey sediments were simulated by montmorillonite. And series of experiments were performed to investigate the gas phase permeability of montmorillonite with different hydrate saturations after gas hydrate formation and reformation. According to the results, it is found that the gas phase permeability of montmorillonite increased significantly with the reformation of gas hydrate. And after reformation of hydrate, the gas phase permeability of montmorillonite is larger than that of montmorillonite after the initial formation of hydrate. In addition, with the increase of initial hydrate saturation, the increasing ratio of montmorillonite gas phase permeability increases gradually after the reformation of gas hydrate. The results of this research play an important role in the safe and efficient exploitation of marine gas hydrates in the future.

Evolution of effective thermal conductivity during hydrate formation and decomposition in natural sediments

Gas hydrates are regareded as a potential alternative energy sources for sustainable development. Thermal properties of gas hydrate-bearing sediments directly govern the heat transfer process during hydrate decomposition which couples with phase transitions and multiphase flows. Latest database on the thermal properties of natural sediments containing gas hydrates still remains limited. Here we report on point heat source measurements of the effective thermal conductivity of hydrate-bearing sediments through a thermistor-based method and evaluation of existing models. Effective thermal conductivity of water saturated natural sediments and partially saturated hydrate-sediment are obtained, showing a significant effect of compaction on the effective thermal conductivity of the sediments. The evolution of thermal conductivity during hydrate formation, decomposition and reformation indicates that the variation of the components in the pores could play a crucial role in the effective thermal conductivity. A higher saturation of hydrates in the pores and a smaller porosity could contribute to a better thermal conductivity. The evolution of the effective thermal conductivity with hydrate formation and decomposition also demonstrates a temperature dependence. Moreover, the reformation process after hydrates are fully decomposed significantly impacts the variation of the effective thermal conductivity, which could be attributed to the solution migration during decomposition and the local inhomogeneity of hydrate reformation. The slightly increase in the thermal conductivity below zero degree Celsius implies the small fraction of ice generation during the cooling process. The results could provide some insights into the effect of hydrate formation and decomposition on the effective thermal conductivity of natural sediments, which would be helpful in the heat transfer evaluation and gas production simulation in the field test.

Thermal Conductivity of Frozen Sediments Containing Self-Preserved Pore Gas Hydrates at Atmospheric Pressure: An Experimental Study

The paper presents the results of an experimental thermal conductivity study of frozen artificial and natural gas hydrate-bearing sediments at atmospheric pressure (0.1 MPa). Samples of hydrate-saturated sediments are highly stable and suitable for the determination of their physical properties, including thermal conductivity, due to the self-preservation of pore methane hydrate at negative temperatures. It is suggested to measure the thermal conductivity of frozen sediments containing self-preserved pore hydrates by a KD-2 needle probe which causes very little thermal impact on the samples. As shown by the special measurements of reference materials with known thermal conductivities, the values measured with the KD-2 probe are up to 20% underestimated and require the respective correction. Frozen hydrate-bearing sediments differ markedly in thermal conductivity from reference frozen samples of the same composition but free from pore hydrate. The difference depends on the physical properties of the sediments and on changes in their texture and structure associated with the self-preservation effect. Namely, it increases proportionally to the volumetric hydrate content, hydrate saturation, and the percentage of water converted to hydrate. Thermal conductivity is anisotropic in core samples of naturally frozen sediments that enclose visible ice-hydrate lenses and varies with the direction of measurements with respect to the lenses. Thermal conductivity measurements with the suggested method provide a reliable tool for detection of stable and relict gas hydrates in permafrost.

Microbial diversity of two cold seep systems in gas hydrate-bearing sediments in the South China Sea

Cold seep is a unique habitat for microorganisms in deep marine sediments, and microbial communities and biogeochemical processes are still poorly understood, especially in relation to hydrate-bearing geo-systems. In this study, two cold seep systems were sampled and microbial diversity was studied at Site GMGS2-08 in the northern part of the South China Sea (SCS) during the GMGS2 gas hydrate expedition. The current cold seep system was composed of a sulfate methane transition zone (SMTZ) and an upper gas hydrate zone (UGHZ). The buried cold seep system was composed of an authigenic carbonate zone (ACZ) and a lower gas hydrate zone (LGHZ). These drill core samples provided an excellent opportunity for analyzing the microbial abundance and diversity based on quantitative polymerase chain reaction (qPCR) and high-throughput 16S rRNA gene sequencing. Compared to previous studies, the high relative abundance of ANME-1b, a clade of anaerobic methanotrophic archaea (ANME), may perform anaerobic oxidation of methane (AOM) in collaboration with ANME-2c and Desulfobacteraceae in the SMTZ, and the high relative abundances of Hadesarchaea, ANME-1b archaea and Aerophobetes bacteria were found in the gas hydrate zone (GHZ) at Site GMGS2-08. ANME-1b, detected in the GHZ, might mainly mediate the AOM process, and the process might occur in a wide depth range within the LGHZ. Moreover, bacterial communities were significantly different between the GHZ and non-GHZ sediments. In the ACZ, archaeal communities were different between the two samples from the upper and the lower layers, while bacterial communities shared similarities. Overall, this new record of cold seep microbial diversity at Site GMGS2-08 showed the complexity of the interaction between biogeochemical reactions and environmental conditions.

Modeling the Mechanical Behavior of Gas Hydrate Bearing Sediments Based on Unified Hardening Framework

Gas Hydrate Bearing Sediments (GHBS) are natural soil deposits that contain methane hydrate inside their pores and they have been considered as a potential energy resource. Deformation behavior and change in shear strength of sediments are not clearly understood on the occasion of methane hydrate production and its modeling poses great challenges. In this study, a new GHBS model based on unified hardening framework has been proposed with less and easily determined model parameters. This analytical model incorporates modified Cam-clay yield surface, plus the effect of hydrate saturation parameters to capture the mechanical behavior of GHBS. Comparisons of tri-axial compression tests with model simulations have been conducted to show the proposed model is able to predict the mechanical behavior of GHBS at different hydrate saturations and confinement conditions.

Lithological properties of natural gas hydrate–bearing sediments in pressure-cores recovered from the Krishna–Godavari Basin

Abstract We investigated the sediment properties and gas hydrate saturations in pressure-cores recovered during the National Gas Hydrate Program Expedition 02 (NGHP-02) in Areas B, C, and E of the Krishna–Godavari Basin (K–G Basin) off the eastern margin of India. These investigations included the analyses of grain size, mineralogy, and grain density and employed scanning electron microscopy (SEM). Sediments with median grain sizes below 90 μm were generally observed within most of the pressure-cores, whereas sediments with median grain sizes exceeding 250 μm were only observed in a few pressure-cores. Higher concentrations of gas hydrate were found in sediment layers with median grain sizes of 10–90 μm; the occurrence of gas hydrates was lower in sediment layers with median grain sizes below 10 μm. Three types of gas hydrate-bearing sediments containing above 50 vol.% gas hydrate saturations were observed: (1) sediments with silt grain size ratios above 70 vol.%, (2) sediments with sand grain size ratios above 35 vol.%, and (3) coarser sandy sediments with gravel. Sandy sediments with high gas hydrate saturations (above 50 vol.%) were very minor constituents in the pressure-core sediments. The gas hydrate saturations were lower in sediments with a clay fraction above 30 vol.%. SEM images revealed microfossil debris, fragments of sedimentary rock, and silt/sand-sized-agglomerated grains throughout the pressure-cores, which indicate that the sediments were deposited at and/or transported to the foot of the continental slope through reworking of upper-slope deposits. The geological setting and SEM observations indicate that the pressure-core sediments are mainly a terrigenous source, with a small component from pelagic or hemipelagic sedimentation. The results of the pressure-core analyses confirm that natural gas hydrates can occur in silt-rich sediments. The occurrence of gas hydrates in the pressure-core sediments recovered during the NGHP-02 is controlled by the lithology of the sediments, especially their clay content.

Stability analysis of submarine slopes in the area of the test production of gas hydrate in the South China Sea

ABSTRACT In this paper, the mechanical properties of gas hydrate-bearing sediments (GHBS) were summarized and the instability mechanism of submarine hydrate-bearing slope (SHBS) was analyzed under the background of the test production of gas hydrate in the northern part of the South China Sea. The strength reduction finite element method (SRFEM) was introduced to the stability analysis of submarine slopes for the safety of the test production. Two schemes were designed to determine the physical and mechanical parameters of four target wells. Through the division of the hydrate dissociation region and the design of four working conditions, the range and degree of hydrate dissociation at different stages during the test production were simulated. Based on the software ABAQUS, 37 FEM models of SHBS were set up to analyze and assess the stability of the submarine slopes in the area of the test production. Necessary information such as safety factors, deformation, and displacement were obtained at different stages and under different working conditions. According to the calculation results, the submarine slope area is stable before the test production, and the safety factors almost remains the same during and after the test production. All these indicate that the test production has no obvious influence on the area of the test production and the submarine slopes in the area are stable during and after the test production.

Modeling of wave reflection in gas hydrate-bearing sediments

The content and distribution pattern of the gas hydrate are the key factors affecting the free surface reflection of the gas hydrate-bearing sediment. In this study, a theoretical model of wave reflection in gas hydrate-bearing sediment is discussed for the incidence of P1-wave and S1-wave to analyze the influences of the gas hydrate. The gas hydrate-bearing sediment is modeled by a Biot-type three-phase theory and assumed to be composed of sediment grains with connected pore occupied by the mixture of fluid and hydrate. The incoming wave is split into three reflected compressional waves and two reflected shear waves at the free surface of a gas hydrate-bearing sediment half-space. The analytical solutions for various reflected waves at open-pore and sealed-pore boundaries are obtained in terms of displacement potentials. Then, two-phase type models based on Biot theory are proposed by assuming the gas hydrate and sediment frame are weakly coupled or essentially consolidated to verify the rationality of the theoretical model. A third two-phase type model is given by assuming there is no gas hydrate in the pore. At last, the influences of the content and distribution pattern of gas hydrate, the frequency of the incident wave, and the permeability condition on the horizontal displacements and displacement ratios are analyzed respectively through numerical calculation. It is revealed that the solutions based on the Biot-type three-phase theory are very consistent with the solutions based on Biot theory if the gas hydrate and sediment frame are weakly coupled or essentially consolidated. Furthermore, the model proposed in this study can be used to analyze the influence of the distribution pattern of the hydrate.

Evaluation of failure modes and undrained shear strength by cone penetrometer for Natural Gas hydrate-bearing pressure-core sediment samples recovered from the Krishna–Godavari Basin, offshore India

Compressional wave velocity measurements and cone penetration tests were carried out for pressure cores recovered from the Krishna–Godavari Basin off the eastern coast of India, using an instrumented pressure testing chamber constructed by the Japanese National Institute of Advanced Industrial Science and Technology. For pressure cores with high gas hydrate saturations (Sh) and exhibited high compressional wave velocities also display a high loading force that resulted in tensile failure. A pressure core sample with low compressional wave velocities and low gas hydrate saturations led to a low loading force, which resulted in radial shear. The cone factor connecting the cone resistance and the undrained shear strength, Nk, was estimated using cone penetration testing and the uniaxial compressional results. We estimated its value as Nk = 45 by using a uniaxial compression loading speed of 0.01%/min and a cone insertion loading speed of 0.25 mm/s. Undrained shear strengths, estimated from cone penetration results using Nk = 45, were assumed to be equal to the tensile strengths between hydrate-hydrate and hydrate-particle interface when Sh is less than 50% and increases when Sh is more than 50% assuming that gas hydrate morphology was matrix supporting.

Lithological characteristics and hydrocarbon gas sources of gas hydrate-bearing sediments in the Shenhu area, South China Sea: Implications from the W01B and W02B sites

The Shenhu area, located in the Pearl River Mouth Basin, South China Sea (SCS), is currently one of the most promising exploration areas for gas hydrates. In this study, the lithological characteristics, as well as the molecular and isotopic composition of hydrocarbon gases are systematically reported for the first time for core sediments obtained from two new drilling sites (referred to as W01B and W02B) in the southeast Shenhu area. Both gas hydrate-bearing and gas hydrate-free samples are characterized by fine-grained sediments (dominantly coarse silt and to a lesser extent, fine and medium silt) and similar median grain-sizes (5.94Φ to 6.49Φ), sorting (2.32 to 2.59), kurtosis (0.82 to 0.97), skewness (−0.12 to 0.03) and mineral compositions. Abundant authigenic pyrites (including euhedral and framboidal pyrites) also occur in these sediments. All these characteristics collectively indicate that the core sediments from the southeast Shenhu area formed in a relatively stable, low-energy and anoxic sedimentary environment. Nevertheless, the hydrate-bearing layers in this study are characterized by lower sand contents and more foraminifera than gas hydrate-free layers. The molecular and isotopic composition of hydrocarbon gases hosted by secondary minerals in the studied sediments indicates that they are of thermogenic origin. Combined with previously published data, we suggest that the abundant foraminifera within the sediments of the Shenhu area play a significant role in controlling the formation of gas hydrates by increasing the porosities of sediments, and that the thermogenic gas is an important source for the hydrocarbon gases of hydrates in the SCS.

Three-dimensional Eulerian modeling of gas–liquid–solid flow with gas hydrate dissociation in a vertical pipe

The pipe transportation of gas hydrate-bearing sediments (GHBS)1GHBS—gas hydrate-bearing sediments;1 is one of the key problems in mechanical–thermal exploitation of gas hydrate (GH)2GH—gas hydrate.2 in marine stratum. It is a gas–liquid–solid (methane gas–seawater–GHBS particles) three-phase flow, accompanied by GH dissociation. In this study, a three-dimensional Eulerian model combined with the kinetic theory of granular flow (KTGF) was adopted to simulate the gas–liquid–solid flow with GH dissociation. The commercial CFD software FLUENT 16.2 was employed, considering the hydrodynamics, heat and mass transfer, and GH dissociation simultaneously. A kinetic model for GH dissociation in the GHBS particles is presented, considering the influence of multiphase flow on dissociation rate. The model can capture the transition from the initial liquid–solid two-phase flow to gas–liquid–solid three-phase flow, describing the distribution of the phase volume fraction, velocity, temperature, and dissociation rate. The interaction between GH dissociation and multiphase flow is discussed. The simulation results indicate that the continuous production of gas bubbles by GH dissociation leads to more violent fluctuations in the pressure gradient and a more marked elevation of the solid particles compared with the liquid–solid two-phase flow without GH dissociation. In addition, the effect of GH dissociation on the multiphase flow under different hydrate saturations was analyzed.

Dissociation Behaviors of CO2 Hydrate-Bearing Sediment Particle during Settling in Water

Dissociation processes of gas hydrate-bearing sediment particles in water flow condition were investigated. Experiments were carried out by observing a spherical CO2 hydrate-bearing sediment (CHBS) particle settling in water. The release process of gas bubbles from CHBS particles was recorded by a high-speed camera and the total time of dissociation was obtained for different particle diameters and water temperatures. An intrinsic dissociation model was presented based on the assumption that the dissociation rate of the CHBS particle is exponential with the concentration of remaining hydrate. The model considered the influences of changing temperature, pressure and the particle concentration on the dissociation rate. The constants in the model were obtained based on fitting the experimental data. The presented model can reveal the dissociation behavior of the CHBS particle.

Multi-component fractal representation of multi-scale structure of natural gas hydrate-bearing sediments

Fractal is a set owning an infinitely fine structure in a self-similar or self-affine way covering all scales, and is widely used to model the structure of porous media. Hydrate-bearing sediments belong to porous media, so this paper attempts to construct a structure model of hydrate-bearing sediments with the three-component fractal extended from the two-component fractal. The three-component fractal is capable to display structural details at arbitrarily small scale. Additionally, in the three-component fractal, any parameter defined as the ratio (such as porosity and hydrate saturation) of two different volumes approaches a determined value when considering the infinitesimal scale. The interface between hydrate elements and pore elements is suggested to be a potential way to classify the hydrate-bearing sediments. Using the proposed three-component fractal model to calculate the interface between one arbitrary hydrate particle and surrounding pores, we obtain the theoretical solutions of the amount of substance, the dissociation rate and maximal dissociation time of one arbitrary hydrate element in sediments. It is found that the effective reacting surface of one hydrate element in sediments could be equal to even greater than that of the pure hydrate element in the late period of hydrate dissociation, during which the amount of substance of one hydrate element in sediments is still greater though. Finally, we further extend the three-component fractal to any-component fractal, and show how to model more complex porous media by using a suitable generator.

Downhole log evidence for the coexistence of structure II gas hydrate and free gas below the bottom simulating reflector in the South China Sea

Abstract Stratigraphic layered pore-filling gas hydrates are identified above the bottom simulating reflector (BSR) using the well log and core data acquired at Sites W11 and W17 during the third gas hydrate drilling expedition conducted by China's Geological Survey/Guangzhou Marine Geological Survey (GMGS3) in the South China Sea. A seismic profile near Site W17, reveal the presence of two BSRs (i.e., double BSR), which we show to relate to zones of structure I gas hydrate (I-BSR) and structure II gas hydrate (II-BSR). Well log data from Site W17 between the “I-BSR” (projected depth of 250 mbsf) and “II-BSR” (projected depth of 330 mbsf) showed anomalous responses for gas hydrate-bearing sediments with high resistivity, high S-wave velocity, and alternating high and low P-wave velocities. Pressure core data support the interpretation that structure II gas hydrate occurs at a depth of 263 mbsf at Site W17. The cross-plot between log-derived neutron and density porosities reveals a free gas-bearing layer at a depth of 258–270 mbsf, suggesting gas hydrate coexists with free gas between the “I-BSR” and the “II-BSR.” Synthetic seismograms generated from the P-wave velocity and density logs further support the presence of free gas in this section. Based on the coexistence of hydrate, free gas and water, the simplified three-phase equation (STPE) was modified to simultaneously estimate free gas and hydrate saturations beneath the “I-BSR” from P-wave and S-wave velocity logs, assuming uniform or patchy distributions of free gas. The estimated free gas and hydrate saturations, together with gas compositions from pressure core samples, collectively indicate that structure II gas hydrate and free gas are interbedded and coexist below the “I-BSR” at Site W17. Our study of the coexistence of gas hydrate and free gas between the double BSR at Site W17 provides new insights into gas hydrate systems in nature that contain more complex gas chemistries.

Permeability variation and anisotropy of gas hydrate-bearing pressure-core sediments recovered from the Krishna–Godavari Basin, offshore India

Abstract Permeability measurements were conducted on gas hydrate-bearing pressure-core sediments recovered from the Krishna–Godavari Basin during India's National Gas Hydrate Program Expedition 02. Pressure cores collected in the deep seabed of the Indian Ocean were cut and stored under high pressure and low temperature on the D/V Chikyu using pressure core analyzing tools. A total of 25, 1.2-m storage chambers were transferred to Japan from India for pressure core studies using pressure-core analysis tools at the National Institute of Advanced Industrial Science and Technology. The sediment core's pressure and temperature were maintained within the hydrate stability conditions during the entire process of transfer and loading into a triaxial testing apparatus called TACTT to preserve the hydrate, which can simulate the in-situ effective stress state under triaxial conditions. The hydrate saturation of the samples was in the range of 50–90%. It was found that the initial effective water permeability was in range of 0.01 mD to tens of mD, depending on the hydrate saturation and the mean particle size of the host sediment. The hydrate pore space morphology is also a critical factor, and results suggest hydrates are forming in the pore centers of sand grains as pore filling, but also as a sediment grain coating. In a first for gas hydrate-bearing pressure cores, the permeability anisotropy was established via vertical and horizontal flow tests combined with a pore fluid flow simulation. The horizontal/vertical permeability ratio was found to be 4. Additionally, the effective stress dependency of permeability was investigated by performing flow tests at different effective stresses. Due to decreasing porosity and particle crushing, the permeability dropped by approximately 90% for an effective stress increase of 10 MPa.

Multi-measurement approach for establishing the base of gas hydrate occurrence in the Krishna-Godavari Basin for sites cored during expedition NGHP-02 in the offshore of India

The 2015 National Gas Hydrate Program of India's second expedition, NGHP-02, acquired logging and coring datasets for constraining the base of the gas hydrate occurrence zone (deepest GH) and the theoretical base of gas hydrate stability zone (BGHS). These data are used here for two primary goals: to constrain the deepest occurrence of gas hydrate relative to predicted stability limits and the observed BSR, and to characterize the nature of the contact between gas hydrate-bearing sediment and the underlying gas hydrate-free sediment. A consensus depth for the deepest GH is derived for each NGHP-02 coring site from downhole indicators of gas hydrate occurrence obtained from well-log electrical resistivity and acoustic data, pressure core compressional wave velocity measurements, and conventional core measurements of anomalously low temperatures. To establish the theoretical BGHS, models of gas hydrate phase stability with depth are compared with downhole temperature profiles derived from: 1) assuming a constant geothermal gradient consistent with downhole temperature measurements, and 2) assuming constant heat flow using a geotherm through the downhole temperature measurements and incorporating thermal conductivity calculated from borehole logging data. Although the deepest NGHP-02 GH occurrences are controlled at several sites by a lithologic boundary, most sites have deepest GH occurrences within a single coarse-grained lithology. Cutoffs within a single coarse-grained lithology, which occur for the primary NGHP-02 Area B gas hydrate reservoir, will inhibit pore-pressure drawdowns used to extract methane from gas hydrate as an energy resource.