Hydrate-bearing Sand Research Articles

Failure Mechanisms in Cemented Hydrate-Bearing Sands

A significant portion of our knowledge on gas hydrate-bearing sands comes from experimental results on laboratory-synthesized specimens. The failure mechanics are often interpreted using components of the stress–strain curves, which capture the specimen’s global (large-scale) response to shear. In this paper, we postulate on the microscale mechanics, which lead to a variety of interesting global behaviors. Two mechanisms of failure during shear are postulated: one involves debonding of the hydrate particle from the soil solid, and the other involves crushing or breakage through the hydrate itself. Both modes of failure lead to similar peak strengths, which arise from both friction and apparent cohesion induced by the hydrate bonding; however, the differences observed in postpeak softening may be attributed to the different failure mechanisms. Global specimen responses such as sudden strength loss, occurrence of double shear banding, and differences in postpeak behavior are manifestations of the microscale...

Development of high-pressure low-temperature plane strain testing apparatus for methane hydrate-bearing sand

A high-pressure low-temperature plane strain testing apparatus was developed for visualizing the deformation of methane hydrate-bearing sand due to methane hydrate production. Using this testing apparatus, plane strain compression tests were performed on pure Toyoura sand and methane hydrate-bearing sand with localized deformation measurements. From the results, it was observed that the methane hydrate-free specimens, despite their relatively high density, showed changes in compressive volume. Marked increases in the initial stiffness and strength of the methane hydrate-bearing sand were observed (methane hydrate saturation of SMH=60%). Moreover, the volumetric strain changed from compressive to dilative. For the specimens with methane hydrate, a dilative behavior above SMH=0% was observed. An image analysis showed that the shear bands of the methane hydrate-bearing sand were thinner and steeper than those of the host sand. In addition, the dilative volumetric strain in the shear band increased markedly when methane hydrate existed in the pore spaces.

Scientific results of the Second Gas Hydrate Drilling Expedition in the Ulleung Basin (UBGH2)

As a part of Korean National Gas Hydrate Program, the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) was conducted from 9 July to 30 September, 2010 in the Ulleung Basin, East Sea, offshore Korea using the D/V Fugro Synergy. The UBGH2 was performed to understand the distribution of gas hydrates as required for a resource assessment and to find potential candidate sites suitable for a future offshore production test, especially targeting gas hydrate-bearing sand bodies in the basin. The UBGH2 sites were distributed across most of the basin and were selected to target mainly sand-rich turbidite deposits. The 84-day long expedition consisted of two phases. The first phase included logging-while-drilling/measurements-while-drilling (LWD/MWD) operations at 13 sites. During the second phase, sediment cores were collected from 18 holes at 10 of the 13 LWD/MWD sites. Wireline logging (WL) and vertical seismic profile (VSP) data were also acquired after coring operations at two of these 10 sites. In addition, seafloor visual observation, methane sensing, as well as push-coring and sampling using a Remotely Operated Vehicle (ROV) were conducted during both phases of the expedition. Recovered gas hydrates occurred either as pore-filling medium associated with discrete turbidite sand layers, or as fracture-filling veins and nodules in muddy sediments. Gas analyses indicated that the methane within the sampled gas hydrates is primarily of biogenic origin.This paper provides a summary of the operational and scientific results of the UBGH2 expedition as described in 24 papers that make up this special issue of the Journal of Marine and Petroleum Geology.

Geomechanical and Thermal Responses of Hydrate-Bearing Sediments Subjected to Thermal Stimulation: Physical Modeling Using a Geotechnical Centrifuge

The geomechanical and thermal responses of sediments can be significantly affected by the dissociation of gas hydrates via various emergent phenomena such as fluid volume expansion, free gas generation, gas migration, and sediment softening. This study explores the geomechanical and thermal responses of hydrate-bearing sediments subjected to thermal stimulation, using physical modeling with a geotechnical centrifuge that enables the simulation of near-seafloor sediment conditions. A water-saturated and CO2 hydrate-bearing sand column was prepared in a large cylindrical pressure vessel, and a linear stress gradient for a near-seafloor condition was created by increasing centrifugal acceleration. The hydrate-bearing sand column was subjected to thermal stimulation, and changes in temperature, pressure, compressional wave velocity (VP), shear wave velocity (VS), and electrical resistance were monitored at various locations across the column. It was found that VP and electrical resistance were good indicators...

Measuring the in Situ Hydrate Saturation from γ-Ray Transmissivity Changes during Local Dissociation

The worldwide abundance of gas hydrates in areas with engineering activities highlights the need for an accurate understanding of how much hydrate the soil contains, before any subsequent assessments are made. For the currently deployed gas hydrate saturation measurement, techniques are based on absolute measurements, derived from either the elastic properties of the formation, its nuclear magnetic resonance, or its electrical resistivity. None of those methods is wholly satisfactory, because of either the required reference measurements or a high dependence upon empirical correlations. This study explores the possibility to determine the hydrate saturation from relative measurements, by deriving its initial value from changes in γ transmissivity during the dissociation process of the contained hydrate. A customized testing apparatus was built, in which hydrate-bearing sand samples of up to 40% pore space saturation were first formed and then tested. The γ transmission from a ⁶⁰Co source was measured by a scintillation counter, while the cylindrical samples were radially dissociated by heating from their axis. Seven independent dissociation tests with an initial hydrate saturation between 20 and 40% were conducted. The results show that the change in local density (γ transmissivity) is an exponential function of the initial hydrate volume in place.

Characterization of elastic properties of near-surface and subsurface deepwater hydrate-bearing sediments

Hydrate-bearing sands and shallow nodular hydrate are potential energy resources and geohazards, and they both need to be better understood and identified. Therefore, it is useful to develop methodologies for modeling and simulating elastic constants of these hydrate-bearing sediments. A gas-hydrate rock-physics model based on the effective medium theory was successfully applied to dry rock, water-saturated rock, and hydrate-bearing rock. The model was used to investigate the seismic interpretation capability of hydrate-bearing sediments in the Gulf of Mexico by computing elastic constants, also known as seismic attributes, in terms of seismic interpretation, including the normal incident reflectivity (NI), Poisson’s ratio (PR), P-wave velocity ([Formula: see text]), S-wave velocity ([Formula: see text]), and density. The study of the model was concerned with the formation of gas hydrate, and, therefore, hydrate-bearing sediments were divided into hydrate-bearing sands, hydrate-bearing sands with free gas in the pore space, and shallow nodular hydrate. Although relations of hydrate saturation versus [Formula: see text] and [Formula: see text] are different between structures I and II gas hydrates, highly concentrated hydrate-bearing sands may be interpreted on poststack seismic amplitude sections because of the high NI present. The computations of elastic constant implied that hydrate-bearing sands with free gas could be detected with the crossplot of NI and PR from prestack amplitude analysis, and density may be a good hydrate indicator for shallow nodular hydrate, if it can be accurately estimated by seismic methods.

The effect of methane hydrate morphology and water saturation on seismic wave attenuation in sand under shallow sub-seafloor conditions

A better understanding of seismic wave attenuation in hydrate-bearing sediments is needed for the improved geophysical quantification of seafloor methane hydrates, important for climate change, geohazard and economic resource assessment. Hence, we conducted a series of small strain (<10−6), seismic frequency (50–550Hz), laboratory resonant column experiments on synthetic methane hydrate-bearing sands under excess-water seafloor conditions. The results show a complex dependence of P- and S-wave attenuation on hydrate saturation and morphology. P- and S-wave attenuation in excess-water hydrate-bearing sand is much higher than in excess-gas hydrate-bearing sand and increases with hydrate saturation between 0 and 0.44 (the experimental range). Theoretical modelling suggests that load-bearing hydrate is an important cause of heightened attenuation for both P- and S-waves in gas and water saturated sands, while pore-filling hydrate also contributes significantly to P-wave attenuation in water saturated sands. A squirt flow attenuation mechanism, related to microporous hydrate and low aspect ratio pores at the interface between sand grains and hydrate, is thought to be responsible for the heightened levels of attenuation in hydrate-bearing sands at low hydrate saturations (<0.44).

Mechanical and dissociation properties of methane hydrate-bearing sand in deep seabed

A series of triaxial tests has been carried out on the mechanical properties and dissociation characteristics of sands containing methane hydrate using an innovative high pressure apparatus which has been developed to reproduce the in-situ conditions expected during proposed methane extraction methods. It was found that the strength of MH sand increased with MH saturation due to particle bonding. Dissociation by heating caused large axial strains for samples with an initial shear stress and total collapse for samples consolidated in the metastable zone. In the case of dissociation by de-pressurization, axial strains were generated by increasing effective stress until a stable equilibrium was reached. However, re-pressurization led to the collapse in the metastable zone.

Experimental Study on Gas Production from Methane Hydrate Bearing Sand by Depressurization

The simulate experiments of gas production from methane hydrates reservoirs was proceeded with an experimental apparatus. Especially, TDR technique was applied to represent the change of hydrate saturation in real time during gas hydrate formation and dissociation. In this paper, we discussed and explained material transformation during hydrate formation and dissociation. The hydrates form and grow on the top of the sediments where the sediments and gas connect firstly. During hydrates dissociation by depressurization, the temperatures and hydrate saturation presented variously in different locations of sediments, which shows that hydrates dissociate earlier on the surface and outer layer of the sediments than those of in inner. The regulation of hydrates dissociation is consistent with the law of decomposition kinetics. Furthermore, we investigated the depressurizing range influence on hydrate dissociation process.

Subsurface gas hydrates in the northern Gulf of Mexico

The northern Gulf of Mexico (GoM) has long been a focus area for the study of gas hydrates. Throughout the 1980s and 1990s, work focused on massive gas hydrates deposits that were found to form at and near the seafloor in association with hydrocarbon seeps. However, as global scientific and industrial interest in assessment of the drilling hazards and resource implications of gas hydrate accelerated, focus shifted to understanding the nature and abundance of “buried” gas hydrates. Through 2005, despite the drilling of more than 1200 oil and gas industry wells through the gas hydrate stability zone, published evidence of significant sub-seafloor gas hydrate in the GoM was lacking. A 2005 drilling program by the GoM Gas Hydrate Joint Industry Project (the JIP) provided an initial confirmation of the occurrence of gas hydrates below the GoM seafloor. In 2006, release of data from a 2003 industry well in Alaminos Canyon 818 provided initial documentation of gas hydrate occurrence at high concentrations in sand reservoirs in the GoM. From 2006 to 2008, the JIP facilitated the integration of geophysical and geological data to identify sites prospective for gas hydrate-bearing sands, culminating in the recommendation of numerous drilling targets within four sites spanning a range of typical deepwater settings. Concurrent with, but independent of, the JIP prospecting effort, the Bureau of Ocean Energy Management (BOEM) conducted a preliminary assessment of the GoM gas hydrate petroleum system, resulting in an estimate of 607 trillion cubic meters (21,444 trillion cubic feet) gas-in-place of which roughly one-third occurs at expected high concentrations in sand reservoirs. In 2009, the JIP drilled seven wells at three sites, discovering gas hydrate at high saturation in sand reservoirs in four wells and suspected gas hydrate at low to moderate saturations in two other wells. These results provide an initial confirmation of the complex nature and occurrence of gas hydrate-bearing sands in the GoM, the efficacy of the integrated geological/geophysical prospecting approach used to identify the JIP drilling sites, and the relevance of the 2008 BOEM assessment.

Applicability of Duncan-Chang Model and its Modified Versions to Methane Hydrate-Bearing Sands

Methane hydrate is ice-like clathrate compound that attracts global attention due to its huge potential as a future energy source. The constitutive law of methane hydrate-bearing sediments remains unknown and becomes a barrier in sustainable exploitation of methane hydrate from marine sediments. The Duncan-Change model is a nonlinear elastic model which was widely accepted by the geotechnical community in approximating the constitutive response of geo-materials. This model and its evolved versions were employed in this study to model the stress-strain response observed in triaxial tests on methane hydrate-bearing sands. Duncan-Chang type models capture well the strain hardening behaviors. However, they fall short of incorporating the dependency of temperature and saturation degree of methane hydrate, which have to be taken into account in future constitutive models of methane hydrate-bearing deposits.

Electrical anisotropy of gas hydrate-bearing sand reservoirs in the Gulf of Mexico

We present new results and interpretations of the electrical anisotropy and reservoir architecture in gas hydrate-bearing sands using logging data collected during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II. We focus specifically on sand reservoirs in Hole Alaminos Canyon 21 A (AC21-A), Hole Green Canyon 955 H (GC955-H) and Hole Walker Ridge 313 H (WR313-H). Using a new logging-while-drilling directional resistivity tool and a one-dimensional inversion developed by Schlumberger, we resolve the resistivity of the current flowing parallel to the bedding, R‖ and the resistivity of the current flowing perpendicular to the bedding, R⊥. We find the sand reservoir in Hole AC21-A to be relatively isotropic, with R‖ and R⊥ values close to 2Ωm. In contrast, the gas hydrate-bearing sand reservoirs in Holes GC955-H and WR313-H are highly anisotropic. In these reservoirs, R‖ is between 2 and 30Ωm, and R⊥ is generally an order of magnitude higher.Using Schlumberger’s WebMI models, we were able to replicate multiple resistivity measurements and determine the formation resistivity the gas hydrate-bearing sand reservoir in Hole WR313-H. The results showed that gas hydrate saturations within a single reservoir unit are highly variable. For example, the sand units in Hole WR313-H contain thin layers (on the order of 10–100cm) with varying gas hydrate saturations between 15 and 95%. Our combined modeling results clearly indicate that the gas hydrate-bearing sand reservoirs in Holes GC955-H and WR313-H are highly anisotropic due to varying saturations of gas hydrate forming in thin layers within larger sand units.

Numerical simulations of depressurization-induced gas production from gas hydrate reservoirs at the Walker Ridge 313 site, northern Gulf of Mexico

In 2009, the Gulf of Mexico (GOM) Gas Hydrates Joint-Industry-Project (JIP) Leg II drilling program confirmed that gas hydrate occurs at high saturations within reservoir-quality sands in the GOM. A comprehensive logging-while-drilling dataset was collected from seven wells at three sites, including two wells at the Walker Ridge 313 site. By constraining the saturations and thicknesses of hydrate-bearing sands using logging-while-drilling data, two-dimensional (2D), cylindrical, r-z and three-dimensional (3D) reservoir models were simulated. The gas hydrate occurrences inferred from seismic analysis are used to delineate the areal extent of the 3D reservoir models. Numerical simulations of gas production from the Walker Ridge reservoirs were conducted using the depressurization method at a constant bottomhole pressure. Results of these simulations indicate that these hydrate deposits are readily produced, owing to high intrinsic reservoir-quality and their proximity to the base of hydrate stability. The elevated in situ reservoir temperatures contribute to high (5–40 MMscf/day) predicted production rates. The production rates obtained from the 2D and 3D models are in close agreement. To evaluate the effect of spatial dimensions, the 2D reservoir domains were simulated at two outer radii. The results showed increased potential for formation of secondary hydrate and appearance of lag time for production rates as reservoir size increases. Similar phenomena were observed in the 3D reservoir models. The results also suggest that interbedded gas hydrate accumulations might be preferable targets for gas production in comparison with massive deposits. Hydrate in such accumulations can be readily dissociated due to heat supply from surrounding hydrate-free zones. Special cases were considered to evaluate the effect of overburden and underburden permeability on production. The obtained data show that production can be significantly degraded in comparison with a case using impermeable boundaries. The main reason for the reduced productivity is water influx from the surrounding strata; a secondary cause is gas escape into the overburden. The results dictate that in order to reliably estimate production potential, permeability of the surroundings has to be included in a model.

Gas hydrate resource potential in the Terrebonne Basin, Northern Gulf of Mexico

The Terrebonne Basin is a salt-withdrawal mini-basin within the northeast portion of the Walker Ridge protraction area in northern Gulf of Mexico continental slope that contains a thick sequence of upper Pliocene and Pleistocene clastic sediment. Data acquired during the 2009 Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) logging-while-drilling (LWD) program confirmed the presence of gas hydrate within a variety of sand and clay units. Integration of the Leg II LWD data with regional seismic mapping allows for the identification of various facies assemblages within the sand units and an initial estimation of the gas hydrate in-place resources throughout the Terrebonne basin. A total of ∼4.4×109m3 (1.55×1011ft3) of gas occurs within highly saturated gas hydrate accumulations within channel, proximal levee, and distal levee facies of four primary Lower Pleistocene sand reservoirs. These sand accumulations occur at the base of gas hydrate stability and locally trap additional, unquantified accumulations of free gas. A number of additional thin hydrate-bearing sand units are also observed to occur at shallower depths. Potential recoverable volumes from this accumulation compare favorably with those realized from conventional deepwater gas reservoirs in the vicinity. In addition, Leg II LWD data delineated the occurrence of a stratal-bound occurrence of gas hydrate-filled fractures at low bulk volume saturations within a thick, shallow, and predominantly fine-grained unit. This unit is estimated to contain roughly 17.0×109m3 (5.87×1011ft3) of gas. The areal gas hydrate resource density within the Terrebonne basin is calculated at 1.183×109m3 per km2 where delineated sand reservoirs are present and 0.32×109m3 per km2 where sands are thought to be absent.

Methane hydrate induced permeability modification for multiphase flow in unsaturated porous media

[1] An experimental study was performed using X-ray computed tomography (CT) scanning to capture three-dimensional (3-D) methane hydrate distributions and potential discrete flow pathways in a sand pack sample. A numerical study was also performed to develop and analyze empirical relations that describe the impacts of hydrate accumulation habits within pore space (e.g., pore filling or grain cementing) on multiphase fluid migration. In the experimental study, water was injected into a hydrate-bearing sand sample that was monitored using an X-ray CT scanner. The CT images were converted into numerical grid elements, providing intrinsic sample data including porosity and phase saturations. The impacts of hydrate accumulation were examined by adapting empirical relations into the flow simulations as additional relations governing the evolution of absolute permeability of hydrate bearing sediment with hydrate deposition. The impacts of pore space hydrate accumulation habits on fluid migration were examined by comparing numerical predictions with experimentally measured water saturation distributions and breakthrough curves. A model case with 3-D heterogeneous initial conditions (hydrate saturation, porosity, and water saturation) and pore body–preferred hydrate accumulations best captured water migration behavior through the hydrate-bearing sample observed in the experiment. In the best matching model, absolute permeability in the hydrate bearing sample does not decrease significantly with increasing hydrate saturation until hydrate saturation reaches about 40%, after which it drops rapidly, and complete blockage of flow through the sample can occur as hydrate accumulations approach 70%. The result highlights the importance of permeability modification due to hydrate accumulation habits when predicting multiphase flow through high-saturation, reservoir quality hydrate-bearing sediments.

Laboratory Measurements of the Effects of Methane/Tetrahydrofuran Concentration and Grain Size on the P-Wave Velocity of Hydrate-Bearing Sand

An experimental apparatus was developed to measure P-wave velocity (VP) of gas-hydrate-bearing sediment. Tetrahydrofuran (THF) was added to quicken the hydrate formation in the porous media and to synthesize hydrate-bearing sediments with uniform distribution. Methane acted as a free gas to participate in the hydrate formation. Five experimental runs were performed to examine the influence of sediment grain size and THF concentration on VP. The P-wave velocity and the amplitude for the first arrival wave signal were collected in real time during hydrate formation process. The experimental data showed that VP increases monotonically with the increase of hydrate saturation in the sediment pore space and finally tends to be a constant value. This final VP value increases with the increase of initial THF content, but the effect of sand grain size on VP is inconclusive. The variations of amplitude for the first arrival wave signal with elapsed time during hydrate formation illustrates that the amplitude increa...

P-wave monitoring of hydrate-bearing sand during CH4–CO2 replacement

The replacement of methane for carbon dioxide in natural gas hydrate-bearing sediments is a promising technology in view of a more sustainable use of fossil fuels. While previous studies have shown successful CH4–CO2 replacement in hydrates, the mechanical response of hydrate-bearing sediments during CO2 injection, CH4–CO2 replacement, and CH4 production needs to be adequately understood in order to avert production problems such as borehole instability, sand production, and buckling of the casing. We take advantage of the characteristics of elastic mechanical wave propagation in sediments to monitor CH4 hydrate-bearing sands before, during, and after CO2 injection. Results show that CH4–CO2 replacement occurs without a loss of stiffness in the granular medium. This implies that CO2-flooded sandy reservoirs can remain mechanically stable during and after CH4 gas production. On the other hand, dry CO2 dissolves hydrate, and continued sediment flushing with dry CO2 reduces the degree of hydrate saturation in the pore space, opens the pore throats, and weakens the granular skeleton. This phenomenon may cause a significant loss of strength near the injection points and regions subjected to high liquid CO2 flow rate. The results of complimentary analyses show a decrease in bulk stiffness as water is displaced by liquid CO2, a stiffening of the granular skeleton during hydrate formation at contacts (diffusion limited), and the implications of water solubility in liquid CO2.

P-wave of hydrate-bearing sand under temperature cycling

P-wave velocity was used to study the formation of gas hydrates that were formed from methane, [Formula: see text], and propane in the pores of sand particles under temperature cycling. During the temperature-cycling tests, additional hydrate or ice formation can be demonstrated by investigating the velocities and amplitude of the P-wave. The experiments indicated that temperature cycling can accelerate hydrate accumulation in the pore space of sediments. Although most of the water in the pore space of sand can transform into gas hydrate after several temperature cycles, the formation of propane hydrate was slower than that of methane hydrate or [Formula: see text] hydrate. To estimate the hydrate saturation, an effective-medium theory was used. The results corroborated the validity of the contact model that the hydrate acted as a mineral grain to support the sediment frame. According to the calculation results, the methane-hydrate saturation can be up to 0.85 after three temperature cycles. The evaluation results accord with those of some natural hydrates in sand layers.

Experimental Study on Gas Production from Methane Hydrate-Bearing Sand by Hot-Water Cyclic Injection

A three-dimensional middle-size reactor was used to simulate gas production from methane hydrate-bearing sand by hot-water cyclic injection. The gas production process and energy efficiency in the whole process, which was divided into injecting hot water, closing well, and producing gas (three steps), were investigated using 16 thermocouples distributed in hydrate-bearing sand samples. The experimental results indicates that the overall temperature trend increases with hot-water injection and decreases with gas production. The temperature distribution and fluctuation in the reactor depend upon the location of the injecting/producing well as well as the porosity and permeability of hydrate samples. Heat transfer is controlled by hot-water seepage flow during the injection of hot water. The affecting factors on the energy efficiency, such as hydrate saturation, hydrate sample temperature, hot-water temperature, mass of hot water injected, and well pressure, were examined. It was found that, when other conditions are similar, the energy efficiency ratio increases with the increase of the hydrate-bearing sand saturation and hydrate sample temperature but decreases with the increase of the hot-water temperature and well pressure.

Documenting channel features associated with gas hydrates in the Krishna–Godavari Basin, offshore India

During the India National Gas Hydrate Program (NGHP) Expedition 01 in 2006 significant sand and gas hydrate were recovered at Site NGHP-01-15 within the Krishna–Godavari Basin, East Coast off India. At the drill site NGHP-01-15, a 5–8m thick interval was found that is characterized by higher sand content than anywhere else at the site and within the KG Basin. Gas hydrate concentrations were determined to be 20–40% of the pore volume using wire-line electrical resistivity data as well as core-derived pore-fluid freshening trends. The gas hydrate-bearing interval was linked to a prominent seismic reflection observed in the 3D seismic data. This reflection event, mapped for about 1km2 south of the drill site, is bound by a fault at its northern limit that may act as migration conduit for free gas to enter the gas hydrate stability zone (GHSZ) and subsequently charge the sand-rich layer. On 3D and additional regional 2D seismic data a prominent channel system was imaged mainly by using the seismic instantaneous amplitude attribute. The channel can be clearly identified by changes in the seismic character of the channel fill (sand-rich) and pronounced levees (less sand content than in the fill, but higher than in surrounding mud-dominated sediments). The entire channel sequence (channel fill and levees) has been subsequently covered and back-filled with a more mud-prone sediment sequence. Where the levees intersect the base of the GHSZ, their reflection strengths are significantly increased to 5- to 6-times the surrounding reflection amplitudes. Using the 3D seismic data these high-amplitude reflection edges where linked to the gas hydrate-bearing layer at Site NGHP-01-15. Further south along the channel the same reflection elements representing the levees do not show similarly large reflection amplitudes. However, the channel system is still characterized by several high-amplitude reflection events (a few hundred meters wide and up to ~1km in extent) interpreted as gas hydrate-bearing sand intervals along the length of the channel.