Gas Hydrate-bearing Sediments Research Articles

Study on the Influence of Sand Production on Seepage Capacity in Natural Gas Hydrate Reservoirs

Sand production has become a common phenomenon in the exploitation of unconsolidated natural gas hydrate reservoirs, which will hinder the long-term production of natural gas hydrate reservoirs. However, there are few literatures reported on the influences in reservoir physical properties such as permeability and porosity, and production laws caused by sand production. This paper provides a numerical model, coupled with reservoir sand-gas-water multiphase flow processes, which is capable to simulate the process of sand production in natural gas hydrate reservoirs. The simulation results indicate that sand settlement is mainly concentrated near the wellbore due to the high concentration of migrated sand. The decrease in reservoir porosity and permeability caused by sand settlement has a significant impact on production. The impact of sand production on reservoir fluid fluidity shows that fluid flow is inhibited near the wellbore, while fluid flow performance increases far away from the wellbore. The numerical model and analysis presented here could provide useful insight into changes in reservoir physical properties and production laws caused by sand production in the natural gas hydrate-bearing marine sediments using depressurization method.

Current states of well-logging evaluation of deep-sea gas hydrate-bearing sediments by the international scientific ocean drilling (DSDP/ODP/IODP) programs

Since deep-sea gas hydrate-bearing sediments were drilled for the first time in the Blake Ridge in 1970, gas hydrates have been discovered at 53 drill sites in the continental margins of global oceans with international scientific ocean drilling (DSDP/ODP/IODP Programs). As a result, massive amounts of geophysical well-logging data have been accumulated, which provide critical information for understanding the in-situ properties of gas hydrates and their host sediments. Gas hydrates have such physical and chemical properties as non-conductivity, low density, high acoustic velocity, and high hydrogen content, which form the basis of identifying gas hydrate reservoirs and predicting their distribution by well-logging data. A series of well-logging evaluation methods have been proposed to estimate gas hydrate saturation of sediments, including Archie equation, combined methods of density and nuclear magnetic resonance well logging, various forms of three-phase acoustic wave equations, and elastic wave velocity simulations based on different rock physical models. The distribution of gas hydrates is highly heterogeneous, which is mainly manifested in the selectivity of hydrate occurrence to the lithology of host sediments and to the nucleation sites within a host sediment of the same lithology. The scientific-ocean-drilling well logging data have also been preliminarily used for evaluating the heterogeneity of gas hydrate distribution and inferring the growth habit of gas hydrates in host sediments. Nevertheless, there still exist some problems. The formation models used in logging evaluation are in general oversimplified, in which only two or three stratal components are involved. The application of high-resolution logging while-drilling (LWD) data remains limited. Log interpretation is not closely integrated with core geology. Therefore, joint inversion of lithologic components, porosity and gas hydrate saturation based on more complex formation models, together with the applications of high-resolution LWD logging data and core calibration, may represent an important direction in future well-logging evaluation of gas hydrate reservoirs.

Rock Physics Model and Seismic Dispersion and Attenuation in Gas Hydrate-Bearing Sediments

A rock physics model was established to calculate the P-wave velocity dispersion and attenuation caused by the squirt flow of fluids in gas hydrate-bearing sediments. The critical hydrate saturation parameter was introduced to describe different ways of hydrate concentration, including the mode of pore filling and the co-existence mode of pore filling and particle cementation. Rock physical modeling results indicate that the P-wave velocity is insensitive to the increase in gas hydrate saturation for the mode of pore filling, while it increases rapidly with increasing gas hydrate saturation for the co-existence mode of pore filling and particle cementation. Meanwhile, seismic modeling results show that both the PP and mode-converted PS reflections are insensitive to the gas hydrate saturation that is lower than the critical value, while they tend to change obviously for the hydrate saturation that is higher than the critical value. These can be interpreted that only when gas hydrate begins to be part of solid matrix at high gas hydrate saturation, it represents observable impact on elastic properties of the gas hydrate-bearing sediments. Synthetic seismograms are calculated for a 2D heterogeneous model where the gas hydrate saturation varies vertically and layer thickness of the gas hydrate-bearing sediment varies laterally. Modeling results show that larger thickness of the gas hydrate-bearing layer generally corresponds to stronger reflection amplitudes from the bottom simulating reflector.

Authigenic Greigite as an Indicator of Methane Diffusion in Gas Hydrate-Bearing Sediments of the Hikurangi Margin, New Zealand

Authigenic ferrimagnetic iron sulfides, essentially greigite (Fe3S4), are commonly found in gas hydrate-bearing marine sediments of active accretionary prisms. Greigite is a by-product, either intracellular or extracellular, of microbial activity, and therefore provides good indication of microbial processes which are closely related to the occurrence of gas hydrate. A high-resolution rock magnetic study was conducted at Site U1518 of International Ocean Discovery Program Expedition 375, located in the frontal accretionary wedge of the Hikurangi Margin, offshore New Zealand. Samples were collected throughout the entire recovered stratigraphic sequence, from the surface to ∼492 m below seafloor (mbsf) which includes the Pāpaku fault zone. This study aims to document the rock magnetic properties and the composition of the magnetic mineral assemblage at Site U1518. Based on downhole magnetic coercivity variations, the studied interval is divided into five consecutive zones. Most of the samples have high remanent coercivity (above 50 mT) and first-order reversal curves (FORC) diagrams typical of single-domain greigite. The top of the hanging wall has intervals that display a lower remanent coercivity, similar to lower coercivities measured on samples from the fault zone and footwall. The widespread distribution of greigite at Site U1518 is linked to methane diffusion and methane hydrate which is mainly disseminated within sediments. In three footwall gas hydrate-bearing intervals, investigated at higher resolution, an improved magnetic signal, especially a stronger FORC signature, is likely related to enhanced microbial activity which favors the formation and preservation of greigite. Our findings at the Hikurangi Margin show a close linkage between greigite, methane hydrate and microbial activity.

Comparison of logging-while-drilling and wireline logging data from gas hydrate-bearing deep-sea sediments, the Ulleung Basin, East Sea

Gas hydrate exploration was conducted using logging-while-drilling (LWD) and wireline logging (WLL) at two sites (UBGH2-6 and UBGH2-10) in the Ulleung Basin, East Sea (Sea of Japan). Coring for gas hydrate sampling was also conducted. Seismic profiles revealed that the upper layers at the logged sites (∼180 m at UBGH2-6 and ∼150 m at UBGH2-10) are dominated by turbidite/hemipelagic sediments, whereas the lower-lying sediments are characterised by thick mass transport deposits. Except in the gas hydrate-bearing interval (140–160 metres below the seafloor) of UBGH2-6, the WLL velocity and resistivity data are a little higher (∼100 m/s and ∼0.4 ohm-m, respectively) than those indicated by LWD. Conversely, the density and porosity do not show a consistent pattern for the two methods. Discrepancies between LWD and WLL can be caused by the methodology, borehole condition, and gas hydrate occurrence type. The high resistivity values (∼100 ohm-m) in the gas hydrate-bearing zone of UBGH2-6 are typical of inclined fracture-filling gas hydrate. In this case, the LWD resistivity may be overestimated as a result of the data acquisition principle. The relationships between the physical properties are significantly controlled by the presence of gas hydrate. Therefore, in fracture-filling gas hydrate-bearing sediments, the WLL resistivity data are more reasonable than the LWD resistivity data. Both methods have advantages and disadvantages in terms of data quality, efficiency, and resolution. Therefore, it is necessary to compare and interpret data measured using both methods to identify the physical properties of gas hydrate-bearing sediments.

Analysis of Hydrate Heterogeneous Distribution Effects on Mechanical Characteristics of Hydrate-Bearing Sediments

It is reported that hydrate distributes inhomogeneously in synthesized gas hydrate-bearing sediments (GHBS). To investigate the effect of hydrate distribution heterogeneity on the mechanical charac...

Dynamic coupling responses and sand production behavior of gas hydrate-bearing sediments during depressurization: An experimental study

Gas hydrates that occur in offshore sediments and onshore permafrost represent a vast source of natural gas. However, current gas production trials indicate that sand production, which refers to the migration of sand into boreholes is an obstacle for long-term safe and efficient gas production from gas hydrate reservoirs. Therefore, understanding sand production behaviors during hydrate mining is especially important. In this study, we conducted laboratory-scale tests to investigate the effect of hydrate saturation and overlying stress on the sand production behaviors of hydrate-bearing sediments with a sand screen before, during, and after hydrate dissociation by depressurization. The synthesized methane hydrate saturations and simulated overlying stresses were ~20%, ~30%, ~40%, ~50%, and ~60% and ~11, ~12, ~13, ~14, and ~15 MPa, respectively. The experimental results indicate that sand production before hydrate dissociation is relatively minor and primarily silty. The amount of sand produced increases and silt, fine sand, and medium sand were observed during and after hydrate dissociation. The mass of sand production increases as hydrate saturation increases under constant overlying stress; however, under constant hydrate saturation, it increases then decreases as overlying stress increases. Friction among particles may become the primary control factor once the overlying stress exceeds a critical value. We considered hydrate saturation and overlying stress to establish a sand production prediction model. Our findings help understand sand production behavior using sand screens during gas production from hydrate reservoirs and provide a reference for sand production control design in future hydrate mining.

Wave Propagation Characteristics in Gas Hydrate-Bearing Sediments and Estimation of Hydrate Saturation

Natural gas hydrate is a new clean energy source in the 21st century, which has become a research point of the exploration and development technology. Acoustic well logs are one of the most important assets in gas hydrate studies. In this paper, an improved Carcione–Leclaire model is proposed by introducing the expressions of frame bulk modulus, shear modulus and friction coefficient between solid phases. On this basis, the sensitivities of the velocities and attenuations of the first kind of compressional (P1) and shear (S1) waves to relevant physical parameters are explored. In particular, we perform numerical modeling to investigate the effects of frequency, gas hydrate saturation and clay on the phase velocities and attenuations of the above five waves. The analyses demonstrate that, the velocities and attenuations of P1 and S1 are more sensitive to gas hydrate saturation than other parameters. The larger the gas hydrate saturation, the more reliable P1 velocity. Besides, the attenuations of P1 and S1 are more sensitive than velocity to gas hydrate saturation. Further, P1 and S1 are almost nondispersive while their phase velocities increase with the increase of gas hydrate saturation. The second compressional (P2) and shear (S2) waves and the third kind of compressional wave (P3) are dispersive in the seismic band, and the attenuations of them are significant. Moreover, in the case of clay in the solid grain frame, gas hydrate-bearing sediments exhibit lower P1 and S1 velocities. Clay decreases the attenuation of P1, and the attenuations of S1, P2, S2 and P3 exhibit little effect on clay content. We compared the velocity of P1 predicted by the model with the well log data from the Ocean Drilling Program (ODP) Leg 164 Site 995B to verify the applicability of the model. The results of the model agree well with the well log data. Finally, we estimate the hydrate layer at ODP Leg 204 Site 1247B is about 100–130 m below the seafloor, the saturation is between 0–27%, and the average saturation is 7.2%.

Numerical simulation on flow characteristics of large-scale submarine mudflow

Gas hydrate (GH) is widely distributed in the world according to the geological survey. Most of GH is located in the ocean. After GH dissociation, the strength of gas hydrate bearing sediments (GHBS) decreases accompanying the rise of pore pressure, which can cause marine landslide. Landslide may transform into mudflows during movement on the seabed. The massive movement of submarine mudflow can result in the serious deformation of (or even damage of) submarine pipelines, the dumping of offshore platforms, and tsunamis. In this paper, the flow characteristics of the large-scale submarine mudflow are investigated based on the Euler-Euler multiphase flow model combining with kinetic theory of granular flow (KTGF). The controlling dimensionless parameters, such as the initial depositional form and the relative depth, are given. The hydroplaning and frontal detachment during the mass movement are captured. The front velocity of the mudflow, and the free surface elevations at the sea level are analyzed. The numerical results show that the large-scale submarine mudflow has a significant impact on the sea level. The submarine mudflow should be taken into consideration during design of submarine pipelines and offshore platforms in GH-distributed sea area duo to its great destructive power.

Insights into the climate-driven evolution of gas hydrate-bearing permafrost sediments: implications for prediction of environmental impacts and security of energy in cold regions.

The present study investigates the evolution of gas hydrate-bearing permafrost sediments against the environmental temperature change. The elastic wave velocities and effective thermal conductivity (ETC) of simulated gas hydrate-bearing sediment samples were measured at a typical range of temperature in permafrost and wide range of hydrate saturation. The experimental results reveal the influence of several complex and interdependent pore-scale factors on the elastic wave velocities and ETC. It was observed that the geophysical and geothermal properties of the system are essentially governed by the thermal state, saturation and more significantly, pore-scale distribution of the co-existing phases. In particular, unfrozen water content substantially controls the heat transfer at sub-zero temperatures close to the freezing point. A conceptual pore-scale model was also proposed to describe the pore-scale distribution of each phase in a typical gas hydrate-bearing permafrost sediment. This study underpins necessity of distinguishing ice from gas hydrates in frozen sediments, and its outcome is essential to be considered not only for development of large-scale permafrost monitoring systems, bus also accurate quantification of natural gas hydrate as a potential sustainable energy resource in cold regions.

Temperature Regulation Effect of Low Melting Point Phase Change Microcapsules for Cement Slurry in Gas Hydrate-Bearing Sediments

Since natural gas hydrates only exist stably at low temperature and high pressure, hydration heat of cement slurry is easy to cause hydrate decomposition. Therefore, low-heat cement slurry system must be used during well cementing in gas hydrate-bearing sediments. In this paper, temperature-regulated microcapsules with binary alkanes as core material and CaCO 3 as shell were prepared based on self-assembly method. In addition, the hydrophilic modified graphite was added into core material to improve the temperature sensitivity. The morphology, thermal property and hydrophilicity of microcapsules were characterized by multi-techniques.The hydration property, setting behavior and compressive strength of cement slurry/stone with microcapsules was studied. The pore distribution of cement stone was analyzed by means of micro-computed tomography (micro-CT). Moreover, the effect of cement slurry hydration on hydrate-bearing sediments was tracked by a hydrate-bearing sediments damage simulation device. Compared with standard group, maximal hydration temperature decreased 3.7 °C by adding microcapsules. The simulation trail showed that the addition of microcapsules can impede the damage to hydrate-bearing sediments which are associated with cement slurry hydrating.

Development of a coupled geophysical-geothermal scheme for quantification of hydrates in gas hydrate-bearing permafrost sediments.

Quantification of hydrates in permafrost sediments using conventional seismic techniques has always been a major challenge in the study of the climate-driven evolution of gas hydrate-bearing permafrost sediments due to almost identical acoustic properties of hydrates and ice. In this article, a coupled geophysical-geothermal scheme is developed, for the first time, to predict hydrate saturation in gas hydrate-bearing permafrost sediments by utilising their geophysical and geothermal responses. The scheme includes a geophysical part which interprets the measured elastic wave velocities using a rock-physics model, coupled with a geothermal part, interpreting the measured effective thermal conductivity (ETC) using a new pore-scale model. By conducting a series of sensitivity analyses, it is shown that the ETC model is able to incorporate the effect of the hydrate pore-scale habit and hydrate/ice-forced heave as well as the effect of unfrozen water saturation under frozen conditions. Given that the geophysical and geothermal responses depend on the overburden pressure, the elastic wave velocities and ETC of methane hydrate-bearing permafrost sediment samples were measured at different effective overburden pressures and the results were provided. These experimental data together with the results of our recent study on the geophysical and geothermal responses of gas hydrate-bearing permafrost sediment samples at different hydrate saturations are used to validate the performance of the coupled scheme. By comparing the predicted saturations with those obtained experimentally, it is shown that the coupled scheme is able to quantify the saturation of the co-existing phases with an acceptable accuracy in a wide range of hydrate saturations and at different overburden pressures.

DEM analyses of cemented hydrate’s effect on the compression behavior of fine-grained sediments

The participation of gas hydrate constitutionally affects the mechanical properties of gas hydrate-bearing sediments. The hydrates contribute to the mechanical properties of hydrate-bearing sediments by pore-filling, bearing and bonding effects. A discrete element model of hydrate-bearing silt was established randomly filled by the hydrate particles into silt-sized granular skeleton reservoir. The cluster & bonding hydrate is generated in the pores of the numerical silt reservoir to study hydrate’s mechanical effects. One-dimensional loading was applied to the numerical sample to investigate its volume change, bond degradation and stress transmission. The main conclusions are as follows. The structural yield stress corresponds with the abrupt bond breakage. Particles and contacts will be redistributed after the widespread bond breakage. The breakage of hydrate-hydrate bonded contacts is more quickly than that of silt-hydrate contacts ascribe to a crushing process of hydrate clusters (especially for SMH=45%).

Mechanical creep instability of nanocrystalline methane hydrates.

Mechanical creep behaviors of natural gas hydrates are of importance for understanding the mechanical instability of gas hydrate-bearing sediments on Earth. Limited by the experimental challenges, intrinsic creep mechanisms of nanocrystalline methane hydrates remain largely unknown yet at the molecular scale. Herein, using large-scale molecular dynamics simulations, mechanical creep behaviors of nanocrystalline methane hydrates are investigated. It is revealed that mechanical creep responses are greatly dictated by internal microstructures of crystalline grain size and external conditions of temperature and static stress. Interestingly, a long steady-state creep is observed in nanocrystalline methane hydrates, which can be described by a modified constitutive Bird-Dorn-Mukherjee model. Microstructural analysis shows that deformations of crystalline grains, grain boundary diffusion and grain boundary sliding collectively govern the mechanical creep behaviors of nanocrystalline methane hydrates. Furthermore, structural transformation also appears to be important in their mechanical creep behaviors. This study provides new insights into understanding the mechanical creep scenarios of gas hydrates.

Analysis of the mechanical properties of methane hydrate-bearing sands with various pore pressures and confining pressures

As a kind of new clean energy, natural gas hydrate has captured headlines around the world. A full study of the mechanical properties of natural gas hydrate-bearing sediments is a prerequisite for the exploitation of hydrate reserves. In this paper, a series of consolidated-drained triaxial tests were conducted to study the mechanical properties of methane hydrate-bearing sediments under different pore pressures and effective confining pressures. The results showed that the strength of samples increased with increasing pore pressure and effective confining pressure. With increasing pore pressure and decreasing effective confining pressure, the stress-strain curves for the samples changed from strain-hardening to strain-softening. By analyzing the Mohr envelops, the cohesion was found to increase with increasing pore pressure. Moreover, the failure stress points lines for the samples were influenced by the pore pressure. A shift point was introduced, and it occurred at a certain pore pressure value between the strain hardening and strain softening behavior of the hydrate-bearing samples. Additionally, the value of the shift point was found to change with effective confining pressure.

Fundamental characteristics of gas hydrate-bearing sediments in the Shenhu area, South China Sea

The basic physical properties of marine natural gas hydrate deposits are important to the understanding of seabed growth conditions, occurrence regularity, and occurrence environment of natural gas hydrates. A comprehensive analysis of the core samples of drilling pressure-holding hydrate deposits at a depth of 1310 m in the Shenhu area of the South China Sea was conducted. The experimental results indicate that the particle size in the hydrate sediment samples are mainly distributed in the range from 7.81 µm to 21.72 µm, and the average particle size decreases as the depth of the burial increases. The X-ray CT analytical images and surface characteristics SEM scan images suggest that the sediment is mostly silty clay. There are a large number of bioplastics in the sediment, and the crack inside the core may be areas of hydrate formation.

Numerical simulation of gas recovery from natural gas hydrate using multi-branch wells: A three-dimensional model

Natural gas hydrate, a potential energy source, is clean and occurs abundantly in nature. Enhancing the gas production efficiency from gas hydrate-bearing sediments (GHBS) is of great significance to promote its industrial development. In this study, the radial jet drilling (RJD) technology is innovatively proposed to stimulate oceanic hydrate reservoirs. With an open-source simulator HydrateResSim, a 3D model is constructed based on the geological data from the SH7 site in the South China Sea. The hydrates exploitation performance using the combination of radial wells and depressurization is studied for the first time. Results indicate that radial wells can significantly enhance the gas production rate in the early stages of production. The hydrate recovery factor and the radial branch length are linearly related. However, the application of radial wells seems incapable of prolonging the gas production life cycle. By analyzing the multi-physical response of GHBS induced by depressurization, the stimulation mechanisms of radial wells are characterized as enlarged drainage area, enhanced pressure drop propagation, and increased geothermal heat flow. This work verified the capability of RJD in promoting hydrate extraction. Also, it provides insights into the potential applications of multi-branch wells in field trials of hydrate production.

Theoretical analysis and numerical simulation of acoustic waves in gas hydrate-bearing sediments* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11974018 and 11734017) and the Strategic Pilot and Technology Special Fund of the Chinese Academy of Sciences, China (Grant No. XDA14020303).

Based on Carcione–Leclaire model, the time-splitting high-order staggered-grid finite-difference algorithm is proposed and constructed for understanding wave propagation mechanisms in gas hydrate-bearing sediments. Three compressional waves and two shear waves, as well as their energy distributions are investigated in detail. In particular, the influences of the friction coefficient between solid grains and gas hydrate and the viscosity of pore fluid on wave propagation are analyzed. The results show that our proposed numerical simulation algorithm proposed in this paper can effectively solve the problem of stiffness in the velocity–stress equations and suppress the grid dispersion, resulting in higher accuracy compared with the result of the Fourier pseudospectral method used by Carcione. The excitation mechanisms of the five wave modes are clearly revealed by the results of simulations. Besides, it is pointed that, the wave diffusion of the second kind of compressional and shear waves is influenced by the friction coefficient between solid grains and gas hydrate, while the diffusion of the third compressional wave is controlled by the fluid viscosity. Finally, two fluid–solid (gas-hydrate formation) models are constructed to study the mode conversion of various waves. The results show that the reflection, transmission, and transformation of various waves occur on the interface, forming a very complicated wave field, and the energy distribution of various converted waves in different phases is different. It is demonstrated from our studies that, the unconventional waves, such as the second and third kinds of compressional waves may be converted into conventional waves on an interface. These propagation mechanisms provide a concrete wave attenuation explanation in inhomogeneous media.

Analyzing the applicability of in situ heating methods in the gas production from natural gas hydrate-bearing sediment with field scale numerical study

For natural gas hydrate (NGH) exploitation, the in-situ electrical heating has been proved as a promising production method in laboratory experimental scale. But the results in this scale are difficult to reliably reflect field trials due to limitations in the size of the reactor. So, by TOUGH+HYDRATE software, this paper stimulates hydrate exploitation in field scale by in situ electrical heating. In addition, depressurization is applied as a comparison. Based on the analysis of several parameters like reservoir temperature and hydrate distribution, results show that although a larger heating power can result in a faster move of the decomposition front, the fast free gas production takes a large amount of sensible heat out of the reservoir. Also, the electrical heating effect is limited by the heat conductivity of formation in the field scale, so the heat promoting distance of the heating method is less than 10 m after five years production even with 1000 W/m of electrical heating power. A large amount of heat loss and the shortage of heating injection make the energy efficiency of it decreased significantly with the production time. So, large heating power cannot always remain fast gas and water production rates. Furthermore, the higher heating power does not mean a better exploitation effect. Thus, it is vital to choose a reasonable heating power in the field exploitation. Under the NGH reservoir and production parameters set in this paper, the most optimal electrical heating power should be 500 W/m with comprehensive consideration of gas production rate, NGH front promoting velocity and energy efficiency.

Multiple physical properties of gas hydrate-bearing sediments recovered from Alaska North Slope 2018 Hydrate-01 Stratigraphic Test Well

Knowledge of the petrophysical and geomechanical properties of gas hydrate-bearing sediments is essential for predicting reservoir responses to gas production from gas hydrate reservoirs. In December 2018, Stratigraphic Test Well Hydrate-01 was drilled in the western part of the Prudhoe Bay Unit, Alaska North Slope, as part of the technical planning effort for a future long-term gas hydrate production test. Side-wall pressure coring was conducted to recover gas hydrate-bearing sediments from two reservoir sections named Unit B and Unit D. A total of 34 cores were successfully recovered during five runs of a wireline deployed pressure corer, and a total of 17 cores were preserved for advanced laboratory analysis. The samples were frozen inside the pressure core autoclave by liquid nitrogen while at high pressure before being removed and stored under liquid nitrogen at atmospheric pressure. High-resolution X-ray computed tomography showed the samples were high-quality, with undisturbed lithological layers. The Unit B and D sediments were categorized as sand or sandy silt with high hydrate saturation. Gas compositions suggest the hydrates formed with thermogenic and microbial mixed gases. Permeability tests and triaxial compression tests were conducted on the hydrate-bearing sediments. Low strengthening and high permeability at hydrate saturation Sh > 80% were observed. There was a small permeability reduction during the triaxial compression tests owing to porosity loss with increasing effective stress in the highly permeable sandy sediment after hydrate dissociation. The apparent minimal changes in porosity and permeability during the tests were due to the low-clay content and low compressibility of the quartz sand grains in the recovered cores. X-ray powder diffraction and thermal conductivity analysis also suggested a high quartz content for the analyzed samples.