Hydrate-bearing Sand Research Articles

An Elastoplastic Constitutive Model Considering Bonding Effects for Methane Hydrate-Bearing Sand

An Elastoplastic Constitutive Model Considering Bonding Effects for Methane Hydrate-Bearing Sand

Application of time domain reflectometry to triaxial shear tests on hydrate-bearing sediments

Various triaxial shearing apparatus are used to quantify mechanical properties of hydrate-bearing sediments which are basically required to the assessment of risks posed by dissociation of natural gas hydrates. However, the key factor, hydrate saturation, has not been well measured during triaxial shearing tests. This study aims to show how a time domain reflectometry (TDR) method nondestructively measures hydrate saturation for triaxial shear tests on hydrate-bearing sands. Hydrate saturation is determined based on the difference of volumetric water contents measured by using a flexible TDR probe with two rods after calibration. The results suggest that values of hydrate saturation acquired by the TDR method are accurate, and the flexible TDR probe can monitor the kinetic process of hydrate formation and avoid any reinforcement effects on triaxial shearing properties of test specimens.

Linear Model for Pore Pressure Predication in Gas Hydrate-bearing Sand Formation of Krishna-Godavari Basin (India) – A Case Study

Linear Model for Pore Pressure Predication in Gas Hydrate-bearing Sand Formation of Krishna-Godavari Basin (India) – A Case Study

Modeling of synthesized hydrate-bearing sands: Effect of hydrate morphology on absolute and relative permeabilities

Absolute and relative permeability of hydrate-bearing sediments are crucial flow parameters to hydrate exploitation processes. However, up to now, grain-coating hydrate-bearing sediments (GC-HBS) with high hydrate saturation (Sh) and pore-filling hydrate-bearing sediments (PF-HBS) are still prohibitive to be prepared in laboratory. In this work, three-dimensional pore structures of GC-HBS with high Sh and PF-HBS were numerically synthesized, based on the CT scanning of porous samples. Then, a state-of-the-art quasi-static pore-network model (PNM) was used to study the absolute and relative permeability of these HBS. Results show that the absolute permeability of both of the GC-HBS and PF-HBS declined rapidly with increasing of Sh, they can be well fitted by the Tokyo model with exponential parameter of 3.5 and 5.3, respectively. And the absolute permeability of PF-HBS is smaller than that of GC-HBS with same Sh for a bigger specific surface area of PF-HBS which increases the flow resistance for flows. It shows that the wetting and non-wetting relative permeability of GC-HBS decreases with the increase of Sh. But the gas-water two-phase relative permeability of PF-HBS has nonmonotonic changes with the increase of Sh.

Stress-strain response and deformation behavior of hydrate-bearing sands under different grain sizes: A particle-scale study using DEM

Natural gas hydrates are crystalline solids found in sediments beneath the seabed and permafrost, which significantly affect the mechanical behavior of sediments. The effect of hydrates often varies for different sediments, which has been captured in some laboratory studies. But the particle-scale information involved is obscure and the underlying mechanisms often rely on assumptions. In this study, discrete element method (DEM) was used to improve understanding of the hydrate effect on the mechanical behavior of different sands. Triaxial compression tests were carried out using DEM on the digital hydrate-bearing sands (HBS) with different grain sizes. Simulation results shows that the large-grain samples often yield earlier and tend to strain-hardening behavior, while hydrate occurrence induces strain-softening behavior. In terms of stress-strain response and porosity changes, the hydrate strengthening on the small-grain samples is much greater than the large-grain samples. Particle-scale observations indicate that particle rotation usually occurs in regions with small particle displacement and is more intense on shear slip surfaces. Hydrate bonds are mainly subjected to tensile failure, and large grain size may hinder bond shear failure. Hydrates restrict the development of radial fabrics and enhance the stress-induced anisotropy, rendering a more stable framework for sediments.

Investigation of particle-scale mechanical behavior of hydrate-bearing sands using DEM: Focus on hydrate habits

Natural gas hydrates are commonly found in onshore permafrost and seabed regions. Hydrates often exhibit different pore habits in different regions, which may affect the mechanical behavior of hydrate-bearing sands (HBSs). In this study, pore-filling, load-bearing and cementing HBSs were prepared using discrete element method (DEM). The numerical biaxial compression tests show that the digital cementing HBS specimen well capture the stress-strain response of the gas-saturated HBS specimen in experiments under σc = 3 MPa, which justify the DEM model in this study. The pore-filling hydrates contribute little to the stress-strain response of sediments, while the load-bearing and cementing hydrates significantly enhance the failure strength and stiffness, and also induce strain-softening behavior and higher volume dilation. Hydrate cementation hinders the lateral fabric development in HBS, and renders orderly particle displacement and abundant strong force chains. The yielding of HBS is dominated by the buckling of strong force chains. Particle sliding, dislocation and rotation are responsible for the formation and development of shear bands, which governs the volume dilation and non-uniform deformation of HBS. Comprehending the geomechanical behavior of HBS under different hydrate habits facilitates the optimization of engineering solutions for hydrate exploitation.

A particle-scale study of the triaxial compression behavior of methane hydrate-bearing sands

A particle-scale study of the triaxial compression behavior of methane hydrate-bearing sands

Reproduction Analysis of Shear Tests Focusing on the Relationship between Formation Process and Mechanical Properties of Gas Hydrate-Bearing Sands by an Elasto-Viscoplastic Constitutive Equation

Reproduction Analysis of Shear Tests Focusing on the Relationship between Formation Process and Mechanical Properties of Gas Hydrate-Bearing Sands by an Elasto-Viscoplastic Constitutive Equation

Thermal Conductivity Variations in Frozen Hydrate-Bearing Sand upon Heating and Dissociation of Pore Gas Hydrate

High-latitude permafrost, including hydrate-bearing frozen ground, changes its properties in response to natural climate change and to impacts from petroleum production. Of special interest is the behavior of thermal conductivity, one of the key parameters that control the thermal processes in permafrost containing gas hydrate accumulations. Thermal conductivity variations under pressure and temperature changes were studied in the laboratory through physical modeling using sand sampled from gas-bearing permafrost of the Yamal Peninsula (northern West Siberia, Russia). When gas pressure drops to below equilibrium at a constant negative temperature (about −6 °C), the thermal conductivity of the samples first becomes a few percent to 10% lower as a result of cracking and then increases as pore gas hydrate dissociates and converts to water and then to ice. The range of thermal conductivity variations has several controls: pore gas pressure, hydrate saturation, rate of hydrate dissociation, and amount of additionally formed pore ice. In general, hydrate dissociation can cause up to 20% thermal conductivity decrease in frozen hydrate-bearing sand. As the samples are heated to positive temperatures, their thermal conductivity decreases by a magnitude depending on residual contents of pore gas hydrate and ice: the decrease reaches ~30% at 20–40% hydrate saturation. The thermal conductivity decrease in hydrate-free saline frozen sand is proportional to the salinity and can become ~40% lower at a salinity of 0.14%. The behavior of thermal conductivity in frozen hydrate-bearing sediments under a pressure drop below the equilibrium and a temperature increase to above 0 °C is explained in a model of pore space changes based on the experimental results.

A state-dependent constitutive model for gas hydrate-bearing sediment incorporating phase change

Extracting energy from hydrate-bearing sediment involves phase change, during which the strength of sediment is reduced and the risk of geohazards may subsequently increase. However, existing mechanical constitutive models cannot capture the responses of hydrate-bearing sediment during phase change since kinetics of phase change is ignored. Besides, most models are not suitable for hydrate-bearing sandy sediment since the dilatancy of sand is not considered well. In the current study, a state-dependent mechanical constitutive model for hydrate-bearing sediment considering phase change of hydrate is developed. The bounding surface plasticity is adopted. Fugacity is incorporated into the model to account for phase change of hydrate and the effects of thermodynamic states on mechanical behaviour of hydrate-bearing sediment. Additionally, a non-linear critical state line and a non-associated flow rule are adopted for hydrate-bearing sandy sediment, which is the most favourable for exploitation due to high hydrate content and permeability. These unique features can improve model performance regarding sediment responses during phase change of hydrate. The model is verified by comparing measured results of hydrate dissociation tests, drained triaxial tests and dissociation tests of hydrate-bearing sand. It is evident that stress-strain and volumetric responses of hydrate-bearing sandy sediments can be well captured by this model over a broad range of effective stresses from 0.1 MPa to 5 MPa. Additionally, the responses of hydrate-bearing sediment during dissociation under different stress states have been successfully reproduced. With an initial axial strain of 5%, dissociation of hydrate results in the development of over 20% axial strain and causes the specimen to collapse.

The Effect of Salinity on the Strength Behavior of Hydrate-Bearing Sands

The first prerequisite for the efficient and safe exploitation of gas hydrate resources is to accurately analyze the primary mechanical performance of hydrate-bearing sediments (HBSs). The mechanical performance of HBSs is complex and affected by many factors, including the reservoir environment in situ (temperature, pore pressure, salinity). Several published studies have demonstrated a correlation of the mechanical behavior of hydrates with temperature and pressure (T-PP). However, the research on the effect of salinity on the mechanical properties of hydrates or HBSs is still a relatively blank field. This study found that the strength of HBSs decreased with increasing salinity. This phenomenon can be attributed to the influence of salinity on the phase equilibrium state of hydrates. NaCl changed the relationship between the phase equilibrium curve of the hydrate and the T-PP conditions. The distance between the T-PP conditions and equilibrium curve was reduced with increasing salinity, which in turn led to a decline in sample strength. Moreover, the effect of the phase equilibrium of hydrates on the mechanical performance of HBSs was further explored. NaCl was added to HBSs to regulate the phase equilibrium state of the hydrate. When the T-PP conditions were on the phase equilibrium curve, the strength behaviors of HBSs showed a high degree of consistency.

Experimental and model study on the weakening of gas hydrate-bearing sand considering dissociation

Hydrate decomposition changes the mechanical behavior of reservoirs, which threatens the safety of mining wells. Therefore, this study determines the weakening behavior of gas hydrate-bearing sands by performing triaxial shear tests considering decomposition based on drilling data from the Shenhu Sea. The results indicate that gas hydrate-bearing sands, both before and after hydrate decomposition, show a strain hardening trend, and the strength and cohesion increase with hydrate saturation. The dilatancy of the nondissociated specimen increases with hydrate saturation, while that of the dissociated specimen decreases. Moreover, hydrate decomposition causes the strength, cohesion and dilatancy of the specimen to decrease, exhibiting significant weakening behavior. Furthermore, a statistical damage constitutive model is proposed by considering that the microunit strength obeys the Drucker‒Prager criterion and Weibull distribution, and the model well describes the deformation and damage of gas hydrate-bearing sands due to decomposition.

Mechanical Characteristics of Underconsolidated Methane Hydrate-Bearing Clayed-Silty Sediments

Natural gas hydrate reserves in the South China Sea are huge, and their commercial prospects have been proved by repeated trial production. It is essential to systematically clarify the mechanical properties of hydrate-bearing sediments for the safe drilling of hydrate reservoirs. To date, most research studies focus on the undrained mechanical behavior of hydrate-bearing sandy or clayed sediments under normal consolidation conditions, but there are few reports on underconsolidated-undrained triaxial shear experiments. In this study, the present research conducts undrained triaxial shear tests on hydrate-bearing clayed-silty sediments. The results show that underconsolidated hydrate-bearing sediments appear to show a strain-hardening phenomenon, which are different from hydrate-bearing sands in Nankai Trough. The excess pore water pressure of hydrate-bearing sediments remains positive during shear. The increasing degree of consolidation will increase the failure strength and the Secant Young’s modulus E50. According to the Mohr–Coulomb criterion, the corresponding predicting formulas are established, and the cohesion increases with the increase in the degree of consolidation, while the internal friction angle is not sensitive to the degree of consolidation. In addition, the failure strength and the Secant Young’s modulus E50 increase with the increase in initial effective confining stress before shearing.

Hydrate-bearing sands in the Terrebonne Basin record the transition from ponded deposition to bypass in the deep-water Gulf of Mexico

We integrated well and seismic data to interpret the stratigraphic architecture and the paleogeographic evolution of two hydrate-bearing sand reservoirs in the Terrebonne Basin in the deep-water Gulf of Mexico. These sands were deposited during the final stage of ponded fill within the Terrebonne Basin. The Green sand filled the final accommodation in the basin as a perched apron; it was formed coincidentally with downstream erosion and sediment bypass which cut through a canyon formed on the southeastern edge of the basin. Subsequently, a low energy turbidite channel continued to bypass sediment through the basin and aggrade. The Orange sand was then deposited as a second perched apron and it was once again incised by the low energy turbidite channel. The Green and Orange sands are regional sheet sands that extend and thicken deep into the Terrebonne Basin. They provide a connected regional aquifer that captured and focused gas migration to the crest of the structure where hydrate was formed. These sands were formed by large, unconfined, turbidity flows. We envision that they will be composed of coarser grained deposits relative to previously explored hydrate reservoirs within levee deposits. As such, they have the potential to be relatively coarse grained and hence have higher permeability: a condition favorable to production of hydrate deposits.

DEM simulations of the creep behavior of cemented methane hydrate-bearing sand

The existence of methane hydrate can greatly influence the creep behavior of sediments, which remains largely unknown. The discrete element method is used to study the creep behavior of cemented methane hydrate-bearing sand. Methane hydrate is simulated as contact bonds between particles, and the bonding ratio is introduced to directly reflect the influence of hydrate saturation. The parallel-bar creep model is proposed to describe the damage development with time in the hydrate crystal, based on which a time-dependent bond strength equation is established to describe the creep behavior of hydrate as interparticle cementation. Reasonable consistency is found between the simulated and experimental results.The results show that methane hydrate bearing sand exhibits significant creep, which is similar to that of frozen sand. The microbehavior during creep is investigated, including the evolution of the local porosity, bond breakage, particle rotation, strong force chain, and deviator fabric, which show strong heterogeneity due to the gradual formation of shear bands. The underlying mechanism is explored, which is related to changes in the microstructure caused by bond breakage with time.

The Influence of Particle Size and Hydrate Formation Path on the Geomechanical Behavior of Hydrate Bearing Sands

Determining the geomechanical properties of hydrate-bearing sands (HBS), such as strength and stiffness, are critical for evaluating the potential for the economic and safe recovery of methane gas from HBS reservoirs. To date, results from numerous independent laboratory studies on synthesized HBS have shown that strength and stiffness are largely influenced by hydrate saturation, the method adopted for hydrate formation, and to a lesser extent, the confining stresses applied during testing. However, a significant scatter is observed in the data even when these conditions are similar. These include recent studies on natural HBS where sands with larger particle size distribution (PSD) exhibited higher strengths despite lower hydrate saturation. To investigate the impact of PSD, and the role that specific hydrate formation conditions might impose, on the strength and stiffness of HBS, a series of laboratory tests were carried out on sand specimens formed with different particle size distributions and utilizing different approaches for forming gas saturated HBS. The laboratory apparatus included a resonant column drive head to measure the small-strain stiffness of the specimen during hydrate formation, and subsequent drained compressional shearing to capture the stress-strain response of the HBS. Results indicate that the PSD significantly affects both the stiffness evolution (during hydrate formation) and peak strength at failure after formation compared to the effect of the methodology adopted for hydrate formation. These observations improve our understanding of the geomechanical behavior of laboratory-synthesized HBS and allow more robust relationships to be developed between them and natural HBS. This may aid in the development of economic and safe methane gas production methods to help realize the energy resource potential of HBS reservoirs.

Fractal analysis on CO2 hydrate-bearing sands during formation and dissociation processes with NMR

Injecting CO2 into submarine sediments to form hydrates is one of the potential methods of CO2 sequestration. The transition behavior of CO2 hydrates in porous media is of great practical significance. In this work, CO2 hydrate formation/dissociation in porous media was monitored in real time by a low-field magnetic resonance (MR) system, and a series of dynamic fractal dimensions of the pore space occupied by converted water during the hydrate formation/dissociation process were obtained based on the transverse relaxation time (T2) distributions. In general, the dimension of the converted water space increases with hydrate formation and decreases with the hydrate dissociation progress. A smaller particle size of porous media and a lower initial water saturation can promote hydrate formation, and the corresponding fractal dimension is higher during the hydrate formation process. There is a special status of the fractal period observed during the hydrate formation/dissociation process, and it is considered the temporally and spatially uniform distribution of hydrate crystal formation/dissociation inside the porous media. These results also indicate the relationships between the hydrate transition progress and the dynamic fractal dimension, which are useful for future works on pore-scale hydrate-bearing transitions during hydrate-based CO2 sequestration.

Influence of Temperature and Pore Pressure on Geomechanical Behavior of Methane Hydrate-Bearing Sand

Geomechanical behavior of methane hydrate-bearing sediment (MHBS) plays a major role in evaluation of the stability of a hydrate reservoir. A series of triaxial compression tests were conducted on MHBS to study the influence of temperature and pore pressure conditions on its mechanical behaviors. The experimental results show that temperature and pore pressure have a significant effect on the stress–strain curve, stiffness, and strength of MHBS. As temperature decreases and/or pore pressure increases, the stress–strain curve manifests an enhanced strain-softening characteristic, stiffness, and strength. Furthermore, MHBS cohesion also tends to exhibit a significant increase, but its internal friction angle almost remains constant with decreasing temperature and/or increasing pore pressure. These findings imply that the change in temperature and pore pressure affects the strength of MHBS, which occurs predominately due to change in its cohesiveness. To describe these impacts, a phase state parameter is introduced to characterize the temperature and pore pressure conditions. Meanwhile, three empirical formulas for relating the secant modulus, strength and cohesiveness to phase state parameter are presented. Good agreement between simulation and measured data indicates that the phase state parameter can effectively describe temperature and pore pressure conditions. The proposed empirical formulas are able to address the influences of temperature and pore pressure conditions on geomechanical characteristics of MHBS.

Modeling the Mechanical Behavior of Methane Hydrate-Bearing Sand Using the Equivalent Granular Void Ratio

For the safe extraction of methane from hydrate reservoirs, modeling the mechanical behavior of the methane hydrate-bearing soil properly is crucial in order to enable designers to analysis hydrate-dissociation-induced geotechnical failures. Hydrate morphology is one of major factors affecting the mechanical behavior of soil containing hydrate. This paper presents a new constitutive model for methane hydrate-bearing sand (MHBS) using the equivalent granular void ratio as a state variable, which can quantify the effects of the pore-filling and load-bearing hydrate morphology under a unifying framework. The proposed model is a combination of generalized plasticity and an elastic damage model so as to take into account the observed frictional and bonding aspects of MHBS, respectively. By using the concept of state-dependent dilatancy, the equivalent granular void ratio is formulated and adopted in the generalized plasticity model. In addition, a nonlinear damage function is implemented to elucidate the degradation of hydrate bonds with respect to shearing. Compared with the basic generalized plasticity model for host sand, only three additional parameters are required to capture key mechanical behaviors of MHBS. By comparing the triaxial test results of MHBS synthesized from a range of host sands with a predicted behavior by the proposed model, it is demonstrated that the new model can satisfactorily capture the stress–strain and volumetric behavior of MHBS under different hydrate saturations, confining pressures, and void ratios.

Investigating the effective permeability evolution as a function of hydrate saturation in the hydrate-bearing sands using a kinetic-theory-based pore network model

Permeability governs the fluid flow of hydrate-bearing sediment and affects the efficiency of natural gas production from hydrate reservoirs. The permeability in hydrate-bearings sediments is estimated empirically and appears to vary widely for sediments. This study focused on the sandy hydrate-bearing sediments and intended to elucidate the evolution of effective permeability by the mean of pore network modeling. A hydrate kinetics theory-based pore network model (KT-PNM) has been developed, in which the simulation of hydrate formation in porous media is implemented by employing two sub-processes, i.e., hydrate nucleation and hydrate growth. This KT-PNM has been applied to simulate hydrate formation in different pore networks. The permeability reduction exponent (N) for sandy hydrate-bearing sediments is determined to fall in the range of 3–4 based on the simulations of seven sandy samples. An empirical equation used for predicting the effective permeability in sandy hydrate-bearing sediments has been further proposed. The developed model and simulations are hoped to provide valuable insights of pore-space effects on hydrate into permeability prediction for studying the fluid seepage in hydrate-bearing sediments and facilitate the numerical simulation of gas production from the hydrate reservoirs.