Hydrate-bearing Sand Research Articles

Creep Behavior of CO<sub>2</sub> Hydrate-Bearing Sand and Its Simulations by an Elasto-Viscoplastic Constitutive Model

Creep Behavior of CO<sub>2</sub> Hydrate-Bearing Sand and Its Simulations by an Elasto-Viscoplastic Constitutive Model

Mechanical behaviours of gas-hydrate-bearing clayey sediments of the South China Sea

To investigate the stability of gas-hydrate-bearing sediments under the seabed, many researchers have studied their mechanical behaviours, including strength and deformation characteristics. However, most researchers have focused on hydrate-bearing sand, and there are few publications on hydrate-bearing clay. In this paper, a series of experiments were conducted using remoulded gas-hydrate-bearing sediments drilled from the South China Sea whose host sediments primarily consisted of clay. The particle size, specific surface area, pore size distribution and scanning electron microscopy images of the host sediments were analysed, and triaxial tests were conducted to study their mechanical behaviours under different hydrate saturations and effective confining pressures. The results indicated that strength and stiffness increase with increasing hydrate saturation under the same effective confining pressure. The effective confining pressure could enhance the strength and stiffness at nearly the same hydrate saturation. The analysis of the Mohr circles and failure envelopes of the hydrate-bearing sediments under four different hydrate saturations revealed that the cohesion and internal friction angle increase with an increase in hydrate saturation.

Particle crushing in hydrate-bearing sands

The depressurization of natural hydrate deposits to produce methane gas can increase the effective stress in hydrate-bearing sediments, which may induce sand particle crushing. Sand crushing may become detrimental to gas production, because the breakage of sand particles can cause sediment contraction, reduction in permeability, and the generation of finer particles exacerbating solids migration and sand production. This study experimentally investigates particle crushing in tetrahydrofuran hydrate-bearing sands. Sandy specimens with various hydrate saturation are loaded to different stresses (up to 40 MPa) in an instrumented oedometer cell to induce sand crushing. The results show that the presence of hydrate crystals inhibits sand particle crushing. More pronounced sand crushing occurs in specimens with lower hydrate saturation and under higher maximum effective stress. Hydrate crystals can cement neighboring sand particles and constrain the particles from rotating and rearranging during loading. Relatively larger particles in hydrate-bearing sediments experience surface grinding with no significant decreases in particle sizes, and shear-off asperities of the larger particles contribute to an increase in the proportion of smaller particles. The total volumetric strain in hydrate-bearing sediments due to loading can be estimated using the known hydrate saturation and the volumetric strain of hydrate-free sediments subjected to an identical load. The changes in hydraulic conductivity due to sand particle crushing are correlated to the breakage index. The results highlight that gas production from hydrate deposits using depressurization increases the effective stress and the loss of hydrate crystals can exacerbate sand crushing.

Investigation of in situ salinity and methane hydrate dissociation in coarse-grained sediments by slow, stepwise depressurization

Abstract Slow (8–15 day), stepwise, degassing of synthesized methane hydrate-bearing sand packs was used to determine in situ salinity to within 0.5 wt%. Rebounds in pressure during shut-in intervals between degassing steps record gradual hydrate dissociation. During successive dissociation steps and pressure rebounds, the pressure decreases more than predicted for a sample with homogenous salinity and temperature at thermodynamic equilibrium. We interpret that hydrate dissociation is limited by local pore water freshening and endothermic cooling around the dissociating hydrate. These results suggest that during production, hydrate dissociation in a reservoir will occur along the freshwater phase boundary even though the bulk salinity of the system may be higher. Pore pressure rebound during shut-in is more rapid in freshwater experiments than in brine experiments, which indicates that salt diffusion plays a role in limiting recovery to homogenous bulk equilibrium during hydrate dissociation. We demonstrate that it is possible to interpret the in-situ salinity from the temperature and pressure at the onset of dissociation. This approach can be applied to actual hydrate-bearing pressure core samples and the results will assist our understanding of how these hydrates form. Depressurization experiments with controlled shut-in periods provide insight into the role of salt and heat diffusion on dissociation in coarse-grained sediments, particularly relevant to pauses in production and in regions of a hydrate reservoir near the hydrate phase boundary.

Thermo-Hydro-Mechanical Coupled Modeling of Methane Hydrate-Bearing Sediments: Formulation and Application

We present a fully coupled thermo-hydro-mechanical formulation for the simulation of sediment deformation, fluid and heat transport and fluid/solid phase transformations occurring in methane hydrate geological systems. We reformulate the governing equations of energy and mass balance of the Code_Bright simulator to incorporate hydrate as a new pore phase. The formulation also integrates the constitutive model Hydrate-CASM to capture the effect of hydrate saturation in the mechanical response of the sediment. The thermo-hydraulic capabilities of the formulation are validated against the results from a series of state-of-the-art simulators involved in the first international gas hydrate code comparison study developed by the NETL-USGS. The coupling with the mechanical formulation is investigated by modeling synthetic dissociation tests and validated by reproducing published experimental data from triaxial tests performed in hydrate-bearing sands dissociated via depressurization. Our results show that the formulation captures the dominant mass and heat transfer phenomena occurring during hydrate dissociation and reproduces the stress release and volumetric deformation associated with this process. They also show that the hydrate production method has a strong influence on sediment deformation.

Micromechanical Investigation of Stress Relaxation in Gas Hydrate-Bearing Sediments Due to Sand Production

Past experience of gas production from methane-hydrate-bearing sediments indicates that sand migration is a major factor restricting the production of gas from methane-hydrate reservoirs. One important geotechnical aspect of sand migration is the influence of grain detachment on the existing stresses. This paper focuses on understanding and quantifying the nature of this aspect using different approaches, with a focus on discrete element method (DEM) simulations of sand detachment from hydrate-bearing sand samples. The investigation in the paper reveals that sand migration affects isotropic and deviatoric stresses differently. In addition, the existence of hydrate moderates the magnitude of stress relaxation. Both of these features are currently missing from continuum-based models, and therefore, a new constitutive model for stress relaxation is suggested, incorporating the research findings. Model parameters are suggested based on the DEM simulations. The model is suitable for continuum mechanics-based simulations of gas production from hydrate reservoirs.

Non-Embedded Ultrasonic Detection for Pressure Cores of Natural Methane Hydrate-Bearing Sediments

An apparatus for the analysis of pressure cores containing gas hydrates at in situ pressures was designed, and a series of experiments to determine the compressional wave response of hydrate-bearing sands were performed systematically in the laboratory. Considering the difficulties encountered in performing valid laboratory tests and in recovering intact hydrate bearing sediment samples, the laboratory approach enabled closer study than the marine environment due to sample recovery problems. The apparatus was designed to achieve in situ hydrate formation in bearing sediments and synchronous ultrasonic detection. The P-wave velocity measurements enabled quick and successive ultrasonic analysis of pressure cores. The factors influencing P-wave velocity (Vp), including hydrate saturation and formation methodology, were investigated. By controlling the initial water saturation and gas pressure, we conducted separate experiments for different hydrate saturation values ranging from 2% to 60%. The measured P-wave velocity varied from less than 1700 m/s to more than 3100 m/s in this saturation range. The hydrate saturation can be successfully predicted by a linear fitting of the attenuation (Q−1) to the hydrate saturation. This approach provided a new method for acoustic measurement of the hydrate saturation when the arrival time of the first wave cannot be directly distinguished. Our results demonstrated that the specially designed non-embedded ultrasonic detection apparatus could determine the hydrate saturation and occurrence patterns in pressure cores, which could assist further hydrate resource exploration and detailed core analyses.

A model to predict the elastic properties of gas hydrate-bearing sediments

A simple and accurate model for predicting the elastic properties of gas hydrate-bearing sediment (GHBS) is proposed and validated against experimental data. It is developed on the basis of the homogenization theory for multiphase composite. Unlike the classical homogenization techniques those fix a homogenization scheme (e.g. self-consistent, Mori-Tanaka, DEM, etc.) for a given microstructure, the proposed model considers a flexible homogenization scheme that adapts with the change of the microstructure of the material. The idea is to modify the elastic properties of the reference matrix with respect to the microstructure change of the mixture. Such modification is ensured by adapting the theoretical method to available experimental data. The derived method is proved to be very powerful as it can fit laboratory measured data on gas hydrate-bearing sand formed by different methods including excess gas and excess water methods and log data taken from various sites. Such feature cannot be satisfied by the existent models those consider the hydrate phase as a cement phase or a pore-filling phase. A very good agreement between the proposed method and experimental data offer a simple and accurate ways to predict the saturation of gas hydrate using sonic log data or 3D seismic data.

Effect of temperature cycle on mechanical properties of methane hydrate-bearing sediment

In this study, methane hydrate-bearing sand (MHBS) was created in the laboratory following two methods in order to obtain two types of gas hydrate morphology in sandy sediment. The hydrate morphology in the sediment was assessed by measuring the compressional wave velocity combined with models to predict the wave velocities of the sediment containing gas hydrates. The mechanical properties of the MHBS were investigated by triaxial compression tests. The results obtained by the compressional wave velocity show that after saturating the MHBS sediment (created by the excess gas method) with water, the methane hydrates are partly or completely converted from grain contacts to pore spaces depending on the hydrate saturation (ranging from 0 to 50%). A subsequent temperature cycle completes this conversion process for high hydrate saturation. The results obtained with the triaxial compression tests show higher shear strength, a higher secant Young’s modulus, and a higher dilation angle at higher hydrate saturation. In addition, the effects of hydrate saturation on the mechanical properties of the MHBS obtained by the two procedures (with and without the thermal cycle) are similar at low hydrate saturation. The effect of gas hydrate morphologies can only be detected in the case where the conversion (and/or redistribution) of gas hydrates from grain contacts to pore spaces is not complete (at high hydrate saturation).

System response to gas production from a heterogeneous hydrate accumulation at the UBGH2-6 site of the Ulleung basin in the Korean East Sea

We investigate the feasibility of production from a layered marine gas hydrate reservoir using the properties and conditions corresponding to the UBGH2-6 site of the Ulleung Basin in the Korean East Sea. The work expands and furthers previous investigations in support of a proposed field test. The target system is location in deep water and consists of 13 m of alternating hydrate-bearing sand and soft mud layers and will be produced using a vertical well. We assess production potential during a 14-day field test, examine sensitivity to heterogeneity in permeability, porosity, and initial hydrate saturation, and assess the geomechanical response of the system to short-term production. Producing gas from the system appears to be technically feasible, however, low production rates and relatively large water production rates are expected during the field test. Expected subsidence and reservoir compaction is limited given the current data and the short timeframes of the production test.

Natural Gas Production from a Marine Clayey Hydrate Reservoir Formed in Seawater Using Depressurization at Constant Pressure, Depressurization by Constant Rate Gas Release, Thermal Stimulation, and Their Implications for Real Field Applications

The depressurization approach of methane production from a natural gas hydrate reservoir has been identified as the most energy-efficient production approach. However, some of the field-scale studies involving constant pressure depressurization (CPD) did not yield significant success. To address this, the constant rate gas release (CRD) depressurization approach was used to overcome the drawbacks of the CPD approach. The experimental investigations of these methods with and without thermal stimulation (TS) have not yet been investigated in detail for marine clayey hydrate reservoirs formed in seawater to understand their comparative effectiveness for methane gas recovery. Although common production approaches have been studied by many researchers on hydrate-bearing sand sediments, energy recovery from hydrate-rich clayey sediments has not yet been investigated in detail, which form the major dominant hydrate reservoirs of the hydrate resource pyramid across the globe. This work investigates in detail the ...

Experimental study of pullout capacity of plate anchors shallowly embedded in hydrate bearing sediments

Plate anchors are commonly used in mooring systems of offshore facilities for gas and oil production in deep and ultra-deep water. They may encounter hydrate-bearing sediments widespread in marine continental margins. The pullout capacity of square anchors embedded shallowly in a hydrate-bearing sand is investigated with small-scale model tests using tetrahydrofuran (THF) hydrate as the substitute of gas hydrate. The results show that the pullout capacity of the anchors increases with the increase of the anchor width and the burial depth. The pullout capacity of the anchors can be estimated by a dimensionless breakout factor. This factor increases with the increase of the burial depth ratio (defined as the ratio of the burial depth to the width of a plate anchor), exhibiting a trend similar to that obtained numerically and experimentally from plate anchors shallowly-embedded in undrained clays. Hydrate dissociation brings detrimental effects on the pullout capacity of the anchors. The pullout capacity of the anchors drops to 0.3% of the initial value after the hydrates dissociate. This pronounced reduction highlights the need for further investigation on the effect of oceanic hydrate dissociation on engineering performance of offshore facilities as a result of natural or anthropologic perturbation.

Numerical simulations of sand migration during gas production in hydrate-bearing sands interbedded with thin mud layers at site NGHP-02-16

The geomechanical behavior of gas hydrate-bearing sands during gas production is complex, involving changes in hydrate-dependent mechanical characteristics and also redistribution of stresses carried by the hydrate. When an interbedded mud layer is present, the complex response of the hydrate-bearing sediments is more pronounced because hydrate dissociation tends to occur preferentially in the sands adjacent to the hydrate-free mud layer, which can act as a heat source. This leads to non-uniform sediment deformation and potentially to shearing deformation along the sand/mud layers. In addition, since the hydrate-bearing sand permeability becomes far greater than the mud permeability with hydrate dissociation, a greater hydraulic gradient may exist along the interface between the sand/mud layers. These two behaviors may contribute to sand migration and thus a thorough study on the effect of a thin interbedded mud layer on the extent of sand migration is necessary. This paper presents numerical sand migration modeling of sand/mud interbedded hydrate-bearing sediments found at Site NGHP-02-16, one of 25 sites drilled as a part of National Gas Hydrate Program Expedition 02 (NGHP-02) in the offshore of India, subjected to depressurization. For a comparison and better understanding, simulation of homogeneous hydrate-bearing sediments is also conducted. It is found that greater sand migration occurs when the interbedded mud layer is present because of greater shearing deformation and greater pressure gradients at sand-mud interface. • This is the first work to analyze in detail the effect of presence of interbeds on sand migration. • Sediment deformation is mostly volumetric when homogeneous but becomes more deviatoric when interbedded. • Interbedded mud layers induce non-uniform hydrate dissociation, leading to greater sand migration in magnitude and duration.

Numerical simulations of depressurization-induced gas production from an interbedded turbidite gas hydrate-bearing sedimentary section in the offshore India: Site NGHP-02-16 (Area-B)

Abstract The recent National Gas Hydrate Program Expedition 02 (NGHP-02) identified the existence of gas hydrate-bearing sand reservoirs at a number of sites in the offshore of India including Site NGHP-02-16 in Area-B of the Krishna-Godavari Basin. The architecture of that gas hydrate accumulation is characterized by thin, gas hydrate-bearing, high quality sand layers interbedded with mud layers within a turbidite sedimentary interval. The lowest gas hydrate-bearing layer contacting a thinly-interbedded saline aquifer designates the base of the gas hydrate stability zone (BGHSZ). The proximity of the BGHSZ and the average temperature around 20 °C make the reservoir a favorable target for hydrate destabilization by means of the depressurization method. The results of the reservoir simulations indicate high gas production potential from this marine gas hydrate deposit with manageable concomitant water production using a well completion design that hydraulically isolates layers with water-saturated sands. Using a detailed geological input model, the predicted cumulative gas rates reach ∼3.1 × 104 m3/day (∼1.1 MMscf/day) after 90 days of continuous depressurization and demonstrate sustained production rates ∼3.0 × 104 m3/day (∼1.0 MMscf/day) after 5 years of production. The interbedded nature of this gas hydrate occurrence promotes the development of horizontal dissociation interfaces between gas hydrate-bearing sand and mud layers. As a result, non-uniform gas production along the horizontal interfaces becomes a primary determinant of reservoir performance. Simulation cases have been executed to determine the impact of the uncertainty in in situ reservoir permeability and the manner in which intrinsic permeability dynamically changes during dissociation in response to the imposed effective stress increase. The cases based on low initial effective permeability and high sensitivity of compaction to stress result in the least favorable production predictions.

Permeability measurements of quartz sands with methane hydrate

The permeability of porous media is one of the critical parameters that determines gas recovery from natural gas hydrate reservoirs. We measured the permeability of quartz sands by injecting water at a certain flow rate. In hydrate-bearing sands, the larger the hydrate saturation, the smaller the shape factor in the presence of hydrate. A kr - SH relationship with the hydrate saturation both lower and higher than 10% was obtained. Based on the experimental results, the saturation exponent n varied from 6.0 to 1.0.

Efficient parameter estimation for a methane hydrate model with active subspaces

Methane gas hydrates have increasingly become a topic of interest because of their potential as a future energy resource. There are significant economical and environmental risks associated with extraction from hydrate reservoirs, so a variety of multiphysics models have been developed to analyze prospective risks and benefits. These models generally have a large number of empirical parameters which are not known a priori. Traditional optimization-based parameter estimation frameworks may be ill-posed or computationally prohibitive. Bayesian inference methods have increasingly been found effective for estimating parameters in complex geophysical systems. These methods often are not viable in cases of computationally expensive models and high-dimensional parameter spaces. Recently, methods have been developed to effectively reduce the dimension of Bayesian inverse problems by identifying low-dimensional structures that are most informed by data. Active subspaces is one of the most generally applicable methods of performing this dimension reduction. In this paper, Bayesian inference of the parameters of a state-of-the-art mathematical model for methane hydrates based on experimental data from a triaxial compression test with gas hydrate-bearing sand is performed in an efficient way by utilizing active subspaces. Active subspaces are used to identify low-dimensional structure in the parameter space which is exploited by generating a cheap regression-based surrogate model and implementing a modified Markov chain Monte Carlo algorithm. Posterior densities having means that match the experimental data are approximated in a computationally efficient way.

Assessing the geomechanical stability of interbedded hydrate-bearing sediments under gas production by depressurization at NGHP-02 Site 16

Abstract Establishing the geomechanical stability of marine sediments in the vicinity of a production well is one of the key design considerations in planning offshore gas production from marine hydrate reservoirs. This paper presents an assessment of the sediment stability at India's National Gas Hydrate Program, Expedition 2 (NGHP-02) Site 16 Area B offshore eastern India, for which gas production is to be carried out by depressurization. One important feature of the study is that extensive calibration of constitutive model parameters has been conducted based on laboratory test data from pressured core samples. From analysis perspective, the site is challenging because the hydrate reservoir consists of thin layers of hydrate-bearing sands interbedded with mud. Moreover, depressurization at the depth of a reservoir more than 2750 m below sea surface will lead to a pore pressure drop, and accordingly an effective confining stress increase as high as 25 MPa. In dealing with thin interbedded hydrate-bearing strata, meshing requirements for flow and geomechanical analysis are quite different from those for reservoirs with thicker massive layers, An axisymmetric model and one-way coupling simulations were thus adopted for this study, in which the geomechanical study utilizes pore pressure and hydrate saturation output from the flow study, but the flow study does not takes the porosity changes from the geomechanical analysis. Instead, the reduction of porosity due to sediment deformation in the flow study is based on a pressure-dependent pore compressibility relationship derived from geomechanical modeling. The rationality is validated through back computing the pore compressibility from the geomechanical deformation results. The study shows that large compression in the reservoir will result in movement of the sediments from above and below, as well as laterally in smaller magnitudes; and the sediment is deemed stable during the gas production period.

Effect of vertical wellbore incorporation on energy recovery from aqueous rich hydrate sediments

Natural gas hydrates, an abundant source of natural gas in nature, have the potential of supplying energy for the future. However, due to the complexity of physical processes that occur simultaneously during the dissociation of hydrates within porous media, further understanding on the dynamic behavior of hydrates dissociation (e.g. heat transfer associated with phase transition, permeability changes, and simultaneous gas/water production) is required to elucidate the process of gas recovery from hydrates. In this work, we investigate the effect of a single vertical wellbore incorporation on the production behavior from hydrates formed in sandy sediments (0.1–0.5 mm). Depressurization approach (3.5, 4.0 and 4.5 MPa) was applied to dissociate the hydrate-bearing sand at a constant surrounding temperature of 281.5 K. Through the incorporation of a vertical wellbore, the gas production rate decreased with increasing bottom hole pressure, and continuous production of gas was observed at 4.5 MPa even after 10 h of dissociation. For water production, a majority of water was produced within the first 2 h, beyond which only a small amount of water was recovered. It was also observed that the incorporation of a vertical wellbore in the current design and apparatus impeded hydrate dissociation, which was attributed to a stronger flow resistance for fluids to exit through the vertical wellbore. Cumulatively, the incorporation of vertical wellbore enhanced gas production by up to 8% and reduced water production by up to 42%. In the future, innovative production schemes, such as heated wellbore and hydraulic fracturing can be applied to further optimize gas production from hydrate-bearing sediments.

Rate effect on the stress-strain relations of synthetic carbon dioxide hydrate-bearing sand and dissociation tests by thermal stimulation

Methane hydrates (MHs) have been recognized as an important material for use as a new energy resource. Recently, not only MHs, but also carbon dioxide hydrates (CO2-hydrates), have been attracting attention from the viewpoint of CO2 storage in the form of CO2-hydrates. It is essential, therefore, to investigate the mechanical behaviour of gas hydrate-bearing sediments in order to achieve safe MH extraction and to ensure the long-term stability of CO2-hydrate-storaged submarine sediments. In order to gain further knowledge of CO2-hydrate-bearing sediments, we carried out three kinds of laboratory experiments on synthetic CO2-hydrate-bearing sand specimens: (1) undrained triaxial compression tests with a constant strain rate, (2) undrained triaxial compression tests with a step-changed strain rate, and (3) dissociation tests on CO2-hydrate-bearing sand specimens using the thermal stimulation method.The main findings obtained from these three experiments are as follows: First, it was found that CO2-hydrate-bearing sand shows larger strength and larger positive dilatancy than water-saturated sand without CO2-hydrates under undrained conditions. Second, CO2-hydrate-bearing sand clearly exhibits strain rate dependency with an increase in hydrate saturation. It is interesting to note that hydrate saturation’s dependency on strength does not appear in cases where the strain rate is quite slow. Third, a drastic increase in pore pressure and extensive tensile strain are observed simultaneously during the dissociation of CO2-hydrates. The pore gas pressure of CO2, produced by the hydrate dissociation, can exceed the liquefied boundary of CO2-gas, and the increase in pore gas pressure can be limited by the liquefaction of CO2-gas.

Impact of formation and dissociation conditions on stiffness of a hydrate-bearing sand

Methane gas recovery from gas hydrate–bearing sands requires dissociation of the hydrate. Understanding changes in the stiffness of the sand is essential if future production scenarios are to be modelled realistically. This paper reports the results of resonant column tests conducted to measure changes in shear and flexural Young’s modulus (stiffness) of sand specimens during the formation and dissociation of hydrate within the pore space. Factors such as hydrate saturation, effective stress, and dissociation method (thermal stimulation and depressurization) were evaluated. Results show a nonlinear relationship between stiffness and hydrate volume, with hydrate formation and dissociation giving markedly different changes in stiffness. Stiffness increases more slowly during the initial stages of hydrate formation, compared to later stages, with the eventual stiffness being independent of the effective stress applied at the start of formation. In contrast, the onset of dissociation leads to a rapid reduction in stiffness, with thermal stimulation giving a greater reduction compared to depressurization for similar changes in hydrate volume. These results highlight the impact of hydrate morphology on changes in stiffness during the hydrate formation process or its dissociation. We present and discuss a conceptual model to explain the differences observed.