Natural Gas Hydrate Dissociation Research Articles

Multi-factor sensitivity analysis on the stability of submarine hydrate-bearing slope

ABSTRACT There are many factors affecting the instability of the submarine hydrate-bearing slope (SHBS), and the interaction with hydrate is very complicated. In this paper, the mechanical mechanism of the static liquefaction and instability of submarine slope caused by the dissociation of natural gas hydrate (NGH) resulting in the rapid increase of pore pressure of gas hydrate-bearing sediments (GHBS) and the decrease of effective stress are analyzed based on the time series and type of SHBS. Then, taking the typical submarine slope in the northern South China Sea as an example, four important factors affecting the stability of SHBS are selected, such as the degree of hydrate dissociation, the depth of hydrate burial, the thickness of hydrate, and the depth of seawater. According to the principle of orthogonal method, 25 orthogonal test schemes with 4 factors and 5 levels are designed and the safety factors of submarine slope stability of each scheme are calculated by using the strength reduction finite element method. By means of the orthogonal design range analysis and the variance analysis, sensitivity of influential factors on stability of SHBS are obtained. The results show that the degree of hydrate dissociation is the most sensitive, followed by hydrate burial depth, the thickness of hydrate and the depth of seawater. Finally, the concept of gas hydrate critical burial depth is put forward according to the influence law of gas hydrate burial depth, and the numerical simulation for specific submarine slope is carried out, which indicates the existence of critical burial depth.

A method to recover natural gas hydrates with geothermal energy conveyed by CO2

A geothermal-assisted CO2 replacement method (GACR) was proposed, in which ambient-temperature CO2 is injected from the well head of a heat exchange well to the wellbore in geothermal reservoir for heating, and the heated CO2 then flows upward into the hydrate bearing layer (HBL) to accelerate the natural gas hydrate (NGH) dissociation. The GACR method, which is low-carbon and environment-friendly, requires no surface heating, recovers NGHs by means of combined thermal stimulation and CO2 replacement, and meanwhile achieves CO2 storage. Then a numerical simulation model was developed to investigate the heat exchange performance of the heat exchange well, the development performance of HBL, the CO2 storage performance and factors affecting such a development process. Calculations indicate that the temperature of the returned CO2 heated by the geothermal reservoir at the entry of HBL can be up to 68.9 °C, far beyond the temperature of HBL. Hence, GACR can fulfill the NGH recovery combining thermal stimulation and CO2 replacement. Compared with the depressurization method and the case with no geothermal reservoir, the cumulative CH4 production after 20 years of development can increase by 305% and 51.9% respectively, with a CO2 storage volume of 2.11 × 107 m3 and a storage factor of 14.5%.

In situ Raman investigation on mixed CH4-C3H8 hydrate dissociation in the presence of polyvinylpyrrolidone

The dissociation of hydrates containing kinetic inhibitors is important for the treatment of gas hydrate blockages in pipelines. In this work, the dissociation of natural gas hydrates in the presence of polyvinylpyrrolidone (PVP) was investigated by Raman spectroscopy and cryo-scanning electron microscope (SEM). The natural gas hydrates were formed from a synthetic natural gas mixture composed of CH4 (95.0 vol%) and C3H8 (5.0 vol%). The results showed that the crystallographic structure of the formed hydrate was structure II (sII) with CH4 encaged in both small and large cages and C3H8 in large cages. Raman spectra showed that PVP could not be captured by the sII hydrate cage and had no influence on the structural type of the hydrate. However, PVP inhibited the occupancy of CH4 molecules in the large cages. The dissociation of hydrates in the presence of PVP was slower than without PVP. No other transient structure was observed during the dissociation and gas hydrate cages were suggested to collapse as an entity, without the interference of PVP. The microstructure of the gas hydrates appeared to change from close-to-spherical to scale-like when PVP was added.

Numerical analysis of depressurization production of natural gas hydrate from different lithology oceanic reservoirs with isotropic and anisotropic permeability

Natural gas hydrate (NGH) is a promising alternative energy and mainly distributes in deep oceanic sediments. The intrinsic permeability of hydrate-bearing layers can potentially influence heat and pressure transfer during depressurization-induced NGH dissociation. By taking siltstone, sand and clay reservoirs in Shenhu area of South China Sea as examples, this study numerically investigates the effects of the magnitude and anisotropy of reservoir permeability on NGH production process and main physical field evolution. Results reveal that permeability anisotropy could impede advective interaction of fluids in vertical direction, significantly changing temperature and pressure evolution during NGH dissociation. Consequently, NGH dissociation slows down, delaying the coming of peak gas production rate. Some degrees of permeability anisotropy even lead to much earlier termination of NGH dissociation. In this case, the more permeable sand reservoir could possibly have a lower gas production potential than the less permeable siltstone and clay reservoirs. The permeable overburden limits the fluid pressure reduction and weakens the efforts of depressurization production. The re-production by further lowering well pressure only lasts for a short time. Comparatively, the depressurization of constant production mass rate is more effective but only provides a small contribution for the total gas yield.

Submarine vortices derived from natural gas hydrate conversion: a mechanism for ocean mixing

We propose that the source water for some abyssal undular vortices cored by cool, low-salinity water identified at depths in excess of 2,500 m in the deepwater region of the Eastern Mediterranean Basin may be related to conversion of natural gas hydrate (NGH) in abyssal marine sediments. The conditions for extensive formation of NGH in the gas hydrate stability zones (GHSZ) of the upper seafloor sediments existed in this region during previous glacial episodes when colder water supported a thicker GHSZ. Seafloor warming during the most recent interglacial caused thinning of the GHSZ at its base and has driven endothermic NGH dissociation that would have released large volumes of low-salinity water and gas that would tend to pond below the base GHSZ. Periodically, trapped low-salinity water and gas would be released into the sea through the overlying sediments. Buoyant low-salinity water masses, supersaturated with gas and locally containing free gas would ascend and introduce a dynamic element into an otherwise generally static environment. As a result of the interaction of the rise of this buoyant plume and Coriolis acceleration the ascending mass would begin to rotate and form a vortex tube in midwater. NGH conversion within the seafloor introduces large coherent masses of low-salinity, lower-temperature water containing a buoyant free gas fraction from near-surface reservoirs into the abyssal depths even where there may only be a weak natural gas petroleum system.

Numerical study on the stratum's responses due to natural gas hydrate dissociation

ABSTRACTNatural gas hydrate (NGH) dissociation can cause many kinds of potential hazards. Dimensional analysis is used first to obtain the controlling factors for NGH dissociation-induced stratum's responses. And then the effects of the controlling factors on the deformation and stress distribution have then been numerically investigated. A simplified critical condition for judging the instability of gentle inclined stratum after NGH dissociation was proposed. It is shown that the maximum settlement is located near the upper part of the slide body while the soil uplifts near the lower part. The fast change of the stress and deformation is located at the interface between hydrate-bearing stratum (HBS) and over layer. The slope angle has the largest effects on the responses of the stratum. The effects of modulus and water depth on the stratum's instability are small. There is a critical extension length of dissociation zone over which landslide will occur which is determined by the strength parameters, slope angle, friction coefficient and height of the over layer.

Rapid formation of dry natural gas hydrate with high capacity and low decomposition rate using a new effective promoter

In this research, the effects of sulfonated Lignin (SL) on formation rate, capacity and stability of natural gas hydrate have been investigated and the results compared with those of hydrate formation in pure water and Sodium dodecyl sulfate (SDS) aqueous solution. Experimental results verified that the reactor pressure is reduced by 63.8% after 15h when hydrate is formed in 500ppm SL aqueous solution, while when using the same amount of SDS it is reduced no more than 57.8% after the same duration of time. It was also noticed that the formed hydrate when using SL had a lower decomposition rate than that of SDS aqueous solution. The reactor pressure increased 0.66MPa during dissociation of SL natural gas hydrate after 70h while in the case of SDS solution, at similar conditions this amount was 0.94MPa. On the other hand, SL solution does not spume and so the formed hydrate is quite dense, brittle and dry which can be easily converted to powder while the hydrate formed in the SDS solution is foamy.

Dissociation of natural and artificial gas hydrate

Dissociation kinetics of natural and artificial methane hydrate were studied experimentally and theoretically. The artificial methane hydrate was produced in the reactor, and the natural clathrate was extracted from the bottom of Lake Baikal. The behavior of the natural gas hydrate decay is markedly different from the artificial one under non-isothermal conditions. For natural methane hydrates there is a much longer self-conservation area and a lower rate of dissociation caused by different initial defectiveness of samples. The proposed model takes into account the area of self-conservation and structural characteristics of the granules that allows qualitatively and quantitatively describing the dissociation. The non-isothermal dissociation was numerically simulated, taking into account heat and mass transfer, kinetics of phase transformation, gas filtering inside the granules and porous characteristics (size and density of pores). Different requirements are applied to technologies for efficient storage and combustion of gas hydrates. A more stable region of self-conservation of natural gas hydrate increases the efficiency of its storage and transportation. In this case, a durable crust of ice with a low concentration of pores is achieved for higher temperatures than for the artificially synthesized hydrate. In the course of dissociation of gas hydrate at negative temperatures of annealing, the density of the pores distribution in the formed ice crust decreases by eight orders that significantly changes the rate of decay during the dissociation. The simulation of dissociation of natural and artificial methane hydrates was implemented with a maximum density of pores and well agreed with the experimental data. Thus, it has been experimentally and theoretically shown that the density and size of pores dramatically affect the kinetics of dissociation. During storage, transportation and combustion of combustible gases solid hydrates it is necessary to determine the heat transfer coefficient and the temperature difference ∆T both for the outer medium (external boundary conditions) and the internal one (a layer of powder or pressed pellets). The dissociation rate of gas hydrates at negative temperatures T<0°С is much more complex than for T>0°С and currently there are no reliable calculation methods. The use of filtration model and structural parameters can effectively predict the behavior of gas hydrates at negative temperatures.

Natural gas hydrate dissociation by hot brine injection

ABSTRACTHot brine injection is one of the most effective dissociation methods of natural gas hydrate, and the movement of thermal front is a key parameter to evaluate the production performance by hot brine injection. Based on the principle of energy conservation, a mathematical model for thermal front movement of gas hydrate dissociation by hot brine injection is developed, which is derived through Laplace transform. In addition, an experimental study on the movement of thermal front is conducted with a self-designed 1D natural gas hydrate dissociation apparatus. The thermal front movement of the experiment is compared with that calculated from the mathematical model established in this paper as well as other two models proposed by Selim and Sloan (1990) and Tang et al. (2006), respectively. It turns out that what fits better with the experimental data is the energy conservation model, which includes the effect of heat used to warm the dissociated reservoir, heat absorption by hydrate dissociation and heat loss to the cap and base sediments. Therefore, this energy conservation model is an effective method to analyze the thermal front of gas hydrate dissociation by hot brine injection.

Productivity and feasibility analysis of gas production from subsea sediment bearing with natural gas hydrate

A dynamics-model of natural gas hydrate (NGH) dissociation was established based on experimental results, which can be used for numerical simulation studies of gas production from subsea sediment bearing with hydrate. Three phases (aqueous, hydrate-solid and gaseous) and three components (water, methane hydrate and methane) were considered in the reservoir simulation model of gas hydrate. Comparisons between the numerical simulation and the experimental results were made to verify the simulation model. The reservoir model was then used to analyse the productivity of typical wells for gas production from hydrate via depressurisation, which was compared with that of conventional gas, tight gas and coalbed methane reservoirs to reveal the main features and feasibility of subsea NGH development. The simulation results indicated that, at similar reservoir scales, NGH reservoirs with hydrate saturation greater than 0.3 can have higher gas productivity than typical tight gas and coalbed methane reservoirs. That means gas production from NGH in subsea sediment can be feasible using the current technologies if reservoirs or formations with high hydrate saturation can be found, while its economics may be of further consideration. [Received: November 26, 2014; Accepted: April 15, 2015]

Mathematical models of the heat-water dissociation of natural gas hydrates considering a moving Stefan boundary

This paper presents mathematical models for radial, quasi-steady state heat transfer in a semi-infinite hydrate reservoir with a moving boundary that is related to the dissociation of natural gas hydrates. The exact solutions of the temperature in the dissociation zone and hydrate zone, using the Paterson exponential integral function, are obtained, and the dissociation frontal brim location of the hydrates is determined by combining the Deaton method with the Clausius–Claperyron equation. A sample calculation shows that the reservoir temperature falls sharply to the dissociation temperature and then drops gradually with increasing distance to the reservoir temperature. With respect to time, the temperature increases slowly to the dissociation temperature, after which, the dissociation temperature falls sharply to the temperature close to that of the injected hot-water. By increasing the temperature of injected hot-water, more hydrates participate in dissociation; with an increase in time, the radius quickly increases, but the radius of hydrate dissociation increases slowly.

Experimental investigation on dissociation driving force of methane hydrate in porous media

As one kind of clean and widely distributed unconventional energy resource, natural gas hydrate (NGH) is attracting increasing attention these years. In this study, the methods of depressurization and thermal stimulation are adopted to dissociate the NGH experimentally, when the initial temperature is 1.0°C, the initial pressure is 3.1MPa, the porosity is 0.35 and initial NGH saturation is 32%. The results reveal that during the depressurization process, there exists a pressure difference threshold (ΔP=0.19MPa), and similarly a temperature difference threshold (ΔT=1.4°C) exists during the process of thermal stimulation. NGH will not dissociate until pressure difference or temperature difference outweighs the corresponding threshold value. In addition, the influencing factors of driving force on the gas production performance have been studied. When thermal stimulation is adopted, the driving force of the temperature difference will grow with the increasing hot-brine injection rate and the brine temperature, in which case the gas production rate will drive up. When depressurization is used, the pressure difference driving force will go up with the increasing depressurizing range and depressurizing rate. Driving force is a major factor during NGH dissociation by depressurization and thermal stimulation.

Analysis of heat transfer effects on gas production from methane hydrate by thermal stimulation

Natural gas hydrate dissociation requires the continuous absorption of heat energy from the periphery, which influences both the gas generation rate and cumulative gas production. To predict the potential impact of heat transfer on the hydrate dissociation process, we previously developed and verified a two-dimensional axisymmetric model. In this study, we build on our previous work to investigate the influence of heat transfer on methane gas production by thermal stimulation. The results show that during hydrate decomposition, increasing the specific heat capacity of porous media containing hydrate inhibits the gas generation rate. However, the effects of the initial water content on the gas generation rate and cumulative gas production are weak. The influence of water and methane convection heat on hydrate dissociation is also weak. Increasing thermal conductivity can initially negatively influence hydrate dissociation, but later promote the process.

A unique Fe-rich carbonate chimney associated with cold seeps in the Northern Okinawa Trough, East China Sea

The East China Sea is an important marginal sea of the Western Pacific Ocean, from which natural gas hydrate sample has not been acquired so far. Recently, copious carbonate chimneys have been discovered in turbidite deposits in the olistostrome zone located on the west slope of the northern section of Okinawa Trough. Here, the petrology, geochemistry and chronology of an iron-rich carbonate chimney were characterized, confirming a close relationship between its formation and the dissociation of natural gas hydrate beneath the chimney in OT. A distinctive relationship has been observed between goethite and total carbonate contents along with a negative correlation between Fe and Ca contents. Conversely, abundant Fe accumulated on carbonate substrate by mineralized microorganisms. The δ13C values of the chimney wall were from −27.56 to −43.66‰ (average: −37.18‰, V-PDB), implying anaerobic oxidation of methane (AOM) as a predominant controlling factor on carbonate precipitation. As no pyrite and organic residues were identified in the iron-rich chimney, it was assumed that AOM was coupled to the iron reduction reaction at least to some extent during the chimney growth owing to the local deficiency of sulfate supply. The δ56Fe values of bulk chimney wall (ranging from −0.316‰ to −0.023‰, average −0.134‰) suggest mass and isotope exchanges between the chimney and ambient environment during its growth history, whereas the enrichment of δ18O of the carbonate implies these carbonate sourcing from hydrate dissociation underlying our sampling site. This assumption has been supported by a distinct bottom simulation reflector (BSR) and a well-developed fault system beneath the sampling site. This is the first report of cold seepage inside the OT and the identified iron-dependent AOM has shed a new light to the Carbon cycle related to the marine methane oxidation, particularly before the Great Oxidation Event ~2.45Ga ago.

Analysis of heat transfer effects on gas production from methane hydrate by depressurization

The dissociation of natural gas hydrates is an endothermic process. This dissociation process requires the continuous absorption of heat energy from the sediment and pore fluid. This heat transfer governs the dissociation rate and affects gas production. In this study, a two-dimensional axisymmetric simulator is developed to model the effects of heat transfer on the process of hydrate dissociation in porous media by depressurization. A series of simulations are performed to study sensible heat effects on the sediment, heat flow transfer in the cap- and base-sediment, and the effects of conductive and convective heat transfer on gas production from methane hydrate depressurization. The results show that the porous media material and the water content are two significant factors that affect the sensible heat in gas hydrate dissociation: the porous media material can increase methane hydrate dissociation, but water inhibits the dissociation process by affecting the pressure on the inner sediment. A high thermal conductivity of the sediment can initially positively affect hydrate dissociation but may later partially inhibit the process. Convective heat transfer in the gas flow increases hydrate dissociation markedly compared to that in water flow.

Impact Factors of Natural Gas Hydrate Dissociation by Depressurization: A Review

Several geological factors can have a potential effect on dissociation of natural gas hydrate (NGH). It is important to understand their roles in safe exploitation of NGH deposits. This paper accordingly reviews experimental and numerical-modeling researches on impact factors of NGHs dissociation by depressurization. NGHs dissociation usually increases with increasing temperature, permeability, water saturation, thermal conductivity of sediments and depressurization rate of production well, whereas it is reverse for pressure, hydrate saturation, production wells pressure, gas saturation and particle size. Comparably, the effect of porosity and the heat conduction of production well can be reasonably ignored at different spatial scales. The heterogeneity of NGH deposits, chemical components of pore fluids, relative permeability and capillary pressure can also play a role in NGHs dissociation. However, their influencing mechanisms still need further studies in future.

Hot-brine Injection for the Dissociation of Natural Gas Hydrates

A new one-dimensional experimental system for natural gas hydrate (NGH) exploitation is designed, which is used to study the formation and dissociation processes of NGH. NGH is formed in the sand-packing tube, and then hot-brine is injected into the tube to study the thermal dissociation characteristics. The injection parameters that influence gas production rate and energy efficiency are analyzed. The results show that the higher the injection temperature and injection rate are, the higher the gas production rate is. In addition, the most sensitive parameter, which influences the energy efficiency of thermal stimulation is the hot-brine temperature, followed by the hot-brine injection rate and injection time. This study provides an experimental basis for further study on NGH exploitation in the future.

Methane combustion in hydrate systems: Water-methane and water-methane-isopropanol

Kinetics of dissociation of synthetic and natural methane gas hydrates, and also double isopropanol-methane hydrate is investigated. Thermal fields of the sample surfaces are measured by means of thermal imaging in combustion of released methane with clathrate dissociation. The dissociation rates of natural hydrate and double hydrate with isopropanol are many times lower than those of synthetic methane hydrate. Methane combustion is accompanied by formation of a thin water film on the powder surface, which has a strong effect on the heat and mass transfer mechanisms. The experiments demonstrated partial self-preservation for methane hydrate and the absence of self-preservation for double isopropanol-methane hydrate. The experimentally observed dissociation rate of double isopropanol-methane hydrate is considerably lower than that of methane hydrate.

Hydrate Formation/Dissociation in (Natural Gas + Water + Diesel Oil) Emulsion Systems

Hydrate formation/dissociation of natural gas in (diesel oil + water) emulsion systems containing 3 wt% anti-agglomerant were performed for five water cuts: 5, 10, 15, 20, and 25 vol%. The natural gas solubilities in the emulsion systems were also examined. The experimental results showed that the solubility of natural gas in emulsion systems increases almost linearly with the increase of pressure, and decreases with the increase of water cut. There exists an initial slow hydrate formation stage for systems with lower water cut, while rapid hydrate formation takes place and the process of the gas-liquid dissolution equilibrium at higher water cut does not appear in the pressure curve. The gas consumption amount due to hydrate formation at high water cut is significantly higher than that at low water cut. Fractional distillation for natural gas components also exists during the hydrate formation process. The experiments on hydrate dissociation showed that the dissociation rate and the amount of dissociated gas increase with the increase of water cut. The variations of temperature in the process of natural gas hydrate formation and dissociation in emulsion systems were also examined.

Physical Properties of Gas Hydrates: A Review

Methane gas hydrates in sediments have been studied by several investigators as a possible future energy resource. Recent hydrate reserves have been estimated at approximately1016 m3of methane gas worldwide at standard temperature and pressure conditions. In situ dissociation of natural gas hydrate is necessary in order to commercially exploit the resource from the natural-gas-hydrate-bearing sediment. The presence of gas hydrates in sediments dramatically alters some of the normal physical properties of the sediment. These changes can be detected by field measurements and by down-hole logs. An understanding of the physical properties of hydrate-bearing sediments is necessary for interpretation of geophysical data collected in field settings, borehole, and slope stability analyses; reservoir simulation; and production models. This work reviews information available in literature related to the physical properties of sediments containing gas hydrates. A brief review of the physical properties of bulk gas hydrates is included. Detection methods, morphology, and relevant physical properties of gas-hydrate-bearing sediments are also discussed.