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KINETIC REPRODUCIBILITY OF METHANE

PRODUCTION FROM METHANE HYDRATES

during the formation or decomposition process. Tracers may also beinjected in order to determine sediment stability during the cycle of

hydrate formation and decomposition.

Phillip Servio & Devinder MahajanResults and Discussion

Energy Sciences and Technology DepartmentBrookhaven National Laboratory

Upton, New York 11973-5000, USA

IntroductionNatural gas is the cleanest of all available fossils fuels and its

use is increasing globally. This increasing demand is raisingconcerns regarding the long-term supply of this precious resource.

Recent estimates have put the amount of stored methane in methanehydrates at several orders of magnitude higher than all presentlyknown sources of methane. In order to pursue this abundant reserve,technological hurdles must be solved which will allow the safe

capture of methane from dispersed hydrate sourcesaround the world. .

The Kinetics of hydrate formation can be described in two steps,

nucleation and growth. Nucleation is characterized as the timerequired for hydrate crystals to achieve a critical size nucleus that cansustain growth. The nucleation time is believed to be a stochastic

phenomenon that cannot be predicted (Parent and Bishnoi, 1996).Hydrate growth is the process where hydrate nuclei have achieved thenecessary critical size and continue to grow and form hydratecrystals. The driving force for hydrate growth is the difference in thefugacity of the dissolved gas and the three-phase equilibrium fugacity

at the experimental temperature. Figure 2 shows a typical plot ofmoles of gas consumed vs. time during a hydrate formationexperiment.

ExperimentalA new experimental apparatus has been developed for the

measurement of hydrate formation and decomposition kinetics, seefigure 1. The main component of the hydrate phase equilibrium

apparatus is a high-pressure reactor fabricated from 316 stainlesssteel. The vessel has rectangular viewing windows constructed from

borosilicate and is immersed in a temperature-controlled bath

consisting of an equal volume mixture of ethylene glycol and water.Experimental gas and water are brought into contact, in the reactor, ina countercurrent fashion. Water enters from the top, while the gasenters from the bottom to help agitate the liquid and clay sediments

and achieve a uniform system. The experimental temperature of boththe gas and liquid phases are measured with the aid of type K

thermocouples. The pressure is measured by differential pressuretransducers. The experimental methane gas flow rate into the systemis measured and regulated by a mass flow controller (Brooks) with arange of 0-2000 ml per minute. A back pressure regulator is used to

ensure constant pressure during both a formation and decompositionexperiment. A dry test meter is used on the back pressure regulator

side to measure the amount of gas exiting the high pressure vessel.

Figure 2. Moles of gas consumed vs. time for a methane hydrateformation experiment (adapted from Englezos et al., 1990).

In figure 2, the onset of hydrate formation occurs at thenucleation point, B, which is known as the turbidity time. Theturbidity time denotes the first appearance of stable hydrate crystalsand is the beginning of the hydrate growth phase. n* is the number

of moles of methane gas corresponding to a vapor-liquid waterequilibrium. This equilibrium is hypothetical because hydrates format these conditions. This would be the solubility of methane in waterin the absence of hydrates. neqis the number of moles of methane

dissolved in liquid water corresponding to a vapor-liquid-hydrateequilibrium pressure at the system temperature (Servio and Englezos,

2002). At the system temperature, the pressure is known as theincipient equilibrium hydrate formation pressure, Peq. The value of

neq is less then n* in the hydrate formation region because itcorresponds to a lower pressure, Peq, then the experimental pressure.Experimental work by Servio and Englezos (2002) validated theseresults. It was hypothesized that the difference in moles of gas

between point B (nB) and neq accounts for the amount of gasconsumed in the formation of hydrate nuclei as postulated byEnglezos et al. (1987).Figure 1. Newly Commissioned Unit for Hydrate Kinetic Study at

BNL.

Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem.2003, 48(2),881

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The hydrate decomposition process has not been as extensivelystudied as that of hydrate growth; fortunately the two processes have

many similarities. A decomposition process can be viewed asdecomposing particles surrounded by a desorption reaction layer,followed by a cloud of evolved gas (Kim et al, 1987). Thedecomposition of solid hydrates is an endothermic process that gives

gas and liquid water as its products. Hydrate decomposition isthought to consist of the following two steps:

1) The destruction of the clathrate host lattice at the surface ofthe particle followed by the desorption of the guestmolecule from the surface into the reaction layer.

2) The diffusion of gas from the reaction layer into the bulkliquid

The driving force is represented by the difference in fugacity ofthe gas at the three-phase equilibrium conditions (equilibriumtemperature and pressure) and the dissolved gas fugacity at the

experimental temperature and pressure. The latter fugacity isassumed to be equivalent to the fugacity of the gas at the solid

particle surface.

Figure 4. Cumulative gas produced from hydrates at various

dissociation pressure drops (adapted from Goel et al., 2001).

The results obtained from this work will give valuable insightinto the current state of existing models for the formation anddissociation of hydrates in porous media. These results will also

provide us with information concerning the stability of sea floorhydrates and sediments under dissociation conditions.

Our current study relates to the methane hydrate formation anddecomposition cycle in a recently commissioned unit. First, the

baseline kinetic study is being carried out with CH4/H2O mixtureunder the temperature/pressure conditions that mimic the methanehydrate stability zone in the subsurface environment to establish datareproducibility. The baseline study will be followed by a kineticstudy in the Euxinic seawater/CH4 mixture to mimic seafloor

environment. The known kinetic models will be reviewed withrespect to their relevance to the measured data.

AcknowledgmentThis work was supported by Brookhaven National Laboratory

under the Laboratory Directed Research and Development (LDRD)Program. A start-up grant (to DM) from the State University of NewYork at Stony Brook is gratefully acknowledged.Typical models that will be evaluated for the production of

methane gas from methane hydrate include those of Yousif et al.

(1991) and Goel et al. (2001). Figure 3 shows the molar ratio ofmethane gas trapped at various dissociation pressures (Goel et al.,2001). Figure 4 gives the cumulative gas produced from hydrates at

References

1. Englezos, P., Gas Hydrate Equilibria, M.Sc Thesis, U. Calgary,1990.

2. Englezos, P., Kalogerakis, N., Dholabhai, P. D., Bishnoi, P. R.,Kinetics of Formation of Methane and Ethane Gas Hydrates,

Chem. Eng. Sci., 42 (11), 2647-2658, 1987.

3. Goel, N., Wiggins, M., Shah, S., Anayltic modeling of gasrecovery from in situ hydrate dissociation,Journal of Petroleum

Sci. & Eng., 29,115-127, 2001.

4. Kim, H. C.,Bishnoi, P. R., Heidemann, R. A., Rizvi, S. S. H.Kinetics of methane Hydrate decomposition, Chem. Eng. Sci, 42

(7), 1645-1653, 1987.

5. Parent, J.S. and P.R. Bishnoi, Investigations into the nucleationbehavior of natural gas hydrates, Chemical Engineering

Communications (CEC), 144, 51-64, 1996.6. Servio, P., Englezos, P.,Measurement of the amount of dissolved

methane in water in equilibrium with its Hydrate, J. Chem. Eng.

Data,47(1), 87-90, 2002.Figure 3. Molar ratio of methane trapped in hydrates at variousdissociation pressure drops (adapted from Goel et al., 2001).

7. Yousif, M.H., Abass, H.H., Selim, M.S., Sloan, E.D.,Experimental and theoretical investigation of methane-gas-hydrate dissociation in porous media, SPE Reservoir Eng., 69-76,

February 1991.

various dissociation pressure drops. It is important to note thatfigures 3 and 4 were produced under the assumption that the pressuredrop is constant through the dissociation of the hydrates. In a

reservoir the pressure drop is variable and is taken into considerationin the model of Goel et al. (2001).

Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem.2003, 48(2),882