Methane Hydrate Stability in Seawater

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METHANE HYDRATE STABILITY IN SEAWATER

Gerald R. Dickens

Department of Geological Sciences, University of Michigan

Ann Arbor, USA

Mary S. Quinby-Hunt

Energy and Environment Division, Lawrence Berkeley Laboratory

Berkeley, CA, USA

Modified for the WWW with permission from the authors

from GEOPHYSICAL RESEARCH LETTERS@, VOL. 21, NO. 19, PAGES 2115-2118,

SEPTEMBER 15, 1994

Abstract. Experimental data are presented for methane hydrate stability conditions in seawater (Salinity

in 33.5 ppt). For the pressure range of 2.75-10.0 MPa, at any given pressure, the dissociationtemperature of methane hydrate is depressed by approximately -1.1 C relative to the pure methane-pure

water system. These experimental results are consistent with previously reported thermodynamic

predictions and experimental results obtained with artificial seawater. Collectively these results provide a

minimum constraint concerning depth ranges over which methane hydrate is stable in the oceanic

environment.

Introduction

Clathrate hydrates of gas ("gas hydrates") are crystalline substances composed of cages of water

molecules that host molecules of gas. The pressure and temperature conditions under which such

hydrates are stable depend on gas composition, dissolved ion concentrations, and possibly surroundingmedia [Katz et al., 1959 Sloan, 1990].

There has been considerable recent interest concerning methane hydrates located along continental

margins because they may represent (1) a future energy resource, (2) a modulator of climate change, (3)

a means to indirectly determine heat flow through accretionary wedges, and (4) a hazard in various

marine operations [e.g., Cande et al., 1987Kvenvolden, 1988aKvenvolden, 1988bMacDonald,

1990 Nisbet, 1990Paull and Ussler, 1991Appenzeller, 1991Hyndman et al., 1992Kvenvolden,

1993]. Such literature often revolves around discussion of the pressure and temperature conditions at

which methane hydrate can form and/or dissociate. However, while pressure and temperature conditions

that govern hydrate stability in pure water have been extensively investigated, pressure and temperature

conditions at which methane hydrate might form and/or dissociate in the marine environment have not

been determined experimentally [Sloan, 1990]. In particular, recent workers [e.g.,Englezos and

Bishnoi, 1988Hyndman et al., 1992] explicitly have stated a need for experimental data regarding

methane hydrate stability in seawater. The objective of this investigation is to determine methane hydrate

stability conditions for the pure methane-seawater system.

Experimental Procedure

The experimental apparatus and procedure used in this investigation are somewhat similar to those of

previous studies of hydrate phase equilibria (seeKatz et al. [1959] Sloan [1990]). An autoclave with a

sight glass and exterior bath chamber was mounted on a rocking device and attached to an externalheating/cooling unit. A Budenberg pressure gauge and an Omega K-type thermocouple then were

connected to the interior of the autoclave via a side port. Methane and water were introduced into the


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autoclave through a second side port. Pressures using the apparatus are limited to below 10.5 MPa.

Prior to and after each experiment the thermocouple was removed from the autoclave and calibrated to

an Omega RTD probe. We estimate errors in reported temperature to be within 0.2 C on the basis of

this calibration. Errors in reported pressure are within 0.07 MPa. Methane used in these experiments was

supplied through Airco Gases and has a reported purity in excess of 99.99 %. Seawater was collected

from the Monterey Bay (off the California coast) and passed through a 0.8 1lm Millipore filter. Thesalinity of this filtered seawater was determined using a refractometer at 33.5 + 0.5 GO. Water was

degassed under vacuum within the autoclave prior to experimentation.

Two sets of measurements were determined in this investigation: (1) hydrate stability conditions for the

pure methane-pure (deionized) water system and (2) hydrate stability conditions for the pure methane-

seawater system. The purpose of the former set of measurements was to compare pressure and

temperature results using our apparatus and techniques to those reported in literature (compiled in Sloan

[1990]). Both sets of measurements were restricted to temperatures above the icewater-hydrate-vapor

quadruple point (Q') because this stability region is of principle interest to earth scientists.

All experiments were conducted under isobaric conditions with pressure held constant by an externalmethane source. Hydrate initially was formed by supercooling mixtures of water and methane below

expected pressure-temperature conditions for hydrate formation/ dissociation. The temperature then was

raised slowly until the hydrate dissociated. This process was repeated several times (with agitation) over

successively smaller temperature increments. Determination of methane hydrate stability conditions at

any given pressure was made visually upon final hydrate dissociation.

Results and Discussion

The temperatures at which methane hydrate dissociates at constant pressure for both the pure methane-

pure water and pure methane-seawater systems are reported in Table 1. Measurements for the pure

methane-pure water system agree well with results from other studies (Figure 1), and indicateconsistency between experimental procedures. All experimental data (compiled in Sloan [1990]) for the

pure methane-pure water system between pressures of 2.56 MPa (quadruple point, Ql) and 100 MPa

show that reciprocal absolute dissociation temperature varies nearly linearly with the logarithm of

pressure (n.b., P > 12 MPa are not displayed in Figure 1). This relation arises because the change in

volume of water and hydrate are negligible compared to the change in volume of methane gas upon

hydrate formation and because the methane hydrate enthalpy of formation and methane compressibility

factor are relatively constant over this pressure/temperature range [Korvezee and Pieroen, 1962].

Addition of simple salts (e.g., NaCI, KCI, CaCI2) and mixtures of these salts to water previously has

been shown to decrease the stability of gas hydrates such that for certain pressure ranges, the

dissociation temperature is depressed by a constant amount relative to the pure water system [Katz et al.,

1959 Barduhn et al., 1962Menton et aL, 1981 de Roo et al., 1983Dholabhai et al., 1991]. This

constant offset presumably results because dissolved ion inhibitors do not affect hydrate enthalpy of

formation, but only decrease the entropy of water molecules i.e., the activity of water is the only

parameter concerning hydrate equilibrium conditions that is affected by introduction of dissolved species

[Mentor et al., 1981Englezos and Bishnoi, 1988].

The net combination of all dissolved dons On seawater also depresses the dissociation temperature for

methane hydrate stability by a constant amount between pressures of 2.75-10 MPa (Figure 1). A

constant temperature offset at any given pressure previously has been observed for dissociation of freon

hydrates in seawater [Bardahn et aL, 1962]. For seawater with salinity of 33.5 ppt, the dissociationtemperature of methane hydrate is offset by approximately -1.1 C relative to the pure water system. The


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temperature offset is within analytical precision of that found for methane hydrate dissociation in a

"synthetic" solution (S = 35 ppt) comprised of only the major ions in seawater [Dholabhai et al., 1991]

(Figure 1). For the seawater used in this investigation, the dissociation temperature at any given pressure

between 2.5-10 MPa further can be described by the following empirical equation (r2 > 0 99)

(1)..........1/T = 3.79 x 10-3- 2.83 x 10-4(logP)

where T is temperature (K) and P is pressure (MPa).

Of interest to earth scientists is the "equivalent water depth" at which methane hydrate is stable

[Kvenvolden and Grantz, 1990Hyndman et al., 1992]. Using a hydrostatic pressure gradient of 0.010

MPa/m (T = 0 C S = 33.5 ppt gravity at sea level = 9.8 m/s2), the above equation can be recast in

terms of depth:

(2)..........d = 100 * exp[ (3.79 x 10-3 - 1/T) /2.83 x 10-4]

where d is depth (m). Assuming dissolved ions have negligible effect on entropy, the heat of formation

of methane hydrate, and/or the compressibility factor of methane with respect to changes in pressure, thetemperature offset will be similar at significantly greater depths, although experimental data concerning

salt inhibition on hydrate stability at high pressures (P > 14 MPa) has not been published.

Past authors [e.g., Claypool and Kaplan, 1974] have extrapolated experimental data for highly saline

NaCI solutions in order to estimate the temperature depression of methane hydrate stability produced by

dissolved ions in seawater. However, such an approach renders a temperature offset greater than the

observed -1.1 C. For example, data from de Roo et al. [1983] indicates the temperature of methane

hydrate dissociation in a 11.75 % NaCI solution is decreased at any given pressure by approximately 5.3

C. A linear extrapolation of this data to a 3.35 % "seawater" solution therefore gives a temperature

offset of -1.5 C. The difference between this latter estimate and the observed value arises because the

temperature offset of hydrate stability in seawater should always be less than that in equivalent salinity

NaCI solutions [Bardahn et al., 1962], and because temperature depressions produced by dissolved

salts vary non-linearly with ionic strength [Katz et al., 1959].

Thermodynamic approaches often have been used to successfully predict the stability conditions of gas

hydrates in pure water and simple salt solutions [e.g., van der Waals and Platteeaw, 1959Holder et al.,

1980Menton et al., 1981].Englezos and Bishnoi [1988] have suggested that such a treatment can be

extended to mixed electrolyte solutions and have predicted that at any given pressure a constant

temperature offset of "about" -1.0 C will be observed for hydrate stability in the pure methane-seawater

(S = 35 %O) system relative to the pure water system. Experimental data presented here (and for

"synthetic" seawater [Dholabhai et al., 1991]) therefore are consistent with their theoreticalexpectations. Moreover, if the predictive method presented byEnglezos and Bishnoi [1988] is assumed

to be accurate (seeDholabhai et al. [1991]), the range of temperature offsets between hydrate

dissociation in pure methane-seawater systems and the pure methane-pure water system at any given

pressure will be less than about 0.14 C for the salinity range (33-37%O) of nearly all ocean water. We

thus suggest that data presented here are a good first approximation of methane hydrate stability

conditions in essentially all oxic seawater.

Many bottom-simulating-reflectors (BSRs) along continental margins are believed to mark the base of

the methane hydrate stability zone i.e., the sedimentary depth at which methane hydrate dissociates

[e.g., Shipley et al., 1979 Cande et al., 1987Hyndman et al., 1992]. A remaining issue is why inferred

pressures and temperatures at BSRs appear to fall closer to the pure methane-pure water stability curverather than the pure methane-seawater stability curve [Hyndman et al., 1992]. A number of hypotheses


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could explain this phenomenon. First, errors in estimated temperatures at BSRs typically are greater than

1.1 C [Hyndman et al., 1992]. Discrepancy between inferred pressure-temperature conditions at BSRs

and those that govern the pure methane-seawater system thus simply may be an artifact of poor

temperature resolution (as noted inHyndman et al. [1992]). There are, however, at least three reasons

why pressures and temperatures at BSRs might be expected to deviate towards the pure methane-pure

water/stability curve: (1) advection and/or diffusion of dissolved salts that are excluded upon hydrate

formation may lower the total salinity of a system, (2) incorporation of other gases (e.g., CO2, H2S,ethane, propane) will enhance the stability of "methane" hydrate relative to pure methane systems [Katz

et al., 1959 Claypool and Kaplan, 1974] and, (3) surrounding sediment may increase the stability of

methane hydrate as suggested by experiments of natural gas hydrate dissociation in bentonite [Cha et

al., 1988]. Results presented here thus provide a minimum constraint concerning pressure and

temperature conditions under which methane hydrate is stable in the oceanic environment. The actual

pressure (depth) range over which "methane" hydrate is stable may be significantly greater than

expected from considerations of the pure methaneseawater system and will depend on pore water

migration, trace gas concentrations (particularly H2S seeNoaker and Katz [1954],Robinson and

Hutton [1967]) and the "effectiveness" of surrounding sediment.

Acknowledgments. Funding for G. Dickens was provided by the U.S. Department of Energy underappointment to Graduate Fellowships for Global Change administered by Oak Ridge Institute for

Science and Education (ORISE). This work was partially funded by Laboratory Directed Program

Research and Development of Lawrence Berkeley Laboratory under the U.S. Department of Energy

Contract No. DEAC03-76SF00098 (M. Quinby-Hunt). We thank P. Stevens, M. Ayers, and A. Hunt

for their help in constructing the apparatus used in these experiments, and R. Hyndman, K. Kvenvolden,

and P. Meyers for suggestions that improved the quality of this manuscript. We also are grateful to D.

Carney for supplying seawater used in this study. This letter is report LBL-35599.

References

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G.R. Dickens, Department of Geological Sciences, University of


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Michigan, Ann Arbor, Ml, 48109-1063.

M.S. Quinby-Hunt, Energy and Environment Division, Lawrence

Berkeley Laboratory, Berkeley, CA, 94720. (e-mail: [email protected])

(Received: May 16. 1994: Accepted June 24 1994

Table 1. Methane hydrate dissociation temperature at constant pressure for the pure methane-pure water

and pure methane seawater systems

P (Mpa) T (K)

Pure methane-pure water

3.45 276.1

4.21 277.9

5.17 280.2

6.21 281.97.31 283.4

8.34 284.5

9.58 285.4

Pure methane-seawater

2.76 272.4

3.45 275.0

4.21 276.9

4.90 278.9

5.58 279.6

6.21 280.7

6.90 281.6

7.65 282.4

8.27 283.3

9.03 284.0

10.00 284.8

Copyright 1994 by the American Geophysical Union.

Methane Hydrate Stability in Seawater

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