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
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 dissociation temperature 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.
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 surrounding media [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., 1987; Kvenvolden, 1988a; Kvenvolden, 1988b; MacDonald, 1990; Nisbet, 1990; Paull and Ussler, 1991; Appenzeller, 1991; Hyndman et al., 1992; Kvenvolden, 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, 1988; Hyndman 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.
The experimental apparatus and procedure used in this investigation are somewhat similar to those of previous studies of hydrate phase equilibria (see Katz et al. ; Sloan ). An autoclave with a sight glass and exterior bath chamber was mounted on a rocking device and attached to an external heating/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 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. The salinity 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 ). 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 external methane 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 indicate consistency between experimental procedures. All experimental data (compiled in Sloan ) 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., 1962; Menton et aL, 1981; de Roo et al., 1983; Dholabhai 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., 1981; Englezos 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 dissociation temperature of methane hydrate is offset by approximately -1.1 °C relative to the pure water system. The 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, 1990; Hyndman 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, the temperature 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.  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, 1959; Holder et al., 1980; Menton et al., 1981]. Englezos and Bishnoi  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 theoretical expectations. Moreover, if the predictive method presented by Englezos and Bishnoi  is assumed to be accurate (see Dholabhai et al. ), 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., 1987; Hyndman 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 curve rather than the pure methane-seawater stability curve [Hyndman et al., 1992]. A number of hypotheses 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 in Hyndman et al. ). 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; see Noaker and Katz , Robinson and Hutton ) and the "effectiveness" of surrounding sediment.
Acknowledgments. Funding for G. Dickens was provided by the U.S. Department of Energy under appointment 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.
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G.R. Dickens, Department of Geological Sciences, University of
Michigan, Ann Arbor, Ml, 48109-1063.
M.S. Quinby-Hunt, Energy and Environment Division, Lawrence
Berkeley Laboratory, Berkeley, CA, 94720. (e-mail: email@example.com)
(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|