Gas hydrate is a crystalline compound consisting of guest gas molecules encaged in water molecules. In nature, gas hydrates occur in the sediments of permafrost regions and continental margins, and methane is the most common guest molecule. The stability of gas hydrate depends on temperature, pressure, salinity, and other factors (e.g., soil characteristics). The difficulty of maintaining recovered natural samples at in situ temperatures (usually 0 to ∼25°C) and fluid pressures (typically 5 to 25 MPa) has been one of the key challenges in studying natural gas hydrate-bearing sediments. At the same time, the analysis of measurements obtained in hydrate-bearing provinces during borehole logging or exploration geophysical surveys (e.g., seismic prospecting) has been limited by the lack of high-quality calibrations of physical properties on synthetic laboratory samples with well-characterized lithology, hydrate saturation, and confining pressures.
Numerous laboratory studies have used sediments containing synthetic gas hydrates to investigate mechanical, thermal, and electromagnetic properties of both pure crystals and hydrate-bearing sediments [Cameron et al., 1990; Waite et al., 2002; Ebinuma et al., 2005; Priest et al., 2005; Santamarina et al., 2005; Winters et al., 2005; Yun et al., 2005, 2007]. Unfortunately, the controlled synthesis of methane hydrate in sediments is challenging owing to methane's low solubility in water and the prolonged time required to form hydrate from aqueous phase methane, which is the way that gas hydrate probably forms in deep sediments that are within (not at the boundaries of) the gas hydrate stability zone [e.g., Buffett and Zatsepina, 2000; Spangenberg et al., 2005]. Forming methane hydrate in this way requires a source of methane within the sediment (e.g., microbes) and/or transport of methane via diffusional or advective processes.
Several alternative methods have been proposed to produce methane hydrate-bearing sediments, including flushing methane gas through partially saturated sediments, exploiting pre-existing ice cages, and using surfactants to increase the availability of the methane gas in water [Handa and Stupin, 1992; Stern et al., 1996; Zhong and Rogers, 2000; Waite et al., 2002, 2004; Lin et al., 2004]. Each of these procedures produces different pore scale growth patterns [Zhong and Rogers, 2000; Ebinuma et al., 2005; Spangenberg and Kulenkampff, 2005] and differences in the macroscale behavior of sediments containing these hydrates [e.g., Yun et al., 2007].
Tetrahydrofuran (C4H8O, hereafter THF) has long been used as a substitute for methane in laboratory studies [e.g., Leaist et al., 1982; Handa, 1984; Rueff and Sloan, 1985]. The main advantage of THF relative to methane is THF's complete miscibility in water, which enables relatively rapid, homogeneous synthesis of THF hydrate in sediments and close control of the hydrate volume fraction. THF hydrate, unlike methane hydrate, also has the advantage of being stable at atmospheric pressure and easily achieved temperatures, making possible the use of standard soil cells and loading frames for laboratory experiments [Bondarev et al., 1996; Santamarina et al., 2005; Yun et al., 2005, 2007]. Finally, because THF hydrate does not dissociate to a gaseous phase, poroelastic complications associated with the formation of highly mobile, compressible gas can be avoided.
In recent years, the use of THF as a proxy for methane in studies of hydrate-bearing sediment properties has also been a source of controversy [Doyle et al., 2004], particularly owing to the polar nature of the THF molecule compared to the nonpolar nature of the methane molecule. This paper examines the differences and similarities between the interactions of methane and THF with water, salt, and mineral grains, the major components of natural sedimentary systems in which gas hydrate occurs.
2. Comparison of Known Properties
The properties of THF and methane molecules, their hydrates, and water ice are compiled in Table 1. We first compare the properties of methane and THF, the hydrate guest molecules (Figure 1). Methane, an alkane, consists of a carbon atom with four attached hydrogens that form a tetrahedral structure. As a cyclic ether, THF has ether oxygen as part of the ring. The THF molecule is ∼1.5 times larger than the methane molecule and is completely miscible in water. Methane is a nonpolar molecule (dipole moment of zero), but the dipole moment of THF is as high as that of water. Yet the permittivity of THF is very low relative to water and approaches a value comparable to that for nonpolar fluids.
|Properties of Guest Molecule|
|Chemical structure||see Figure 1||see Figure 1||see Figure 1|
|Molecular size, Å||4.36 (1)a||6.3 (2)||∼1.8 (1)|
|Dipole moment, D||0 (3)||1.63 (4)||1.85 (3)|
|Molecular polarizability, Å3||2.6 (5)||7.9 (6)||1.5 (5)|
|Permittivity||1.7 (7)||7.5 (4)||80 (3)|
|Density, kg m−3, at 293.5 K||N/A||888 (4)||1000 (3)|
|Viscosity, cP, at 298.5 K||N/A||0.46 (4)||0.89 (3)|
|Surface tension, N m−1, at 293.5 K||0.00676 at 140K (8)||0.028 (4)||0.0728 (3)|
|Solubility in water at 293.5 K||0.04 × 10−3 [mole fraction] (9)||miscible (7)||N/A|
|Hydrate cavity diameter, Å||7.9, 8.66 (1)||7.82, 9.46 (1)||n/a|
|Ideal hydrate stoichiometric ratio||CH4.6H2O||C4H8O.17H2O||n/a|
|Slope of phase transformation boundary at 10 MPa, K MPa−1||+0.96 (1)||−0.08 (10)||−0.01 (11)|
|Thermal Properties of the Frozen State|
|Heat capacity, kJ kg−1 K−1, at 270 K||2.07 (12)||2.07 (13)||2.10 (13)|
|Heat of dissociation, kJ kg−1, at 273 K||338.7 (12)||262.9 (13)||333.5 (14)|
|Thermal conductivity, W m−1 K−1||0.5 @ 270 K (15)||0.5 @ 270 K (16)||2.2 @ 263 K (1)|
|Thermal diffusivity, m2 s−1||3 × 10−7 @ 270 K (15)||2.8 × 10−7 @ 270 K (17)||8.43 × 10−7 @ 273 K (18)|
|Thermal linear expansivity, K−1, at 200 K||77 × 10−6 (1)||52 × 10−6 (1)||56 × 10−6 (1)|
|Mechanical Properties of the Frozen State|
|Density, kg m−3, at 273 K||910 (1)||∼910 (1)||917 (1)|
|Interfacial Tension, J m−2||0.017 (19) −0.032 (20)||0.016–0.031 (21)||0.029 (19) −0.032 (20)|
|Adiabatic bulk compressibility, Pa, at 273 K||∼14 × 10−11 (1)||∼14 × 10−11 (1)||12 × 10−11 (1)|
|Isothermal Young's modulus, Pa, at 268 K||∼8.4 × 109 (1)||∼8.2 × 109 (1)||9.5 × 109 (1)|
|Shear wave speed Vs, m s−1||1950 (22)||1890 (23)||1950 (24) ∼1990 (25)|
|Compressional wave speed Vp, m s−1||3370 (23) ∼ 3800 (22)||3670 (23)||3890 (24) ∼3910 (25)|
|Strength, MPa||2 to 10 (26)||0.9 to 44 (27)||0.6 to 1 (26)|
|Electrical Properties of Frozen State|
|Electrical conductivity, S/m||∼0.01 (28)||0.01||0.01 (29)|
|Dielectric constant at 273 K||∼2.5 (28)||4.3||2.8 (29)|
In terms of hydrate properties, THF hydrate forms as Structure II, with THF filling only large cavities. In contrast, methane hydrate most commonly occurs as Structure I with methane filling both large and small cavities. Despite these structural differences, a comparison of the macroscale mechanical and electrical properties and some thermal properties (e.g., heat capacity, thermal conductivity) of the two hydrates reveals gross similarities (Table 1), particularly when each property is considered within the range of values attained in marine or permafrost sediments. On the other hand, there are important differences in thermal expansivity, the heat of dissociation, and the degree to which equilibrium temperature depends on pressure for the two hydrates.
3. Dipole Moment Effects
The polar nature of the THF molecule (relatively large dipole moment) compared to the nonpolar methane molecule has led to concerns about the potential interaction of THF with water and THF hydrate with sediments. We next discuss each of these issues in detail.
3.1. THF-Water Interaction
As mentioned above, the dipole moment of the THF molecule is similar to that of water, but its permittivity is much smaller. The molecular dipole moment μ [Debye = C · m] is determined by the geometric arrangement of charges in a molecule while the permittivity is a measure of macroscale polarizability per unit volume [Santamarina et al., 2001]. The orientational polarization P [C m−2] reflects μ and the number of molecules per unit volume N, as well as the competing effects between the externally imposed electric field E[N C−1] and the randomizing thermal motion at temperature T [K]. The relationship among these parameters is captured in [Atkins, 1978]
where k is Boltzmann's constant [1.38 × 10−23 J K−1], ɛo is the permittivity of free space [8.854 × 10−12 F m−1], and κ′ is the relative permittivity. The permittivities of water and THF are related as
Using values from Table 1, the polarization ratio is Pwater/PTHF ≅ 11, which is similar to the ratio of measured permittivities (κ′water − 1)/(κ′THF − 1) = 12, as shown for microwave complex permittivity spectra in Figure 2. Therefore molecular size, which is captured by N in (1), must be considered in conjunction with the molecular dipole moment for the analysis of the macroscale behavior of THF.
Hydrogen bonds between water molecules are responsible for the interaction between water and other fluids (e.g., THF) and for the activity of water, which is closely related to phase equilibrium conditions [Luck, 1973]. Among organic compounds, alcohol and ether form hydrogen bonds with water, which explains the high miscibility of these fluids. As a cyclic ether, the oxygen of a THF molecule uses one of its unshared electron pairs to accept a proton from a water molecule to form a hydrogen bond [Carey, 1987]. However, the interaction between THF and water molecules is relatively weak compared to water-water interaction, and water clusters formed by the hydrogen-bonded water network are preserved around THF molecules [Fukasawa et al., 2004; Ohtake et al., 2005; Takamuku et al., 2003]. Thus THF's capacity to form hydrogen bonds does not play a crucial role in hydrate formation, and THF molecules essentially behave as nonpolar molecules.
3.2. Fluid-Salt Interaction
Salts are often present in natural systems as precipitates in sediments or in a dissolved phase in pore fluids. Polar solvents like water can dissolve ionic compounds through thermal agitation and ion-dipole interactions, even though the ion-dipole interaction is weaker than ion-ion interaction. The dissolution of salts by water starts with the alignment of the polar water molecules near ions, with H-ends toward anions and O-ends toward cations. The small water molecules gradually position themselves between surface anions and cations, weaken the ion-ion attraction forces, and eventually pull ions away from the crystal. This sequence of events shows the relevance of polarity and molecular size for the ability of a fluid to dissolve salt. For reference, Figure 3 illustrates the relative sizes of the sodium chloride, tetrahydrofuran, and water molecules.
A key question is whether fluids that contain large polar molecules (e.g., THF) can also dissolve salt. We devised a simple, yet robust, experiment to evaluate the effect of molecular polarity and size on the ability of a fluid to dissolve salt. Three fluids were considered for the study of salt-fluid interactions: water (small polar molecule, high permittivity), THF (large polar molecule, low permittivity), and benzene (nonpolar molecule, low permittivity). The relevant properties of benzene are molecular size of 5.36 Å, zero dipole moment, molecular polarizability of 10.32 Å3, permittivity of 2.28, density of 879 kg m−3 at 293.5 K, viscosity of 0.65 cp at 298.5 K, surface tension of 0.0289 N m−1 at 293.5 K, and solubility of 0.18 (mole fraction) in water at 293.5 K [Atkins, 1978; Smallwood, 1996; Yoshida et al., 2005].
Table salt (7g of NaCl) was thoroughly mixed in 50 ml of each liquid. The mixture was then filtered through P5 (5–10 μm) filter paper, and the retained salt was oven-dried and weighed. Within the precision of these measurements, the results show that THF (κ′ = 7.52) and benzene (κ′ = 2.28) do not dissolve NaCl (>99% of the salt retained in solid form) while water (κ′ = 80) dissolves NaCl completely (no salt retained).
Under the microscope, the salt crystals that had been mixed with benzene had intact crystal surfaces (Figure 4b), indicating that the benzene did not react with the salt. The crystal surfaces of the salt mixed with THF showed flakes of precipitated butylated hydroxytoluene (BHT, C15H24O), which is added to THF at 0.2g/L to prevent peroxide formation during storage (Figure 4c), but revealed no indication of dissolution. To confirm whether the observed flakes were BHT, the same experiment was repeated with excess BHT (250 g/L) added to the THF, producing extensive BHT precipitation on salt grains (Figure 4d).
Unformatted text preview: (53/3 Stoichiometry-Hydrates Prelaboratory Assignment Name and Drawer Number Q9 Q}: n I SCAN g‘ﬁ ‘” £93 1. The data listed below was determined from the dehydration of the hydrate of magnesium sulfate. Mass of empty crucible = 35.794 g Mass of crucible and magnesium sulfate hydrate = 37.255 g Mass of crucible and anhydrous magnesium sulfate = 36.504 g 3. Write the chemical formulas for water and anhydrous magnesium sulfate. From the formulas calculate the molar masses of water and anhydrous magnesium sulfate. van-MU '23 \AarO ; Fiona 3imo‘ Our\\-“:§_\re'c$ \‘Ynsxnegwcm $‘0\\:O_5‘ Q— i\$ 1 at) o 5 l m0" b. Calculate the mass of water removed from the hydrate during the drying process from the difference between the mass of the crucible and hydrate and the mass of the crucible and the anhydrous salt. 0 a'\ ‘53\ E) be 0 3%“; guitalri‘wj / c. Use the mass of water (part lb) and the molar mass of water to calculate the moles of water in the sample. CncﬁM-r W‘hu\ use (:1. Determine the mass of anhydrous magnesium sulfate by subtracting the mass of the empty crucible from the mass of the crucible containing the anhydrous compound. 0.110 5 /( e. Use the molar mass of the anhydrous compound and the mass of the anhydrous compound (part 1d) to determine the number of moles of anhydrous salt. ODOD%%$ mo\ M5504 f. Determine the ratio of the number of moles water in the hydrate to the number of moles of anhydrous salt in the hydrate by dividing the answer from part 1c by the answer from part 1c. This value is the number of moles of water in the hydrate for each mole of the anhydrous salt. Use this value to write the chemical formula for the hydrate. O I“ N W : 7.0a me" “BO/MM “as” 090°5%% chmvixgg} Qou W -41- "‘ Prelaborator)‘ Assignment - continued 2. A11 unknonn hydrated salt has the general formula Salt~nH20 where “n” is the number of moles of water for each mole of the hydrated compound. Although the chemical formula for the anhydrous salt is not known, it is known that the molar mass of the anhydrous salt is 151.92. The data listed below were obtained for a portion of the salt. (3) Determine the number of moles n of water in the hydrate for each mole of the anhydrous salt. (b) Write the formula for the salt, i.e., substitute the value of n into the general formula written above. Hint: It might be useful to follow the general solution outline used in question 1 of the Prelaboratory Assignment. Show the calculations. Mass of empty crucible = 35.107 g Mass of crucible and hydrated salt = 36.274 g Mass “crucible and anhydrous salt = 35.745 g _ “is “ (“Maﬁa Sn“ Ema. MA as.“ I c F - 3‘5. \0‘1 5 ml vaxix‘ow. ’ 0 .t; 35% so. \ - \\.o "t :5 \\\5qu¥: tux 93CL\¥- 3) mm of: \—\a0 \ﬁ‘b\ N30: 3.x\.,00%-\\ + unmet: -(3 \q .Q:\\o % \XQD \\ (mask -»a\ A a o k 0 , muse 0'53 9&0 Mngb — O, oacvkwxcﬁQsaG Ff, ‘ _ H \a \w'] S\\%d\ro}st¢3~ bc‘\¥ x \% ic\\o$ 9‘30 -W o .594; g) ‘R'axo f— so‘\1r -— O. QQQ‘AO ’mo\ 3Q\* O‘KPE’Q $30“ \:3\_O\';%SCLU¢ ® V&k\0 \e\ 0 0ean «WA 9‘30 A: "\ r“‘°_\______.—9‘ _—————r*""" mw-\ ww Ado - DO‘VBD “‘5‘ aﬂk , —42- 930A": ‘_l \'\ 9‘0 ...
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