Hydrate stability

Solids that form specific crystal hydrates sorb small amounts of water to their external surface below a characteristic relative humidity, when initially dried to an anhydrous state. Below this characteristic relative humidity, these materials behave similarly to nonhydrates. Once the characteristic relative humidity is attained, addition of more water to the system will not result in a further increase in relative humidity. Rather, this water will be sorbed so that the anhydrate crystal will be converted to the hydrate. The strength of the water-solid interaction depends on the level of hydrogen bonding possible within the lattice [21,38]. In some hydrates (e.g., caffeine and theophylline) where hydrogen bonding is relatively weak, water molecules can aid in hydrate stabilization primarily due to their space-filling role [21,38]. 

ABSTRACT The Mallik gas hydrate field represents an onshore permafrost-associated gas hydrate accumulation in the Mackenzie Delta on the coast of the Beaufort Sea, Northwest Territories, Canada. This deposit contains a high concentration of natural gas hydrate with an underlying aquifer or free-gas zone at the base of the hydrate stability field. The physical and chemical properties of CH4 and C02 hydrates indicate the possibility of coincident C02 sequestration and CH4 production from the Mallik gas hydrate bearing zones. This study presents a numerical assessment of C02 sequestration and the recovery of CH4 from the gas hydrates at the Mallik site, Mackenzie Delta, Canada. 

KEYWORDS CH4 hydrate hydrate kinetics, hydrate stability, C02 sequestration numerical simulation... 

Uddin et al. (2008b) conducted several depressurization simulations for the Mallik 5L-38 well. Their results showed that the Mallik gas hydrate layer with its underlying aquifer could yield significant amounts of gas originating entirely from gas hydrates, the volumes of which increased with the production rate. However, large amounts of water were also produced. Sensitivity studies indicated that the methane release from the hydrate accumulations increased with the decomposition surface area, the initial hydrate stability field (P-T conditions), and the thermal conductivity of the formation. Methane production appears to be less sensitive to the specific heat of the rock and of the gas hydrate. 

Fig. 2. CH4 hydrate stability curves (laboratory data) showing CH4 hydrate dissociation paths (schematic) in the depressurization method.

Fig. 3. C02 hydrates stability curves (lab data) showing C02 hydrate formation paths (schematic) by injection of C02 gas.

Fig. 4. CH4 - and CO2 hydrates stability curves showing C02 enhanced CH4 hydrate dissociation zone.

After allowing for a statistical correction for the number of protons that may be lost from the cation, the order of acid strengths (per NH-proton) of these cations remains unchanged. The initial trend of decreasing acidity with methyl substitution is reversed in the trimethylammonium ion, because with a decreasing number of protons in the cation, its hydration stabilization by hydrogen... 

Hydrate structure Methane storage in hydrate [m3/m3] Hydrate stability condition ... 

The guest molecule fit within each cavity determines the hydrate stability pressure and temperature. 

Hester, K.C., Probing Hydrate Stability and Structural Characterization of both Natural and Synthetic Clathrate Hydrates, Ph.D. Thesis, Colorado School of Mines, Golden, CO (2007). 

In a review of the thermodynamics of water, Franks and Reid (1973) showed that the optimum molecular size range for maximum solubility was similar to hydrate stability. Franks and Reid noted, this is not intended to imply that long-lived clathrate structures exist in solution—only that the stabilization of the water structure by the apolar solutes resembles the stabilization of water in a clathrate lattice. Glew (1962) noted that, within experimental error, the heat of solution for ten hydrate formers (including methane, ethane, propane, and hydrogen sulfide) was the same as the heat of hydrate formation from gas and ice, thereby suggesting the coordination of the aqueous solute with surrounding water molecules. 

A typical chart for water content from this period is presented in Figure 4.21. In Figure 4.21 the water content chart at temperatures above the hydrate stability conditions is based primarily on the data of Olds et al. (1942) while the data of Skinner (1948) were the basis for extrapolations to temperatures below the hydrate formation point. A summary chart is given by McKetta and Wehe (1958). However, below the initial hydrate formation conditions, Figure 4.21 represents metastable values, as observed in gas field data by Records and Seely (1951). Kobayashi and Katz (1955) indicated that such concentration extrapolations across hydrate phase boundaries yield severe errors. 

Similarly, if channels are available, biogenic gas may migrate to regions within the hydrate stability envelope. Most of the gas was of biogenic origin in the hydrate core recovered from the Northwest Eileen State Well Number 2, one of the first wells to recover hydrates (Collett, 1983). The biogenic source is likely to predominate for hydrates in permafrost (Kvenvolden, Personal Communication,... 

Because free gas (or gas-saturated water) is less dense than either water or sediments, it will percolate upward into the region of hydrate stability. Kvenvolden suggested that a minimum residual methane concentration of 10 mL/L of wet sediment was necessary for hydrate formation. The upward gas motion may be sealed by a relatively impermeable layer of sediment, such as an upper dolomite layer (Finley and Krason, 1986a) or the upper siltstone sequence, as in the North Slope of Alaska (Collett et al., 1988). Alternatively, permafrost or hydrate itself may act as an upper gas seal. These seals can also provide traps for free gas that has exsolved from solution, and the seals can subsequently act to provide sites for hydrate formation from the free gas. 

Xu and Ruppel (1999) solved the coupled mass, heat, and momentum equations of change, for methane and methane-saturated fluxes from below into the hydrate stability region. They show that frequently methane is the critical, limiting factor for hydrate formation in the ocean. That is, the pressure-temperature envelope of the Section 7.4.1 only represents an outer bound of where hydrates might occur, and the hydrate occurrence is usually less, controlled by methane availability as shown in Section 7.4.3. Further their model indicates the fluid flow (called advection or convection) in the amount of approximately 1.5 mm/yr (rather than diffusion alone) is necessary to produce significant amount of oceanic hydrates. 

There are four requirements for generation of natural gas hydrates (1) low temperature, (2) high pressure, (3) the availability of methane or other small nonpolar molecules, and (4) the availability of water. Without any one of these four criteria, hydrates will not be stable. As indicated in both the previous section and in Section 7.4.3, the third criteria for hydrate stability—namely methane availability—is the most critical issue controlling the occurrence of natural gas hydrates. Water is ubiquitous in nature so it seldom limits hydrate formation. However, the first two criteria are considered here as an initial means of determining the extent of a hydrated reservoir. 

Figures 7.11a,b are arbitrary examples of the depths of hydrate phase stability in permafrost and in oceans, respectively. In each figure the dashed lines represent the geothermal gradients as a function of depth. The slopes of the dashed lines are discontinuous both at the base of the permafrost and the water-sediment interface, where changes in thermal conductivity cause new thermal gradients. The solid lines were drawn from the methane hydrate P-T phase equilibrium data, with the pressure converted to depth assuming hydrostatic conditions in both the water and sediment. In each diagram the intersections of the solid (phase boundary) and dashed (geothermal gradient) lines provide the lower depth boundary of the hydrate stability fields.

Figure 7.11 Envelopes of methane hydrate stability (a) in Permafrost and (b) in Ocean sediment. (Reproduced from Kvenvolden, K.A., Chem. Geol., 71, 41 (1988). With permission from Elsevier Science Publishers.)...

Ginsburg and Soloviev (1998, pp. 150-151) state that the BSR is the most widely used indirect indication of gas hydrates. The most important evidence of the hydrate caused nature of the BSR is the coincidence of temperature and pressure calculated at it s depth with the equilibrium temperatures and pressure of gas hydrate stability. The association with the base of the hydrate stability zone is beyond question. ... 

As shown in Figure 7.13 the pressure and temperature limits to the hydrate stability exists from the seafloor (because hydrates are less dense than seawater) to the intersection of the geotherm (BGHS). The solubility limit, however, imposes a further depth restriction because the methane concentration must equal the solubility limit to be in equilibrium with hydrates. It is assumed that the sediment provides sufficient nucleation sites so that there is no methane metastability, so hydrate forms in the narrow depth region where methane concentration lies atop the methane solubility line. As illustrated in the Leg 311 case study, the GHOZ is always smaller than the GHSZ. 

On ODPLeg 164, three sites were drilled below the base of hydrate stability over a short distance (9.6 km) in the same stratigraphic interval. Figure 7.21b shows the three Leg 164 holes Site 994 without a BSR, Site 995 with a weak BSR, and... 

FIGURE 7.31 Gas hydrate stability envelope at Messoyakha. (Reproduced courtesy of U.S. Dept, of Energy (Sheshukov, 1972).)... 

Figure 7.35 Mallik 2002 geothermal gradient and hydrate stability curve for pure water and water containing 40 ppt salt. Note the depths of the thermal stimulation test and the six pressure stimulation (MCT) tests. (From Wright, J.F., et al., in Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, Geological Survey of Canada Bulletin 585, including CD (2005). With permission.)...

Carter KN, Greenberg MM (2001) Direct measurements of pyrimidine C6-hydrate stability. Bioorg Medic Chem 9 2341-2346... 

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