Gas hydrate saturation

Fig. 1. Log data showing the variation of porosity, permeability, and gas hydrate saturation for the Mallik Gas Hydrate Research Program Well-5L-38 (Uddin et al. 2008b).

Kleinberg et al. (2005) and Takayama et al. (2005) show that NMR-log measurement of sediment porosity, combined with density-log measurement of porosity, is the simplest and possibly the most reliable means of obtaining accurate gas hydrate saturations. Because of the short NMR relaxation times of the water molecules in gas hydrate, they are not discriminated by the NMR logging tool, and the in situ gas hydrates would be assumed to be part of the solid matrix. Thus the NMR-calculated porosity in a gas-hydrate-bearing sediment is apparently lower than the actual porosity. With an independent source of accurate in situ porosities, such as the density-log measurements, it is possible to accurately estimate gas hydrates saturations by comparing the apparent NMR-derived porosities with the actual reservoir porosities. Collett and Lee (2005) conclude that at relatively low gas... 

Figure 7.41 Mallik 5L-38 gas hydrate saturations at point of thermal stimulation test. (From Hancock, S.H., 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 (2005b). With permission.)...

Fig. 2. Gas isotope compositions, gas hydrate saturation and TOC in the Nankai Trough, (a) Depth-trends of of CH4 and CO2. Open symbols, PTCS samples solid symbols, headspace gas samples.

Some of the data are from Waseda et al (1998) and Waseda and Uchida (2002). (b) Depth-trend of gas hydrate saturation estimated from Cl anomalies. Data are from Uchida et al (2004). (c) Depth-trend of TOC. Some of the data are from Waseda et al (1998). 

Triaxial compression test of SI hydrate bearing coal used gas 1, and the one of SII used gas 2. We adopt heating and cooling of repeated during the course of the experiment, to make distribute hydrate in coal uniformly. We adopt gas consumption calculating and resistance measuring method to controlling gas hydrate saturation at about 70%. 

The procedure for calculating methanol usage can best be explained by an example. Given a flowing temperature for one well of our example field of 65°F (as could occur with a remote well and subsea flow line), calculate the methanol required to prevent hydrates from forming. Assume that at the high flowing pressure there is no free water, but the gas is saturated. 

Such special atmosphere is often necessary in decomposition and rehydration studies of hydroxides, hydrates etc. Furthermore also the catalytic effect of water vapor on certain reactions is of interest. For such studies the gas is saturated with water, or other vapors e.g. D20, alcohol, CS2, etc. When higher water vapor concentrations are required special furnaces are available (see Sect. 2.4). 

Of special interest in recent years has been the analysis of natural gas hydrates that form in marine sediments and polar rocks when saline pore waters are saturated with gas at high pressure and low temperature. Large and 5D-variations of hydrate bound methane, summarized by Kvenvolden (1995) and Milkov (2005), suggest that gas hydrates represent complex mixtures of gases of both microbial and thermogenic origin. The proportions of both gas types can vary significantly even between proximal sites. 

A group of substances which are closely associated with the gas hydrates, are the compounds of urea (and thiourea) with a large number of organic substances with long-chain molecules, such as normal saturated hydrocarbons and olefins, alcohols, acids, esters, ketones, halogenated hydrocarbons, etc.3, which were discovered accidentally by Bengen in 1940. These compounds are only produced with unbranched, non-cyclic molecules4. This is even so very specific that a method of separation can be based on it for normal and iso hydrocarbons in mixtures. 

Elvert M., Suess E., and Whiticar M. J. (1999) Anaerobic methane oxidation associated with marine gas hydrates superlight C-isotopes from saturated and unsaturated C20 and C25 irregular isoprenoids. Naturwissenschaften 31, 1175-1187. 

This work presents experimental data on the CO2 hydrate formation in gas saturated wet samples under cooling conditions as well as on the hydrate decomposition kinetics in frozen C02-hydrate saturated samples. 

Our approach for studies of gas hydrate formation and decomposition in sedimentary pore space consists of two steps. The first one is devoted to the hydrate accumulation kinetics in pore space of frozen soils to obtain frozen hydrate-saturated samples. The second one concentrates on the pore hydrate dissociation kinetics in frozen soils under non-equilibrium conditions. 

Experimental modelling of CO2 gas hydrate formation in pore space of wet gas saturated soils has been carried out using an experimental installation consisting of the following basic elements a pressure chamber (about 420 cm ), a refrigerator to provide temperature variability of the pressure chamber, a converter of electric signals of the... 

Apart from a direct observation of the gas content of frozen samples in the course of time, the study of gas releases from the frozen hydrate saturated samples at atmospheric pressure were carried out with the help of DC-1 gas meter. 

The analysis of thermo-baric changes in the wet soil samples saturated with CO2 as a function of time under condition of cyclic cooling and heating permits to follow the kinetic and thermo-baric indicators of phase transitions within the pore space of the samples. On cooling of wet gas-saturated soils under gas pressures higher than the three-phase equilibrium line gas - water - CO2 hydrate , conditions for gas hydrates nucleation in pore space of soils are created. Pressure stabilization marks the end of the phase transition of water into hydrate. Upon further cooling below 0°C the remaining, untransformed liquid turns into ice. 

An vigorous CO2 hydrate dissociation was observed in frozen hydrate saturated samples after the pressure release in the pressure chamber. The hydrate coefficient decreased 1.5-3.0 fold in 30 minutes after a pressure drop to atmospheric values. The maximum decrease was observed in the sand sample with 14% of kaolinite particles, the minimum decrease in the sand sample with 7% montmorillonite particles with 17% of initial water content. In the course of time the intensity of CO2 hydrate dissociation in frozen samples dropped sharply with even a complete stop of the dissociation process as a consequence of gas the hydrates self-preservation effect at sub-zero temperatures A... 

The mineral composition of the soil will also influence the kinetics of gas hydrates dissociation in frozen soils. Our results show, that gas hydrate formations in pore space of samples with montmorillonite particles dissociate less markedly as compared to the samples with kaolinite admixture. This influence may be explained by microstructural specificities of pore hydrate saturated samples but undoubtedly requires additional micro-morphological studies for a full understanding. 

Methane Hydrate-bearing Sediments. SEM images of methane hydrates in gas-saturated sediments display gas hydrates between the quartz grains like a glue or cement (Figure 4a). This cement should contain almost pure gas hydrate after the complete transformation from water (see Sect. 3.1). 

Figure 4 SEM images of methane hydrate formed due to the transformation of liquid water in gas-saturated porous media, a, b porous medium I (quartz (Qz) and frost having mass content of 10 %) showing dense (DGH) and porous gas hydrate (PGH) cement, c, d medium II (Qz, kaolinite (KaJ) presenting kaolinite particles on the gas hydrate cement surface, d, e medium 111 (Qz, montmorillonite (Mm)) showing montmorillonite flakes on porous gas hydrate cement between quartz grains, f overview of medium III.

There is another, very important and large repository of methane methane hydrates (also known as gas hydrates or clathrates Kvenvolden 1988).They comprise ice in which the interstices of the lattice house small molecules, such as methane, ethane, carbon dioxide and hydrogen sulphide. In fact, enough gas needs to be present to fill 90% of the interstices in order for the hydrate to form, and it has a different crystal structure from normal ice (Sloan 1990). If fully saturated, the most common crystalline structure can hold one molecule of methane for every 5.75 molecules of water, so lm3 of hydrate can contain 164 m3 of methane at STP (see Box 4.8).The solubility of methane in water is insufficient to account for hydrate formation, and a major nearby source is required, typically methanogenesis, based on the dominance of methane (99%) and its very light isotopic composition (813C generally <—60%o see Section 5.8.2). 

Gas hydrates form wherever appropriate physieal conditions exist and eoneentrations of low moleeular weight gases, mostly methane, exeeed saturation. The P/T factors for the presence of methane hydrates (Fig. 

SCH] Schumb, W. C., The dissociation pressures of certain salt hydrates by the gas-current saturation method, J. Am. Chem. Soc., 45, (1923), 342-354. Cited on pages 269, 328. 

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