Sediment, hydrate formation

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. 

Using innovative experiments, Tohidi and coworkers (2001) and Anderson et al. (2001) have shown that hydrates can be formed in artificial glass pores from saturated water, without a free gas phase. They found that with significant subcooling the amount of hydrate formation was proportional to the gas solubility carbon dioxide formed more hydrates from a saturated solution than did methane. Further, the maximum amount of methane hydrate formation was fairly low— about 3% of the pore volume—a value consistent with the amount of hydrates in sediment. 

Figure 7.9 (from Roberts, 2001) gives an overview mechanism of hydrate formation in the Gulf of Mexico as a function of the flux of methane through sediments ... 

However, other researchers suggest that considerably less methane can be generated by in situ hydrate production. Hyndman and Davis (1992) indicated that an unaccountably high concentration of gas was required for hydrate formation. Minshull et al. (1994), Pauli et al. (1994), and Klauda and Sandler (2005) suggest that for in-place formation, under the best conditions the maximum amount of hydrate that can fill the sediments is 3%. 

In a recent ocean hydrate formation state-of-the-art summary, Trehu et al. (2006) listed the effects of fluid flow and sediment lithology. Ocean hydrate deposits are distributed on a spectrum between two types in ocean sediments (1) focused high flux (FHF) gas hydrates, and (2) distributed low flux (DLF) gas hydrates. In FHF hydrates the gas comes from a large sediment volume channeled through a high-permeability sand to the point of hydrate formation, and these hydrates are typically in the upper tens of meters of the sediment. In contrast, the DLF hydrates are generated near where the hydrates are formed, and fluid flow is responsible for movement of the gas within the gas hydrate occurrence zone (GHOZ). 

Sediment stratigraphy controls the hydrate distribution at Hydrate Ridge. The methane-rich migration pathway of Horizon A provides enriched hydrate formation relative to other sediments. In Figure 7.26, the remote sensing logs (gamma ray, density, RAB, and Archie water saturation) are most sensitive to hydrates. 

Figure 3 Different methods of hydrate formation (1-5) each can give a distinct perspective on certain aspects of hydrate formation l-ice + gas at T< 273K 2- ice with the temperature ramped above T = 273K 3 - amorphous ice +gas at T < 13 OK 4 and 5, either quiescent or with agitation are usually used for phase equilibrium studies and the effect of inhibitors. Simulation of natural gas hydrate requires the addition of sediment at various levels of water-sediment content.

In order to get a better idea of the process of hydrate formation and growth in the pore space of water saturated sediment we realised an experimental set-up to study these processes under the microscope. 

Water Absorption, Transport Process of Water. An NMR microscopy study has been performed to measure the imbibition of water into natural cork, extractives-free cork and de-suberized cork. It was clearly indicated that suberin is the key constituent which determines the ability of cork to resist water uptake. Hydrates were generated in synthetic sediments in a laboratory cell. After hydrate formation took place and the sediments were solidified, the samples were investigated both visually and by use of H NMR imaging. 

Gas hydrate formation involves the removal of water molecnles from the surrounding pore water, as they are seqnestered in the clathrate lattice. Removal of water, with the exclusion of the dissolved ions, leads to changes in the concentration of salts in the pore water. Becanse chloride is an abnndant and nsnally conservative ion in pore waters of shallow marine sediment, changes in dissolved chloride content are... 

Fig. 14.13 Cartoon illustrating how gas hydrate formation increases the salinity of the adjacent interstitial pore fluid, and subsequent dissipation of the chloride anomaly via diffusion over time. A. Shows system before hydrate formation, sodium and chloride ions homogeneously distributed in the pore fluid. B. When gas hydrate forms, ions are excluded from the crystal lattice, and the pore fluids become saltier at the foci of hydrate formation. Right panel illustrates a 56 mM anomaly created by formation of gas hydrate that occupies 9% of the pore space. C. Over time the excess ions diffuse away, as illustrated by the diffusional decay model showing dissolved chloride profiles at 1,000 and 10,000 years. D. After 100,000 years, the chloride anomaly is smaller than that which can be detected with current analytical techniques. The 1-dimensional model assumes that the half width of the concentration spike to be 5 meters, a sediment porosity of 50% and the free solution diffusion coefficient for the chloride ion of 1.86 x 10 cm s" at 25 °C (modified from Ussier and Pauli 2001).

There is to date no reliable data on the amount of Cl sequestered by the hydrate cage because the physical separation of the water released by natural hydrate dissociation from pore water contamination can be very difficult. Suess et al. (2001) suggest that there may be residual chloride trapped within the hydrate pore space. Nevertheless, since this number is small and very poorly defined, most estimates of hydrate abundance in marine sediments assume that hydrate formation excludes all dissolved ions. 

The water sequestered in the hydrate lattice is preferentially enriched in 0 and deuterium (D), thus the isotopic composition of the water in the pore spaces collected from gas hydrate bearing sediment can provide additional information on the abundance and the characteristics of these deposits. Pore fluid samples that had been modified by hydrate decomposition upon core recovery during ODP Legs 146 (Kastner et al. 1998), and 164 (Matsumoto and Borowski 2000) provided the first field data to derive the oxygen isotope fractionation factor for in situ hydrate formation. A more comprehensive sampling... 

Discuss different methane sources in sediments. Is there a difference in hydrate formation whether the gas is biogenic or thermogenic in origin ... 

Since chlorides and other ions from marine pore water cannot be incorporated into the clathrate structure, those constituents are enriched in residual pore fluids when hydrate is formed in marine sediments. When hydrate formation is faster than the rate of salt removal by diffusion and advection, the excess salt would accumulate in the pore water resulting in the formation of a brine. 

In 1995, Japan s Ministry of Economy, Trade and Industry (METI), formerly Ministry of International Trade and Industry (MITI), launched a project to explore for marine gas hydrate accumulations around Japan. From late 1999 to early 2000, an exploratory hole MITI Nankai Trough was drilled on the landward side of the eastern Nankai Trough, offshore Japan (Fig. 1) by Japan National Oil Corporation (JNOC) along with Japan Petroleum Exploration Co., Ltd (JAPEX) as the well operator. The water depth at the drill site was 945 m and the sub-bottom depth of the hole was 2355 m. The seismic bottom simulating reflector (BSR) is present at around 295 mbsf. There were two exploration objectives one was a gas hydrate survey in shallow Quaternary sediments and the other was conventional oil and gas exploration in deeper Tertiary sediments. In addition to the main hole, seven short holes (two site survey, two pilot and three post-survey holes) were also drilled for the gas hydrate survey around the main hole. In this paper, we will clarify the origins of methane in gas hydrates found in the MITI Nankai Trough Well and discuss gas migration and hydrate formation in the sediments. 

Drilling and production through hydrate formations above oil and gas reservoirs can cause dissociation and well blowouts. Similarly, the possible instability of the sea floor sediments over hydrate deposits where oil and gas is extracted raises concerns over the collapse and loss of engineering structures. 

Masui A., Haneda H., Ogata Y. et al. 2005. Effect of methane hydrate formation on shear strength of synthetic methane hydrate sediment. Proc. 15th Int. Offshore and Polar Eng. Conf, Seoul, Korea, 364-369. 

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