EXPERIMENTAL ESTIMATION OF GAS HYDRATES ACCUMULATION AND EXISTENCE CONDITIONS IN SEDIMENTS

E. M. Chuvilin and E. V. Kozlova
Geological Faculty, Moscow State University, Leninskie Gory, 119992, Moscow, Russia
[email protected]

Natural gas hydrate as hydrocarbon potential south, which superiosr traditional ones by resources, are very interested for many specialists and explorers. For present moment it is known that gas hydrate content in dispersed sediments may vary from per cent tenth (in porous space) to monolitic accumulations. Their distribution is limited by geological agents. Gas hydrates are stable within certain temperature, pressure and geochemical ranges. But mechanisms and peculiarities of gas hydrates formation in nature are very poorly studies. Experimental investigations which allow to extend our perceptions about gas hydrates formation and existence in sediments.

In given paper some methodic approaches for study of gas hydrate contained sediments are proposed. These approaches are received on the base of experimental investigations of gas hydrate formation and dissociation in sediments. Physical simulation of gas hydrate formation and dissociation has been conducted within a 420 cm³ test cell. Cyclic temperature fluctuations were imposed on a 4,5 cm diameter by 9 cm high specimen according to the procedures of Chuvilin et al. (2).

The methodic approaches include an estimation of some parameters characterizing gas hydrate accumulation in porous media of sediments and their existence conditions. Among these are hydration coefficient (K_H), which characterizing the part of porous water transformed into hydrate; volume hydrate content (H_v); degree of pores filling by hydrate (G_h). Besides, we propose a method for estimating the relative stability of natural hydrate based on the calculation of a stability coefficient (K_st).

The important characteristic for understanding of process of hydrate accumulation in modelling specimen porous space is the hydration coefficient – K_Н. As a rule during cooling methane saturated specimen only a part of porous water transfers in hydrate. Quantitatively portion of porous water, transformed into hydrate, may be designated as a parameter, so-called hydration coefficient. Experimental researches show, that the hydration coefficient depends on structure porous spaces of dispersed sediments, connection energy of water with a surface of mineral particles, the “gas-water” contact surface and gas permeability of porous media and also pressure/temperature regime of hydrate formation.

Using this method we estimated K_H for hydrate containing thickness of Mallik 2L-38Gas Hydrate Research well based on data (1). As showed calculations K_H varies from 0,03 at gas hydrate content 5% to 0,7 at gas hydrate content near 80%.These data mark that only a part of porous water transformed into hydrate.

Experimental study of the thermodynamic condition of form and dissociation of the methane hydrate in porous space make it possible to propose a method for estimating the relative stability of natural hydrate (fig. 1) based on the calculation of a stability coefficient (K_st):

where: Td is the dissociation temperature (K) of hydrate in recovered sediment porous space; T_sed is the estimated in situ temperature (K) of recovered hydrate containing sediment; H_v – volume content of gas hydrate in recovered sediment (unit fraction). This method afford to take into consideration natural characteristics of hydrate saturated sediments and from other side to use of the experimental estimating which indicative conditions of the methane hydrate existence in situ. Note that the stability of natural gas hydrate accumulations increases as K_st approaches 1 and decreases as K_st approaches 0 (3).

The stability coefficient may be below zero. In this case the gas hydrate forms are metastable. This situation can be occured for permafrost-bearing sediments regions, where the gas hydrate accumulations exist above stability threshold due to self-preservation effect under negative temperature. At that ice component acts as preservation agent, which increases the gas hydrate stability. In this instance the coefficient stability is expressed by the follow view:

where Hi – ice volume content in sediment (unit fraction); T_th – melting temperature of sediment.

This method was used for estimating the thermodynamic stability of natural hydrate from gas research well Mallik 2L-28 (1) and from site 2569 located in the western Mississippi Canyon region in the northern Gulf of Mexico (3).

We propose to use for the quantitative characteristic gas hydrates metastable the coefficient of self-preservation (Кsp) based on a ratio volume gas hydrate content under nonequilibrium and equilibrium conditions:

where Нv - volume gas hydrate content under equilibrium conditions; H_v* - residual volume gas hydrate content under nonequilibrium conditions (fig. 2). Metastable condition of gas hydrates at negative temperatures, and, accordingly, the coefficient of self-preservation, depends on temperature conditions, sediment characteristics and existence forms of gas hydrate in sediments. With other things being equal, at lower temperature it is necessary to expect the greater coefficient of self-preservation. The hydrate in sediment fineer form and more dispersed (in comparison with monolithic form) the К_sp less.

Reference

1. Dallimore S.R., Uchida T., Collett T.S (ed). (1999). Scientific Results from JAPEX/JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada, (ed.), Geological Survey of Canada, Bulletin 544.

2. Chuvilin E.M., Kozlova E.V., Makhonina N.A., Yakushev V.S., Dubinyak D.V. (2002) Peculiarities of methane hydrate formation/dissociation P/T conditions in sediments of different composition. Proceeding of the Fourth International Conference on Gas Hydrate, Yokohama, pp. 433-438.

3. Chuvilin E.M., Kozlova E.V., Boldina O.M., Winters W.J. (2004) Stability of methane gas hydrate formed in marine sediment from the northern Gulf of Mexico. International Conference on Minerals of the Ocean - Integrated Strategies, St. Petersburg, Russia. pp. 216-218.

Figure 1. The method for estimating stability coefficient (K_st) of natural gas hydrates.