INTRAPERMAFROST GAS HYDRATES AT THE NORTH OF WEST SIBERIA
Research Institute of Natural Gases and Gas Technologies (VNIIGAZ), Gazprom, Russia
Laboratory simulation of frozen hydrate-bearing sediments conducted in the middle and the end of 80-ties of 20th century  allowed predicting the existence of relic (metastable hydrates) in nature. Once formed in ancient time, when thermodynamic conditions in geological section were favorable for hydrate formation, relic hydrates can exist at subzero temperatures even if thermodynamic conditions are not safe more for hydrates. This phenomenon named gas hydrate self-preservation allows hydrate to exist due to ice film formation at the hydrate surface after pressure drop. Or hydrate could be sealed by outside ice when hydrate stability conditions period and then these conditions disappeared, but temperature remains subzero.
Gas hydrate self-preservation phenomenon allowed to predict much wider spreading of gas hydrates in the Universe than it was expected earlier. In particular, it helped to explain some unusual features of ice comet behavior in cosmic space . This phenomenon was the base for new natural gas industry technologies development (see, for example ). In geology, this phenomenon resulted to re-consideration of general view of gas hydrate existence in permafrost regions and to introduction of new depth interval of gas hydrate existence – Hydrate Metastability Zone (HMSZ) . HMSZ includes all frozen sediments from the bottom of seasonal defrost layer down to the depth of the top of usual hydrate stability zone (HSZ). If sediment does not contain ice within this zone, hydrate can not be safe, so only hard-frozen sediments can contain metastable hydrates. Thus, theoretically all the interval of permafrost section can be favorable for gas hydrate existence within HMSZ and upper part of conventional HSZ (fig.1). But in reality, hydrates can be safe only on sediments containing ice and permeable for gas (now or in past before ice and hydrate formation).
First indications about relic hydrates existence in permafrost of West Siberia (Yamburg gas field area) have been documented at the end of 80-ties – beginning of 90-ties of 20th century [5, 6]. These indications were visible gas liberations from permafrost drill cores from depths less than 150 m when thawing in kerosene or warm water. Drill cores were represented by fine-grained sand and had very small empty space in pores for free gas. Volume of gas liberated when thawing was many times over this space volume. The same situation was a little bit later with drill cores recovered from depth 119 m at the well 92GSC Taglu at the North of Canada . There also supposed presence of relic (metastable) gas hydrates.
The most advanced studies of relic gas hydrates have been conducted at Bovanenkovo gas field area in Yamal peninsula at the North of West Siberia [4, 8]. These studies included well drilling, permafrost cores recovery, permafrost gas liberations study (at wells), and hydrate content of drill cores measurement (in laboratory). Gas hydrates were revealed not at all the drill cores selected for study, but some of them contained hydrates in volumes 0,5 – 3% of pore space volume according to the volume of gas liberated during the sample thawing in water. The most interesting observation was that hydrate-containing sediments often neighbored with intrapermafrost gas-bearing layers. Gas flow rates at wells reached more than 10000 m3/day from depth 60-120 m. Gas analysis showed microbial genesis of methane in hydrates and in free gas liberations from permafrost. This data gave a certain base to suppose that free gas accumulations could be in particular the result of gradual decomposition of intrapermafrost relic (metastable) gas hydrates.
According to isotopic and chemical analysis, gas in permafrost in Bovanenkovo gas field area is completely different from the gas of upper productive reservoir. The same situation is at Yamburg gas field. This means, that gas hydrate and free gas accumulations within permafrost interval can have their own mechanism of gas generation, accumulation and conservation. Probably, microbial gas was generated before freezing of section and was partially dissolved in pore waters. Freezing of geologic section could concentrate the gas under freezing front and in certain situations result to local hydrate formation. Formed hydrates then came to metastable state and this process was accompanied by free gas accumulation in neighboring permeable layers. We can not evaluate precisely total gas resources of intrapermafrost relic gas hydrates and free gas accumulations at Yamal peninsula, but according to first estimations, specific density of these resources at Bovanenkovo gas field area in the interval 60-120 m should be no less than 100 000 m3/km2. This value is received by measurement of total volume of gas liberated at wells in the certain area. Taking into account low permeability of studied drill cores, we can assume that only small part of resources in this interval was touched by wells and real value of resource density is much more greater (may be order or two orders more).
Other study of intrapermafrost gas hydrates and free gas accumulations was begun recently in other gas fields areas of West Siberia: Zapolyarnoe and Kharvuta (fig. 2). The same procedure was applied there as it was in Bovanenkovo gas field area. Although strong gas releases from permafrost have been observed sometimes when drilling, by now only 2 drill cores from more than 15 recovered and transported to Moscow have shown gas liberations when thawed in water. Nevertheless the study is continuing and new wells drilling are expected.
Other target of studies is drill cores recovery from low part of permafrost, from sediments situated in conventional HSZ. This part of West Siberia is characterized by deep cryolithozone – about 400-450 m and considerable HSZ – about 400 m. Upper part of HSZ in depth interval 250-350 m is situated in permafrost. In the same depth interval there is regionally spread sandy layer, generating weak gas liberations when wells drilling with warm drill mud. This layer is not studied for gas yet and there are no drill cores from it in this area. So its testing for gas hydrates could be of interest.
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Figure 1. Methane Hydrate Stability Zone (HSZ) and Hydrate Metastability Zone (HMSZ) in permafrost regions.
Curves: 1- equilibrium conditions of methane hydrate; 2- P/T conditions in geologic section.