Relic Gas Hydrate and Possibility of Their Existence in Permafrost Within the South-Tambey Gas Field (Russian)

Abstract Isolated gas and gas hydrates in the permafrost are serious geological danger in the process of oil and gas field development in Arctic. The particular hazard is the largegas accumulations confined to the sand and loamy sand horizons in the permafrost at depths down to 200 meters. Such gas accumulations are found in a number of Yamal gas fields and South-Tambey gas field among them. There are some indirect signs that they may be relic gas hydrates formed earlier in specific hydrate accumulation conditions. Up now they might have been preserved in the permafrost due to the effect of gas hydrate self-preservation at temperatures below zero. These gas hydrates lying above the modern gas hydrate stability zone are in a metastable state and very sensitive to various anthropogenic influences. While drilling and during borehole operation of in the areas of relic gas hydrates locations various technical complications up to bow out may occur. To evaluate the possibility of formation and existence of relic gas hydrates on the territory of the South-Tambey gas field the mathematical simulation and experimental modeling were performed. The aim of mathematical simulation was to understand the dynamic of the permafrost thickness and of the zone of gas hydrates stability (GHSZ) in the Late Pleistocene and Holocene. The simulation allow to present the evolution of the permafrost during which the zone of gas hydrates stability started from the earth’s surface being located within permafrost. As the permafrost has never fully thawed, near surface horizons may still contain relic gas hydrates to this day. Experimental researches were undertaken to study the possibilities of preservation of relic gas hydrates in the permafrost of the South-Tambey gas field up to now. The experiments consisted of two stages. The first part was the artificial saturation of field samples by methane hydrate. At the second stage we studied processes of self-preservation of pore gas hydrate in the frozen samples under nonequilibrium conditions close to the reservoir conditions such as temperatures –5…-6 0C and pressure of 0,6–1,3 MPa. These experiments highlignt the high stability of the pore gas hydrates in the frozen samples at the given nonequilibrium conditions. In general we can make a conclusion that the existence of relic gas hydrates on the territory of the South-Tambey gas field at depths of less than 200 m in the permafrost is possible.