Effects of pore gas hydrate dissociation on physical properties of frozen soils due to thermobaric conditions change

Abstract

Permafrost soils can contain pore gas in the hydrate (clathrate) form. Intrapermafrost gas hydrates were recovered from deep boreholes drilled in cold regions. There are indirect indicators that gas hydrates can exist in shallow permafrost (<150 m) above the present zone of their stability due to the self-preservation effect. Shallow permafrost gas hydrates may be responsible for methane emission and gas blow hazard and thus pose serious risks to engineering structures and facilities for oil and gas production in the Arctic. Metastable gas hydrates are extremely sensitive to various external factors (pressure drop and temperature increase) and, in turn, affect the physical properties of permafrost.

Changes in strength, thermal conductivity and permeability of frozen hydrate-bearing sandy soils caused by partial dissociation of pore methane hydrates upon pressure drop below the equilibrium and subsequent heating are investigated in laboratory experiments. The experiments reveal shear strength reduction and gas permeability increase due to phase transitions in pore space. The observed consequences of methane hydrate dissociation include formation of cracks and grain defects, changes in relative percentages of pore hydrate and ice, increasing amount of unfrozen liquid water, formation of porous ice and its follows melting (temperature rise). The increase of thermal conductivity after pressure drop is due to the larger share of pore ice which is more thermally conductive than gas hydrate, with follows thermal conductivity decrease cause by pore ice melting and unfrozen water content increasing due to heating.

The obtained experimental results have implications for geotechnical prediction and monitoring in permafrost and can be include in state standards and recommendations for geotechnical monitoring and civil engineering operations at oil and gas fields in the Arctic.