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Abstract

Permeability is known as a key factor affecting the gas production effectiveness from the natural gas hydratebearing reservoir. We studied the permeability behavior of natural clayey sand core samples from a natural hydrate-bearing reservoir in the Qilian Mountain permafrost before and after hydrate formation, as well as after hydrate decomposition. We found a substantially lower permeability after hydrate decomposition and assumed a formation damage process involving fines mobilization, migration and deposition at pore throats. The assumption was proved by SEM analysis of the filter paper separating the sample and the end caps containing the fluid ports. The analysis showed fines trapped in the paper from the outlet side. Fines migration and resulting formation damage is known from enhanced oil recovery by low salinity water flooding, but was unexpected for hydrate decomposition. The underlying mechanism was identified by a series of different permeability tests. The results indicate that fresh water released from the hydrate dissociation causes the fines mobilization, migration and redeposition at pore throats leading to the observed permeability decrease. Obviously the large volume of released methane gas displaces the remaining saline water and separates it from the fresh water released from the hydrate. The fresh water in contact with parts of the grain framework causes the detachment of clay particles by increased electrostatic forces and clay swelling, if swellable clays are present. This is an important mechanism that has to be taken into account in the planning of gas production from low-permeability clayey hydratebearing formations.