Physical Modeling of Hydrate Dissociation in Sandy Sediment by Depressurization under Hypergravity and Normal Gravity Conditions

Abstract

Geomechanical and heat transfer characteristics of gas hydrate-bearing sediment (GHBS) are significantly affected by hydrate dissociation during gas production from reservoirs, which is typically tens of meters in thickness. This paper presents the development of an innovative in-flight apparatus that is capable of modeling hydrate dissociation in GHBS on a geotechnical centrifuge, by which a series of model tests are conducted under normal gravity (1g) and hypergravity (100g and 80g). The effects of the hypergravity field on the development of pore pressure, soil deformation, as well as particle migration and gas production during hydrate dissociation are explored. Results show that gas released from hydrate dissociation increases excess pore pressure and changes the soil pore structure. During hydrate dissociation, the accumulation of excess pore pressure leads to the development of gas-driven fractures and the subsequent formation of a dominant seepage channel. The critical excess pore pressure of fracture formation in the 100g test is higher than that in the 1g test. The dominant seepage channel promoting fluid seepage and fine particle migration is more likely to be formed into obvious fracture structures under 100g compared with slender pipe structures under 1g. Two peaks are witnessed in gas production in the 100g test, corresponding to the stage at maximum pressure difference and at the complete formation of dominant seepage channels, which is consistent with that in the field trails. These results indicate that the formation of fracture during hydrate dissociation is beneficial to efficient gas production, while the problem of particle migration should be carefully paid attention to.