Study on influences of geological and gas source conditions on gas-chimney hydrate accumulation using a reservoir numerical simulation method

Abstract

The Shenhu Area in the South China Sea is rich in oil and gas resources and has many vertical gas chimneys, making it an excellent geological environment for hydrate accumulation. This paper examines the geological conditions governing these gas-chimneys. A numerical simulation method based on the partial-equilibrium reaction model of hydrate was applied to simulate the migration of methane gas and the resultant hydrate formation when the gas enters the hydrate stability zone under the seabed through gas-chimneys. The dynamics of this gas-chimney hydrate accumulation were analyzed, and the influences of different factors—namely, the fluid supply time, rate, and temperature—on the formation temperature and ultimate distribution of the hydrate reservoir were evaluated. The simulation results indicate that the accumulation of hydrate via gas-chimneys is significantly affected by the temperature of the gas source, the transfer state of the methane gas, and the number of cycles of alternating gas–water invasion. Hydrate accumulation takes shape in an annular or semi-annular distribution pattern divided by fluid state as follows: a two-phase gas–water zone, a three-phase gas–water–hydrate zone, a two-phase water–hydrate zone, and a phase of water passing from the inside to the outside. Formation inclination and reservoir heterogeneity can greatly affect the distribution shape and abundance of the hydrate. A high fluid supply temperature, frequent alternating invasions of gas and water, and long-term pore-water invasion at a high rate can jointly cause a large central hydrate-free zone. In contrast, a long-term supply shutdown during the alternating gas–water invasion process, and a high gas rate with a low water rate in the gas-dominant invasion stage, foster the accumulation of hydrate in great abundance and with considerable thickness. The results of this study can help us understand the accumulation of hydrate through gas chimneys in the Shenhu Area.