Numerical Modeling of Gas Hydrate Accumulation in the Marine Sediments of Shenhu Area, Northern South China Sea

TLDR: Wang et al. as mentioned in this paper designed a 2D numerical model to simulate the formation and accumulation in the marine sediments, aiming to understand this unique feature of gas hydrate, as well as to study the relationship between gas chimneys, mud diapirism and gas hyrate reservoir.

Abstract: Shenhu is located in the middle of northern South China Sea. The bottom simulating reflector (BSR) exists widely in the area, and the strong BSR appears mostly in the diapiric structure or at the top of gas chimney. Drilling showed that gas hydrate was only distributed in the upper 10~25 m range above the bottom of hydrate stability zone (BHSZ), but with the maximum saturation of up to 48%. We designed a 2D numerical model to simulate the formation and accumulation in the marine sediments, aiming to understand this unique feature of gas hydrate, as well as to study the relationship between gas chimneys, mud diapirism and gas hydrate reservoir. Modeling results indicate that, the methane from deep moves upward vertically and is confined to a rather narrow range, which is shown as gas chimney in the seismic section and BSR at its top. During the process of slow movement of gas, the BSR (top of gas chimney) is always lower than the BHSZ controlled by temperature and pressure. Unless it crosses the BHSZ hydrate cannot form. In case the hydrate forms, the stratum becomes a sealing layer, thereby preventing the continued migration of methane. Consequently, the hydrate only forms within a very thin layer above the BHSZ, but the saturation can be very high. After that, if the gas supply from deep stops, the gas chimney will gradually disappear, leaving only the hydrate. So, the existence of gas chimney does not necessarily correspond to hydrate. In addition, the modeling results also show that the fracture fluid activity (diapir) is not always favorable to the hydrate formation. If the fluid flow from deep is very large, all the methane may dissolve in fluid once produced, and is taken away. Thus, no free gas and hydrate is able to form. Only when the methane flow matches the fluid flow can the hydrate form. Therefore, the larger the fluid flow, the more difficult to form hydrate.