| Summary: | Classification on natural gas hydrate deposits is summarized in this paper, and a dynamic theory of gas hydrate reservoirs formation is put forward. Based on this theory, there have three types of reservoirs: growing type(including seeping type and diffusing type), matured type and regressive type. Some experiments have been done by simulating in-situ hydrate formation system when gas migrating in seepage system in a glass bubble cell. The phenomena in experiments indicate that gas migration can change the quality of mass and heat transfer when hydrate forming. There may exist a “pseudo” buffer layer which has some distance of thickness in the interface between gas and liquid. The overburden stress in sediments capillary will gradually breed because of continuously hydrate formation in the course of gas migration, and causes faults or fractures of the stratum matrix. Therefore, it must build up new hydrate phase diagram in order to adapt the new characteristic of natural hydrate system brought by gas migration. Assuming there has a methane venting zone of 1500m, and the methane flux is 1000kmol/a, and the temperature of methane hydrate-bearing sediments ranges from 3℃ to 20℃, then according to the hydrate film growth theory, by numerical simulation,the results show that the cementation velocity in sediments matrix of methane hydrate is about 0.2nm/s, and the seepage system will evolve into diffusion system over probably 35ka. Meanwhile, the methane hydrate growth velocity in leakage system is about 20-40 times faster than in diffusion system. A multi-phase flow model including water - gas (free gas) –salt- hydrate has been eatablished to describe this flow-transfer-reaction process. Combinating informations gotten from in-situ drilling samples datum, the methane formation process in Shenhu area of South China Sea has been deducted. It has been speculated that there has a moderate methane flux over this seafloor sediment. If the flux is about 0.5kg/m2·a, then it has evoluted about 4800a through simulation, and ...
|
|---|