(174cj) Multilayer Hydrate Sediment Gas Production Behavior by Depressurization with CO2-Enriched Gas

Large amounts of naturally occurring gas hydrates can be found in continental margin zones around the world and in and in permafrost areas. Efficiently and safely producing this natural gas stored as gas hydrates remains an urgent and as yet unsolved challenge. Among several natural gas hydrate production methods, CO2 replacement has the potential benefit of combining methane production and carbon dioxide storage. The introduction of a second gas with CO2 has been shown to increase the recovery of methane from natural gas hydrate. Injection of CO2-enriched air is viable and economical compared with transportation of flue gas. Further, a number of field trials have demonstrated that depressurization can successfully be employed in producing gas from hydrate reservoirs. This method has the drawbacks of geological risk during gas production as huge volumes of water are produced and sediments are potentially loosened by the removal of gas.

In this study, a series of experiments were conducted where air and CO2-enriched air were injected into simulated multilayer hydrate sediment. The experiments were conducted using a method that combines depressurization and CO2-enriched air injection for methane recovery. Recovery rates are shown to increase when a three-stage depressurization method was introduced in the process at three different initial injection pressures at the same gas hydrate reservoir temperature. Compared with injecting air, injecting CO2-enriched air promotes gas hydrate production. A multi-layer hydrate cap mechanism is proposed to describe the methane hydrate production combined with CO2 storage.