Assessing the prospect of long-term exploitation of Class 1 hydrate deposit with horizontal well in the South China Sea from energy, environmental, and economic perspectives

Economical gas recovery from hydrate reservoirs with underlying gas, namely Class 1 hydrate deposits, still remains a challenge. Although the mining potential of the Class 1 hydrate deposits have been investigated by some researchers, the knowledge of the production responses under different exploitation schemes in horizontal-well systems is insufficient. Additionally, few studies have systematically evaluated the long-term mining potential of the Class 1 hydrate deposits from economic perspective. On this basis, in this study, we investigated the characteristics of the gas–water production stage and numerically analyzed the risk of methane leakage from a typical Class 1 hydrate deposit in the South China Sea (SCS). Then, the economic benefits and the energy utilization efficiency were evaluated. The results revealed that the depressurization method with dual wells exhibited the best performance, with cumulative produced gas values 1.1–1.5 times higher than those in the other simulated cases at the end of production. In all of the studied cases, extremely low energy utilization was achieved in the initial stage of production, and it is concluded that considerable economic benefits can be generated in the production stage dominated by free gas. Only a small amount of dissolved methane effused during the entire production process. Additionally, the permeability of hydrate-bearing layer and the initial gas saturation were the key factors influencing the gas production. However, the average production level of the Class 1 hydrate deposits in the SCS was still an order of magnitude away from that required for commercial exploitation.