Numerical investigation on the long-term gas production behavior at the 2017 Shenhu methane hydrate production site

In 2017, an offshore methane hydrate production test was successfully conducted at well SHSC-4 in the Shenhu Area of the South China Sea, but the long-term gas production behavior is still unknown and requires further investigation. In this study, a multi-layered methane hydrate reservoir model with three sublayers of the hydrate-bearing layer (HBL), three-phase layer (TPL), and free gas layer (FGL) was built based on the actual geological conditions at this site, and a short-term simulation was initially conducted to verify the validity of the reservoir model. Afterwards, the long-term simulations were conducted to predict the hydrate dissociation and gas production behaviors in the reservoir and investigate the contributions of each sublayer to the total gas production, and the effects of the intrinsic permeability of each sublayer on the gas production were fully examined. The simulation results indicated that the average gas production rate (1.83 × 103 ST m3/d) was less than half of that confirmed during the 2017 Shenhu production test (5.15 × 103 ST m3/d). The majority of the total gas production originated from the free gas in the FGL (56.5%), followed by the methane gas released from hydrate dissociation in the HBL (24.1%), and the TPL contributed the least to the gas recovery (19.4%). In addition, if the method of permeability enhancement was applied to the methane hydrate reservoir at well SHSC-4, the gas production could be greatly promoted, but the mechanisms were different. Finally, the following application priority was recommended: HBL > FGL > TPL.