Long-Term Analysis of the 2020 Gas Hydrate Production Test in the South China Sea by Full Implicit Simulator of Hydrate

China conducted an offshore gas production test at the site SHSC2-6 in the Shenhu Area of the South China Sea in 2020. The geosystem and key parameters of this site, including the water depth, thickness of the hydrate and the free gas layers, pressure and temperature at key locations, and sediment porosity and permeability, are currently available. The hydrate dissociation is induced by depressurization through a single horizontal well located in the hydrate-bearing layer (HBL). The underlying free gas layer (FGL) below the HBL makes it a "two-gas coproduction" (natural gas hydrate and shallow gas) process. A novel numerical simulation code, the full implicit simulator of hydrate (FISH), is employed to develop the mathematical model and reproduce the 42 day field test in this marine hydrate reservoir. During the simulation, a permeability reduction model is used to fit the hydrate-bearing and the hydrate-free permeability in the field test. After strict history matching of the field test results (the mean absolute percentage error of gas production is 2.264%), we predict the long-term gas production behavior, including the gas and water production and the spatial distribution of pressure, temperature, phase saturations, etc. The results of the 8 year simulation indicate that the average gas production rate is 2.323 x 104 m3/day and the methane/water ratio is in the range of 50-120. This preliminary evaluation suggests that the hydrate deposits in the Shenhu Area are an unattractive production target with current technology. The main reason for this poor performance of gas production is the limited saturation of the free gas (smaller than 15%) in the hydrate reservoir. A future geological survey should focus on finding hydrate reservoirs with high gas saturation and large gas effective permeability. A key finding of this study is that the bottleneck of hydrate dissociation in the deposits is the low temperature caused by its own cooling effect. A thermal-assisted depressurization method is recommended as a potential strategy of gas recovery from marine hydrate reservoirs.