Numerical Analysis of Production Behaviors and Permeability Characteristics on the Second Gas Hydrate Production Test in the South China Sea

The dissociation of hydrate plays an important role in restructuring the pore space and controlling the permeability, which determines the gas recovery from offshore hydrate reservoir. Based on the second production test in the Shenhu area, China South Sea (2020), this study investigates the production behaviors and permeability characteristics of the hydrate reservoir by using the depressurization method with a single horizontal well. A permeability adjustment model is proposed to revise the absolute permeability, and then the changes in permeability characteristics with hydrate dissociation are analyzed on a field scale for the first time. The dissociation of the hydrate can significantly improve the intrinsic permeability, thus promoting fluid flow and gas production efficiency. The simulation results show that it achieves a cumulative gas production of 89.41 × 104 m3 for 30 days, which is very close to the second production test of 86.14 × 104 m3. Also, the produced gas-to-water ratio is in the range of 15–55, and the ratio of dissociation-originating gas to the cumulative gas production reaches 0.50 for the 30 day simulation. The invasion of the underlying water with relatively high temperatures can promote the rate of hydrate dissociation in the bottom of the hydrate-bearing sediment due to the poor thermodynamic stability. The sensitivity analyses also indicate that the parameters of absolute permeability, irreducible gas saturation, and irreducible water saturation have significant influences on gas production. These insights verify the feasibility of the depressurization with the horizontal well and can be beneficial for future hydrate exploration.