Numerical Evaluation of Long-Term Depressurization Production of a Multilayer Gas Hydrate Reservoir and Its Hydraulic Fracturing Applications

In this study, a multilayer gas hydrate reservoir model was implemented based on the geological conditions of the Shenhu area in the South China Sea (SHCS) to predict the production performance of the reservoir during long-term depressurization. Hydraulic fracturing technology was introduced to boost production, and its positive/negative impact on the production behavior of the hydrate reservoir was evaluated. Results show that hydrate dissociation is severely constrained by pressure propagation and fluid flow in the low reservoir. During production, almost half of the wellhead gas production is from the dissolved gas in seawater and the free gas contained in sediments. Massive secondary hydrate forms and gathers in the hydrate layer I and near the interface of hydrate layers. Underlying free gas is conducive to reservoir production, in which the cumulative wellhead gas production can be increased by ∼59% compared to the reservoir lacking underlying free gas. On one hand, hydraulic fracturing can significantly promote hydrate dissociation and increase the capacity of production, especially for long-distance fracture implemented in the middle part of the hydrate layer. On the other hand, high permeability in the fractured zone also provides a convenient channel for water in the sedimentary layer. After hydraulic fracturing, the production efficiency of the reservoir is still low due to the involvement of more pore water. In future, the combination of hydraulic fracturing and other auxiliary means can be considered to develop hydrate reservoirs.