Numerical Study on Multiphase Production Dynamics and Sand Migration in Deepwater Unconsolidated Shallow Gas Reservoirs with Hydrate-Bearing Layer

Deepwater shallow gas reservoirs possess substantial resource potential. However, because they are commonly hosted in unconsolidated sandstones and overlain by a hydrate-bearing layer (HBL), their development is accompanied by complex fluid–solid interactions and significant sand production risks. In this study, a three-dimensional radial multiphase and multicomponent numerical model incorporating sand migration was established using CMG STARS to investigate the production behavior and spatiotemporal evolution of gas, water, and sand during depressurization production of a shallow gas layer (SGL) beneath an HBL. The results show that gas, water, and sand production rates rapidly reach peak values at the early production stage. Thereafter, gas production gradually declines with reservoir pressure depletion, whereas the gradual restoration of water-phase mobility leads to a slow increase in water production rate and water–gas ratio. Hydrate dissociation remains limited to the gas–water interface, while the sand production zone evolves from rapid expansion to gradual contraction. increases in pore volume and flow capacity induced by sand migration contributes little to gas production enhancement, with a maximum increase of only 1.2%. Higher reservoir permeability results in greater gas and water production, a higher water–gas ratio, and higher sand production. In contrast, a higher initial water saturation of the SGL reduces gas production but increases water production and water–gas ratio while suppressing sand production. A larger overall pressure drawdown enhances gas and water productivity but aggravates sand production. A smaller single-step pressure reduction causes a more pronounced delay in productivity release and induces alternating expansion–contraction fluctuations of the sand production zone. These findings provide guidance for sand-control design and production optimization in deepwater shallow gas reservoirs.