Numerical investigation of geomechanical responses from a marine hydrate accumulation at the Ulleung basin in the Korean East Sea

Gas hydrate reservoirs are currently considered as one of the clean future energy resources. Gas hydrates, which mainly consist of methane, are solid crystalline compounds in which gas molecules are trapped within the lattice of ice crystals. From their thermodynamic characteristics, they are typically found in the oceanic sediments or permafrost regions. One of the main concerns related to gas production from the hydrate deposits is geomechanical instability. When the hydrates, which support part of the stress in the formations, are dissociated or formed, the stress and strain fields are significantly changed. In particular, the oceanic hydrate sediments are highly affected by geomechanics, because of huge overburden. In this study, we investigate geomechanical responses from a marine hydrate accumulation at the Ulleung basin in the Korean East Sea, by numerical simulation, when gas is produced by depressurization. Depressurization is widely used for gas production from the hydrate deposits, dissociating the hydrates. We employ two-way coupling between flow and geomechanics, proposed by Kim et al. (SPE Journal 2012 17(2):485-501), in order to model variations in porosity and permeability more rigorously. From the numerical results, the geomechanical situation appears challenging, as significant subsidence (exceeding 3.5 m near the sea floor and 1.5 m at the top of the hydrate deposit) is estimated to occur along a large part of the wellbore, and geomechanical failure within the 20m-thick system is possible early in the production process. However, there is still significant uncertainty in geomechanical data, because the data are incomplete. Further simulations based on the experimental and field data will be required.