Numerical Study on Stratum Deformation during Dissociation Process of Methane Hydrate by Depressurization

Abstract

Depressurization process is regarded as the most effective process for gas recovery method from the viewpoints of gas productivity and economic efficiency among in-situ dissociation processes of Methane Hydrate (MH) existing in marine sediments. However, it is supposed that the stratum deformation occurs due to MH dissociation and increase of effective stress in the stratum during operation of depressurization. When stratum consolidation is progressed, the occurrences of damage on production well and destabilization of seabed in addition to decrease of gas productivity are concerned. Therefore, for optimization of gas production process by depressurization, it is necessary to perform numerical simulation in consideration of a series of phenomenon during MH dissociation in porous media and predict the behavior of stratum deformation quantitatively. In this study, using the geo-mechanical simulator named as COTHMA developed under MH21 research consortium, we carried out the field-scale numerical simulation to clarify a series of behaviors during MH dissociation process by depressurization. On the basis of field data for Nankai Trough area, reservoir model was constructed as alternation strata consisting of sand and mud layers. From calculation results, it was found that consolidation process during depressurization consisted of three stages such as stage-1: consolidation only due to decrease of water pressure in pore space, stage-2: consolidation due to both MH dissociation and decrease of water pressure and stage-3: consolidation only due to MH dissociation under a constant pressure condition. The second stage involving increase of effective stress by depressurization and decrease of elastic modulus due to MH dissociation has large effect on consolidation behavior of each layer. In addition, larger vertical stress was applied to the casing and cementing of production well corresponding to the interval of depressurization. Furthermore we discussed the effects of degree of depressurization, MH saturation and absolute permeability on the behavior of stratum deformation and the integrity of the production well during depressurization process.

Keywords

• methane hydrate
• depressurization
• dissociation
• stratum deformation
• consolidation
• distribution of stress
• numerical simulation