Analysis of Stratum Subsidence Induced by Depressurization at an Offshore Hydrate-Bearing Sediment

Abstract

Gas production from an offshore hydrate-bearing sediment (HBS) by depressurization will reduce the strength of cementation and increase the effective stress of hydrate reservoirs, which can result in some potential geohazards, such as wellbore instability and stratum subsidence. In this study, a thermal−hydrological−mechanical−chemical coupling numerical simulation model was established and applied in the example of the first offshore hydrate production test of Nankai Trough, Japan (2013). On the basis of the actual data of gas and water production, the evolution of pressure, temperature, and stratum subsidence during depressurization was analyzed. The factors influencing stratum subsidence were studied, such as producing pressure difference, permeability, and Young's modulus. The results have shown the following: (1) As a result of the combined effect of hydrate dissociation and increased effective stress, there is stratum subsidence near the production well. (2) The maximum subsidence (0.085 m) appeared at the upper part of the HBS near the wellbore after 6 days of production. The subsidence weakened rapidly far away from the production well. As production went on, the stratum subsidence near the wellbore at the upper part of the hydrate reservoir continued to deteriorate. After 1 year, the stratum subsidence rate was obviously slower. (3) The increase of production pressure difference and permeability and the decrease of Young's modulus led to the deterioration of stratum subsidence. The stratum subsidence should be considered during the production of the offshore hydrate reservoir.

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