A semi-analytical model considering two-phase flow and full thermal–hydraulic–mechanical (THM) coupling for the mechanical response of formation in hydrate production

Abstract

Marine natural gas hydrate exploitation involves complex thermal–hydraulic–mechanical–chemical (THMC) coupling. However, the influence of the full hydraulic–mechanical (HM) coupling on formation stability in hydrate exploitation is absent in the published analytical/semi-analytical models. This study proposed a new semi-analytical model with full THM coupling and two-phase flow to analyze the influence of the full HM coupling and some key factors on formation stability in hydrate production by depressurization, heat injection and their combination, focusing on the full THM coupling, especially for the influence of the mechanical field on pore pressure due to volumetric strain change rate and variations in permeability/porosity. The semi-analytical model is validated by a finite element model with the same conditions, experimental results and complex numerical model. The full HM coupling has a significant influence on the mechanical response of the formation. Compared with the solutions of partial HM coupling, the pore pressure increases by 23.43 % relative to the pressure gradient, incremental radial displacement decreases by 46.89 %. Wellbore stability is minimally influenced by HM coupling due to the constant production pressure, whereas stability at the dissociation front is improved as a result of HM coupling.