Wellbore Integrity and Drilling Risk Evaluation During Depressurization in Gas Hydrate-Bearing Reservoir (GHBR)

Abstract

Abstract Wellbore integrity in gas hydrate-bearing formations is an important issue during drilling and production. Excessive pore pressure increase and formation strength reduction may occur during a wellbore a depressurization, the most popular and economic approach for hydrate production. Dynamic thermally induced hydraulic-mechanical (HM) effects may be triggered due to the solid hydrate decompositional process once the supercritical pressure is exceeded, despite an isothermal operation condition is maintained. A comprehensive model is developed to incorporate the phase change, porosity increase, and formation-weakening due to the hydrate decomposition process triggered once a critical equilibrium pressure or temperature is surpassed during a wellbore depressurization or heating. A thermal-hydraulic-mechanical-decompositional hydrate (THMD) model is proposed and a poro-elastoplastic model is imposed, in which a linear Mohr-Coulomb (M-C) criterion is introduced to characterize plastic deformation. The onset of wellbore integrity loss is assumed to occur if an equivalent plastic strain (EPS) exceeds a critical strain measured from experiments. The peak unconfined compression strengths (UCS), depending on the solid hydrate saturation and collected from several GHBR fields in the world, is assumed to dictate the critical EPS. The onset of wellbore collapse are calculated theoretically and validated by experimental hollow cylinder tests subject to isothermal condition. The critical wellbore pressure and temperature are calculated, the solid hydration saturation effects on formation cohesion, the onset and applications of wellbore integrity risk during wellbore depressurization are highlighted.