An analytical model of time-dependent elastoplasticity with hydraulic–mechanical coupling for wellbore stability in hydrate exploitation

Abstract

Natural gas hydrates have potential economic and environmental significance. However, wellbore instability likely occurs during drilling due to the deterioration in the geomechanical properties of formation caused by hydrate dissociation. In this study, a time-dependent elastoplastic analytical model for wellbore stability is proposed under overbalanced and underbalanced drilling, considering the hydrate dissociation-induced changes in the geomechanical properties and full coupling between hydraulic–mechanical fields. The analytical solutions are verified with the finite element results under the same conditions and validated with the results from complex numerical simulations. According to the analytical solutions, the difference in pore pressure relative to the initial pore pressure decreases, and accordingly, the incremental displacement decreases (by approximately 39.54% for the maximum displacement) after the influence of the mechanical field on pore pressure is considered. The influence is greater far from the wellbore and smaller around the wellbore due to the constant drilling fluid pressure. The risk of wellbore instability increases with time, and the safe drilling window of drilling fluid pressure will be narrowed by hydrate dissociation (by −5.90%), the increase in the drilling fluid temperature (by −12.64% from the temperature of 20 °C to 30 °C) and the reduction in cohesion (by −47.43% at most).

Keywords:

• Methane hydrate
• wellbore stability
• time-dependent
• elastoplastic analytical model
• thermal-hydraulic-mechanical coupling

Data availability statement

The authors confirm that all relevant data are included in the article and that no supplementary information files are available.

Disclosure statement

No potential conflict of interest was reported by the authors

Additional information

Funding

This study was supported by the National Natural Science Foundation of China (Grant Nos. 11872281, 51890911), Hainan Province Science and Technology Special Fund (ZDYF2021SHFZ264), Hainan Research Institute of China Engineering Science and Technology Development Strategy (No. 21-HN-ZD-02-5) and the State Key Laboratory of Disaster Reduction in Civil Engineering (SLDRCE19-A-06).