| Summary: | Diagenetic natural gas hydrate (DNGH) is a metastable material that is widely distributed in the frozen formations of Qinghai–Tibet Plateau in China. Drilling boreholes in such frozen formations can lead to the decomposition of DNGH, which is deleterious for wellbore stability as this process can lead to a complex and uncontrollable changes in the structure of the frozen formation. In this study, a fluid–solid–heat coupling mathematical model was developed for evaluating the wellbore deformation of a DNGH reservoir while considering the effect of DNGH decomposition. This mathematical model includes the kinetic equations describing DNGH decomposition, rock skeleton deformation equations, seepage field equations, temperature field equations, dynamic porosity equations, and dynamic permeability equations. COMSOL Multiphysics software was used to solve this new mathematical model and the deformation of a real wellbore in the permafrost of Qilian Mountains was analyzed. Two drilling methods were considered in the numerical simulation. The results indicate that: 1) the stress redistribution that occurs immediately after drilling the DNGH reservoir stabilizes rapidly. The maximum stress observed was at an angle of 135° with respect to the minimum horizontal principal stress. 2) The stress in the wellbore surrounding rock under the condition of micro-overbalanced drilling stabilizes more rapidly, and the values are higher compared with the results of micro-underbalanced drilling. 3) The most significant compression deformation occurs in the direction of minimum stress and the displacement in the wellbore surrounding rock simulated by micro-overbalanced drilling increases more rapidly and produces lower stress compared to that simulated with micro-underbalanced drilling.
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