ABSTRACT:

The influence of temperature on wellbore stability in geothermal drilling has attracted wide attention, especially the delayed damage of borehole wall rock in complex formation caused by borehole temperature recovery cannot be ignored. The traditional model only considers the effect of temperature reduction on wellbore stability, but does not consider the stress distribution and mechanical parameters deterioration of wellbore surrounding rock caused by temperature recovery. Based on the transient heat transfer mechanism, the finite difference method is used to simulate the temperature field of wellbore/reservoir system, and then the time-dependent temperature boundary on the wellbore is coupled into the temperature-stress field by Duhamel principle, the stress field around the wellbore in transversely isotropic formation during drilling fluid circulation is obtained. In addition, the modified Mohr-Coulomb criterion is used to establish the time-dependent analysis model of wellbore instability which considers the effect of temperature and hydration. The results show that: 1) If we don’t consider the effect of temperature and hydration on the mechanical properties of rock, the abnormal high pressure caused by fluid-solid coupling at the time of wellbore opening makes the wellbore in the most dangerous state. 2) After the drilling fluid stops circulating, the formation temperature around the wellbore gradually recovers, and the pore pressure and additional stress generated by the temperature recovery are not conducive to the wellbore stability, in addition, due to the deterioration of rock mechanical properties caused by temperature rise and hydration, the risk of wellbore collapse is greater than that at the time of wellbore opening.

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