The dynamic wellbore pressure induced by the frictional pressure loss and swab/surge pressure may lead to wellbore instability in the process of drilling. However, the traditional wellbore stability analysis assumes a steady wellbore pressure, rather than a dynamic wellbore pressure. Therefore, to investigate the influence of dynamic wellbore pressure on wellbore stability, an elastoplastic model was proposed by involving the dynamic wellbore pressure, and the finite element method was employed to solve this model. The normalized yield zone area (NYZA) was utilized to characterize the stability of wellbore. The influences of waveform, mean pressure, amplitude, period, and cycle number of the dynamic wellbore pressure wave on wellbore stability were simulated and discussed. The results indicated that the equivalent plastic strain and NYZA increased significantly when the dynamic wellbore pressure is applied, so that the dynamic wellbore pressure has a significant impact on the wellbore stability. Both waveform and wave period almost have no impact on wellbore stability, while the mean pressure, wave amplitude, and cycle number have a significant impact on wellbore stability. Overall, the rank of influence degree on wellbore stability are as follows: cycle number > wave amplitude > mean pressure > waveform > wave period. The primary cause of wellbore instability is the accumulation of plastic damage induced by shear or compressive stress around the wellbore.
Wellbore instability is a classic rock mechanics problem, it is generally caused by excessive stress concentration that exceeds rock strength. Wellbore instability costs drilling industry certainly more than $100 million per year worldwide (Fjar et al., 2008; Ma et al. 2018a; Aadnoy and Looyeh, 2019; Yang and Liu 2021).