Dynamic stress distribution around the wellbore influenced by surge/swab pressure

The calculation of dynamic stress distribution around the wellbore under surge/swab pressure is an unsolved issue. The conventional method to calculate wellbore stress is only applicable to the steady state without considering the coupled effect of deformation-diffusion. In this paper, an elastodynamics model and a poroelastodynamics model have been established to overcome these deficiencies. The numerical solution is provided by the implicit finite difference method, and it is verified by the analytical solution achieved from Laplace transform and Crump inversion. Through sensitivity analysis, it shows that the period of the sine function (inner boundary condition), Biot coefficient, hydraulic diffusivity, and rock types could influence the stress distribution. Then the dynamic surge/swab pressure predicted based on Lubinski's method is implemented into these two models. Two field cases with different depths have been analyzed, and it demonstrates that the surge/swab pressure is more significant for a shallow formation because the static formation pressure and in-situ stresses are relatively small. If the tripping velocity is high, the stress distribution will be significantly changed. Finally, the stress calculated by these two models is compared with the stress obtained using the conventional solution. The comparison shows that the poroelastodynamics solution is different from the elastodynamics solution because pore pressure is significant in the poroelastodynamics solution. The poroelastodynamics solution is also significantly distinguishable from the conventional solution, which demonstrates the importance of the poroelastodynamics model. Overall, it fills the gap in calculating dynamic wellbore stress under tripping conditions, which can be applied to wellbore stability analysis and contribute to a reduction in both tripping time and drilling costs.