A transient pressure analysis for wellbore strengthening

Wellbore strengthening treatments have been widely applied to improve formation bearing capacity and to mitigate lost circulation during drilling fractured formations with narrow mud-weight window. However, the mechanisms and mechanics of wellbore strengthening are still not completely understood since the fluid-solid coupling process between plugging zone and drilling fluid is previously simplified. In this paper, a fully coupled numerical model that includes rock elastic deformation, fluid flow and plugging mechanics is employed to study the evolution of plugged fracture system after wellbore strengthening. The numerical result is in good agreement with the published ones, verifying the accuracy of the present model. Symmetric bi-wing fracture geometry is considered to investigate the near-wellbore stress distribution and internal pressure profile in fracture before and after wellbore strengthening. The numerical results indicate that the hoop stress can be enhanced in compression after plugging the fractures. The evolution of fracture opening and internal pressure in fracture suggest that wellbore strengthening treatment can prolong the time to reach the fracture growth state. The existence of the plugging zone can also significantly change the normal stress distribution. In addition, the impact of plugging zone permeability on its shear failure pattern is analysed, and the relations of wellbore pressure and plugging zone failure time to the controlling parameters are discussed in detail. The limitations of the present model are also pointed out.