The Wellbore Stability Study in Bedding Shale Formationon the Condition of Plasticity

Horizontal drilling is an effective way to increase the capacity of shale formations. However, in the horizontal shale drilling, the major difficulties are the problems of the wellbore stability and the well collapse prevention. In this paper, a jointed-material model is adopted to simulate the strength anisotropy and the permeability anisotropy of the bedding shale. The model is used to obtain the distribution of the plastic regions in the vicinity of the well under the condition that the stress state matches the failure criterion. The shale permeability anisotropy also contributes to the nonuniform propagation of pressure in the formation. The model is applied to study the wellbore stability law of a shale gas well in the Longmaxi formation in Sichuan Basin, China. It is shown that the drilling fluid preferably invades in the bedding plane, resulting in a higher pore pressure in the bedding plane than that in the adjusting layers. The lower strength of the bedding shale weak planes could create four plastic shear failure locations around the wellbore. This tendency is different from the normal dog-ear-shaped breakout. The study also shows that the sealing performance of the drilling fluid plays an important role in maintaining the wellbore stability. If the wellbore wall is perfectly penetrated, the increased mud weight will enlarge the plastic zone area around the well and increase the drilling risks.