Abstract
In the process of drilling, wellbore instability in shale formations is frequent and makes it hard to drill. In order to find causes of wellbore instability in shale formations, this paper proposed a wellbore instability analysis model to reveal the combined effects of anisotropic strength of shale, occurrence of stratification, strength reduction, and seepage stresses caused by drilling fluid on wellbore instability. The results show that when the occurrence of bedding planes is certain, it can maintain wellbore stability in the condition of lower mud weight to choose a reasonable azimuthal angle. With changes of shale strength and porous flow, the wellbore would increase the level of instability with drilling time, which means mud weight needs to be increased to keep the wellbore from collapsing.
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