Chapter 10 - Borehole stability

Abstract

Borehole instabilities pose significant challenges to drilling and completion operations. In this chapter, borehole failure types and their identifying methods are introduced. Wellbore stresses in inclined boreholes are studied; elastic and poroelastic solutions are given for determining the minimum required mud weight for borehole stability. The impacts of bedding planes and rock anisotropy are also considered in the wellbore solutions to improve borehole stability modeling. This improved model enables to calculate borehole failures and minimum required mud weight along borehole trajectories with various drilling orientations versus bedding directions. The slip failure gradient in the weak planes is derived, which can be used to model wellbore shear failures in the planes of weakness. Time-dependent rock compressive strength is examined to analyze wellbore failure evolution with time. The mud weight applied to prevent salt creep and borehole closure in salt formation is also examined. Chemical reaction between shales and drilling fluids is considered in wellbore stability analysis to select right mud salinity.