Break-Out Prediction as a Chip Cutting by Milling Machine for Wellbore Stability in Geomechanics

Wellbore instability is caused by formation break-out or drilling-induced fracture. Formation break-out and drilling-induced fracture depend on the formation strength and in situ stresses near the borehole. The main objective of this study is to use the concept of milling machine theory to predict the break–out depth of the formation. A new approach is proposed for the prediction of formation break-out dimensions for maximum wellbore stability. The formation break-out is due to the stress concentration around the borehole. This stress concentration acts as an additional shear cutting force to break a formation. The shear cutting force is similar to the milling cutting tool force for the same formation to produce an equal dimension chip. The formation cutting force causes break-out by an imaginary milling tool. This additional shear cutting force plays an important role in the calculation of the break-out dimension. The concept of milling machine theory is applied to develop the mathematical model for the prediction of break-out dimension. Through this study, it has been observed that the depth of the break-out is the function of the break-out angle and yield strength of the formation. Shear cutting force around the wellbore is the maximum at a point near the minimum horizontal stress direction that produces break-out. Break-out angles can be calculated theoretically, empirically (using failure criterion) and actual (using image log). It has been shown that the radial stress acting on the boundary of the wellbore is not responsible for the shear failure of the formation. In this study, a relationship between break-out and stresses around the borehole has been established. The new relation established through this study may help to predict break-out dimension from the shear strength of the formation and stress profile. Till date, no such literature is available for the prediction of break-out dimensions for the wellbore stability in geomechanics. The proposed mathematical model helps us to design a cost-effective and safe well in the geologically complex reservoir for exploitation and exploration of hydrocarbon.