Chapter 4 Stresses around boreholes. Borehole failure criteria

When a well is drilled into a formation, stressed solid material is removed. The borehole wall is then supported only by the fluid pressure in the hole. As this fluid pressure generally does not match the in situ formation stresses, there is stress redistribution around the well. This may lead to deviatoric stresses greater than the formation can support, and failure may result. Knowledge of the stresses around a well is therefore essential for the discussions of well problems. This chapter illustrates that, to examine the stresses in the rock surrounding a borehole, one need to express the stresses and strains in cylindrical coordinates. The hollow cylinder model is a simple example of a borehole in a stressed formation. The model is important in itself, as laboratory tests concerning well stability often are carried out on such samples. Finally, the hollow cylinder model also provides a model for vertical wells through formations where the horizontal stresses are equal.