Abstract
"Well control" is an activity to keep the static equilibrium between the formation and well conditions, as well as between the wellhead and bottom hole. In this paper, the former equilibrium is focused. The drilling can be interpreted as an operation in which rock mass is replaced by drilling fluids. By this operation, the fluid pressure has to support the stress and formation pressure that have been kept balanced in the original condition. However, there are two difficulties to keep the equilibriums by hydrostatic drilling fluid pressure. One difficulty is that hydrostatic pressure cannot support all direction of the anisotropic stresses. The other one is that two equilibrium, pore fluid pressure and radial stress, should be satisfied at the hole surfaces simultaneously. Then, generally, original condition cannot be achieved. Those unbalances lead to the deformation of solid and stress concentrations, and the fluid exchange between borehole and formation. Both phenomena can cause wellbore instability and kicks. Therefore, it is useful for drilling engineers to recognize how forces by stress and formation pressure act on their wells.
At the beginning of the workshop "Well control", we discuss what forces work on the wellbore. Also basic mechanics of porous media, stress concentration, and stress measurement methods are summarized.
