Wellbore Stability and Wave Velocities Near a Borehole
During oil-field drilling operations, the rock that originally was in the volume occupied by the wellbore is replaced by drilling fluid that exerts pressure on the borehole wall. In general, that pressure differs from the stress that was present in the rock before the well was drilled, and the stresses in the rock near the wellbore are redistributed. That redistribution can lead to yield or failure of the rock close to the borehole, which results in decreased stress near the borehole wall.
In this chapter, the redistribution of stresses in the vicinity of the wellbore is illustrated first by using an elastic model. Then the deformation and failure of rock that might occur in response to a redistribution in stress are discussed, using a computational model that accounts for rock deformation and plastic strain in the near-wellbore region. Stress perturbations around the borehole lead to altered elastic wave velocities in the rock, which can be used to monitor the changes in stress that occur. Because the changes in elastic wave velocities are sensitive to the mechanical properties of the rock, those velocity changes also can be used to calibrate mechanical earth models (MEMs) that help in prediction of rock failure caused by production-altered stresses.
At the outset, when drilling fluid replaces the rock removed from the wellbore, it exerts pressure on the borehole wall. That pressure can cause significant changes in the stress field near the borehole.