The in situ stress state and rock strength are key inputs for all modeling work of wellbore stability. Many techniques have been utilized to determine the in situ stress and rock strength by the observation of wellbore failures in combination with the leak-off-tests. The analyses of data from these techniques assume the rock formation to be elastic. However, when drilling through HPHT rocks coupled hydrothermomechanical processes result in a time dependent stress and pore pressure distribution around the wellbore. In this work, the initial shape and extension of the wellbore failure are determined for elastic, poroelastic and porothermoelastic models as a function of the in situ stress, wellbore orientation , rock strength, and drilling fluid pressure and temperature. It is found that the elastic model underestimates the compressive failure zone. Whereas heating increases the compressive failure zone and decreases the tensile failure zone, cooling decreases the compressive failure zone while increasing the tensile failure zone. Also, the in situ stress and rock strength are estimated according to the orientation of wellbore failure using poroelastic and porothermoelastic models. The upper bound of the rock strength obtained by a poroelastic analysis is larger than the elastic case, indicating that rock strength may be underestimated by an elastic analysis. When the rock is heated, the upper bound of rock strength obtained by a porothermoelastic analysis is larger than the poroelastic (and hence elastic) case so that the rock strength will be further underestimated if thermal effects are not considered.
The in situ stress and rock strength are two of the key considerations in prediction and prevention of wellbore instability. The minimum horizontal stress sh and the vertical stress sv can be determined by leak-off tests and density measurements, respectively.