Validation of Soil Models for Wellbore Stability in Ductile Formations Using Laboratory TWC Tests.

This paper describes numerical analyses of laboratory thick-walled cylinder (TWC) tests used to study the stability of vertical wellbores in hard clays. The TWC experiments were designed to simulate the drilling process by reducing the internal cavity pressure although maintaining the external radial stress and measuring internal changes in volume within the model wellbore. Radial deformations of the cavity (borehole squeezing) attributable to plastic deformations of the clay represent a critical condition for the design of wells drilled through ductile formations (i.e., very hard soils and poorly lithified rocks). The current analyses simulate coupled flow and deformation in the TWC soil specimens using a finite element (FE) program and the MIT-E3 model to represent the effective stress-strain-strength properties of the reference clay, resedimented Boston blue clay (RBBC). The model is freshly calibrated using results of laboratory triaxial element tests performed at the same stress levels as the TWC experiments (s'_vc = 1-10 MPa). The numerical simulations are compared with data from experiments performed at different levels of confining stress, rates of depressurization, and wall thickness. The numerical model provides good estimates of the instability measured in the TWC tests and provides an accurate estimate of the undrained shear strength controlling cavity deformations. These validations provide confidence in the application of the model for estimating behavior of prototype (i.e., vertical and deviated) wellbores in similar hard clay formations. [ABSTRACT FROM AUTHOR]