This paper utilises a recently modified 3D geo-mechanical constitutive model for wellbore stability analyses. The modified constitutive model using two independent yield surfaces composed of shear failure and a strain rate-dependent pore collapse yield surfaces in order to describe dilation and compaction mechanism of deformation was developed based on the combination of non-associated shear failure and an associated viscoelastic consistency model relying on empirical formulation originally developed by J.A. de Waal and co-workers in the late 1980s. A fully implicit stress-update algorithm for multi-mechanism consistency model is used to determine the viscoplastic strain components. In this study, a series of single lateral hole (SLH) experiments were conducted on chalk samples. For this purpose, a horizontal wellbore was drilled laterally at the center of the specimen and loaded under triaxial condition with constant stress ratio. The SLH samples were scanned after the experiments using Computed Tomography scanning to precisely identify the damaged zones formed around the borehole. The results from the experiments and CT analysis were compared with the model prediction and a good agreement was observed.
Pore collapse, shear failure and tensile failure are major deformation mechanism involved in constitutive model of soft rock which makes their behaviour quite complex to describe. An additional complication specific to soft rocks is that their deformations have been repeatedly reported to be significantly influenced by the loading / deformation rate to which they are subjected (Hueckel et al, 2001. Priol et al, 2007). Evidence from laboratory exhibits rate dependent strength and time dependent deformation called creep in many geological formations such as clays, sand and soft rocks.
A widely used formulation for visco-plastic materials is the Perzyna model (Perzyna, 1963). The main feature of this model is that the stress position can go beyond the rate-independent yield function called reference surface used for describing the viscoplastic strain, which effect is known as overstress (DeGennaro et al, 2003. Datcheva et al 2001). Several models are developed based on rate-lines formulation (Hickman and Gutierrez, 2007. Gutierrez and Hickman, 2011). In this case, all viscoplastic strain components are scaled from the viscoplastic volumetric strain which is inversely proportional to distance in stress space between stress point and the pore collapse yield surface.