Wellbore stability is affected by thermal and chemical reaction between drilling fluid and pore fluid in formation, in order to analyze the thermal and chemical effect on pore pressure and effective stresses, a generalized plain strain finite element model is developed, the model is also available for any inclined well drilled in transversely isotropic formation and could greatly save computing time comparing to three dimensional model. The results indicate that temperature and solute concentration of drilling fluid states different influence on wellbore stability: though cooling effect of drilling fluid could reduce wellbore shear failure risk by decreasing pore pressure and effective stresses, tensile fracture is easier to form in maximum principle stress direction due to sharply decreased effective compressive tangential stress. Lower solute concentration could increase pore pressure greatly because drilling fluid tends to flow into formation under chemical potential difference, then higher pore pressure significantly decreases effective radial stress in turn, so wellbore "spalling" instability may happen when effective radial stress turns to be tensile. Meanwhile, the developed model reveals that Young's modulus and permeability anisotropy ratios play an important role in the pore pressure and effective stresses distribution for the directional well in transversely isotropic formation, however, the effect of Poisson's ratio could be ignored.

1. Introduction

Wellbore instability is a universal problem in drilling operation, especially in shale formations, the interaction between shale and water based drilling fluid is studied a lot by many researchers, Detournay and Cheng (1988) introduced coupled poroelastic theory and the first analytic solution is given, then poroelastic theory is extended by taking into consideration thermal and chemical effects. Ekbote and Abousleiman (2006), Ghassemi and Diek (2003) investigated fully coupled chemo-poroelastic problem and eventually obtained analytical solutions that are suitable for isotropic and transversely isotropic formations, Ekbote (2002), Tao and Ghassemi (2006) have studied thermo-chemo-poroelastic problem. Meanwhile, some numerical methods are applied, Cui et al. (1996) used finite element method to analyze pore pressure and stress distribution around directional well in anisotropic formation based on poroelasticity. Ghassemi and Zhang (2004) applied the boundary element method to study poroelastic problem, Zhou and Ghassemi (2009) presented a 2D finite element model based on nonlinear chemo-poro-thermoelasticity for isotropic formation.

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