Wellbore Failure Mechanisms in Shales: Prediction and Prevention
Authors: Gazaniol Didier , Forsans Thierry , Boisson M.J.F. , Piau J-M. 
Published: December 2007

Summary
Shale stability is still one of the most important problems faced during drilling. Until recently, stability problems were most often attributed to shale swelling; however, recent research shows that several mechanisms are involved and that their relative importance can be estimated. This paper presents a review of these mechanisms, including pore-pressure diffusion, plasticity, anisotropy, capillary effects, osmosis, and physicochemical alteration. Pore-pressure diffusion into the rock in the vicinity of the wellbore (transition from undrained to drained behavior) appears to be of major importance in these very-low-permeability rocks. Plasticity is discussed in terms of modeling. Compared with simple elastic models, modeling of plasticity can simulate the actual behavior of wellbore better.
Rock anisotropy can influence failure either by its effect on stress redistribution or through rock-strength anisotropy. Analytical studies show that the second effect is more important. Capillary effects can significantly enhance the use of oil-based muds by effectively supporting the borehole wall. On the other hand, the same kind of effect between air and pore fluid can lead to misinterpretation of laboratory results on unsaturated outcrop material.
Osmosis is a very controversial topic. Although industry publications do not give a clear idea of the extent of osmosis effects in shales, some guidelines can be established to compare it with other mechanisms. Physicochemical interaction between a shale and the drilling mud can lead to the dissolution of a mineralogical phase of the rock. Subsequent alteration of the rock cohesion can explain shale failure or dispersion. Thermal effects between cooling of the bottom part of the well and heating of the upper part can also be very significant.
The behavior of different types of muds is discussed while taking these phenomena into consideration, and the practical use of rock-mechanics models is also addressed.

Introduction
For a long time, drilling through shales has been one of the most troublesome issues for the drilling industry. Despite the real progress achieved in the area of borehole stability, shale problems are still reported frequently (including long trip times to lost holes and sidetracks, the inability to log, or important washouts leading to poor log quality or poor cementations). Moreover, stability problems become more crucial as wells become deeper and boreholes more deviated. These wells, in which a limited number of casings and mud systems can be used (because of environmental problems with oil-based muds), result in open holes of increasing lengths, especially for larger diameters. Therefore, the open hole is exposed to drilling fluids for longer periods of time, which seriously impairs the resistance of borehole walls.
The problems encountered in drilling in shales are caused by the large variety of rocks called shales and to the complexity of the mechanisms involved (mechanical, porosity-related, thermal, and physicochemical). Being able to characterize shales correctly and having a good knowledge of the mechanisms that can actually occur are key points in making an appropriate diagnosis and in solving or reducing problems associated with shales to acceptable levels.
In this paper, several mechanisms or particular shale properties are discussed. Pore-pressure diffusion, plasticity, and anisotropy are the mechanical or poromechanical mechanisms, and capillary effects, alteration, and osmosis are the physicochemical mechanisms. Some other mechanisms are also briefly discussed, as well as effects of different mud types and use of rock-mechanics models.