WELLBORE FAILURE MECHANISMS IN SHALES - PREDICTION AND PREVENTION

Shale stability is still one of the most important problems faced duri ng drilling. Until recently, stability problems were most often attrib uted to shale swelling; however, recent research shows that several me chanisms are involved and that their relative importance can be estima ted. 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 roc k in the vicinity of the wellbore (transition from undrained to draine d behavior) appears to be of major importance in these very-low-permea bility rocks. Plasticity is discussed in terms of modeling. Compared w ith simple elastic models, modeling of plasticity can simulate the act ual behavior of wellbore better. Rock anisotropy can influence failure either by its effect on stress redistribution or through rock-strengt h anisotropy. Analytical studies show that the second effect is more i mportant. Capillary effects can significantly enhance the use of oil-b ased muds by effectively supporting the borehole wall. On the Ether ha nd, the same kind of effect between air and pore fluid can lead to mis interpretation 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, s ome 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. Subseque nt alteration of the rock cohesion can explain shale failure or disper sion. Thermal effects between cooling of the bottom part of the well a nd heating of the upper part can also be very significant. The behavio r of different types of muds is discussed while taking these phenomena into consideration, and the practical use of rock-mechanics models is also addressed.