5/19/06
Figure 6-17 – The area in which wellbore breakouts form around a cylindrical well can be modeled using a total plastic strain criterion rather than a stress criterion. These finite element calculations indicate the zone of expected breakouts assuming a critical strain level at which failure occurs (courtesy S. Willson). (a) Strain around a wellbore assuming a strain softening model of rock deformation (red indicates high strain). (b) Failure zone predicted using a strength of materials approach and Mohr Coulomb failure criterion. Chemical Effects - Chemical interactions between drilling mud and clay rich (shaley) rocks can affect rock strength and thus exacerbate wellbore failure. While the utilization of oil based muds can mitigate such problems, it can be very expensive to implement or precluded by regulatory restrictions. In the context of the theory described by [Mody and Hale, 1993], there are three important factors that affect wellbore stability when chemical effects may be important: First is the relative salinity of the drilling mud in relation to the formation pore fluid. This is expressed as the water activity Am, which is inversely proportional to salinity. If the activity of the mud is greater than that of the formation fluid (Aw), osmosis will cause the formation pore pressure to increase and the wellbore to be more unstable. Second, the change in pore pressure is limited by the membrane efficiency, which describes how easily ions can pass from the drilling mud into the formation. As oil has perfect membrane efficiency it prevents ion exchange. Finally, the ion exchange capacity of the shale is important as the replacement of cations such as Mg++ by Ca++ and Na+ by K+ weakens the shale. The impact of these effects are illustrated in Fig. 6-18. In Fig. 6-18a, we show the portion of the wellbore wall that goes into failure is shown as a function of membrane efficiency and water activity of the mud. As discussed at some length in Chapter 10, a convenient rule-of-thumb is that if less than half the wellbore wall goes into failure, wellbore stability will not be particularly problematic. As can be seen,
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