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Highly inclined, extended-reach wellbores may have to remain open for prolonged time periods, not only during the drilling stage but also over the life of a reservoir. New challenges also emerged since the increasing use of horizontal wells, drilling in naturally fractured media, in very deep formations, and in difficult geological conditions, where wellbore stability is of major concern (Willson and Willis 1986). For example, a 8,715 m deep well was drilled in crystalline rock in Germany and some types of wellbore instabilities (breakouts, washout, undergauged sections) were observed (Hoffers et al. 1994).
Some wellbore instabilities associated with complex geologic conditions, where the stress regime was controlled by active faults, are reported in the Cusiana field (Colombia), the Pedernales field (Venezuela), the Alberta Basin (Canada), the Tarim Basin (China), certain areas of the Norwegian Sea, and offshore Indonesia (Willson et al. 1999, Plumb et al. 1998, Wiprut and Zoback 1998, Ramos et al. 1998).
When a borehole is drilled in a naturally fractured formation, excessively high mud density allows the drilling fluid to penetrate into fractures, mobilizing the rock blocks, and intensifying ovalization (Charlez 1999). When this occurs, the fractured blocks are no longer subject to the mud overbalance pressure, and the destabilized blocks can cave into the well bore as a result of swabbing the formation when tripping (Willson et al. 1999).
When a borehole crosses a fault, drilling mud may invade the discontinuity plane. Apart from mud losses, penetration of the fluid reduces the normal stress and induces a displacement along the crack planes which shears the well, as shown in Fig. 7.1. The consequences can quickly become dramatic and could lead to partial or even total loss of a well.
Two case histories in Aquitaine, France were described that resulted in the loss of the wells and the need for the drilling of two new wells, costing in the range of US$30 million (Maury and Zurdo 1996). Wellbore instability can result in lost circulation where tensile stresses have occurred due to high drilling mud pressure (Fig. 7.2a); breakouts and hole closure in case of compressive and shear failures (Fig. 7.2b).
During drilling stage an open hole is supported by drilling mud pressure to keep wellbore from collapse. If the mud weight is lower than the shear failure stress or collapse stress, the shear failure and compressive failure occur in the wellbore in the minimum far-field stress (Sh) direction, causing hole collapse or breakout. If the mud weight exceeds the rock tensile strength, the tensile fracture is induced in the maximum far-field stress (SH) direction.
Consequently, this may cause drilling fluid losses or lost circulation. Figure 7.3 shows a typical wellbore instability due to breakout and drilling induced tensile fracture. For a circular opening with large diameter the hole/tunnel breakout has a similar behavior as small boreholes. Figure 7.4 presents hole breakout in a circular tunnel with a radius of R = 1.75 m in the Underground Research Laboratory of Canada (Martino and Chandler 2004). The fully developed notch (breakout) in the roof was caused by stress redistribution due to excavation.
The notch is stable, owing to its naturally formed shape, which develops a confining pressure at the notch tip. The notch will remain stable unless disturbed by changing conditions, such as increased temperature, small stress changes caused by nearby excavations. In severe cases the borehole instability can lead to loss of the open hole section. The borehole stability problem can be considered by separating the potential rock failure mechanisms into the following four categories (Roegiers1990)