Wellbore stability while drilling

The drilling for oil and gas exploration and production under increasingly difficult geological conditions has revealed a need for better understanding of borehole stability issues. It is estimated that wellbore instability results in substantial economic losses of about US$ 8 billion per year worldwide. Many innovative technologies have been applied in the oil and gas industry, such as underbalanced drilling, high pressure jet drilling, re-entry horizontal wells, and multilaterals from a single well. These have definitely increased the demand for wellbore stability studies. Recently, technological advances have been pushing the boreholes to reach beyond 34,100 ft (10,400 m) below the sea level in deepwater of the Gulf of Mexico. 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):

- failures related to pre-existing or drilling-induced formation damage;

- failures caused by the induced stress, pore pressure concentrations, and temperature redistributions;

- failures attributed to deliberate or unintentional additional stresses; and,

- failures related to shock-wave loading.

Borehole instabilities are the main cause of drilling difficulties, resulting in an expensive loss of time, sometimes in a loss of part of or even whole boreholes. Wellbore instabilities make logging very difficult to perform and to interpret (Maury and Sauzay 1987). A bad condition of the borehole wall alters artificially the annulus zone corresponding to the depth of investigation of most of the logging tools.

The shape of the borehole can be strongly modified giving an elongated hole in one direction, diameter reduction in the other direction and also almost circular cavings in places. In the Cusiana field in Colombia, even though some measures to prevent borehole instability were taken, extensive breakouts in fissile and naturally fractured shales – of up to 44" in 121/4" hole – occurred (Willson et al. 1999). Approximately 10% of the well costs in the Cusiana field were spent coping with bad holes, mainly because of abnormally high tectonic stresses induced by an active thrust-faulting environment (Addis et al. 1993, Last and McLean 1995). In addition to the cost associated with borehole instability while drilling, borehole stability also has a substantial impact on reservoir productions (Bradley 1979).

There are several stages in the life of a well, i.e., drilling, completion, stimulation, flow tests, production, and depletion. Borehole instabilities can be encountered in all these stages (Ramos et al. 1996). In the drilling stage, the main concerns are to determine the mud composition and density (mud weight) which will maintain the integrity of the well, without the loss of drilling fluids. During the completion and stimulation stage, the reservoir must be connected to the well via perforations.

This operation could fail if the rock adjacent to the cemented casing is non-brittle. Prior to full production, downhole tests include open-hole logging, fluid sampling, build-up, drawdown, injection, and deliverability tests. It is possible to induce wellbore failure and casing collapse during these processes (Peng et al. 2007). As hydrocarbons are depleted, the drained region compacts which could induce solids production, casing damage, surface subsidence and wellbore failure. All these stages in the life of a well, integrated borehole stability analyses are important to ensure the reservoir economical production and minimize the costly problems induced by the wellbore instabilities.

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