Image analysis has revealed three fracture patterns associated with incipient well bore breakouts. Knowledge of these patterns will improve our ability to discriminate natural from drilling induced fractures on borehole imagery. Diametrically opposed failure zones on the well bore surface are located at the azimuth of the minimum horizontal stress. One type of failure is characterized by straight or diffuse breakout margins. In crystalline rock, these breakouts are first detected as small pits in the well bore surface. In sedimentary rock fracture zones at breakout margins are the first manifestations of failure. One margin is formed by small fractures inclined in the direction of the maximum horizontal stress. The other margin consists of larger fracture planes inclined in the direction of the minimum horizontal stress. Observations suggest that failure initiates on the wellbore surface in crystalline rocks and inside the formation in sedimentary rocks. The three breakout types are consistent with three modes of failure predicted by wellbore stability analysis assuming that incipient breakout margin fractures are shear fractures.
This paper presents new observations of the initiation of wellbore breakouts. Well bore breakouts are a type of well bore failure that develops when stress concentrations on or near the well bore surface exceed the strength of the rock. This failure produces a wellbore profile with symmetric elongations aligned with the minimum horizontal stress (Figure 1) (Gough and Bell, 1982; Plumb and Hickman 1985; Zoback et al., 1985). The systematic alignment of breakouts with the stress field has been exploited as a low cost method of determining stress directions. Stress directions determined from breakouts have been used to predict the azimuth of massive hydraulic fractures; to design optimal placement of production and injector wells; and to better understand regional tectonics (Plumb and Cox, 1987; Mount and Suppe, 1987; Lacy 1984). A better understanding of the well bore failure process will improve our ability to distinguish between natural and drilling induced fractures using borehole images. The purpose of this paper is to present new image data showing examples of incipient well bore failure and to explore some implications of the observed failure patterns. Full coverage ultrasonic imagery of the well bore surface has revealed three modes of failure that clearly precede the development of wellbore breakouts as described in Plumb and Hickman (1985); Hickman et al., (1985) and Zoback et al., (1985). Image examples come from wells located in three sedimentary basins and crystalline basement in different tectonic settings. Well depths range from 1 km to 3 kms (Table 1).
Ultrasonic images used in this study are produced by a digital version of the borehole televiewer (BHTV) described by Zemanek and Caldwell (1969). Our digital BHTV (DBHTV) consists of a rotating ultrasonic transducer operating in a pulse echo mode which is oriented with respect to magnetic North. The transducer is pulsed 250 times per rotation and a complete scan of the wellbore is made for every 7.6 mm (0.3 in) of tool travel.
