This paper presents field evidence of triple breakouts occurring around vertical boreholes drilled in a poorly consolidated formation in Kuwait. Three distinct breakouts, 120-degrees apart, developed at the borehole wall in shallow oil reservoirs. The breakouts were identified on acoustic and resistivity wireline image logs from three independent imaging tools. This manuscript does not fully explain why these breakouts were formed 120-degrees apart, or how to predict breakout sizes; instead, it shows real examples of this unique and very rare borehole shear failure. Triple breakouts have been created in the laboratory by drilling small boreholes in cubic rock samples under isotropic horizontal stress conditions. Furthermore, triple breakout failures under isotropic stress boundary conditions have been also predicted from discrete element modeling by previous researchers. To our knowledge, triple breakouts have never been observed in vertical boreholes drilled into subsurface formations, and no field evidence of their existence has been published. The evidence for triple breakout seen in the acoustic and electrical borehole images was examined in detail to discount other possibilities for the origin of the observed features.
1. INTRODUCTION
Numerous articles have been published on investigating borehole breakouts. These articles are mainly based on the elastic theory of material strength, which was first introduced by Kirsch in 1898 [1]. Borehole breakouts are considered very important for characterizing the farfield in-situ stresses, and they are commonly used in vertical wells to estimate the magnitudes and orientation of the horizontal stresses. Typically, borehole breakouts are created 180-degrees apart at the high-stress concentration zones around the wellbore defined by the Kirsch?s solutions [2]. When the borehole is drilled vertically and parallel to one of the principal stress directions, the orientation of the breakouts corresponds to the direction of the minimum horizontal stress, i.e., along the least far-field horizontal stress direction (Gough and Bell, 1981) [3]. There are different rock-failure mechanisms which create borehole breakouts of diverse forms and shapes. The most common mechanism associated with borehole breakouts is shear failure occurring at the maximum compressive stress zones near the borehole, (e.g., Zobacket. al., 1985 [4]; Haimson and Song, 1993 [5]; Ong and Roegiers, 1993 [6]). Another phenomenon of borehole breakout failure is rock splitting parallel to the direction of maximum stress concentration (Fairhurst and Cook, 1966) [7]. Tensile failure was also found to be another failure mechanism causing borehole breakouts in brittle rocks. Laboratory evidence (Lee and Haimson, 1993) [8] and modeling studies (Germanovich et. al., 1994) [9] also suggested that breakouts could be formed by tensile buckling. There is limited understanding on the failure mechanism of triple breakouts, and the unique equilateral-triangular geometry around the borehole. Although triple breakouts have recently been produced in the laboratory (Haimson, 2007) [10] and predicted by discrete element modeling (Cook et. al. 2004) [11], it is not possible to predict the location and size of triple breakouts by linear elastic solutions. The triple breakout images presented in this paper were all observed with wireline borehole imaging tools, even though images of the borehole wall can also be recorded while drilling.
