A series of physical simulations was conducted on large cubical specimens of an artificial, weak, sedimentary rock as part of an investigation into the mechanisms involved in borehole breakouts. A partially penetrating borehole, 44.5 mm in diameter, was drilled under a triaxial state of stress (up to 24 MPa in magnitude) thereby closely following the field stress path. Measurements of radial strain around the borehole by extensometers and post test sectioning of each specimen facilitated failure mode analysis. The case histories reported herein serve as a credible database upon which numerical models can be developed or verified in addition to identifying some factors to be considered when drilling deep boreholes in rock.
Wellbore instability, especially in the form of breakouts, has received much attention in recent years for two reasons. First, the costs to operators in the oil and gas sector associated with wellbore instability are too high given the reduction in revenues (reported by Woodlands [1988] at $60 million for Western Canada alone). Second, breakout geometries are being utilized as indicators of magnitude and orientation of the in situ horizontal stress field (Gough and Bell, 1982; Zoback et al., 1985; and Haimson and Herrick, 1986 amongst others). Whereas many factors contribute to the formation of breakouts, the problem is primarily stress related. Stress redistribution due to the creation of the hole results in local overstressing of the rock and the formation of zone of yielded rock or a breakout. The resulting stress distribution, in turn, depends upon the absence or presence of local weaknesses such as fissures, joints or bedding planes. Studies by Kwong and Kaiser (1989) indicate that the number of localized weaknesses, their proximity to the opening and their orientation relative to the stress field influence the development of different types of failure mechanisms and, hence, cannot be neglected. In order to gain a better understanding of the influence of stress and local weaknesses on the stability of underground openings in brittle rock, including boreholes, a series of physical simulations was undertaken. These tests served to provide a database for the verification of numerical models. Due to equipment capacity limitations and the moisture dependent strength of available natural rocks, an artificial sandstone was developed and employed in these investigations. This paper describes this model material, its fabrication and mechanical properties, and the test procedures adopted.
In order to facilitate the investigation of the influence of specific variables on the response of the rock structure to the drilling of a borehole, a model rock was used. As it was impractical to simultaneously satisfy all similitude conditions for particular rock prototypes, it was decided to model a general rock class. In this study, weak sedimentary rocks, such as sandstone or limestone, exhibiting brittle fracture under low confining pressure and ductility when confined, were simulated. According to Stimpson (1970), model rocks of this class can be fabricated from sand combined with a cementing agent.
