We conducted laboratory experiments aimed at studying the effect of several drilling variables on the shape, size, and failure mechanism of borehole breakouts in Mansfield sandstone. The objective was to acquire a better understanding of the counterintuitive long and narrow fracture-like breakouts observed in this sandstone, and to examine whether the dimensional characteristics can be controlled by manipulating the drilling operation. Initial tests conducted with two common types of drill-bits proved inconclusive as to whether an affect on breakout characteristics exists. Larger-diameter boreholes produced longer breakouts, suggesting that field-scale wellbore drilling could induce breakouts approaching several meters in length. Boosting the drilling-fluid flow rate was found to cause significant breakout lengthening under high far-field stress conditions. On the contrary, drill-bit penetration rate increases shortened the breakout length under the same far-field stress conditions. Heavier drilling-fluids were found to improve borehole stability by yielding shorter breakouts due to a thin "mud cake" deposited on the borehole wall and along the breakout. Despite the varying drilling conditions, in most cases the breakout width remained surprisingly constant.
1. INTRODUCTION
Stress-induced breakouts in vertical holes are the product of compressive failure at the borehole wall around the points of intersection with the diameter aligned with the least horizontal principal far-field stress (sh springline), where the maximum tangential stress concentration occurs [1]. Bell & Gough [2] were among the first to discover this in the field, when they realized that the average breakout orientation obtained from four-arm dipmeter logs in a large number of oil wells in Alberta, Canada coincided with the presumed far-field sh direction. This finding led to the growing use of borehole breakouts as indicators of the in situ principal stress azimuths.
Laboratory drilling investigations under critical far-field stress conditions conducted in crystalline and fine-grained sedimentary rocks produced dog eared breakouts (Figure 1a), comparable to those observed in the field [3-7]. These breakouts resulted from dilatant tensile intra- and intergranular microcracks that develop in the zone of the maximum tangential stress along the sh springline. The microcracks were sub-parallel to the borehole wall and to the maximum far-field principal stress (sH). In addition, a distinct correlation was discovered between breakout dimensions and the magnitude of the far-field sH (when the other two principal stresses are kept constant) suggesting a potential additional use of breakouts as crustal stress magnitude indicators.
More recently, experimental studies conducted in high-porosity (25%) Berea sandstone revealed a significantly different type of breakout that varied in both size and shape from that observed in previously tested rocks [8]. The breakouts were so unusually long and extremely narrow that they resembled fractures. Counterintuitively, these fracture-like breakouts were perpendicular to the sH direction (Figure 1b). The completely different borehole breakout shape appeared to be the result of a previously unrecognized failure mechanism. Further borehole drilling experiments in St. Peter,
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Figure 1. (a) Typical dog eared breakout in Westerly granite. (b) Typical fracture-like breakout in Mansfield sandstone.
Mansfield, and Aztec sandstone, which varied in porosity between 10-26%, have also yielded fracture-like breakouts [9-11].
