Laboratory experiments were conducted to study the effect of grain bonding on the formation of borehole breakouts in sandstones of varied porosity and grain cementing characteristics. The experimental resultsuggest that grain bonding in sandstone, as well as porosity, determine the shape of borehole breakouts. Samples of St. Peter sandstone and high porosity Berea sandstone, which have prominent quartz overgrowth grain suturing, develop long, tabular breakouts perpendicular to the maximum far-field horizontal principal stress direction, having virtually constant widths of approximately 5-8 grain diameters. Samples with considerable amounts of iron oxide cementing material, such as the 17% porosity Berea sandstone, develop dog-cared breakouts in the same orientation. Samples of high porosity (n=29%) Tablerock sandstone, which has clay mineral cementing material, develops broad, rounded breakouts that also extend perpendicular to the maximum far-field horizontal principal stress.
Breakouts in vertical boreholes result from compressive failure at the borehole wall, and occur at diametrically opposed points where the stress concentration is greatest, i.e. along the least far-field horizontal stress (o?,) springline. Gough & Bell (1981) first documented this type of borehole failure in many Alberta, Canada wellbores, and confirmed from four-arm dipmeter logs that breakout orientations are orthogonal to the maximum horizontal principal stress (OH).
Laboratory experiments reported by Haimson & Herrick (1986), Lee & Haimson (1993), and Haimson & Song (1993), revealed breakout geometries in crystalline and fine-grained sedimentary rocks identical to those described by Gough & Bell. Typically, these breakouts had a dog-cared shape extending from the borehole wall in the direction of the least principal far-field horizontal stress.
Similar laboratory breakout experiments done by Haimson & Song (1998) described two distinct breakout shapes in Berea sandstone. Samples of Berea sandstone with 17% porosity developed breakouts having the typical dog-cared shape and orientation. However, 22% porosity Berea sandstone samples developed elongated, fracture-like breakouts, which also formed parallel to the least principal far-field horizontal stress. Bessinger et al. (1997) documented such fracture-like breakouts in lightly sintered glass bead bricks subjected to uniaxial stress. These fracture-like openings, counterintuitively oriented perpendicular to the maximum farfield stress direction, suggest that they form by a different mechanism than breakouts studied previously. The goal of the research reported here is to provide some insight into the development of fracturelike breakouts, and the different material properties that determine the shape of borehole breakouts. We report here some initial breakout experiments in St. Peter sandstone and subsequent thin section analyses, and compare the results to previously published breakout studies in Berea and Tablerock sandstones 0talinson & Song 1998, Kovacich 2000).