Borehole breakouts in Berea sandstone reveal a new fracture mechanism

Vertical drilling experiments in high-porosity (22% and 25%) Berea sandstone subjected to critical true triaxial far-field stresses, in which sigma(H) (maximum horizontal stress) > sigma(v), (vertical stress) > sigma(h) (least horizontal stress), revealed a new and non-dilatant failure mechanism that results in thin and very long tabular borehole breakouts that have the appearance of fractures, and which counterintuitively develop orthogonally to sigma(H). These breakouts are fundamentally different from those induced in crystalline rocks, as well as limestones and medium-porosity Berea sandstone. Breakouts in these rocks are typically dog-eared in shape, a result of dilatant multi-cracking tangential to the hole and subparallel to the maximum far-field horizontal stress (sigma(H), followed by progressive buckling and shearing of detached rock flakes created by the cracks. In the high-porosity sandstone a narrow layer of grains compacted normal to sigma(H) is observed just ahead of the breakout tip. This layer is nearly identical to compaction bands observed in the field. It is suggested that when a critical tangential stress concentration is reached along the sigma(h) spring line at the borehole wall, grain bonding breaks down and a compaction band is formed normal to sigma(H). Debonded loose grains are expelled into the borehole, assisted by the circulating drilling fluid. As the breakout tip advances, the stress concentration ahead of it persists or may even increase, extending the compaction band, which in turn leads to breakout lengthening.