Borehole breakouts and compaction bands in two high-porosity sandstones

We compare the shape and mechanism of failure around vertical boreholes drilled in blocks of two high-porosity sandstones subjected to unequal far-field principal stresses. Tablerock sandstone has a porosity of 28%, and is composed of 55% quartz and 37% weaker feldspar grains. Grain cementation is substantial through microcrystalline quartz. Critical far-field stresses induce failure around boreholes in the form of V-shaped (dog-eared) breakouts, the result of dilatant intra and trans-granular microcracking subparallel to both the maximum horizontal far-field stress and to the borehole wall. On the other hand, boreholes in Mansfield sandstone, which has similar porosity (26%), but contains mainly quartz grains (90%) held together primarily by spot-sutured contacts, fail by developing fracture-like breakouts. We show evidence suggesting that Mansfield sandstone breakouts are preceded by the development of a narrow (several grain diameters) localized compaction zone along the direction of the least horizontal stress, which is where the maximum compressive tangential stress concentration occurs. Failure mechanism here is simply the removal by the circulating drilling fluid of mainly intact grains loosened during the formation of the compaction band. The type of cementation, mineral homogeneity, grain strength, and sphericity appear to be major factors in the formation of compaction bands. Some breakout dimensions in both rocks are correlatable to the far-field principal stresses, and can be potentially used (in conjunction with other information) as indicators of their magnitudes.