Stress orientations from oil-well fractures in Alberta and Texas

Abstract

Many oil wells in Alberta exhibit spalling of the walls (known as breakouts), which elongates the holes with the longer axis aligned northwest–southeast. This alignment is observed over an area in excess of 4 × 105 km2, in siltstones, sandstones, carbonate sediments, and one shale formation, through the stratigraphic column from Devonian to Cretaceous. It is unrelated to dip of the beds. We have elsewhere suggested that these breakouts are produced through stress concentration near the hole walls, in a stress field having large, unequal horizontal principal stresses and with the larger compression oriented northeast–southwest. It is probable that this northeast–southwest principal stress is σ1. This paper adds new data from oil wells in Alberta and northern British Columbia and shows that the breakouts, and by inference the stress orientations, are consistent through much of the western Canadian sedimentary basin. It also uses evidence from hydraulic fracturing in west-central Alberta and from steam-injection fracturing in eastern Alberta to support the view that σ1 is aligned northeast–southwest. The breakouts are consistent with either a thrust stress field or a strike–slip stress field, but the fractures formed by excess pressures in wells favour the latter. Recently Schafer has reported oil wells systematically elongated with a mean azimuth of N39°E in the Austin Chalk of southern Texas. We regard these elongations as formed by breakouts in a stress field with greater horizontal stress near N51°W, and this view is supported by overcoring measurements by Hooker and Johnson in granite of the Llano Uplift, 225 km from Schafer's wells, that reveal large horizontal compressions at the surface with the larger oriented N33°W.