In Situ Stresses in Borehole Blanche-1/South Australia Derived from Breakouts, Core Discing and Hydraulic Fracturing to 2 km Depth

Abstract

The development of Hot-Dry Rock (HDR) geothermal energy in Australia with drillings to some kilometres depth yields an impetus for deep stress logging. For the Olympic Dam HDR-project, borehole Blanche-1 was drilled to almost 2 km depth and provided the possibility to estimate the in situ stresses within the granitic borehole section by the analysis of borehole breakouts and core discing, as well as by hydraulic fracturing combined with acoustic borehole televiewer logging for fracture orientation determination. Although the stress magnitudes derived by the different methods deviate significantly, they clearly indicate for the depth range between 800 and 1,740 m a compressional stress regime of Sv ≤ Sh < SH and a consistent East–West orientation of maximum horizontal compression in agreement with existing stress data for Australia. The minor horizontal stress Sh derived from the hydraulic fracturing closure pressure values is about equal to the overburden stress and may be regarded as most reliable.