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This paper describes a study of estimating the magnitude of in situ horizontal stresses from borehole breakout data in a deep geothermal well in Australia. The knowledge of in situ dominates the shape and orientation of the underground heat exchange reservoir.
Extensive borehole breakouts occurred in the granite section (depth = 3650–4421 m) of Habanero No. 1 well in Cooper Basin during drilling. The direction and dimension of borehole breakouts were measured using borehole geophysical logging. No direct stress measurements (e.g. hydraulic fracturing) could be conducted in the well due to extraordinary temperature (240(C) and over-pressure (35 MPa above hydrostatic pressure conditions). Therefore, the breakout data were the only information available for evaluating the in situ horizontal stresses.
The study was conducted using fracture mechanics numerical code (FRACOD) which can simulate rock fracture initiation and propagation in both intact and jointed rocks. Firstly, a quantitative relation between the breakout dimensions and the magnitude of the in situ stresses in the granitic rock was established by means of the numerical modelling. Then, the in situ stresses were back-analysed using this relation and the measured geophysical borehole breakout data at various borehole depths.
The predicted stress ratios from this study were consistent with the seismic monitoring results obtained during the subsequent reservoir stimulation tests. The prediction and monitoring results indicate that both horizontal principal stresses are higher than the vertical principal stress, which is the most favourable stress condition for geothermal reservoirs.
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