In-situ stress field and fault reactivation in the Mutineer and Exeter fields, Australian North West Shelf

Abstract

This study evaluates the in-situ stress field and the potential risk of fault reactivation and seal breach in the Mutineer and Exeter fields, Australian North West Shelf. Stress determinations are undertaken using pumping pressure test, rock mechanical, and log data from twelve wells. Subsequent geomechanical modelling uses the stress data to assess pore pressure changes that may induce slip on mapped faults cutting the region. The principal stresses are assumed to be the vertical stress (SV), and a maximum and minimum horizontal stress, SH and Sh respectively. Borehole breakouts and drilling-induced tensile fractures (DITFs) interpreted from image logs indicate SH has an average orientation of 107°N and Sh is orientated 017°N. Leak-off test data compiled from well completion reports reveal the magnitude of Sh increases with depth at a rate of 17.1 MPa/km. Density log data show SV can be approximated by a power law function. An upper bound to SH is calculated using the frictional limit to stress beyond which faulting occurs when using a frictional coefficient of 0.6. Better constraints on the magnitude of SH are gained using rock mechanical data, knowledge of Sh and SV, mud weights, and the occurrence of borehole breakouts and DITFs. Stress magnitudes show that the tectonic regime is strike-slip (Sh < SV < SH). The likelihood of fault reactivation in Mutineer-Exeter is expressed as the increase in pore pressure required for fault slip. Results show that faults are non-optimally orientated for reactivation by the stress field. The likelihood of brittle seal failure due to fault reactivation is low, primarily because of non-optimally orientated faults. The creation of new faults requires greater increases in pore pressure than reactivation and is thus seen as being more unlikely. The results have implications for seal integrity, well bore stability, and the safe and successful production of the fields.