ABSTRACT:
Knowledge of the contemporary stress field is vital to the petroleum industry for assessing trap integrity and establishing drilling directions and mud weights to optimise wellbore stability. High quality image log data from 63 petroleum wells in the Carnarvon Basin were analysed for borehole breakouts and drilling- induced tensile fractures to ascertain the orientation of the contemporary horizontal stresses. Orientations were consistent across the basin, with a regional mean maximum horizontal stress orientation of approximately 105°N. Over 80% of the mean borehole breakout and/or drilling-induced tensile fracture orientations of each well, showed mean stress orientations within 15° of the 105°N mean orientation. Stress orientations were also consistent in the vicinity of faults, contrary to previous interpretations from caliper logs, where faults in the area locally perturb the stress field. A preliminary investigation into the magnitude of the contemporary stress field of the region suggests the Carnarvon Basin is in a strike-slip faulting environment, implying that the most stable drilling direction is horizontal. The majority of faults in the Carnarvon Basin are steeply dipping and strike north-south and northeast-southwest. These fault orientations are not at risk of reactivation in the regional stress field. Results from this study are included in the Australian Stress Map database, which is becoming increasingly recognised for its vital importance to petroleum professionals as a source of contemporary stress information throughout the Australian continent.
1 INTRODUCTION
The key driver of this study was to investigate the occurrence of anomalous northeast-southwest maximum horizontal stress (SH) orientations derived from caliper logs (Mildren 1997) in the otherwise consistently oriented east-west regional stress field. The extent to which faults perturb the stress field is particularly significant to the pre-drill prediction of stress orientations.
