In Situ Stress and Fracture Characterisation in the Fairview Area, Bowen Basin
Muriel N. Schadler
Bachelor of Science (Petroleum Geology and Geophysics) 2013
Australian School of Petroleum
The University of Adelaide
Knowledge of the in situ stress tensor and natural fracture occurrence is necessary to assess the contribution of natural fractures to structural permeability. The feasibility of a brine injection program relies on an understanding of the in situ stress tensor, fracture orientation and structural permeability.
The in situ stress tensor in the Fairview area was obtained from image log interpretation of borehole breakouts and drilling induced tensile fractures in eight wells. Fractures and faults acquired from interpretation of borehole images were classified according to strike orientation, electrical conductivity, dip angle and depth. A comparison between fracture data collected from image log interpretation and fractures obtained from core analysis was undertaken. Subsequently, the fracture data was correlated with regional fault networks visible on seismic images. Fracture orientation, rock strength data and knowledge of the in situ stress tensor were used to construct structural permeability diagrams.
The interpretation of borehole breakouts and drilling induced tensile fractures indicated that the regional SHmax orientation strikes east-west to northeast-southwest. Heterogeneity visible in the in situ stress orientation could be attributed to local stress perturbations caused by geological structures such as faults. The broad northeast-southwest trend is in agreement with previous studies conducted in the Bowen Basin. Natural fractures observed on borehole images were oriented in all directions. However, a dominant fracture set striking northwest-southeast and a secondary fracture set striking northeast-southwest were identified. North-south trending fractures were mainly observed in older units and in the basement. Formation of the fractures in the Fairview area reflects both the basin forming paleo stress environment, and also later stage deformation resulting from uplift and sediment loading.
In wells with a northeasterly SHmax orientation, fractures striking northeast-southwest were most suitably oriented to be hydraulically conductive in the contemporary in situ stress field. In wells with an east-west trending SHmax orientation the east-west fracture set was most favourable to be open and hydraulically conductive. In the Fairview area, most faults are hydraulically resistive and not optimally oriented in the present day stress regime.
The fracture density in the basement interval is relatively high. Thus the hydraulically conductive fractures may be able to assist permeability in the area and enhance fluid flow.