Natural fractures and in situ stresses in the Bowen-Surat Basins, Australia: implications for structural permeability and fluid flow

Abstract

In situ stress orientations and natural fractures have been observed on image logs from ten wells in the Bowen-Surat Basins in Queensland and New South Wales, Australia. The in-situ orientation of the maximum horizontal stress (σHmax) at the location of each well was inferred from Drilling Induced Tensile Fractures (DITFs) and Borehole Breakouts (BOs) indicating variable orientations throughout the basins, however overall trends show a rotation of the σHmax from NW-SE in the northern regions to NE-SW in the southern regions. 1708 fractures were identified and divided into twelve fracture sets based on their electrical characteristics, orientations, and degree of dip. Resistive and Conductive Set A exhibited a WNW-ESE orientation, Set B was oriented NW-SE, Set C was oriented N-S, Set D was oriented NW-SE and Set E was oriented ENE-WSW. Conductive and Resistive Set F contained approximately horizontal fractures of any orientation. The variability in the orientations of fractures and in-situ σHmax orientations were likely due to influences from regional scale structures in the basins. Fractures that were electrically resistive on image logs were assumed to be closed to fluid flow, while electrically conductive fractures were taken to be hydraulically conductive, i.e able to serve as pathways for fluid flow. In order to test this, daily drilling reports for each well were analysed to identify depths at which significant loss of drilling fluid were observed. These fluid loss zones corresponded with an abundance of electrically conductive fractures, suggesting that those fractures were open and acting as pathways for fluid flow. These fractures also tended to belong to a set which was aligned with the in situ σHmax, further suggesting that electrically conductive fractures that are optimally aligned with the in situ σHmax are likely to be open and hydraulically conductive.