Subsurface fractures, in-situ stress and permeability variations in the Walloon Coal Measures, eastern Surat Basin, Queensland, Australia

Abstract

In-situ stress pattern and subsurface fractures control the production of Coal Seam Gas (CSG) reservoirs and, hence, comprehensive analyses are required for better development of these reservoirs. The Surat Basin is a prolific CSG province with major economic impacts in eastern Australia. Recent studies have reported significant stress variations in this basin due to local geological structures. However, a systematic analysis of localised stress perturbations within the basin, and their impacts on reservoir permeability have not been presented thus far in the published literature. In this paper, we combine different sets of data including conventional log data, wellbore image logs, 2D seismic lines and interpreted permeability (from well test data) in order to characterise the role of geological structures in the subsurface fluid flow of the Walloon Coal Measures in eastern Surat Basin.Analysis of 7.3 km of borehole image log across 33 vertical wells revealed a regional ENE-WSW orientation of the maximum horizontal stress (SHmax) in the study area. However, there are massive variability in the SHmax orientation, spatially and with depth. Analysis of 2D seismic lines highlight the role of geological structures in stress perturbation, and the interpretation of natural fractures using image logs show that most of the open fractures are oriented sub-parallel to the acute angle of local SHmax orientation. Interpreted well test data in the study wells demonstrates that most of the high permeability data points are located where in-situ stress rotation is observed. This indicates the influence of local SHmax orientation responding to geological structures on the subsurface fluid flow in the eastern part of the Surat Basin.