In-situ stress and geological structure influence on the coal fractures and initial reservoir permeability within Coal Seam Gas reservoirs, eastern Surat Basin

Mukherjee, Saswata1,2, Rajabi, Mojtabaa and Esterle, Joan1,2

1School of Earth and Environmental Sciences, University of Queensland, Brisbane, Australia; 2Centre for Natural Gas, University of Queensland, Brisbane, Australia

The development of Coal Seam Gas (CSG) reservoirs in the Surat Basin has highlighted the basin as a prolific gas province and one of the significant economic successes in eastern Australia. An improved understanding of the controls on CSG reservoir behaviour contributes to better reservoir management for determination of production area targets. In-situ stress patterns and subsurface fractures within CSG reservoirs control the reservoir permeability and, hence, comprehensive studies are required for better exploration and development of these unconventional reservoirs. This is particularly important for the Surat Basin, where significant stress variations have been reported due to local geological structures. However, a detailed and 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. Herein, 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 fracture character and subsurface fluid flow of the Walloon Coal Measures in eastern Surat Basin.

Analysis of 8.4 km of borehole image log in 33 vertical wells revealed a regional ENE-WSW orientation of the maximum horizontal stress (SHmax) in the study area. However, there are massive stress rotations, spatially and with depth. Analysis of 2D seismic data highlights the role of geological structures in stress perturbation. In addition, 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. Herein, we show some examples of the complexity of major deformation underlying the Surat Basin and the controls on later fracture development in the Walloon Coal Measures. The greater concentration of net negative graben features recognized as “keystone structures” in the eastern part of the Surat Basin, significantly influenced fracture development within the Walloon Coal Measures. 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. In addition, the connection between different structural domains and the in-situ stress and/or fracture relationship, in previously unidentified areas of oblique-slip faulting is discussed.


Mr. Saswata Mukherjee is a PhD candidate at the University of Queensland. His PhD research incorporates geological characterisation of Coal Seam Gas reservoirs, in-situ stress and fracture characterisation from image logs and understanding the impact of stress and fracture on the reservoir flow behaviour.

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