The Surat Basin Coal Seam Gas (CSG) is maturing from exploration to production to meet sales targets for several Liquefied Natural Gas projects. This requires effective and predictable reservoir performance that is controlled by permeability, gas saturation and matching the well completion technique to the ground conditions. Permeability in coal has major dependence on in-situ stress and natural fractures, and these will have characteristics that vary from field scale to larger regional scale faults and folds. This study investigates the spatial and stratigraphic variability of stress and fracture orientation relative to measured permeability within the Walloon Subgroup (WSG) that hosts CSG resources within the Surat Basin.
Open file wireline data from 33 vertical wells across the eastern part of the Surat Basin are used to calculate the in-situ stress orientation and fracture distributions within the WSG. Present day in-situ stress orientations were identified from the borehole breakout and drilling induced tensile fractures. Permeability data from 13 wells are compared using the in-situ stress and fracture data from image log interpretation.
CSG reservoir permeability is, in general sensitive to the changes in effective stress and primary permeability is reduced exponentially with increasing effective vertical stress. However, this relationship does not always hold true. Permeability within the WSG is also affected by the angle between present day in-situ stress orientation and fracture or joint trends and fracture dip angle. Wells in the relatively simple structural areas with subtle faulting show higher permeability when the coal fracture trends are parallel and up to 40 degrees to the present day in-situ stress. Permeability is significantly less outside this range. Most of the high permeability zones tend to occur in the areas where there is significant rotation of in-situ stress from the regional mean stress orientation (commonly ENE – WSW). The observed stress rotation is attributed to the stress perturbations mainly from folds and faults within the Surat sedimentary succession, suggesting local structural influence on the permeability distribution within the WSG. Few analyses of permeability versus stress and fracture properties are in the public domain for the Surat Basin. This study illustrates the connection between different structural domains, stress and fractures in previously unidentified areas of oblique-slip faulting, in some cases reactivating and in others creating new features from basement through the Jurassic to Tertiary succession.