Subsurface high-grading potential in Coal Seam Gas in eastern Australia

Abstract

A case study from eastern Australia is used to demonstrate methods for subsurface high-grading and development-concept optimisation in Coal Seam Gas resource plays. We discuss two aspects of subsurface high-grading: (1) improved prediction of net coal using 2D seismic data, and (2) structure-geological analysis and mapping of the fracture network. Net-coal predictions across the study area focused on the upper Juandah Coal Measures interval of the Jurassic Walloon Coal Measures. There is a positive relationship between extracted and normalised upper Juandah Coal Measures seismic-amplitude strength and net coal in the wells. Areas with considerable amplitude dimming commonly coincide with reflector ‘washout’ areas mapped from seismic data and are interpreted as channel incisions into otherwise continuous coal seams. Integration of seismic and well data can improve the mapping of net coal and help optimise the placement of appraisal wells, reducing the number of unexpected or difficult-to-interpret well finds. It may also guide delineation of resource-classification domains in a more robust manner than mapping from well data alone. Fractures in coal seams of the study area are mostly bed-perpendicular joints and cleats. Three sets of fractures/cleats are identified in the coal beds: a dominant set and two more localised sets. Fracture intensity was mapped per set using well data and then combined. Fracture maps are deemed useful as an indication of permeability anisotropy, and hence the expected shape of well drainage areas, as opposed to indicating a range of permeability values. Based on the fracture interpretations, it may be possible to optimise the development concept of specific areas, i.e. varying well spacing and/or drilling wells slanted perpendicular to dominant fracture strike.