Petrophysical properties for CSG reservoirs from 3D imaging at multiple scales

Abstract

CSCSG reservoirs are intrinsically heterogeneous on every scale and the permeability and producibility of CSG is decreased when the pores and fractures are filled with minerals. The 3D characterisation and quantification of pore connectivity, cleat/fracture aperture and spacing, and extent of mineral infilling in coal is required for CSG reservoir evaluation of gas storage and flow characteristics. A technique has been developed to determine petrophysical properties of coal using data from a large-field, 3D microfocus X-ray computed tomography (µCT) at multiple scales, combined with SEM imaging, and automated mineralogy by QEMSCAN. µCT is a non-destructive technique and the X-ray densities of coal components are distinct; therefore, the pore/fracture, mineral, and coal matrix can be differentiated and quantified in 3D. The high resolution 3D image data can then be used to measure petrophysical properties. Specifically, this technique characterises porosity and its connectivity, cleat/fracture networks (aperture and spacing), cleat/fracture permeability, and mineral occurrences in 3D to better describe CSG reservoirs. The technique has been tested on samples of bituminous coal from a number of coalfields in the Sydney and Bowen Basins, Australia. The samples imaged were from 110–114 mm in diameter, yielding voxels ranging from 54–63 µm in size. The results can determine the depositional and post-depositional history of coal seams, in coal preparation and use, and in seam gas studies.