Multi-scale formation evaluation of tight gas resources

Tight unconventional rocks have become an increasingly common target for hydrocarbon production. Exploitation of these resources requires a comprehensive reservoir description and characterization program to accurately estimate reserves and identify properties which control production. In particular this requires mapping the porosity at multiple scales and understanding the coupled contributions of fractures, variable pore types, microporosity and mineral heterogeneity to petrophysical response and reserves assessment. This paper describes the application of a formation characterization study based on the integrated analysis of data in 2D and 3D at multiple scales on plugs from two sets of unconventional tight gas samples. Heterogeneity and geological rock typing is considered at the core scale via classical 3D imaging techniques. Mineralogy and secondary microporosity characterization is mapped at the plug scale with different modes of 3D X-ray micro-CT analysis coupled with SEM and SEM-EDS analysis. In particular the pore connectivity and production potential is probed. FIBSEM imaging can then used to reveal the porous microstructure of the key phases at the nano-scale. This information, collected at multiple scales, is integrated to provide an understanding and quantification of the pore structure and connectivity of these complex rocks. Petrophysical properties which impact the storage capacity and production characteristics are then computed for each key phase and data up-scaled to the plug scale using standard procedures. Results compare favourably with available core analysis data. The methodology illustrates the value of integrating conventional geological rock typing with plug/core scale petrophysical characterization to better understand rock properties characteristic of heterogeneous "unconventional" resources.