Tight unconventional reservoirs have become an increasingly common target for hydrocarbon production. Exploitation of these resources requires a comprehensive reservoir description and characterization program to estimate reserves, identify properties that control production and predict fracturability. Multiscale imaging studies from the whole core to the nanometer scale can aid in understanding the multiple contributions of heterogeneity, natural fracture density, pore types, porethroat connectivity, mineral and organic content and distribution to petrophysical response and production characteristics.
In this paper we present three examples of the application of multiscale imaging to challenging unconventional reservoirs: a deep, clastic tight-gas reservoir; a fractured basement reservoir; and coal-seam gas reservoir. All these samples exhibit features at multiple scales, which present major challenges to petrophysical evaluation and understanding of reservoir engineering properties. In all cases, characterization of heterogeneity and geological rock typing is undertaken at the core scale. Mineralogy and porosity/microporosity characterization is then mapped at the pore scale with varying modes of microcomputed tomography (µCT) 3D imaging. Focusedion- beam scanning electron microscopy (FIBSEM) imaging can then be used to reveal the nanoporous structure of the key phases controlling hydrocarbon movement within the core material. Petrophysical properties (porosity, permeability, elastic moduli) can also be computed for each key phase and the data upscaled using standard techniques. The presented case histories demonstrate that multiscale imaging and modeling provides a complimentary method to existing core-measurement techniques via characterization of the distribution and nature of different pore types and matrix components. This can enable improved classification and aids the prediction of elastic and dynamic rock properties even on rock fragments that are not suitable for conventional core analysis. In addition, the results have the potential to enhance our understanding of petrophysical, fracturing and multiphase flow processes in challenging unconventional reservoirs with low porosities and permeabilities.