Multi-Scale Shale Imaging Challenges: Examples From U.S. and European Plays

Abstract

Quantitative 2D and 3D imaging datasets are of great value to the petroleum industry as they can provide the basis for geological reservoir models, which aid calculation of in-place volumes and modelling of permeability flow. Because of the high resolution required to image small scale features of interest (nm), the areas and volumes of data sets are inevitably small when compared to the typical scale of geological variability. Here we document, using examples from US and European shale plays, pertinent issues to consider when imaging mudstones, and in particular the limits of resolution and the representative nature of 2D and 3D imaging data sets. We present data taken from multi-scale X-ray CT and SEM observations (through four orders of magnitude in resolution; from 10s microns to 10s nanometres). Imaging nanoscale porosity in mudstones requires the highest resolution techniques available either in 2D (standard SEM) or 3D (FIB-SEM). Connected pore networks within and between mudstone components are overwhelmingly small (nm-um length scale). The small size of pores measured attests to the low permeability of mudstones, but also highlights the undefined pore volumes beyond the limits of resolution (∼5-10nm), which are confirmed by petrophysical measurements. At lower resolutions (um) and larger areas and volumes (mm³), x-ray nanotomography can be used to provide 3D volumes. Mesoscale features such as sedimentary textures can be observed allowing nanoscale datasets to be contextualised. However, discrimination of mudstone components is a challenge, with differentiation between organic matter, pores and clay minerals difficult or impossible due to similar attenuation characteristics of these phases. Another challenge comes from relating nanoscale and microscale datasets; ideally numerous nanoscale datasets should be generated to statistically confirm that measurements are representative across scales. At the lowest resolution (10s um) and sample volume (cm³), x-ray microtomography can produce 3D datasets that approach the core scale. This creates an opportunity to bridge from imaging dataset to petrographic or core studies. However, the low resolution of these data means that only large features such as fractures and fossils are observable. We use our data to highlight the scales and methods of observation that best provide reservoir quality-specific information in shale plays, and to propose an initial upscaling approach for such data.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016