Abstract

Multiscale characteristics of anisotropic, heterogeneous pore structure and compositional (e.g., kerogen, clay, cement, etc) distribution profoundly influence the hydro, mechanical, and chemical response of shale materials during stimulation and production. In this work the impact of these lithologic heterogeneities on physical, chemical, and mechanical properties is investigated over a micron to core scale of shale samples for Cretaceous Mancos Shale. Principal macroscopic lithofacies at a decimeter scale are petrographically examined. Thin sections (~2-3cm ) impregnated with fluorochromes are examined using laser scanning confocal microscopy and optical microscopy with different filters to characterize micro-facies (i.e., texture patterns) and using electron microprobe to identify the mineralogical distribution. Advanced multiscale image analysis for texture classification will be used to identify key features of samples which will be further analyzed using dual focused ion beam-scanning electron microscopy, aberration corrected-scanning TEM and energy dispersive X-ray spectrometry for nano-pore and organic-pore structures and mineralogies at nano scale. This characterization will be examined against experimental data including acoustic emission and nano-indentation measurements of elastic properties using focused ion-beam milled pillars. Finally, multiscale 3-D image stacks will be segmented to rigorously test the scale of a representative elementary volume based on multiple measures from image analysis and pore-scale simulations.

1. INTRODUCTION

Shale gas recovery relies on the combined horizontal drilling and hydrofracturing, or "fracking" technologies, required to produce sufficient permeable pathways for gas to flow from these low permeability formations. Production rates progressively decline from slow diffusive transport in nanometer-sized pores, closure/collapse of fractures, and other factors [1]. Shales are unique among rock types and pose challenges for continuum scale modeling. This is both due to the low permeability, scaling of pore sizes, and compositional heterogeneity at all scales [2,3]. Multiscale characteristics of anisotropic, heterogeneous pore structure and compositional (e.g., kerogen, clay, cement, etc) distribution are critical to improving our understanding of large heterogeneity and variability between shale formations, uncertainties in the nature of the stimulated volumes, and the physics of what underlies the rapid decline in production rates. In this work the impact of these lithologic heterogeneities on physical, chemical, and mechanical properties is being investigated over a micron to core scale of shale samples for Cretaceous Mancos Shale.

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