Technology used to examine solid materials at the nanometer scale has existed for decades, but the interest in applying these methods to reservoir rocks has increased dramatically in recent years. Shale gas and shale oil reservoirs require this level of imaging to resolve the pore space, and other tight rocks benefit as well, particularly when clay-hosted porosity is common or wettability is difficult to characterize. A significant aspect of organic-rich reservoirs that is unique to them is the common occurrence of organic-hosted pores, which are portrayed in several chapters of this Memoir.
The chapters in this volume include a few focused on imaging techniques and interpretation, several on comparison and integration of analysis methods, and some case studies. Several link new methods to new interpretations of specific reservoirs. Applications of pore-scale imaging showcased here include a fluid sensitivity study to determine the cause of formation damage, characterization of pore types and associated organic matter, and evidence of wettability alteration. Relationships between mudrock pore systems, lithofacies, and sequence stratigraphy are also addressed in two of the chapters.
Fabric and composition play key roles in generating and preserving pore systems as well as exerting control on mechanical properties. Maps of mineralogy with micron-scale resolution are featured in a few chapters, and the importance of fabric to mineral-hosted pores is discussed in another one.
Several chapters address porosity quantification with scanning electron microscope (SEM) imaging, in 2-D and/or 3-D (FIBSEM). One key drawback of this approach is the tiny areas or volumes of investigation. One chapter (Goral et al.) points out that micro- (and nano-) CT (x-ray) images provide a useful bridge between more representative volumes and SEM images that more fully resolve the pore network in nanoporous systems. Another drawback, occlusion of porosity in the imaged surfaces, results from the types of artifacts described in the chapter on mineral precipitates and re-deposited amorphous material.
Methods compared include SEM imaging of milled and fresh surfaces for pore types, organic textures, and fabric; helium ion microscopy versus electron microscopy for “shale” pore systems; and porosity determination techniques as a function of pore size that can be captured by various methods. The common conclusion reached in all of these comparisons is that integrating multiple methods, preferably at multiple scales, provides more and better information about these challenging reservoirs than any single method alone.
This volume will prove useful to anyone interested in the methods for observing and quantifying the pore systems that control hydrocarbon storage and flow in unconventional reservoirs. Unconventional reservoirs studied include Bakken, Barnett, Bossier, Eagle Ford, Geneseo, Green River, Horn River, Marcellus, Mississippi Lime, Monterey, Niobrara, Wolfcamp, and Woodford formations.