Nanochemo-mechanical signature of organic-rich shales: a coupled indentation–EDX analysis

Abstract

10The organic-inorganic nature of organic-rich source rocks poses several challenges for 11 the development of functional relations that link mechanical properties with geochemical 12 composition. With this focus in mind, we herein propose a method that enables chemo-13 mechanical characterization of this highly heterogeneous source rock at the micron and 14 submicron length scale through a statistical analysis of a large array of Energy Dispersive 15 X-Ray Spectroscopy (EDX) data coupled with nanoindentation data. The ability to 16 include elemental composition to the indentation probe via EDX is shown to provide a 17 means to identify pure material phases, mixture phases and interfaces between different 18 phases. Employed over a large array, the statistical clustering of this set of chemo-19 mechanical data provides access to the properties of the fundamental building blocks of 20 clay-dominated organic-rich source rocks. The versatility of the approach is illustrated 21 through the application to a large number of source rocks of different origin, chemical 22 composition and organic content. We find that the identified properties exhibit a unique 23 scaling relation between stiffness and hardness. This suggests that organic-rich shale 24properties can be reduced to their elementary constituents, with several implications for 25 the development of predictive functional relations between chemical composition and 26 mechanical properties of organic-rich source rocks such as the intimate interplay between 27 clay packing, organic maturity, and mechanical properties of porous clay/organic phase. 28

Creep Deformation in Vaca Muerta Shale From Nanoindentation to Triaxial Experiments

“…In general, the highest values were obtained when inclusions of pyrite, dolomite, calcite, and quartz were indented, while moderate and low values corresponded to the clayish matrix forming the bulk of the rock. The correlation between nanoindentation moduli and local mineralogical composition has also been established in previous studies (Abedi, Slim, Hofmann, et al, 2016;Slim, 2016;Vandamme, 2008). The compilation of moduli and hardness values exhibited relatively well-defined power law relations, H ∝ E 1.8 ± 0.2 , C ∝ E 2.9 ± 0.8 , and C ∝ H 1.6 ± 08 (uncertainties are 95% confidence interval; Figure 7).…”

Nanomechanics of organic-rich shales: the role of thermal maturity and organic matter content on texture

“…The focus of our study will be the porous kerogen/clay phase in the samples that were isolated by means of a coupled nanoindetation and Energy dispersive x-ray spectroscopy technique developed in detail in Abedi et al [2016] for the considered samples. The technique which extends earlier work by Ulm and co-workers [29,30] for shales and other heterogeneous materials to organic-rich shale samples consists of assessing mechanical and chemical properties at micrometer and sub-micrometer length scales by clustering grid nanoindentation results and Energy dispersive x-ray spectroscopy results for phase identification.…”

“…The results of nanoindentation and EDX are coupled by averaging EDX elemental intensities over 2 µm diameter area centered at each indentation point and then by feeding mechanical and chemical data into clustering analysis [1]. Through such clustering analysis, one can identify the most likely number of clusters in a data set as well as the uncertainty of observations belonging to a cluster based on statistical criteria.…”

“…Depending on the material phase of interest, different elemental intensities required for proper phase identification are incorporated into statistical analysis. For the investigated samples in this study, in order to identify clay-rich phases, maps of "Si" and "Al" from EDX were used [1]. The coupled indentation-EDX clustering technique thus provides a means of isolating among the organic and inorganic phases in shale, a distinct porous kerogen-clay mixture phase at a length scale of 1-4 m. This length scale is defined in indentation tests by the indentation depth, as an indentation test carried out to an indentation depth of h, probes roughly the material response of a volume 3-4 times the indentation depth.…”

“…One of the sources of complexity in organic-rich shales is the intricate interplay between micro-texture evolution and kerogen maturity [1,18,20,23,24]. There is increasing evidence that the maturation of organic matter changes the micro-texture of organic-rich shales [18-20, 23, 25] contributing to the changes in mechanical properties of the rock.…”