We report on a nano-indentation study of shales from the Barnett, Woodford, Ordovician, Eagle Ford and Haynesville plays. Careful selection of load and displacement during nano-indentation testing yields micro to macro-mechanical properties, Young's modulus and hardness, of shale. Scanning Electron Microscope coupled and nano-indentation were used to study the mechanical behavior of kerogen. The measured Young's modulus of kerogen varied from 5 to 9 GPa. Mineralogy is found to play an important role in controlling mechanical properties of shales; an increase in carbonate and quartz content is correlated with an increase in Young's modulus whereas, an increase in TOC, clay content and porosity decreases Young's modulus. Close agreement is found between indentation moduli measured on small samples (mm scale) and dynamic moduli calculated from velocity and density measurements made on larger samples (centimeter scale). Tests conducted on cuttings provided results comparable to measurements made on larger core samples. Nano-indentation can provide a viable means of assessing quantitative measure of shale "fraccability." IntroductionThe increase in price of conventional oil and decline in petroleum reserves make shale reservoirs an attractive alternative source of hydrocarbon fuel (Eseme et al., 2007). Success in hydrocarbon exploitation from different shale plays, like Barnett, Haynesville, Woodford, Marcellus, Fayetteville and Eagle Ford shale in North America has resulted in companies vying for shale assets (Aoudia et al., 2010). The development of shale gas is made possible by horizontal drilling and multistage hydraulic fracturing. Hydraulic fracture design can be optimized by measuring the mechanical behavior of shales. However, mechanical properties of shales have been poorly sampled primarily because of their chemical and mechanical instability. It is difficult to recover suitable samples of shale for conventional mechanical property measurements. Organic rich shales are intrinsically heterogeneous and complex, consisting of dispersed organic matter in an inorganic framework. Organic matter present in shales can either act as an effective source (i.e., it is mature enough to be in the oil-gas generation window) or a potential source (i.e., can produce hydrocarbon if adequately matured) (Jarvie, 1991). Shale heterogeneity and compositional variability present serious challenges in quantifying the mechanical properties of shale. These can only be overcome with adequate statistical sampling. Nano-indentation technology can eliminate the need of larger samples thus allow improved statistical sampling.

Shales are one of the most heterogeneous and complex natural materials found. Recent spike in the activities in shale gas and oil plays has been possible through horizontal drilling and hydraulic fracturing, which requires better understanding of mechanical properties. Complexities associated with elastic properties of shale are amplified with presence of wide range of organic fraction present in them. There is a need to understand the mechanical properties of organics and their associated impact on bulk mechanical properties. Scanning Electron Microscopy with focused ion beam milling and nano-indentation have been employed to calculate mechanical properties of kerogen at the submicron level in Woodford shale samples of different maturities. A displacement of 500 nm was applied to investigate mechanical properties of kerogen and force in the range of 400–500 mN was applied to measure average mechanical properties of shale. Young’s modulus of kerogen was found to be linked to localized porosity as well as maturity. Kerogen in different samples with vitrinite reflectance range of 0.5–6.36 % and almost no porosity showed Young’s moduli in the range of 6–15 GPa, whereas, kerogen with significant porosity showed values as low as 1.9–2.2 GPa. Young’s modulus measured by nanoindentation on small shale samples (~ 5–10 mm) was found to be in good agreement with dynamic modulus measured on core plugs (~cm). Young’s modulus is most sensitive to the Total Organic Carbon present. Increase in organics is found to qualitatively reduce both Young’s modulus and hardness. Measurement of elastic properties of shale is significant for optimizing hydraulic fracture design, for well stability study and better seismic velocity prediction in shale. This technique requires small sample dimension, on the order of millimeters, for experiment and thus eliminates the requirement of larger, centimenter, size samples. This is particularly significant for shale as they are mechanically and chemically unstable which makes retrieval of larger core samples challenging.

We report a nanoindentation study of shales on 144 samples from Barnett, Eagle Ford, Haynesville, Kimmeridge, Ordovician, and Woodford plays. Mineralogy is found to play an important role in controlling mechanical properties of shales: An increase in carbonate and quartz content is correlated with an increase in Young’s modulus, whereas an increase in total organic content, clay content, and porosity decreases Young’s modulus. We had a close agreement between indentation moduli measured on small samples (millimeter scale) and dynamic moduli calculated from velocity and density measurements made on larger samples (centimeter scale). By taking an average of a large number of indentation Young’s moduli, 100 indentations in our case, and using an appropriate penetration force, nanoindentation technology measured an acceptable average Young’s modulus even for heterogeneous samples such as shale highlighting the potential of applying this technology to plug and perhaps field-scale problems.

No abstract

Tip-enhanced Raman spectroscopy revealed the nanoscale chemical properties of organic molecules encapsulated in single walled carbon nanotubes (SWNTsanalysis focuses is more on structural information, such as intermolecular interactions, molecular orientations, and symmetry distortions of each species. Moreover, vibrational spectroscopy can be used for identifying molecular species, which is not possible by TEM. Therefore, Raman spectroscopy is a powerful tool for studying the chemical composition of matter. Conventional confocal Raman spectroscopy techniques are limited to sub wavelength spatial resolution, and localized vibrational features of molecules encapsulated in SWNTs have not been resolved so far. This limitation has been overcome by the development of tip-enhanced Raman spectroscopy (TERS) [8][9][10][11][12][13][14][15][16]. By introducing a sharp metal tip to the focus of a laser beam, we were able to localize Raman excitation to an area of 30 nm 2 [17][18][19]. This technique has recently enabled the molecular nanoimaging of a single carbon nanotube [20] and double-stranded DNA network structures [21].