Nano to Macro Mechanical Characterization of Shale

Abstract

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.

Mechanical properties of shale-gas reservoir rocks — Part 1: Static and dynamic elastic properties and anisotropy

“…Previous studies have also emphasized that the bulk rock anisotropy is influenced by the amount of organic content in the rock (Vernik and Nur, 1992;Vernik and Liu, 1997). Solid organic materials are anomalously compliant compared with the surrounding minerals (Ahmadov et al, 2009;Kumar et al, 2012), and if their shape or distribution is anisotropic, they can become strong sources of mechanical anisotropy. In our data set, it is difficult to distinguish the effect of clay and kerogen on the bulk rock anisotropy given that clay and kerogen volume tend to have a positive correlation in our samples (Figure 2a).…”

Effect of thermal maturity on elastic properties of kerogen

“…The technique uses a diamond indenter to apply small-scale forces to a material while measuring the resulting displacement. Recently, this technique has gained tremendous attention for the characterization of geomaterials, especially finegrained mudrocks (Prasad, 2001;Zeszotarski et al, 2004;Ulm and Abousleiman, 2006;Bobko and Ulm, 2008;Ahmadov et al, 2009;Kumar et al, 2012;Shukla, 2013;Zargari et al, 2013). The accuracy of measurements using this technique is limited by the sharpness of the tip, the size of the grains (in granular materials), or the thickness of the films (in thin films) in the substrate.…”

Nanoindentation Measurements on Rocks

“…Mechanical properties of shales have been studied extensively by Kumar, ( [7], [8]) and Shukla, [9]. They studied samples from Wolfcamp, Woodford, Barnett, Hayensville, Eagle Ford, Collingwood, and Kimmeridge shales.…”

“…They studied samples from Wolfcamp, Woodford, Barnett, Hayensville, Eagle Ford, Collingwood, and Kimmeridge shales. A more detailed analysis of mechanical properties of shale and its dependence on mineralogy, porosity and TOC can be found in Kumar, ( [7], [8]) and Shukla, [9]. In this study, we present results of Fourier Transform Infrared Spectroscopy (FTIR), low pressure pycnometer porosity (LPP) and dynamic pulse transmission measurements along with nanoindentation measurements on a specific sample of Wolfcamp shale.…”