Estimation of Elastic Properties of Organic Matter and Woodford Shale Through Nano-indentation Measurements

Abstract

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.

Modeling the Kerogen 3D Molecular Structure

“…Without an isolation process, they found Young's modulus of kerogen was dependent on localized porosity and maturity and the Young's moduli of samples ranged from 1.9 to 15 GPa. While the measured Young's modulus is dependent on the experimental conditions and sample sources, our result (3.50GPa) of Green River kerogen is comparable to previous experimental results reported by Kumar et al (2012). Yan et al (2013) measured the bulk modulus of the type I Green River kerogen and the values were around 3.5-5 GPa which are very close to our calculation result of 2.63 GPa.…”

“…However, it was not indicated whether the target is kerogen, clay or some other minerals. Recently, Kumar et al (2012) measured the Young's modulus of kerogen samples from Woodford shale play based on the nano-indentation method. Without an isolation process, they found Young's modulus of kerogen was dependent on localized porosity and maturity and the Young's moduli of samples ranged from 1.9 to 15 GPa.…”

Brazilian Tensile Strength Test of Orgainc-rich Shale

“…The mineralogy influences on shale compressive strength have been studied by a number of researchers (Marouby et al 2011;Akrad et al 2011;Kumar et al 2012;. Generally, uniaxial compressive strength and Young's modulus are reported to decrease significantly with increasing clay content and TOC (Sone and Zoback 2011;Gupta et al 2012;Kumar et al 2012). In this work, XRD and pyrolysis were used to quantitatively determine the mineralogy and TOC of 37 Mancos and 12 Eagle Ford samples; the detailed composition is listed in Table A2.…”

High-Resolution Mapping of the Distribution and Connectivity of Organic Matter in Shales

“…Mechanical properties of a shale reservoir are affected by the distribution and connectivity of the organic matter which influences hydraulic fracture design. AFM and nanoindentation studies of shale have determined that Young's modulus for organic matter is -2-15 GPa which is significantly lower than other mineral constituents such as quartz and calcite (Zeszotarski et al, 2004;Ahmadov et al, 2009;Kumar et al, 2012a;Kumar et al, 2012b;Shukla et al, 2013). Young's modulus appears to have a dependence on the porosity and maturity of the organic matter (Kumar et al, 2012b;Shukla et al, 2013), and the connectivity of the organic matter lowers the overall elastic stiffness of the shale (Sayers et al, 2001).…”

“…AFM and nanoindentation studies of shale have determined that Young's modulus for organic matter is -2-15 GPa which is significantly lower than other mineral constituents such as quartz and calcite (Zeszotarski et al, 2004;Ahmadov et al, 2009;Kumar et al, 2012a;Kumar et al, 2012b;Shukla et al, 2013). Young's modulus appears to have a dependence on the porosity and maturity of the organic matter (Kumar et al, 2012b;Shukla et al, 2013), and the connectivity of the organic matter lowers the overall elastic stiffness of the shale (Sayers et al, 2001). In addition, the shape, distribution, and connectivity of organic matter, as well as the thermal maturity (Sone, 2012), may be a factor in the elastic anisotropy observed in shale (Vernik andNur, 1992, Sondergeld et al, 2000).…”