Kumar, Vikas (University Of Oklahoma) | Curtis, Mark Erman (University of Oklahoma) | Gupta, Nabanita (University of Oklahoma) | Sondergeld, Carl H. (University of Oklahoma) | Rai, Chandra Shekhar (University of Oklahoma)
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.
Since the discovery of pores in the organic matter of gas shales, the conventional thought has been that the pores formed as a result of hydrocarbon generation during the thermal maturation of the organic matter. However, thermal maturity alone may not be the only factor in determining the formation and preservation of pores in the organic matter. In this paper we report on a study of organic porosity in Woodford Shale samples with vitrinite reflectances ranging from 0.51% Ro to 6.36% Ro. Using focused ion beam (FIB) milling and scanning electron microscopy (SEM), it is observed that while the first appearance of porosity for the samples occurs by 1.23% Ro, there are anomalies. One anomaly is the complete lack of organic porosity in the 2.00% Ro sample. In addition, some samples with a vitrinite reflectance = 1.23% Ro exhibit regions of porous organic matter adjacent to non-porous organic matter regions that are separated by a few microns Observations show that while some regions of porous organic matter appear protected by grains others appear stress-supporting. These observations have important consequences for using indicators such as thermal maturity in predicting the occurrence of pores in the organic matter of shales.