Structural Characterization of Gas Shales on the Micro- and Nano-Scales

Abstract

Abstract The structure of shale samples from 9 different formations, the Barnett, Woodford, Eagle Ford, Haynesville, Marcellus, Kimmeridge, Floyd, Fayetteville and Horn River, has been investigated on the micro- and nano-scales using a combination of focused ion beam (FIB) milling and scanning electron microscopy (SEM). Using the FIB, a 10 nm thick cross-sectional slice (5 μm tall by 5 μm wide) is made into the shale. The newly exposed cross-section surface is then imaged using the SEM; this process is then repeated 300 - 600 times with each new slice progressing 10 nm deeper into the shale. The SEM images from this process form a 3-dimensional data set which is used to reconstruct a volume of the sectioned shale material. The reconstructed shale volumes show significant variations in the micro-structure (mineral content, micro-texture, clay platelet distribution, porosity, etc.) depending on the shale. Of particular importance to gas production is the variation in kerogen content and porosity within each shale. Assigning gray scale ranges to features, volumes can be constructed which show three dimensional distributions of these features. Some shale samples show significant kerogen content with connectivity of the kerogen observed throughout the reconstructed volume. Porosity in the shale samples is observed to be prevalent in either the kerogen, inorganic matrix, or both depending of the origin of the shale. When present, the kerogen in some shales has been found to have porosities of up to 50%. Observed pores within the shale samples range from a few to hundreds of nanometers in diameter, and the cross-sectional geometry of the pores varies depending on whether the pores are located in the kerogen or the inorganic matrix. Furthermore, the calculated porosity from the reconstructed shale volumes varies among the different shales. In addition to the calculated porosity, initial results for pore size distributions extracted from the reconstructed shale volumes will be presented in comparison to those found by mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR) measurements. The SEM observed dimensions require consideration of new flow mechanisms for the transport of gas through shale.