Electron Microscopic Imaging of shale samples has come a long way in the past 3–5 years, primarily because of the increased activity in unconventional reservoirs and advances in electron microscopy, especially on the hardware end. This has manifested itself in imaging resolution being in the nanometer (nm) range on a routine basis. Sample preparation utilizing advanced Argon Ion Milling techniques have also helped to render an essentially artifact free surface to examine. This results in the ability to image surfaces in 2 dimensions (2D), followed by a 3 dimensional (3D) data collection and reconstruction of specific areas. (Use of Digital Imaging for Improved Evaluation of Unconventional Reservoirs, Mike Dixon et.al, SPE RCSC, Abu Dhabi, UAE, September 2013)
In this paper we will outline a protocol by co-relating HCT, Micro CT and Optical images with electron microscope images to determine locations for FIB/SEM cubes that are representative of the sample. End trims/plugs from two cores were examined for this paper. A proprietary core from the Middle East was submitted to Weatherford Laboratories in Houston, TX. The slab was then examined using Helical Computed Tomography (HCT), followed by extracting a core plug for further analysis. This plug was imaged using a Micro-CT. Subsequently, an end trim from the plug was obtained and the entire end trim was broad beam Argon Ion Milled, and imaged optically.
The second plug was from a US (state side) formation from an anonymous client. This sample followed a similar protocol to extract plugs, image them using a Micro-CT, and cut end trims for subsequent imaging and analysis. The samples were examined in a Field Emission Scanning Electron Microscope (FE-SEM). High magnification and high resolution images were obtained to further delineate depositional features and the degree of fabric variability present in the sample. All imaging modalities (CT, optical, SEM) were used together to define the range of lamina types present within the sample. Image mosaics of increasing magnification were constructed in 2D within each lamina type. These mosaics were used to select a representative region within each lamina for FIB volume generation using both Secondary and Backscatter imaging modes. These two modes provide complimentary information regarding the sample itself. Using this method and the volumetric abundance of lithotypes present in the sample allows for more accurate up scaling of FIB/SEM image analysis results.