A multiscale imaging solution for special core analysis

Porosity is the fine-scale network of cavities in the rock typically occurring at micrometer scale and smaller. The rock matrix, which supports the porosity, tends to vary on larger length scales of millimeters to centimeters. Rock matrix properties, such as mineralogy or grain size and shape, are referred to as the fabric.

The closest analog to illustrate the unconventional reservoir core-to-pore imaging requirement is Google Earth, which provides a coherent uninterrupted viewing experience of earth features over 5-6 orders of magnitude, from hundreds of kilometers satellite view, down to street level with sub meter resolution.

MAPS images can either be viewed using Microsoft freeware (HDView), or using on- or off-line MAPS software provided with the solution. The MAPS software offers additional key features such as a scale bar, and the ability to navigate back to a particular area of interest (on-line only).

Core-to-pore imaging requirements

Reservoir core samples are typically imaged for two distinct properties: porosity and rock fabric. A typical imaging requirement is preserving the spatial relationship between porosity and fabric in one coherent imaging dataset, often described as "core-to-pore" imaging. In the case of conventional reservoirs, porosity typically occurs at the micrometer scale, and fabric heterogeneity at the millimeter to centimeter scale, spanning about three orders of magnitude of length scale (from 10 µm to 1 cm). Imaging systems typically found in petrology laboratories such as optical petrographic microscopes, micro-CTs and scanning electron microscopes (SEMs) are all sufficient for spanning these pore-to-core length scales.

However, the recent discovery of nanometer-scale porosity in unconventional gas shale core samples, along with textures suggesting in-situ fracturing and fluid flow occur at the nanometer-scale, indicate that the core-to-pore length scale that needs to be bridged has increased significantly, to 5 or 6 orders of magnitude (from 10 nanometers to 1 millimeter or 1 centimeter) for unconventional core samples.

Even high resolution field emission scanning electron microscopes (FEG-SEM), capable of resolving nanometer scale porosity, and unconventional core imaging solutions such as FIB/SEM (DualBeam™) instruments do not meet the core-to-pore requirement of a providing a coherent and representative view of the rock fabric.

Large-scale, high-resolution imaging

The approach FEI Natural Resources has adopted to address the pore-to-core requirement for unconventional core samples is similar to that employed by Google Earth. SEM imaging automation software (MAPS) acquires multiple, high-resolution backscattered electron images, covering the entire required field of view, followed by image tiling and stitching. Backscattered electron intensity scales with the average atomic number of the material exposed by the electron beam – revealing minerals by different shades of grey. Organic matter typically appears dark in the backscattered image, whereas pyrite framboids appear bright.

Samples from gas shale plays such as Marcellus and Barnett have validated that an image pixel resolution up to ten nanometers is adequate to reveal nano-pores in organic matter. At a typical image size of 4000 by 4000 pixels, this means a single tile covers approximately 40 by 40 micrometer field of view at most. In order to capture a field of view that spans millimeters, typically 100 tiles in either direction are required. This is achieved by a 10,000 tile image mosaic, stitched to become a 400,000 by 400,000 pixel digital image that takes tens of gigabytes of storage space. With acquisition times of approximately one hour per gigabyte of raw data, the process may take multiple days of instrument time but not operator time, as the entire acquisition process is automated and unattended.

Available soon, FEI's Core-to-Pore Petrography Workflow delivers a comprehenisve platform for confident positioning of QEMSCAN, high-resolution SEM and DualBeam areas of interest, and a framework for correlated and coherent viewing of all unconvetional reservoir imaging datasets.

Learn more about our Core-to-Pore petrography workflow

> Core-to-Pore Product Brochure
> Multiscale Imaging of Shale Samples in the SEM (application note)