Third source of controlled experimental data

Laboratory experiments and well data serve as main sources of controlled experimental data where a number of physical properties are measured on the same samples at varying conditions, such as saturation and pressure. Chapter 2 discusses how these data are used to derive theoretical models as well as establish the relevance of these models to rock types.

Computational rock physics, also called digital rock physics or DRP, is the third such source. The principle of this technique is “image and compute”: image the pore structure of rock and computationally simulate various physical processes in this space, including single-phase viscous fluid flow for absolute permeability; multiphase flow for relative permeability; electrical flow for resistivity; and loading and stress computation for the elastic properties.

The principle of DRP is simple but its implementation is not. It requires at least three main steps: imaging; image processing and segmentation; and physical property simulation.

Three-dimensional imaging of a rock sample is usually performed in a CT scanning machine by rotating the sample relative to an X-ray source. The actual 3D geometry is reconstructed tomographically from these raw data and the image appears in shades of gray. The brightness of a voxel in such a 3D image is directly affected by the effective atomic number of the material and is approximately proportional to its density. For example, dense pyrite will appear bright while less dense quartz will appear light gray. The empty pore space will be black and parts of it illed with, for example, water or bitumen will be dark gray. To image very small features present in shale or micrite in carbonates, even the sharpest CT resolution may not be enough. A different technique, the so-called FIB-SEM, is used where the focused ion beam gradually shaves off thin slices of the sample and the exposed 2D surface is imaged (photographed) by the scanning electron microscope to produce a stack of closely spaced 2D images.