Pore space imaging techniques

Extensive experimental techniques have been developed for porous material characterization [1], including direct imaging [2-5] and bulk measurement techniques for the statistical properties of the pore space. NMR is one such bulk measurement that is both non-destructive and compatible with large samples. 

The technique of DDIF provides a quantitative characterization of the complex pore space of the rocks to supplement conventional mineralogy, chemistry and petrology analyses. A combination of DDIF, Hg intrusion, NMR T2 and image analysis has become the new paradigm to characterize porous rocks for petroleum applications [62, 61]. 

In this section, two examples are presented for the application of a technique of low-melting-point alloy (LMPA) impregnation that provides for a visualization of the invasion of a nonwetting fluid into the pore spaces in a typical porous article. The visualization can be linked to the modeling of mercury porosimeter curves using 3-D stochastic pore networks. This makes the quantification of pore structure more direct. Quantified structures can be visually examined against sample particle sections. The visual comparison can be made more precise by image analysis of the accessible porosity made visible by metal penetration over a series of pressures. 

Some information can be obtained on porous media from conventional NMR spectroscopy, and this is discussed in Section 2. Relaxation time measurements have been widely used to characterize porous solids, and this technique is discussed in Section 3. Pulsed field gradient (PFG) methods may be used to probe the local structure of the pore space and to characterize transport within it, and these are discussed in Section 4. Magnetic resonance imaging (MRI) techniques can also be used to characterize the pore space and to measure transport, and applications are discussed in Section 5. The bulk of this review will be concerned with mesoporous and macroporous materials, as it is for these systems that NMR is particularly useful in characterizing the pore space. However, some applications of NMR techniques to probe the pore space and transport within microporous materials will be mentioned in Section 6. Finally, some general conclusions are given in Section 7. 

In this section, we shall briefly consider the principles behind MRI, and then go on to consider applications of the technique to the characterization of porous media. These include characterization of the pore space and components within it, time-resolved imaging studies, and the measurement of velocity images. 

NMR methods offer a noninvasive method of characterizing porous media. A variety of different techniques may be used to obtain useful information on the pore space. For instance, pore sizes may be measured using the freezing point depression technique for mesoporous solids or by relaxation time measurements for macroporous solids. Other pore space information comes from PFG techniques, while direct imaging of the pore space is possible for large pores. The information from studying the pore space can then be incorporated into appropriate pore network models. 

Pore space properties are important for the description and characterization of pore volume and fluid flow behaviour of reservoirs. Laboratory techniques (standard and special core analysis) deliver fundamental properties. Thin sec-timis and microscopic or scanning electron microscopic (SEM) investigations are used for description and computer-aided analysis. Sophisticated techniques result in digitized core images (Ams et al., 2005 Kayser et al., 2006) and the development of a virtual rock physics laboratory (Dvorkin et al., 2008). 

Blunt et al. (2013) describe pore-scale imaging and modelling as imaging of the pore space of rocks from the nanometer scale upwards, coupled with a suite of different numerical techniques for simulating single and multiphase flow and transport through these images . 

Step 1 Creation of the digital rock sample Modem imaging methods deliver a three-dimensional reconstmction from a series of two-dimensional projections taken at different angles of a rotating sample. Micro-CT scanners have an extremely high resolution—the image samples have voxel sizes as low as 2.5 pm. With this technique, it is possible to study the pore spaces and pore connectivities in great detail. 

Spectral analysis, which is a standard technique for detection and quantification of orientation and/or periodicity of images, has been used for the characterization of pore orientation on gray level images [16]. This technique has been mainly applied to characterize the surface structure studied by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) [16]. This method is based on the calculation of the Fourier power spectra, defined in the frequency or k space, as S(k) = F( k) /1 k, where F(k)Fourier coefficients of the image. The... 

The basical theories, equipments, measurement practices, analysis procedures and many results obtained by gas adsorption have been reviewed in different publications. For macropores, mercury porosimetry has been frequently applied. Identification of intrinsic pores, the interlayer space between hexagonal carbon layers in the case of carbon materials, can be carried out by X-ray dififaction (XRD). Recently, direct observation of extrinsic pores on the surface of carbon materials has been reported using microscopy techniques coupled with image processing techniques, namely scarming tunneling microscopy (STM) and atomic force microscopy (AFM) and transmission electron microscopy (TEM) for micropores and mesopores, and scanning electron microscopy (SEM) and optical microscopy for macropores [1-3],... 

---