## 2 Characterisation of the Structural Evolution of Rocks Following Exposure to Carbon Dioxide
### Void Space Segmentation Schemes  
The void spaces of rocks are complex and, thus, in order to make them amenable to the understanding and prediction of the processes occurring therein, some sort of classification and characterisation scheme is required. This typically consists of breaking down the complex whole into a large assemblage of more manageable segments, which can be more easily classified and assigned descriptive parameters, which may then quantify change during exposure to CO\({}_{2}\)-rich phases. For example, for indirect methods, this segmentation process breaks the void space down into elements called 'pores', which possess a characteristic pore size as their key parameter. However, the exact definition of what is meant by a 'pore' is problematic [61]. For the traditional analysis of mercury porosimetry, this process involves effectively converting the discontinuous (as it consists of a series of small steps up in pressure) mercury intrusion curve into a histogram of pore volume (derived from measured intruded volume for a pressure step) as a function of pore size (derived from intrusion pressure and the Washburn equation) [61]. This treats the void space as if it consisted of a parallel bundle of cylindrical pores (a 'wine-rack'-like structure). For more direct imaging methods, such as computerised X-ray tomography (CXT) or magnetic resonance imaging (MRI), the segmentation process of the reconstructed image usually depends upon some geometrical criterion, such as a pore-pore boundary being defined by a local minimum in some measure of characteristic size [61; 73]. However, it is not clear whether these often arbitrarily chosen segmentation processes break up the void space into zones that actually matter in the physical process of interest occurring within the rock. For example, the advanced condensation, or cascade effect, during capillary condensation means that many pore elements, defined in a similarly simplistic way to that described above for imaging, would fill with liquid phase, together, during the same pressure-increase step, thus making the supposed distinction amongst pores superfluous [61; 74]. However, segmentation methods based upon particular reference processes, occurring within the void space itself, have been developed, which are also of wider physical meaning since they are useful in predicting other physical processes besides the one used to define them. One such method is gas sorption scanning curves [61]. Figure 8 shows an example of this method applied to Nordland shale. The particular forms of the nitrogen sorption scanning curves obtained for this shale suggested that the void space could be considered a composite of two thermodynamically independent networks, denoted 1 and 2, in order of increasing pore size. This particular segmentation is indicated because, although the desorption scanning curve initiated from the adsorption boundary curve at a relative pressure of approximately 0.8 was converging on the lower hysteresis closure point, those initiated further up the isotherm, at relative pressures higher than 0.8, converged instead on a specific point on the boundary desorption isotherm at a relative pressure of 0.8. This latter finding indicated a concealed lower hysteresis closure point for a network of larger pores, denoted Network 2. The ascending scanning curves, initiated at this upper convergence point on the desorption branch, crossed directly to the adsorption branch, where they indicated the particular pressure (and thus the pore size from the Kelvin equation) that delimits the two networks. This form suggests they may be like the top flat plateau of the hysteresis loop for a network containing smaller mesopores, denoted Network 1, filling completely below relative pressures of ~0.8. The gentle rise in the crossing, ascending scanning curve arose from multi-layer adsorption occurring in the empty pores of Network 2.