## 2 Characterisation of the Structural Evolution of Rocks Following Exposure to Carbon Dioxide
### Pore Volume Determination  
Rocks, especially shales and carbonates, often have networks of micro-, meso-, and macro-pores, which makes them difficult to characterise with a single technique, even using standard indirect methods. For example, micropores are generally too small to intrude mercury during porosimetry, whereas nitrogen will not capillary-condense in macropores in standard adsorption experiments. Nonetheless, mercury porosimetry is popular for characterising rocks because it is one of the very few techniques that can probe the large range of length-scales, from \(\sim\)100s \(\upmu\)m to nanometres, found therein with just one experiment. However, high pressures of \(\sim\)400 MPa are needed to probe nanoporosity, and thus there is a risk of crushing the sample. In contrast, the gas overcondensation method allows the probing of sizes from micro- to macro-pores with one technique and does not require high external pressure to probe smaller pores [61; 71]. The gas overcondensation method has shown that conventional gas sorption experiments tend to miss much of the macroporosity present in shales [71]. Figure 7a-c indicate what might be expected from gas sorption for a range of idealised porous structures, and Figure 7d shows some experimental data for a real shale. As seen in Figure 7, in contrast to the conventional adsorption experiment, gas overcondensation can be used to probe both macroporosity and microporosity. Comparing the experimental data for the shale sample from conventional adsorption, given in Figure 7d, with the idealised isotherm in Figure 7a, and by comparing the experimental overcondensation data in Figure 7d with the idealised isotherm in Figure 7c, it can be seen that the combined experimental datasetsreflect what would be expected if connected macroporosity in the shale was being missed in the conventional sorption experiment but detected by overcondensation. Figure 7: Schematic diagram depicting pore geometry and results anticipated from gas sorption experiments with (**a**) a nanopore (rectangle)-connected macroporc (circle) in a conventional adsorption experiment, (**b**) a disconnected macroporc in an overcondensation experiment, and (**c**) a nanopore-connected macroporc in an overcondensation experiment. The dark (blue) shading indicates the presence of the condensate, whereas the light (blue) shading indicates an empty pore or vapour only. The dotted line in the idealised isotherm line plots indicates bulk condensation pressure. Reprinted from [PERSON] et al. [70] under Creative Commons CC-BY Licence. (**d**) Ascending adsorption scanning curves (starting at relative pressures of 0.6 and 0.8) and corresponding descending desorption curves (indicated by a descending arrow) for scanning loops starting on the overcondensation boundary desorption isotherm (solid line), and conventional isotherms for a thermochemically-treated Rempstone shale sample (\(\bullet\)). It can be seen that the conventional isotherm misses a lot of porosity and leads to a misleading hysteresis loop shape. Reprinted from [PERSON] et al. [71] under a Creative Commons CC-BY Licence. The boundary overcondensation desorption experiment can also incorporate ascending gas adsorption scanning curves that can probe pore morphology. A scanning curve is defined as an isotherm, whereby the progressive change in pressure is halted and the direction of change reversed, before reaching one of either hysteresis closure points in the full boundary isotherms, which are the isotherms where complete pore-filling with condensate is achieved at the upper end, in terms of the amount adsorbed. For example, the crossing ascending scanning curves, even for the very wide hysteresis shown in Figure 7d, suggest that large voids with narrow windows are present in thermochemically-treated Rempstone shale, as seen within the macroporosity of the organic (carbon) phase observed in complementary electron microscopy data [71].