## 2 Characterisation of the Structural Evolution of Rocks Following Exposure to Carbon Dioxide
### Adsorption Calorimetry  
A very rarely used technique for rock characterisation involves combining microcalorimetry with gas adsorption simultaneously. In addition to the usual thermodynamic parameters, such as the distribution of the heat of adsorption across different adsorption sites, and how these may change following exposure to scCO\({}_{2}\), adsorption calorimetry can also be used to study mass transfer effects within rocks. As mentioned above, adsorption calorimetry has been used to show the impact of narrow micropores guarding access to much larger meso- and macro-pores in Bowland shale [75]. Figure 9 shows typical examples of the type of raw data obtained from adsorption calorimetry for carbon dioxide adsorption on the Above Marine Band Bowland shale, originally taken from Preese Hall Well 1 in north-western England. In Figure 9, it can be seen that the form of the heat-flow for a typical adsorption step exhibits a skewed, asymmetric peak shape. There is an initial relatively steep rise in heat released, followed by a slower decrease. The decreasing part of the data in Figure 9 has been fitted to an exponential decay function with a time constant denoted \(\tau\). This characteristic parameter for heat-flow is said to be related to the rate of mass transport of the adsorbate [76]. Furthermore, Figure 9 shows the corresponding mass uptake response with time, for the same pressure step, along with a fit of the experimental data to the exponential growth functional form of the linear driving force (LDF) model, with a characteristic time constant, denoted \(1/k\), where \(k\) is a mass transfer coefficient [61]. Figure 10 shows the correlation of the characteristic mass transfer rate parameters, derived from the two aforementioned data-set types obtained from adsorption microcalorimetry (\(\tau\) and \(1/k\)), with each data-pair being for the same adsorption step over a range of saturation levels of carbon dioxide during adsorption experiments on an Above Marine Band sample of Bowland shale. It is noted that the observed LDF mass transfer coefficients, \(k\), as obtained from plots such as Figure 9, have been corrected for adsorption before plotting in Figure 10. From Figure 10, it can be seen that, initially, at lower values of the time constants, there is a good fit to a straight line, suggesting that both methods are probing the same mass transfer process. However, at the highest values of \(1/k\), which correspond to the highest carbon dioxide saturation levels tested, there is some deviation from the linear fit. Thereafter, the time constant for heat flow, \(\tau\), shows a tendency to plateau around a constant value, instead of continuing to increase. Figure 8: A series of nitrogen sorption scanning curves for as-received Nordland shale of particle size \(>\)150 \(\upmu\)m, with (**a**) desorption starting at ultimate relative pressures of 0.99 (\(\blacksquare\)), 0.95(\(\blacktriangle\)), and 0.93 (\(\bullet\)) on the adsorption boundary isotherm; (**b**) a descending scanning curve starting at an ultimate relative pressure of 0.82 on the adsorption boundary isotherm; and (**c**) an ascending scanning curve springing from an ultimate relative pressure of 0.8 on the conventional desorption boundary isotherm. Reprinted from [PERSON] et al. [68] under a Creative Commons CC-BY Licence. Figure 9: Typical raw datasets for a particular pressure point of the adsorption isotherm from an adsorption calorimetry experiment on a sample of Above Marine Band Bowland shale, showing heat-flow (**(a)**, blue crosses) and mass uptake (**(b)**, green squares). Also shown (solid red lines) are fits of the experimental data to an exponential decay function (**a**) and the LDF model (**b**). The deviation from linear behaviour in Figure 10 suggests that, at higher CO\({}_{2}\) saturation levels, the mass transfer processes probed by mass uptake and heat-flow data are different.