Large-scale structure analysis methods such as small and ultra-small angle neutron scattering (SANS and USANS, respectively), yield unique, pore-size-specific insights into the kinetics of CO2 sorption in a wide range of pores (nano to meso). These methods also provide data that may be used to determine the density of adsorbed CO2
through the evolution of microstructure and adsorption capacity. This approach has been applied to the analysis of CO2 in geological samples, including coal. By studying coal exposed to CO2 at subsurface-like temperature and pressure the phase behaviour of the confined CO2, particularly the densification occurring on changing from the gaseous to the liquid phase, was found to have significant operational and reservoir capacity ramifications when assessing the suitability of unmineable coal seams for use as CO2 sequestration reservoirs [51]. The results show that the sorption capacity of coal is sample-dependent and strongly affected by the phase state of the injected fluid (subcritical or supercritical). Subcritical CO2 densifies in the coal matrix, with details of CO2 sorption differing greatly between different coals and dependent on the amount of mineral matter dispersed in the coal. A purely organic matrix was found to absorb more CO2 per unit volume than one containing mineral matter, although the mineral matter markedly accelerated the sorption kinetics [52].

Figure 3.6 shows SANS and USANS data for coal from Seelyville (Indiana, USA) exposed to several pressures of CO2, which could be described using a power law for the scattered intensity with an exponent of -3, indicating the fractal character of the scattering. The scattering intensity shows Q-dependency as a result of the CO2 in the pores. After completion of the pressure cycling, the neutron­scattering curves returned to their original shapes within 1 %, implying that the microstructure was not permanently affected by exposure to CO2 over a period of days. This result indicated that the phenomenon of coal plasticization upon expo­sure to CO2 may be less widespread than thought previously. The work also found that the small pores within coal are filled preferentially over larger void-spaces by the invading CO2, a result echoed by MOFs [32]. Apparent diffusion coefficients for CO2 in coal are thought to vary in the range 5 x 10 7 to more than 10 4 cm2min 1 according to the CO2 pressure and location. At higher pressures CO2 is shown to

diffuse immediately into the coal matrix, swelling the coal and changing its mac­romolecular structure, where it is postulated to create microporosity through the extraction of volatile components [53]. Injection of CO2 into model subsurface geologic formations has been identified as a key strategy for CO2 storage. Key to the success of such a strategy is the prevention of leakage from the host by an effective cap with low porosity and permeability characteristics. Shales comprise the majority of caps encountered in subsurface injection sites with pore sizes typ­ically less than 100 nm and whose surface chemistries are dominated by quartz and clays. Analysis of simple, well-characterized fluid-substrate systems can provide details on the thermodynamic, structural, and dynamic properties of CO2 under conditions relevant to sequestration. In particular, the behaviour of CO2 interacting with model silica substrates can act as proxies for more complex mineralogical systems. SANS data for CO2-silica aerogel (95 % porosity; *7 nm pores) indicates the presence of fluid depletion for conditions above the critical density [54].