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Top seals and overburden above reservoirs at geological carbon capture and storage (CCS) sites can be major concerns when they are at risk of being mineralogically and texturally unstable in the presence of high-pressure CO2. Here we report on the pore systems, mineralogy, and surface area attributes of the Palaeocene Lista Shale, the caprock to the Mey Sandstone at the UK’s planned East Mey CCS site. The core was logged, and then mineral quantification was undertaken with X-ray powder diffraction mineralogy, light optics and electron microscopy analyses. Laser particle size analysis was used for grain size determination. Porosity, pore throat diameter, surface area and pore body size were measured via mercury intrusion porosimetry and nitrogen adsorption analyses. The mudstone facies from the Lista Shale are dominated by smectite-rich matrix and silt-grade quartz, with small quantities of chlorite and sodic-plagioclase. Chlorite, sodic-plagioclase, and even smectite are known to be capable of reacting with, and potentially leading to mineral sequestration of CO2. The mean pore throat and pore body diameters are 17 and nearly 18 nm, respectively, showing that the Lista is mesoporous; the similarity of pore body and pore throat dimensions reveals a predominance of plate and slit pores. Gas adsorption analyses revealed that the overall pore structure is complex, with a high tortuosity of fluid movement through a complex clay-rich matrix (this equates to a mean fractal dimension D2 value of 2.67). Gas adsorption analyses have also shown that grain surfaces are moderately complex (rough) due to the dominance of clay aggregates (this equates to a mean fractal dimension D1 value of 2.56). D2 being higher than D1 suggests that there is a relatively low potential to physically store CO2 gas on grain surfaces. Conversely, the ability of the CO2 to react with minor quantities of chlorite and sodic plagioclase, or even with smectite, could lead to increasing surface area of the remaining shale minerals with newly exposed reactive silicates leading to further enhanced mineral trapping of the injected CO2. The restricted pore throat size linked to small grain size and poor sorting, and reflected by the high fractal D2 value, plus limited grain surface complexity, reflected by the low fractal D1 value, collectively suggest that mineral trapping of the injected CO2 would be relatively slow (on the order of 1000s of years) if CO2 penetrated the top seal.
AlNajdi et al. (Thu,) studied this question.