Supercritical methane adsorption and storage in pores in shales and isolated kerogens

Shale gas is an important hydrocarbon resource in a global context. It has had a significant impact on energy resources in the US, but the worldwide development of this methane resource requires further research to increase the understanding of the relationship of shale structural characteristics to methane storage capacity. In this study a range of gas adsorption, microscopic, mercury injection capillary pressure porosimetry and pycnometry techniques were used to characterize the full range of porosity in a series of shales of different thermal maturity. Supercritical methane adsorption methods for shale under conditions which simulate geological conditions (up to 473 K and 15 MPa) were developed. These methods were used to measure the methane adsorption isotherms of Posidonia shales where the kerogen maturity ranged from immature, through oil window, to gas window. Subcritical methane and carbon dioxide adsorption studies were used for determining pore structure characteristics of the shales. Mercury injection capillary pressure porosimetry was used to characterize the meso and macro porosity of shales. The sum of the CO2 sorption pore volume at 195 K and mercury injection capillary pressure pore volumes (1093–5.6 nm) were equal to the corresponding total pore volume (< 1093 nm) thereby giving an equation accounting for virtually all the available shale porosity. These measurements allowed quantification of all the available porosity in shales and were used for estimating the contributions of methane stored as ‘free’ compressed gas and as adsorbed gas to overall methane storage capacity of shales. Both the mineral and kerogen components of shale were studied by comparing shale and the corresponding isolated kerogens so that the relative contributions of these components could be assessed. The results show that the methane adsorption characteristics were much higher for the kerogens and represented 35–60% of the total adsorption capacity for the shales used in this study, which had total organic contents in range 5.8–10.9 wt%. Microscopy studies revealed that the pore systems in clay-rich, organic-rich and microfossil-rich parts of shale are very different, and also the importance of the inter-granular organic-mineral interface

Influence of Clay, Calcareous Microfossils and Organic Matter on the Nature and Diagenetic Evolution of Pore Systems in Mudstones

Mudstones exert a fundamental control on the flow of both aqueous and non‐aqueous fluids in sedimentary basins. Predicting their flow and storage properties requires an understanding of pore size and connectivity, yet there are very few quantitative descriptions of pore systems of mineralogically and texturally well‐characterised mudstones. We use a combination of electron microscopy, mercury injection capillary pressure porosimetry, and CO2 sorption methods to generate a quantitative description of the size distribution, connectivity, and evolution of pore systems in a sequence of Posidonia Shale mudstones buried to 100‐180 °C. We place the pore data into a detailed mineralogical, petrographical and textural context to show that the nature and evolution of porosity and pore systems can be described in terms of associations with clay‐rich, microfossil‐rich and organic matter‐rich domains, common to many mudstones. Pore size distributions are described by power laws and pore systems are well connected across the full nanometer‐micrometer spectrum of pore sizes. However, connected networks occur primarily through pores < 10 nm radius, with typically 20‐40% of total porosity associated with pores with radii < ca. 3 nm, within both organic matter and the clay matrix. Clay‐rich, microfossil‐rich and organic matter‐rich domains have distinct pore size distributions which evolve in very different ways with increasing thermal maturity. We suggest that the flow of aqueous and non‐aqueous fluids depends not only on the overall connectivity of pores but also the larger‐scale connectivity and wetting state of clay‐rich, microfossil‐rich and organic matter‐rich domains