Understanding pore structure of mudrocks and pore-size dependent sorption mechanism using small angle neutron scattering

To quantitatively analyse the pore structure at a broad pore scale range (∼ 2 nm to ∼ 2 μm), low pressure sorption (LPS) and small angle neutron scattering (SANS) were conducted on several mudrocks originating from radioactive waste storage sites, hydrocarbon seals and shale gas reservoirs across the globe. These include Opalinus Clay Switzerland, Posidonia Shale, Germany, and Carmel Claystone, Bossier Shale, and Eagle Ford Shale, USA. Furthermore, upon injection of supercritical fluids (deuterated methane, CD₄) into the pore space of mudrocks, the phase behaviour depending on pore size was investigated with subsequent neutron scattering. The results have revealed a vast heterogeneity, which can be related to the high clay contents. Due to the high clay contents, pores smaller than 10 nm constitute a large fraction of total porosity (25-30 %) and up to 80 % of specific surface area (SSA). Moreover, total porosity and SSA are not significantly affected by thermal maturation. However, thermal maturity contributes to different pore size distribution (PSD) related to meso- and macro-pores. Thermal maturation is likely to develop porosity at macroscale range, which can enhance the permeability for continuum flow in organic rich mudrocks. Results obtained from supercritical fluid sorption within SANS experiments demonstrated the formation of an adsorbed phase characterised by a higher density than predicted for the bulk fluid by the equation of state. The effect of sorbed phase is pore size dependent. It implies that the density as well as the volume fraction of the adsorbed phase are influenced by the pore structure; sorbed phase tends to fill small pores, followed by progressively filling larger pores. Mineralogy and maturity interplays contribute to a pore network of few-to-several nano-Darcy permeability in which pore size dependent transport mechanisms can vary from diffusional transport in small pores to slip flow in progressively larger pores. In order to improve pore network models, the incorporation of SANS PSD as well as pore size dependent sorption are important to more realistically understand storage capacity and/or transport phenomena in mudrocks.