Using small angle neutron scattering (SANS) for a systematic understanding of the pore structure of Mudrocks of different origin

Abstract

Technologies utilising the subsurface are impacted by the presence and properties of mudrocks. This includes the petroleum industry evaluating top seals and shale reservoirs, as well as applications related to the low carbon energy transition plan for a net zero future: CO2 and energy storage (CH4, H2, and compressed air) and radioactive waste disposal. To assess the feasibility of mudrocks for technical and geotechnical applications, the pore structure needs to be characterised in detail. Mudrocks are fine-grained siliciclastic sediments, characterised by complicated textures (size and sorting of grains) and fabric (arrangement of grains). The pore structure of mudrocks consists of matrix inter- and intra-particle void space and organic matter intraparticle pores, ranging from micrometers down to the nanometers in size. With type, size, and organisation of pores being different, inorganic- and organic-related pore networks lead to the natural permeability pathways for fluid transport in mudrocks. To quantitatively analyse the pore structure at a broad pore scale range (~ 2 nm to ~ 5 μm), small angle neutron scattering (SANS) and low-pressure sorption (LPS) were conducted on 13 sets of mudrocks originating from radioactive waste storage sites, hydrocarbon seals and shale gas reservoirs across the globe. These include Opalinus Clay, Switzerland, Boom Clay, Belgium, Våle Shale, Norway, Posidonia Shale, Germany, Carboniferous Shale, Belgium, Jordan Oil Shale, Jordan, and Carmel Claystone, Big Hole Carmel, Entrada Siltstone, Bossier Shale, Haynesville Shale, Eagle Ford Shale, and Newark Shale, USA. The results have revealed a vast heterogeneity of porosity, which can be related to the high clay contents and organic matter. Due to the high clay contents and various types of organic matter, pores smaller than 10 nm constitute a large fraction of the total porosity (15-50 %) and up to 98 % of the specific surface area. The results indicate that pore structure varies with a change in the depositional environment. The homogeneous pore structure of Opalinus Clay reflects a stable, low energy depositional environment. Boom Clay features variable pore size distribution resulting from variations in grain sizes due to variable depositional processes. Furthermore, diagenesis and compaction evolve pore structure of mudrocks during and after sedimentation. The complex geometry of the pore networks in Carmel stems from dissolution of dolomite by the geochemical interaction between carbonic acid, resulting in increased macroporosity. The effective pore structure evolves from the oil to the gas window in organic rich mudrocks. A progressive amalgamation towards larger pores indicates thermal maturity and generates secondary organic-matter pores when coarse grained minerals preserve primary porosity. Thermal maturation develops porosity at macroscale range more effectively, which can enhance the permeability for continuum flow in organic rich mudrocks. Based on 71 individual samples, we developed a normal cumulative density function model to predict porosity as a function of maximum burial depth. The model allows understanding porosity reduction independent of depositional and post-depositional processes, like compaction and diagenesis. We further conducted multivariate statistical analyses consisting of principal component analysis and constrained multilinear regression to identify and assess the contribution of controls on the pore structure e.g., clay content, coarse grained minerals, TOC, and compaction. Principal component analysis suggests that clay content and compaction are key controls on the pore structure of organic lean mudrocks. Coarse grained minerals, compaction, and TOC are key characteristics of organic rich mudrocks. The constrained multilinear regression shows that the controlling components, compaction and tortuosity, are statistically significant predictors of the pore structure in all mudrocks. Clay content and coarse-grained minerals have secondary control on porosity. The statistical analyses do not show a correlation between porosity and TOC content in both oil window and gas window mudrocks. Nevertheless, the interplay between mineralogy, organic matter, and compaction contributes to a pore network of few-to-several nano-Darcy permeability in which pore size dependent transport mechanisms can vary from transitional transport in micro- and mesopores to slip flow in meso- and macropores and continuum/Darcy flow in progressively larger macropores. We developed fractal models to integrate Darcy permeability with continuum/Darcy flow and apparent permeability and effective diffusion coefficients with diffusion flow for the relevant pore sizes in mudrocks. The results indicate that the majority of mudrocks possesses higher apparent permeability than Darcy permeability that results in a greater total permeability, leading to a better flow conductivity. On the other hand, the increased macroporosity results in low diffusivity. To improve fluid dynamic models at pore scale, the incorporation of SANS PSD, porosity, and SSA as well as pore size dependent fluid flow are necessary to understand storage capacity and/or transport phenomena more realistically in mudrocks.