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.