(675f) Modeling Illite Within Shale: From a Density Functional Theory Perspective

MODELING ILLITE WITHIN
SHALE: FROM A DENSITY FUNCTIONAL THEORY PERSPECTIVE

Dawn Geatches*, Jennifer Wilcox^%

Department of Energy Resources Engineering, 367 Panama
Street, Stanford University, Stanford, CA 94305-2220, USA: *geatches@stanford.edu

^%wilcoxj@stanford.edu      

The
recent resurgence of interest in gas shale as a rich source of methane has
raised many research questions at all length scales, from the seismic effects
of hydrofracturing to interactions within the shale
matrix at the atomistic level. Our research falls into the latter category,
specifically investigating the interaction of clay minerals with H₂O,
CH₄ and CO₂ within the wider research context of enhanced
methane extraction and carbon dioxide storage and sequestration in exhausted
shale beds. To identify possible chemical interactions between the clay mineral
surfaces and H₂O, CH₄ and CO₂, we investigated
the electronic structure of clay mineral models from first principles using
density functional theory (DFT), plane waves and pseudopotentials.  We focused on illite
due to its prevalence within mature gas shale, and began by building the two
main polytypes – 1M and 2M₁ - based
on crystallographic data, experimental formulae and illite-sample
analyses. Illite (general formula (Ca_0.059,
K_0.655)(Si_3.597, Al_0.403)(Fe_0.628,
Al_0.969, Mg_0.428) per O₁₀(OH)₂)
is classed as a dioctahedral interlayer-deficient
mica characterized by non-swelling behavior and an interlayer dominated by
potassium ions. Each layer consists of tetrahedral-octahedral-tetrahedral (TOT)
sheets composed of (mainly) Si₂O₅ in the tetrahedral
sheets and Al₂O₅ in the octahedral sheets_.
Substitutions of the Si⁴⁺ by Al³⁺ and octahedral Al³⁺
by Fe²⁺/Fe³⁺ and/or Mg²⁺ produce a negatively
charged TOT layer. The negative charge is balanced by positive interlayer cations, which, in the case of 1M illite, are mainly K⁺, and less commonly
Na⁺ and Ca²⁺. Within the interlayers of the 2M₁polytype it is also possible to find NH₄⁺
and H₃O⁺. The octahedral layer cations
fill two of three possible octahedral sites, hence the description 'dioctahedral'. There are two types of occupation of these
octahedral sites, the M1 and the M2 positions, which are identified by their
vacancies and the positions of the hydroxyl groups around the vacant site.
Where pairs of hydroxyl groups lie on opposite apices of the vacant cation site this is `trans-vacant', and where the pairs lie
on the same side of the vacant cation site this is `cis-vacant'. This short description of the possible
variations in polytype, structure and composition of illite, explains why there were so few ab
initio studies of illite available before we built
our models.

In
this presentation we show how, to account for these structural and
compositional variations, we have explored cis- and
trans-vacant structures of two main polytypes of illite found within shale and their various cation substitutions represented in the general formula. We
also show how we described the strongly correlated d-electrons of iron, by testing
a range of Hubbard (U) values within the DFT+U formalism, and when investigating clay
mineral surface/H₂O/CO₂/CH₄ interactions we compared
models with and without van der Waals forces, i.e. within the DFT+D formalism. Finally
we interpret the nature of the surface/molecule interactions within the wider
research area of enhanced methane extraction and carbon dioxide storage.