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 2M₁ polytype 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.