(675f) Modeling Illite Within Shale: From a Density Functional Theory Perspective Conference: AIChE Annual MeetingYear: 2013Proceeding: 2013 AIChE Annual MeetingGroup: Computational Molecular Science and Engineering ForumSession: First-Prin?ciples Simulation?s of Condensed Phases Time: Thursday, November 7, 2013 - 2:10pm-2:30pm Authors: Geatches, D. Wilcox, J., Stanford University 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: *firstname.lastname@example.org %email@example.com 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 (Ca0.059, K0.655)(Si3.597, Al0.403)(Fe0.628, Al0.969, Mg0.428) 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.