Swelling of clay minerals is an important phenomenon that is relevant to many problems in geoscience, oil and gas reservoirs, geophysics, and engineering. The phenomenon is even more complex when carbon dioxide is also present. Since sequestration of CO2 in sedimentary rock has been under active consideration as a way of alleviating the increasing concerns over climate change for which CO2 is a prime culprit, studying swelling of clays minerals in the presence of CO2 has taken on added urgency. The swelling changes the permeability and porosity of porous formations, which in turn may lead to a reactivation of dormant fractures, triggering seismic activities, and opening up new pathways for CO2 to return to the porous formations’ surface. Although the large majority of sedimentary rocks contain various types of mixed-layer clays (MLCs) with intermixed stacking sequences of several types of distinct layers within a single crystal, the vast majority of the past experimental and computational studies of the swelling was focused on pure clays. We present the results of extensive molecular dynamics simulation of hydration and swelling of illite-montmorillonite (I-MMT) MLCs, the most common type of mixed clays, in the presence of both water and CO2 and various combinations of interlayer cations, Na+ and K+. To understand the differences with pure clays, we also report on the same phenomenon in the MMT only. At low CO2 concentrations in Na-MMT, which has its layers’ charge concentrated in its octahedral sheet, weak ion–surface interactions result in fully hydrated ions and, therefore, more extensive swelling than in I-MMT. Without CO2, adsorption of the cations at the illite surface increases the hydrophobicity of its surface. Thus, in the asymmetric interlayer of I-MMT, illite with stronger surface–ion interactions causes accumulation of cations near its surface, which limits their hydration and, therefore, controls swelling of the MLCs. Further inhibition of swelling of Na-MLC can be achieved by increasing the concentration of K+ in the interlayer. At higher CO2 concentrations, however, intercalation of water and CO2 results in a completely different behavior, since in the 2W and 3W hydration states, the hydrophobicity of the MMT surface is stronger than that of illite. Therefore, the density distributions of the intercalated molecules are considerably different from that with water only, which together with disruption of the water network by CO2 reduces the difference between the extent of swelling of the MLC and pure MMT.

Swelling of clay minerals is important to a broad class of problems in science and engineering. While the problem has been extensively studied experimentally, past molecular modeling of the phenomenon was focused on pure clays of one type or another. In practice, however, there is a diverse class of mixed-layer clays (MLCs) in sedimentary rock with intermixed stacking sequence of two or more types of distinct layers within a single crystal. In fact, more than 60% of sedimentary rocks in the U.S. contain various types of MLCs. We present the results, to our knowledge, of the first molecular dynamics simulation of hydration energetics and swelling of illite–montmorillonite (I–MMT) MLCs, the most common type of mixed clays. The swelling is studied as a function of the water concentration with three combinations of interlayer cations, namely, Na+ and K+. The hydration energies, the radial distribution functions, and the density profiles in the interlayer region are computed. For regular Na–MMT with layer charge concentrated in the octahedral sheet, weak cation–surface interaction results in fully hydrated ions and significant swelling. In the asymmetric interlayer of the MLC, however, the illite sheet with stronger interaction of surface and ions causes adsorption of the cations deep in the ditrigonal cavities of the siloxane surface. Given that the hydration enthalpy of K+ is smaller than that of Na+, its hydration shell is quite unstable compared with that of Na+. Therefore, swelling is inhibited as the ratio K+/Na+ increases. The results demonstrate the significant differences between the hydration and swelling properties of pure clays and the mixed ones, which have important implications in practice, particularly for sequestration of CO2 in sedimentary rock.

No abstract

Understanding the properties of clay minerals is crucial to various phenomena in geophysics and environmental science and engineering. Although about 70 percent of all clays are of mixed-layer (ML) types, such as illite-montmorillonite (I-MMT) and chlorite–montmorillonite (CH–MMT) clays, the vast majority of the previous experimental and simulation studies were focused on pure clays. This paper reports on a study of important properties of ML clays by molecular dynamics simulations. One goal of the study is to understand the differences between the behavior of CH–MMT and pure chlorite and MMT. Another goal is to understand the effect of cations on the swelling behavior of the ML clays, for which we use Na+, K+, and Cs+. In both CH–CH and CH–MMT, the strong octahedral substitution of the chlorite layer results in, respectively, increasing both polarization and adsorption of water near, and onto the clay surface. The latter reduces hydration of the interlayer cations and, consequently, swelling of the CH–CH and CH–MMT clays, a conclusion that is supported by the computed density profiles and the radial distribution functions. Compared with the octahedral substitutions, the interlayer cation concentration and tetrahedral substitutions are shown to have a substantially weaker effect on swelling, whose pattern is also a function of the type of the interlayer cation. We find that the differences in the size and hydration energy of the cations have strong implications for the distribution of the interlayer species and, thus, their swelling, and that the higher the hydration energy and the smaller the atomic radius of the cation, the more swelling of the clay interlayers occurs. We also show that the layer substitution, the location, density, type of interlayer cations, and asymmetry of the ML clays play important roles in swelling of ML clays.

No abstract