We perform grand-canonical molecular simulations to study the molecular mechanism of clay swelling hysteresis as a function of the relative humidity. In particular, we focus on the transition from the one-to the two-layer hydrate and the influence of three types of counterions (Li + , Na + , and K + ). Our results cover the experimental relative humidity region where swelling and shrinking usually occur. We show that the thermodynamic origin of swelling hysteresis is a free-energy barrier separating the layered hydrates. This free-energy barrier is dominated by breaking and formation of hydrogen bonds between and within water layers. This network of water molecules is similar for all counterions, but the positions of these counterions depend upon their size. The relatively large K + counterions show more affinity for clay surface adsorption, which increases the free-energy barrier and inhibits swelling. On the other hand, the relatively small Li + counterions are quite well-accommodated in the water network, and thereby, they can form a new swelling state with a basal spacing of approximately 13.5 Å. This new swelling state is an alternative explanation for the widely accepted simultaneous occurrence of two or more swelling phases.

We have carried out molecular simulations in the grand-canonical ensemble of water and cations in Wyoming and Arizona montmorillonite clay minerals, with varying relative humidity. Several water models and cations are used to investigate the swelling of these clays. We show how the water content depends on the type of clay, type of cation, the basal spacing, and the relative humidity. Related to the layering of water molecules in the interlayer space, the pressure normal to the clay sheets oscillates as a function of the basal spacing. Minima in corresponding free energy curves indicate the presence of dehydrated states and layered hydrates. The development of these stable states and the corresponding basal spacings are in agreement with experimental data. Density profiles show significantly different interlayer structures depending on the type of clay and models used. We show a relation between formation of two-layer hydrates and the position of the cations. The simulations with the MCY water model underestimate the spacings of two-and three-layer hydrates, whereas our simulations with the TIP4P model produce a better agreement. Therefore, we recommend the TIP4P model for simulating clay minerals. In addition, we report remarkable ordering of cations and water molecules in a one-layer Arizona montmorillonite hydrate.

CommunicationsSwelling and shrinking in clay minerals, such as the NA-montmorillonite shown, occurs in a stepwise fashion and by different pathways. Through simulations it has been shown that a free-energy barrier separating stable layered hydrates is the cause of this hysteresis. For more information see the Communication by T. J. Tambach et al. on the following page.

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Coalbed methane (CH 4 ) is an important energy source. With increasing climate change concerns, coalbeds are also considered potential sinks for underground carbon dioxide (CO 2 ) storage. 1,2 Roughly twice the amount of CO 2 can be adsorbed than CH 4 in most bituminous coals. 1 Another advantage of CO 2 injection into coalbeds is the additional production of CH 4 . This process is known as CO 2 -enhanced coalbed methane (ECBM) and is very attractive considering the increasing energy demand. Several ECBM pilot programs have been implemented in several parts of the world; 3,4 however, its full potential has yet to be exploited. 5 ECBM field operations can be better managed by an improved understanding of laboratory measurements and coalbed reservoir simulations. 6 Both CH 4 and CO 2 are predominantly stored in the micropores of the coal matrix, 7 which appear as "isolated" locations. 8 No consensus exists on measuring the total surface area, 9 the heat of adsorption, 1 or the theoretical description of adsorption, 10 partially because coal is difficult to characterize as a result of its heterogeneity 11 and complex response to CO 2 adsorption. 12 In this study, molecular simulations are used to identify energetically favorable adsorption regions, thereby testing the molecular exchange of CH 4 by CO 2 . 13,14 Our goal is to discuss the adsorption distribution at a molecular scale. † Progress in Coal-Based Energy and Fuel Production.

Disorder of intercalated surfactant molecules in clay minerals causes gradual swelling, rather than commonly assumed swelling in discrete steps.Nanocomposites of clay minerals and surfactants are of practical importance in many fields.1 Beside the use of these systems for pollution removal from waste water, 2 they can also be used as catalysts, 3 to stabilize clay formations while drilling for hydrocarbons, 4 or for improving the strength of polymers. 5,6The nanocomposites are obtained through exchange of natural counter ions by ionic surfactants, such as alkylammonium. 7Despite the importance of such nanocomposites, their behaviour is not understood. It is commonly assumed that the distance between the clay platelets (basal spacing) increases stepwise or linearly with increasing alkylammonium chain length. 8 The traditional explanation for this observation is that the surfactants form all-trans conformations between the clay platelets, 8 either in layers or tilted in a liquid crystalline ordered phase. However, this explanation has recently been debated. 9 X-Ray diffraction measurements show a complex intercalation behaviour.7 For example, when basal spacings are measured, corresponding to values in-between monolayer and bilayer structures, then these values are usually attributed to layer charge heterogeneity.10 In this case, the explanation is that different layer charges within a clay sample cause a varying surfactant loading in the interlayers between two clay platelets and therefore an ensemble of monoand bilayer structures, but this has never been directly confirmed experimentally. As X-ray diffraction peaks for such states overlap, the resulting peak is broad and difficult to interpret.Molecular simulations are ideally suited for obtaining more insight in the intercalation behaviour of surfactants in clay minerals. Several simulation studies have been carried out to compute the basal spacing for some clay minerals and some alkylammonium surfactants. 6,[11][12][13] These simulation studies show increasing disorder (phase transitions) of the surfactants in mica clay minerals with increasing temperature.6,12 Our simulations are complementary to the previous studies, in the sense that they cover an extensive range of surfactant chain lengths. Also, in contrast with mica, we consider clay minerals that are able of swelling. We find, for the first time, full quantitative agreement between the experimental and simulated swelling behaviour of alkylammonium in montmorillonite and vermiculite clay minerals. Based on our simulations, we give a new explanation for the gradual increase of the clay platelets distance. We show evidence that this is a true physical phenomenon, caused by disorder of the surfactant molecules.Out of several available molecular clay models, 6,14,15 we choose a clay model 15 that has proven to be successful in predicting the basal spacing of natural clay minerals with water, 15,16 the swelling capabilities of clay minerals, 17,18 and hysteresis in clay swelling. 17,19 In our simulations, we ...

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Well abandonment in the context of CO2 storage operations demands a mitigation strategy for CO2 leakage along the wellbore. To prevent possible CO2 transport toward the surface and to protect the wellbore material from contact with acid brine, we propose forming a salt seal around the wellbore at reservoir level, after CO2 injection into a depleted gas field. The mechanism of water evaporation into dry gas and subsequent salting‐out of the dissolved halite is a well‐known issue in hydrocarbon production. We propose alternating injection of brine and CO2 to facilitate intentional salt clogging of the reservoir. Salt clogging was studied with TOUGH2, simu‐lating multiple cycles of brine and CO2 injection. We developed two near‐well reservoir models – a homogeneous 2D model and a more complex heterogeneous 2D model – using certain characteristics of the K12‐B depleted gas field. The homogeneous 2D model yields a 50‐cm thick salt bank around the well and complete permeability impairment after eight cycles of brine‐CO2 injection. Addition of vertical permeability heterogeneities causes variation in the lateral extent of salt precipitation and hence vertical discontinuities in the salt bank. A carefully designed injection strategy – with more injection cycles and lower injection rates – improves the vertical sealing. Predictions of the amount of required clogging for effective sealing would benefit from more accurate porosity‐permeability relationships and insights in the long‐term stability of the salt bank regarding re‐dissolution. We conclude that alternating brine‐CO2 injection could be a promising method for intentional salt‐clogging of the near‐wellbore area. © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd