Shale gas, which predominantly consists of methane, is an important unconventional energy resource that has had a potential game-changing effect on natural gas supplies worldwide in recent years. Shale is comprised of two distinct components: organic material and clay minerals, the former providing storage for hydrocarbons and the latter minimizing hydrocarbon transport. The injection of carbon dioxide in the exchange of methane within shale formations improves the shale gas recovery, and simultaneously sequesters carbon dioxide to reduce greenhouse gas emissions. Understanding the properties of fluids such as methane and methane/carbon dioxide mixtures in narrow pores found within shale formations is critical for identifying ways to deploy shale gas technology with reduced environmental impact. In this work, we apply molecularlevel simulations to explore adsorption and diffusion behavior of methane, as a proxy of shale gas, and methane/carbon dioxide mixtures in realistic models of organic materials. We first use molecular dynamics simulations to generate the porous structures of mature and overmature type-II organic matter with both micro-and mesoporosity, and systematically characterize the resulting dual-porosity organic-matter structures. We then employ the grand canonical Monte Carlo technique to study the adsorption of methane and the competing adsorption of methane/carbon dioxide mixtures in the organic-matter porous structures. We complement the adsorption studies by simulating the diffusion of adsorbed methane, and adsorbed methane/carbon dioxide mixtures in the organic-matter structures using molecular dynamics.
We employ grand canonical Monte Carlo and molecular dynamics simulations to systematically study the adsorption and diffusion of C to C alkanes in hierarchical ZSM-5 zeolite with micropores (∼1 nm) and mesopores (>2 nm). The zeolite is characterized by a large surface area of active sites on the microporous scale with high permeability and access to the active sites, which arises from the enhanced transport at the mesoporous scale. We model this zeolite as a microporous Na-exchanged alumino-sillicate zeolite ZSM-5/35 (Si/Al = 35) in which cylindrical mesopores with a diameter of 4 nm have been built by deleting atoms accordingly. We use the TraPPE and Vujić-Lyubartsev force fields along with the Lorentz-Berthelot combining rules to describe adsorbate-adsorbate and adsorbate-adsorbent interactions. The performance of the force fields is assessed by comparing against experimental single-component adsorption isotherms of methane and ethane in microporous ZSM-5/35, which we measured as part of this work. We compare the adsorption isotherms and diffusivities of the adsorbed alkanes in the dual-porosity zeolite with those in microporous ZSM-5/35 and discern the specific behavior at each porosity scale on the overall adsorption, self-diffusion, and transport behavior in zeolites with dual micro/mesoporosities.
Organic-shale formations are unconventional gas reservoirs with broad pore size distributions. Shale consists of two distinct components: organic matter and clay minerals. The size of pores in the organic matter is mostly concentrated at less than six nanometers, and these micropores and small mesopores provide the majority of adsorption surface area and gas storage volume. In these nanometer-sized pores, the geofluid behavior becomes significantly different from the bulk behavior due to the strong solid− fluid interactions and other confinement effects. Understanding the properties of fluids such as methane, ethane, propane, and carbon dioxide in narrow shale pores is critical for identifying ways to deploy shale gas technology with reduced environmental impact. Specifically, methane is a proxy of the shale gas, and ethane and propane are minor shale-gas components. Further, carbon dioxide is used for enhanced shale-gas recovery. We employ molecular-level simulations to explore adsorption, diffusion, and transport of methane, ethane, propane, and carbon dioxide in realistic models of organic-shale materials at a representative shalereservoir temperature and pressures. We first use Hybrid Reverse Monte Carlo with experimental pair distribution functions to build dual-porosity kerogen models corresponding to an immature marine kerogen from the Eagle Ford Play and a mature marine kerogen from the clay-rich Marcellus Play. We then employ Grand Canonical Monte Carlo simulations to study the fluid adsorption in the porous kerogen structures. We complete the adsorption studies by simulating the self-diffusivity, collective diffusivity, and transport diffusivity of the adsorbed fluid molecules in the shale kerogens using equilibrium and nonequilibrium molecular dynamics.
The manuscript describes a computational study that provides molecular-level insight into shale gas adsorption and transport in shale rocks, which are composed of organic and inorganic matter. Atomistic simulations were used to generate realistic models of the organic matter structures with both micro-and mesoporosity, and correspond to mature and overmature type-II kerogens. These porous material models are unique to most other previous kerogen models since they contain other components (asphaltene/resin, hydrocarbons and carbon dioxide/water fractions) that are typically not modeled. The inclusion of these additional components significantly influences the resulting porous structure characteristics. The adsorption and diffusion behavior of methane (as a shale gas proxy) and methane/carbon dioxide mixtures were simulated in the model structures.Several key industrial-relevant findings are described in the manuscript.
Understanding the microscopic behaviour of aqueous electrolyte solutions in graphene-based ultrathin nanochannels is important in nanofluidic applications such as water purification, fuel cells, and molecular sensing. Under extreme confinement (<2nm),...
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