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Effects of molecular level forces on the diffusion and adsorption behavior of confined hydrocarbons: a molecular dynamics approach
Coskuner, Yakup Berk
Coskuner, Yakup Berk
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2022
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Abstract
It has been shown that, in shale reservoirs, heavy hydrocarbons tend to be hindered due to nano-sized pore structures, while the lighter ones pass through. This phenomenon is explained by the molecular sieving, preferential adsorption, and anomalous diffusion mechanisms and affects the primary recovery performance as well as the composition of the oil retained in the reservoir. To recover the retained oil in shale reservoirs, CO2 and CH4 injection have been applied as an enhanced oil recovery technique (EOR). Although the mechanics of gas injection EOR has been fairly well known for conventional applications, the understanding is mostly based on the physics of bulk flow. For applications in unconventional reservoirs, a detailed study is required to explain the molecular level physics, which becomes dominant when the pore sizes drop down to nanometers.
The primary objective of this dissertation is to understand molecular level flow phenomena in nanopores to identify key process parameters to design more appropriate compositional flow models for ultra-tight, unconventional oil reservoirs. Moreover, for the construction and application of new compositional models, this dissertation provides definitions and quantifications of the phenomenological relationships to account for hindrance, preferential adsorption, and anomalous diffusion. To meet the objectives of this dissertation, I conducted molecular dynamics (MD) simulations and investigated the changes in the diffusivities and adsorption of hydrocarbon components as a result of molecular interactions between the fluid and the wall, as well as the interactions between the fluid molecules.
The MD simulations provided both qualitative and quantitative insights. The quantitative results were given as empirical equations modeling the diffusion and adsorption as a function of pressure, composition, and confinement. The qualitative results provided the evidence of anomalous diffusion, delineated the mechanisms of molecular level flow in tight formations (effect of fluid composition and adsorption on diffusivity), and explained the physics behind the EOR applications in ultra-tight reservoirs.
The previous studies in the oil and gas literature focused on the anomalous diffusion in the bulk level, which occurs as a result of fractal geometries of reservoirs. One of the contributions of this dissertation is to document the diffusion behavior of hydrocarbons at molecular level using the approaches suggested in the molecular biology and material science literatures. Due to the adsorption of heavy hydrocarbons, deviations from the Fickian diffusion at specific time regions were observed in the mean square displacement vs. time graphs. This finding suggests that confinement causes anomalous diffusion at the molecular level.
The quantitative results of the MD simulations were used in a compositional simulator, which can take the diffusion coefficients into account. Although the simulator was not capable of considering all the mechanisms discussed in this dissertation (adsorption and changes in the diffusion coefficients due to composition and pressure), it was useful to indicate considerable effects of the changes in diffusion coefficients because of confinement on the production rate and produced fluid composition. These findings show the importance of the development of new compositional fluid-flow models to incorporate more detailed representation of adsorption, sieving, and diffusion in nanoporous unconventional reservoirs.
Through detailed MD simulations, I showed that the use of CH4 and CO2 as EOR agents caused enhancements in the diffusivities of hydrocarbons trapped in a tight pore. The mechanisms causing the enhancements, however, were different. While the desorption of hydrocarbon molecules was dominant in the case of CO2-EOR, the dilution of oil was the main cause of diffusivity enhancements for CH4-EOR.
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