Author: Mingshan Zhang
Publisher:
Published: 2022
Total Pages: 0
ISBN-13:
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Atmospheric CO2 concentration has been gradually growing since the industrial revolution, leading to climate change and global warming. As a result, carbon capture, utilization, and sequestration (CCUS) has become utterly important for human society. CO2 geological sequestration in depleted shale gas reservoirs is regarded as a promising strategy to mitigate the emission of CO2. As one of the typical clay minerals in shale reservoirs, kaolinite presents two structurally and chemically distinct basal surfaces known as siloxane and gibbsite surfaces which can significantly affect CO2 adsorption in kaolinite nanopores, especially in the presence of water. Nevertheless, due to the complicated surface properties and pore structures, it is practically impossible to distinguish the contributions from two distinct kaolinite surfaces for CO2 adsorption. In addition, to the best of our knowledge, the effect of moisture on CO2 adsorption in different kaolinite nanopores is rarely reported. We systematically explored CO2 adsorption in partially water-saturated kaolinite nanopores by molecular dynamics (MD) and Grand canonical Monte Carlo (GCMC) simulations using the flexible clay model. In the absence of water, CO2 presents a stronger adsorption ability on gibbsite surfaces. In gibbsite pores, the water tends to spread out on the surface forming a thin film while water bridges are observed in siloxane pores. In siloxane mesopores, a more CO2-wet surface appears as pressure increases, while it is not obvious in micropores because of stronger confinement effects. In general, the presence of water will result in the reduction of CO2 sequestration in both gibbsite and siloxane pores, while a slight enhancement is observed in siloxane mesopores when the pressure is quite low. CO2 utilization for enhancing gas recovery has been attracting extensive attention as it can greatly alleviate the financial burden from CO2 capture while it can also achieve CO2 sequestration in the deep formations. Compared with the conventional reservoirs, shale has heterogeneous rock compositions consisting of organic and inorganic matters and some shale formations contain anextensive number of heavier alkanes, such as ethane (C2) and propane (C3). While CO2 huff-n-puff is proved to be an effective method to enhance recovery of methane (C1), competitive adsorption between shale gas mixtures (C1-C2-C3) and CO2 in organic and clay minerals remains unexplored. On the other hand, the different recovery mechanisms of hydrocarbon mixtures during pressure drop, CO2 huff, and CO2 huff are still unclear. We used Grand Canonical Monte Carlo (GCMC) simulations to study competitive sorption of C1-C2-C3 and C1-C2-C3-CO2 mixtures in shale organic and inorganic nanopores under different production schemes. We found that while C1 in the adsorption layer can be readily recovered during pressure drawdown, C2 and C3 are trapped in pores, especially in organic micropores. CO2 injection can effectively recover each component in the adsorption layer in organic pores, while in inorganic pores, the adsorption layer is dominated by CO2 molecules, displacing all hydrocarbon components. Additionally, application of CO2 responsive surfactants provides a novel idea for economical and sustainable oil production. While the experimental work can test and design a promising smart surfactant formula for efficient O/W emulsification and demulsification processes, the microscopic structural properties and interface hydration structures related to CO2 switching mechanisms from molecular perspectives remain unclear. MD simulations are employed to carefully study the interfacial properties of n-heptane/water emulsion before and after purging CO2 using lauric acids (LA) as the surfactant. Before purging CO2, the deprotonated lauric acid (DLA) help to form and stabilize O/W emulsion droplets in aqueous solution due to high interface activity and strong surface electrostatic repulsion, whereas the protonation of lauric acid (PLA) arising from CO2 injection results in the coalescence of emulsion droplets thanks to the increased IFT and surface charge neutralization, which is also in line the potential mean force (PMF) calclation resutls.
