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Pantelides, Sokrates T. (Vanderbilt University) | Prabhakar, Sanjay (Vanderbilt University) | Liu, Jian (Vanderbilt University) | Zhang, Yu-Yang (Vanderbilt University) | Lai, Chia-Yun (Masdar University) | Chiesa, Matteo (Masdar University) | Alhassan, Saeed (Petroleum Institute)
Abstract Extraction of oil from wells is hampered by the fact that oil sticks to rock surfaces and water does not pry it loose easily. Technically, this is an issue caused by the relative wettability of rock surfaces. Experiments have shown that Na ions that are present in sea water have a negative effect on oil extraction, while Ca, Mg, and other ions enhance oil extraction. However, only limited understanding of the pertinent mechanisms has been achieved. Atomic-scale modeling of wettability is usually pursued using classical molecular dynamics based on empirical potentials. Only limited research based on quantum mechanical calculations has been reported so far. Here we describe the development and implementation of parameter-free, quantum-mechanical approaches, at different levels of approximation, that can provide detailed understanding of relative wettability and have predictive capabilities. At the lowest level of approximation, we calculate the binding energies of water and prototype oil molecules to calcite surfaces in vacuum as indicators of relative wettability. At the next level, we calculate binding energies in the presence of liquid water using quantum molecular dynamics. We find that the binding energy of Na acetate is larger than the binding energy of acetic acid, a prototype oil molecule, which suggests that, upon reacting with Na ions, a layer of oil becomes stickier on calcite rocks. On the other hand, Ca and Mg acetate desorb easier than acetic acid, facilitating oil extraction, as observed. At a much more sophisticated level of approximation, we calculate the wetting angle, a measurable quantity that serves as a measure of relative wettability. We applied this method to water on graphene and graphitic surfaces, which has been studied extensively and for which we have obtained new experimental data.
Abstract Gas hydrates are an energy resource composed of natural gas in a solid state, in which water molecules, are in a relatively stable composition, surround the gas molecules. One volume of gas hydrates is equivalent to about 164 volumes of methane. Gas hydrates may represent more than twice the energy content of all other hydrocarbon resources. Gas hydrates are in equilibrium under conditions of high pressures and low temperatures; they occur in arctic regions (permafrost) and on the continental shelf - in marine surface and subsurface deposits. The importance of gas hydrates is related to their potential for exploration and production as a source of natural gas; to the known problems they cause in drilling and production systems; to their climate change effects - negative (GHG) and positive (CO2 sequestration). Presently, depressurization, thermal stimulation, inhibitor injection, or a combination of these methods have been considered as possible means of producing gas hydrate. Another method for gas hydrate production involves the injection of CO2. The idea of swapping CO2 for CH4 in gas hydrates was first advanced by Ohgaki et al. (1996) and then for ethane hydrate by Nakano et al. (1998). Their concept involves injecting CO2 gas, which is then allowed to equilibrate with methane hydrate along the three-phase equilibrium boundary. Because of the difference in chemical affinity for CO2 versus methane in the sI hydrate structure, the mole fraction of methane would be reduced to approximately 0.48 in the hydrate and rise to a value of 0.7 in the gas phase at equilibrium. The concept for enhanced gas hydrate recovery (EGHR) discussed in this paper takes advantage of the physical and thermodynamic properties of CO2 combined with control of multiphase flow, heat, and transport processes hydrate mass in porous media carriers.