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Interfacial Regulation for Enhanced Gas/Oil Recovery and CO2 Geo-sequestration from Molecular Perspectives
- Author / Creator
- Li, Wenhui
The interfaces between two immiscible fluids or between fluid and solid are ubiquitous. The interfacial properties between aquifers and hydrocarbons have significant effects on the distribution and production of gas and oil, whereas understanding the fluid-solid interfacial phenomena is important to CO2 sequestration in the saline aquifers and depleted reservoirs.
The interfacial properties between aquifers and hydrocarbons are affected by the hydrocarbon and aquifer’s compositions. For natural gas, it consists of ethane (C2), propane (C3) and so on besides methane (C1), and the effects of heavier components (C2 and C3) on gas-water interfacial tension (IFT) are explored in this study by molecular dynamics (MD) simulations. We compare the IFTs of C1water system, C1+C2water system, C1+C3water system, and C1+C2+C3water system. It is found that heavier components can lower natural gas-water IFT because of stronger adsorption capacity on the interface. For crude oil, we use N-, S-, and O-bearing compounds as polar components and n-decane as non-polar component to represent oil. We found that polar components can lower oil-water IFT, especially O-containing components. The mechanism behind this phenomenon is the polar components accumulate on the oil-brine interface by forming hydrogen bonds with water. The O-bearing components have the highest adsorption and hydrogen-bond density, corresponding to the lowest oil-water IFT. For the aquifer (i.e., formation water), it contains various monovalent and divalent cations (Na+, K+, Ca2+, and Mg2+) over a wide range of salt concentrations (up to ~4.5 M). We investigate the effects of cation type and salt concentration on gas-brine and oil-brine interfacial properties. We found that cation type has a negligible effect on IFTs, but divalent ions generally have a more prominent double layer at the interface than that of monovalent ions. As salt concentration increases, gas-brine IFT increases obviously but oil-brine IFT only slightly increases. These works should provide a guidance for enhanced gas/oil recoveries.
On the other hand, the interfacial properties between fluid and solid are determined by the solid surface properties and fluid compositions. Therefore, for the purpose of CO2 sequestration, the solid surface wettability effects on CO2 solubility in water under confinements are studied. The confinement surfaces are represented by two different kaolinite basal surfaces, respectively. It is found CO2 solubility decreases as the confinement surface becomes more hydrophilic. The effects of aquifer properties can also be reflected by the confinement surface properties. For example, the deprotonation degree of silica surface varies as aquifer’s pH changes. In this study, we investigate the effects of pH (represented by silica surface deprotonation degree) and salinity of aquifer on CO2 solubility in silica nanopores. It is found that the brine in silica nanopores with low salinity and pH has a relatively high CO2 storage capacity in terms of the solubility mechanism. The pore size effect on CO2 storage is also studied in kerogen in shale reservoirs. Type II kerogen with different degrees of maturity (II-A, II-B, II-C, and II-D) is chosen and three pore sizes (1, 2, and 4 nm) are designed. The results showed that, in the large pores (2- and 4-nm pores), CO2 distributes by dissolution form in the middle of the pore, but it forms some nano-sized clusters adsorbed on the surface. However, in the small pore (1-nm pore), CO2 occupies the pore space by displacing the original water, inducing an extremely high storage capacity. These works should shed some lights on CO2 storage evaluation in tight/shale formations.
- Graduation date
- Fall 2021
- Type of Item
- Doctor of Philosophy
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