Browsing by Subject "Temperature effect"

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Modeling of temperature effect on low salinity waterflooding
(2016-05) Fu, Wensi; Sepehrnoori, Kamy, 1951-
Recent studies have shown that additional oil can be achieved by modifying the composition and/or salinity of the injection water. Low salinity waterflooding gains popularity due to its low cost, availability of water, and high displacement efficiency of light to medium gravity oil. Various mechanisms behind low salinity waterflooding have been proposed. However, the dominant underlying mechanism is still under debate due to the complex nature of the interaction between crude oil/brine/rock (COBR). Temperature has been reported to play a significant role in the process of low salinity waterflooding, particularly in carbonates. Temperature may affect geochemical reactions between rock surface, crude oil, and water and consequently alter the rock wettability. Investigating the temperature effect not only helps identify optimum condition to achieve additional oil recovery but also contributes to understanding the mechanisms behind low salinity waterflooding. In order to investigate the temperature effect on low salinity waterflooding, we implemented an energy module in the UTCOMP-IPhreeqc simulator. Hereafter, we refer to the improved simulator as “non-isothermal UTCOMP-IPhreeqc.” UTCOMP-IPhreeqc is capable of modeling non-isothermal, multi-dimensional, and multi-phase transport process with geochemical calculations between water, minerals, gases, ion exchangers, kinetics, and surface complexes. Non-isothermal UTCOMP-IPhreeqc was then applied to study the temperature effect on low salinity coreflood experiments of sandstone and carbonate rocks based on the laboratory work of Kozaki (2012) and Chandrasekhar (2013), respectively. Our simulation results revealed that for the sandstone case, changing the temperature from 30 to 120 ºC has insignificant effect on the oil recovery. We believe the reason is due to the fact that for this specific case the total ionic strength and the viscosity ratio (water viscosity over oil viscosity) did not change with increasing temperature. Noteworthy, double-layer expansion is assumed to be the underlying mechanism for low salinity waterflooding in sandstones in non-isothermal UTCOMP-IPhreeqc. On the other hand, the total ionic strength is the main controlling parameter for the double-layer expansion. For the carbonate case, with increasing temperature from 120 to 150 ºC, oil recovery increased for both formation brine and low salinity water injection. The reason: while the viscosity ratio remained constant, calcite dissolution increases as the temperature increases. The calcite dissolution is assumed to be the underlying mechanism for low salinity water in carbonates in non-isothermal UTCOMP-IPhreeqc. Hence, as more calcite dissolves the wettability of the rock changes towards more water-wet. As a result, oil recovery improves.
Thermomechanical and interfacial properties of monolayer graphene
(2014-08) Gao, Wei, active 21st century; Huang, Rui, doctor of civil and environmental engineering
The thermomechanical properties of monolayer graphene and the interfacial interactions between graphene and an SiO₂ substrate are investigated in this dissertation using a multiscale approach. The temperature dependent mechanical behavior of graphene with thermal fluctuations is studied by statistical mechanics analysis under harmonic approximation, which is then compared to molecular dynamics simulations. It is found that the amplitude of thermal fluctuation depends nonlinearly on the graphene size due to anharmonic interactions between bending and stretching modes, but a small positive pre-strain could suppress fluctuation amplitude considerably and results in very different scaling behavior. The thermal expansion of graphene depends on two competing effects: positive expansion due to in-plane modes and negative expansion due to out-of-plane fluctuations. The in-plane stress-strain relation of graphene becomes nonlinear even at infinitesimal strain due to the entropic contribution. Consequently, the modulus of graphene depends on strain non-monotonically, with strain stiffening followed by intrinsic softening. Moreover, it is found that the thermomechnical behavior of graphene is dependent on its interactions with environment such as supporting substrate. The interfacial interactions between graphene and SiO₂ substrate is investigated in terms of three perspectives. Firstly, the interaction mechanisms between graphene and SiO₂ substrate are studied by density functional theory (DFT). The dispersion interaction is found to be the predominant mechanism, and the interaction strength is strongly influenced by changes of SiO₂ surface structures due to surface reactions with water. The adhesion energy is reduced when the reconstructed SiO₂ surface is hydroxylated, and further reduced when covered by a monolayer of adsorbed water molecules. Next, we study the interfacial interactions between graphene and a wet substrate that is covered by a liquid-like water film. During the separation of graphene from the wet substrate, MD simulations show evolution of the water from a continuous film to discrete islands. The water bridging effects are further described by continuum models. Finally, a continuum model is developed to predict how the surface roughness may affect the adhesion between graphene membranes and their substrate.