Enhanced Recovery of Oil Mixtures from Calcite Nanopores Facilitated by CO2 Injection

Slow production, preferential recovery of light hydrocarbons, and low recovery factors are common challenges in oil production from unconventional reservoirs dominated by nanopores. Gas injection-based techniques such as CO2 Huff-n-Puff have shown promise in addressing these challenges. However, a limited understanding of the recovery of oil mixtures on the nanopore scale hinders their effective optimization. Here, we use molecular dynamics simulations to study the recovery of an oil mixture (C10 + C19) from a single 4 nm-wide calcite dead-end pore, both with and without CO2 injection. Without CO2 injection, oil recovery is much faster than expected from oil vaporization and features an undesired selectivity, i.e., the preferential recovery of lighter C10. With CO2 injection, oil recovery is accelerated and its selectivity toward C10 is greatly mitigated. These recovery behaviors are understood by analyzing the spatiotemporal evolution of C10, C19, and CO2 distributions in the calcite pore. In particular, we show that interfacial phenomena (e.g., the strong adsorption of oil and CO2 on pore walls, their competition, and their modulation of transport behavior) and bulk phenomena (e.g., solubilization of oil by CO2 in the middle portion of the pore) play crucial roles in determining the oil recovery rate and selectivity.

Insights into Waterflooding in Hydrocarbon-Bearing Nanochannels of Varying Cross Sections from Mesoscopic Multiphase Flow Simulations

Waterflooding is one of the geotechniques used to recover fuel sources from nanoporous geological formations. The scientific understanding of the process that involves the multiphase flow of nanoconfined fluids, however, has lagged, mainly due to the complex nanopore geometries and chemical compositions. To enable the benchmarked flow of nanoconfined fluids, architected geomaterials, such as synthesized mesoporous silica with tunable pore shapes and surface chemical properties, are used for designing and conducting experiments and simulations. This work uses a modified many-body dissipative particle dynamics (mDPD) model with accurately calibrated parameters to perform parametric flow simulations for studying the influences of waterflooding-driven power, pore shape, surface roughness, and surface wettability on the multiphase flow in heptane-saturated silica nanochannels. Remarkably, up to an 80% reduction in the effective permeability is found for water-driven heptane flow in a baseline 4.5-nm-wide slit channel when compared with the Hagen-Poiseuille equation. In the 4.5-nm-wide channels with architected surface roughness, the flow rate is found to be either higher or lower than the baseline case, depending on the shape and size of cross sections. High wettability of the solid surface by water is essential for achieving a high recovery of heptane, regardless of surface roughness. When the solid surface is less wetting or nonwetting to water, the existence of an optimal waterflooding-driven power is found to allow for the highest possible recovery. A detailed analysis of the evolution of the transient water-heptane interface in those nanochannels is presented to elucidate the underlying mechanisms that impact or dictate the multiphase flow behaviors.

Fluoropolymer Microemulsion: Preparation and Application in Reservoir Wettability Reversal and Enhancing Oil Recovery

Reservoir wettability is an important factor in the process of reservoir reconstruction. Especially in hydrophilic formation, it is easy to cause a water-locked phenomenon. A new type of fluoropolymer microemulsion was prepared by emulsion polymerization, and its structure and properties were characterized. The average particle size in the prepared emulsion was about 2.0 μm. The emulsion had good stability and wettability reversal performance for the storage of 30 days. After the treatment of 2.0 wt % emulsion, the contact angle between the core and water changed from 26 to 128°, the core surface free energy decreased from 66 to 2.6 mN/m, and the saturated water imbibition amount of the core decreased from 1.38 to 0.15 g. The ability of the fluoropolymer microemulsion to enhance oil recovery was evaluated by the visual displacement experiment. The fluoropolymer microemulsion can increase the displacement efficiency by more than 10%. The wettability of the core changed from hydrophilicity to hydrophobicity, and wettability reversal was achieved.

Spontaneous propulsion of a water nanodroplet induced by a wettability gradient: a molecular dynamics simulation study

The directional propulsion of liquid droplets at the nanoscale is quite an interesting topic of research in the fields of micro/nano-fluidics, water filtration, precision medicine, and cooling of electronics. In this study, the unidirectional spontaneous transport of a water nanodroplet on a solid surface with a multi-gradient surface (MGS) inspired by natural species is modeled and analyzed using molecular dynamics (MD) simulations. There are three different MGSs considered in this study containing different wedge angles of the hydrophilic region of the solid surface. The MGSs contain two regions: a hydrophilic wedge-shaped region with a constant surface energy parameter equal to 50 meV and a hydrophobic region with a tuned surface energy parameter. The energy parameter of the hydrophobic region is set equal to 1, 5, 10, 20, 30, and 40 meV in order to alter the intensity of the wettability gradient of the two surfaces and its effect on the propulsion of the water nanodroplet is analyzed. Furthermore, three different sizes of water droplets containing 6000, 8000, and 10 000 water molecules are also used in this study and their effect on the transport behavior of the water nanodroplet is also measured. Moreover, two different designs on a solid surface with a continuous wettability gradient are modelled and the impact of solid surface geometry on the transport of the water droplet is calculated by means of mean square displacement (MSD) and average velocity data. In addition, the wedge-shaped surface is found to be more superior to the parallel-shaped surface for the spontaneous propulsion of the water droplet.

Invasion of gas into mica nanopores: a molecular dynamics study

The invasion of gas into liquid-filled nanopores is encountered in many engineering problems but is not yet well understood. We report molecular dynamics simulations of the invasion of methane gas into water-filled mica pores with widths of 2-6 nm. Gas invades into a pore only when the pressure exceeds a breakthrough pressure and a thin residual water film is left on the mica wall as the gas phase moves deeper into the pore. The gas breakthrough pressure of pores as narrow as 2 nm can be modeled reasonably well by the capillary pressure if the finite thickness of residual liquid water film and the liquid-gas interface are taken into account. The movement of the front of the liquid meniscus during gas invasion can be quantitatively described using the classical hydrodynamics when the negative slip length on the strongly hydrophilic mica walls is taken into account. Understanding the molecular mechanisms underlying the gas invasion in the system studied here will form the foundation for quantitative prediction of gas invasion in practical porous media.

Evaporation dynamics of a sessile droplet on glass surfaces with fluoropolymer coatings: focusing on the final stage of thin droplet evaporation

The evaporation dynamics of a water droplet with an initial volume of 2 μl from glass surfaces with fluoropolymer coatings are investigated using the shadow technique and an optical microscope. The droplet profile for a contact angle of less than 5° is constructed using an image-analyzing interference technique, and evaporation dynamics are investigated at the final stage. We coated the glass slides with a thin film of a fluoropolymer by the hot-wire chemical vapor deposition method at different deposition modes depending on the deposition pressure and the temperature of the activating wire. The resulting surfaces have different structures affecting the wetting properties. Droplet evaporation from a constant contact radius mode in the early stage of evaporation was found followed by the mode where both contact angle and contact radius simultaneously vary in time (final stage) regardless of wettability of the coated surfaces. We found that depinning occurs at small contact angles of 2.2-4.7° for all samples, which are smaller than the measured receding contact angles. This is explained by imbibition of the liquid into the developed surface of the "soft" coating that leads to formation of thin droplets completely wetting the surface. The final stage, which is little discussed in the literature, is also recorded. We have singled out a substage where the contact line velocity is abruptly increasing for all coated and uncoated surfaces. The critical droplet height corresponding to the transition to this substage is about 2 μm with R/h = 107. The duration of this substage is the same for all coated and uncoated surfaces. Droplets observed at this substage for all the tested surfaces are axisymmetric. The specific evaporation rate clearly demonstrates an abrupt increase at the final substage of the droplet evaporation. The classical R² law is justified for the complete wetting situation where the droplet is disappearing in an axisymmetric manner.