Molecular insight into the oil charging mechanism in tight reservoirs

Molecular Insights into the EOR Mechanism of Water, CO2, and CO2-WAG Flooding in Heterogeneous Nanochannels: A Molecular Dynamic Simulation

In tight oil exploitation, the water flooding, CO2 flooding, and CO2 water gas alternate (CO2-WAG) flooding are three commonly adopted methods to enhance the oil recovery (EOR), and the heterogeneous of reservoir has crucial influence on the oil sweep volume and oil displacement efficiency. In this work, molecular dynamic simulation was employed to investigate the displacement behavior in these three flooding methods in the heterogeneous tight reservoir. First, a single nanochannel was used to investigate the different displacement performances in these three flooding methods. Then a double nanochannel model were constructed to mimic the heterogeneous tight reservoir. The threshold injection pressure of three flooding modes was calculated. The number of displaced oil molecules was used to evaluate the oil displacement efficiency. Simulation results showed that the threshold injection pressure gave the following order: CO2 flooding < CO2-WAG flooding < water flooding. In double nanochannel systems, the injecting water passed through the large-sized nanochannel, and the oil inside the small-sized nanochannel could not be displaced in water flooding and CO2-WAG flooding. In CO2 flooding, some oil molecules inside two nanochannels were displaced, and the oil displacement efficiency in the large-size nanochannel was higher than that in the small-size nanochannel. The comparison of these three flooding methods showed that the CO2-WAG flooding has priority over the other two flooding methods, exhibiting both low threshold injection pressure of gas flooding and high oil displacement efficiency of water flooding; consequently, high EOR could be achieved. Our work was helpful to deeply understand the microscopic oil displacement processes of different flooding methods, and it has reference value for the development of tight oil reservoirs.

Unraveling the adsorption dynamics of asphaltene molecules on silica surfaces

Understanding the adsorption behavior of heavy oil components on reservoir solids is crucial for improving oil recovery, yet the molecular mechanism remains unclear. This study used molecular dynamics simulations to explore the adsorption kinetics and thermodynamics of asphaltene molecules on silica surfaces. The adsorption process was divided into three stages: free, adsorption, and equilibrium. In the adsorption stage, asphaltenes must pass through two dense hydration layers and adhere to the silica surface in a flat configuration. Carboxyl groups increase asphaltene hydrophilicity, raising interaction energy with water molecules and hindering adsorption. In addition, two distinct hydration layers of water molecules on the silica surface. The first hydration layer, with a peak density of 2000 kg m-3, was located around 0.6 nm from the surface, driven by hydrogen bonding between Si-OH groups and water molecules. The second layer, found at 1.44-1.80 nm, had a lower density of 1200 kg m-3, formed through hydrogen bonding between water molecules. This study aims to enhance the understanding of the physicochemical mechanisms governing oil droplet adsorption on silica surfaces, potentially informing the design of improved oil recovery strategies.

Breakthrough pressure of oil displacement by water through the ultra-narrow kerogen pore throat from the Young-Laplace equation and molecular dynamic simulations

As one common unconventional reservoir, shale plays a pivotal role to compensate the depletion of conventional oil resources. There are numerous nanoscale pores and ultra-narrow pore throats (sub 2 nm) in shale media. To displace oil through ultra-narrow pore throats by water, one needs to overcome excessively-high capillary pressure. Understanding the water-oil two-phase displacement process through pore throats is critical to numerical simulation on tight/shale oil exploitation and ultimate oil recovery estimation. In this work, we use molecular dynamics simulations to investigate oil (represented by n-octane) displacement by water through a ~2 nm kerogen (represented by Type II-C kerogen) pore throat. Besides, the applicability of the Young-Laplace equation to the ultra-narrow kerogen pore throat has been assessed. We find that although the Type II-C kerogen is generally oil-wet, water has an excellent displacement efficiency without the oil film on the substrate, thanks to the hydrogen bonding formed between water and heteroatoms (such as O, N, and S) on the kerogen surface. Unlike previous studies, the capillary pressure obtained from the widely used Young-Laplace equation shows a good agreement with the breakthrough pressure obtained from MD simulations for the ∼2 nm kerogen pore throat. Our work indicates that explicitly considering intermolecular interactions as well as atomistic and molecular level characteristics is imperative to study the two-phase displacement process through ultra-narrow pore throats.