Mixed wet pores are abundant in shales, yet there is little to no information on the behavior of fluids confined within pores characterized by one hydrophilic and one hydrophobic surface. This mixed wettability can impact fluid storage, distribution, capillary pressures and transport through these pores. In this study, I use molecular dynamics simulations to describe the initial distribution of reservoir fluids, such as multicomponent oils and water, in pores of mixed wettability. This is an essential pre-requisite to additional studies documenting fluid transport in such pores.
The molecular model of the mixed wet pore used in this study consists of kerogen separated by some distance from a clay surface. Molecular dynamics (MD) provides the spatial distribution of water and the individual hydrocarbon species, at varying values of water concentration. In a series of additional sensitivity studies, I also evaluate the impact of salinity on the distribution of these fluids for two different arrangements of charged clay surface that are denoted as hydroxyl-potassium (H-P) and potassium-potassium (P-P) surfaces.
Throughout the entire study, the results from the equilibrated system indicate a high affinity between the heavy components, such as the asphaltene/resin fraction and the kerogen surfaces, which is to be expected. The more surprising result is the hydrogen bonding observed between the polar constituents in the asphaltene/resin fraction and water. This creates a situation where the asphaltene/resin fraction shows an affinity towards water and resides adjacent to the water adsorbed on to the clay surface. When this happens, the hydrophilic clay surface effectively becomes hydrophobic. A pore bounded by an asphaltene layer on one side and kerogen on the other is more oil-wetting than mixed-wet. The presence of asphaltenes can therefore expect to create conditions of modified wettability that will impact oil recovery, oil transport and distribution.
Another surprising result in this work is that water forms structures that bridge between opposing surfaces of the model. These water bridges are seen to happen for water concentration values larger than 20%. In other words, water is not merely just adsorbed on to the clay surfaces, but also forms these bridge-like structures. However, when the salinity is even moderately increased, the water bridges dissipate, and water only occurs as an adsorbed phase or as a free fluid droplet. Nevertheless, I still observe a strong affinity between the asphaltene/resin fraction and water leading to a lesser degree of mixed wettability.
This study is the first, to the best of my knowledge, that considers water and multicomponent hydrocarbon mixtures in mixed-wet pores with realistic surface chemistry and constitutes a necessary first-step towards additional studies related to water and hydrocarbon transport and EOR processes in shale nanopores
