Nanofl uid injection into oil reservoirs is a novel chemical enhanced oil recovery (EOR) method and has been the subject

of many researches in recent years. Despite its increasing applications, there is not enough information on the mechanisms

and microscopic aspects of nanoparticle performance in EOR processes. Among nanoparticles, Janus nanoparticles (JNPs),

which have two distinct hydrophilic and hydrophobic sides, can play an eff ective role in oil recovery enhancement applications.

In the present study, molecular dynamics (MD) simulations were performed to provide a molecular-scale insight into

the working mechanisms of silica Janus nanoparticles in oil recovery enhancement by considering the presence of sodium,

chlorine, magnesium and sulfate ions. The calcite surface interacts with the mixture of heptane, decane, and toluene as the

oil phase. Based on the simulation results, the mechanism of oil detachment from the calcite surface involves several steps.

Due to the electrostatic interactions between the nanofl uid and the calcite, the formation of a water channel towards the

calcite surface begins, and the nanofl uid reaches and spreads over the calcite surface, which is infl uenced by two factors:

hydrogen bonds between water and calcite; the presence of ions in the nanofl uid, which can increase the hydrophilicity of

the calcite surface. Thus, the oil molecules remain as a droplet on the rock surface. Subsequently, the JNPs approach to the

oil–water interface near the calcite surface and push the oil droplet upward so that the oil phase completely detaches from the

surface. The presence of ionic compounds around the JNPs increases their electrostatic interactions with each other and also

increases the probability agglomeration of JNPs, which is a negative factor. On the other hand, they increase the electrostatic

interactions of JNPs with calcite, which is a positive factor. Therefore, it is necessary to choose the optimal concentration

of the ionic compounds in the injected nanofl uid. According to the simulation results, JNPs could increase the viscosity of

the water phase by 60% and reduce the surface tension of water–oil by 33%. Under the reservoir temperature and pressure

conditions, the diff usion coeffi cient of 1nm JNPs has increased from 3.33 × 10 –10 to 6.67 × 10 –10 m 2 /s. The results of this

study may be useful for designing favorable conditions for nanofl uid injection in the EOR applications.