Atomistic Insight into Nanofluid Enabled Enhanced Oil Recovery

Abstract

Nanoparticles (NPs) possess great potential in applications to enhanced oil recovery (EOR) due to their outstanding features of ultra-small size, great surface and interface effect, high sustainability, etc. The underlying mechanisms of which however remain to be explored. Especially, revealing the displacement mechanisms of residual oil by NPs and the transport stability of NPs stabilized Pickering emulsion is of great significance. In this thesis, the trapped oil displacement in rough channels by nanofluids and the transport dynamics of NPs stabilized Pickering emulsion across the pore throat have been investigated by atomistic simulation.

The displacement dynamics of residual oil trapped in rough channels by different nanofluids are elucidated initially. The results indicate that both hydrophilic nanoparticles (NPs) and Janus NPs have a highly obvious oil displacement effect. Specifically, hydrophilic NPs increase the viscosity and enlarge the sweeping scope of injected fluid. Janus NPs alter the local surface wettability by sliding along the surface and remobilize the oil by sliding along the oil/water interface. Conversely, hydrophobic NPs further lock the trapped oil and impede the displacement by entering the oil phase. In addition, the oil displacement effect is found to be less significant with low pumping force. Yet, Janus NPs maintain a relatively great effect under low pumping force, thanks to sufficiently long contact time between Janus NPs and the oil phase. Further analysis of the capillary number not only verifies the simulation results but also highlights the applied prospect of Janus NPs in actual oil reservoirs.

Then, the local enlarged surface system is built for elucidation of the displacement dynamics and mechanisms of Janus NPs. It’s found that Janus NPs with 25% and 50% hydrophobicity significantly recover more oil from the rough surface. The number, position, and orientation of adsorbed nanoparticles on the side wall of the groove determine the final amount of extracted oil. ‘Adsorption invasion process’ dominates the formation of Janus NPs adsorption structure, which is mainly contributed by the pinning effect and the collision between NPs. To modify such an ‘adsorption invasion process’, identification of the residual oil, displacement pressure, and the morphology of the inside oil-water interface are crucial factors. Furthermore, the effect of rough surface parameters on such displacement dynamics is studied. The applicable surface for JNPs to efficiently displace residual oil is neutrally wetted or weakly hydrophobic. For the geometric parameters, the smaller entry angle and exit angle, the larger aspect ratio of the groove, and the big enough tip length of the bulge contribute to a considerable EOR effect. Separately, the tip length of the bulge affects the formation of the adsorption film; A smaller entry angle means a larger adsorption film area and better local wettability alteration effect, while a smaller exit angle limits the amount of trapped oil that cannot be covered at the outlet side; Given the certain JNPs adsorption film, a larger aspect ratio of the oil trapping grooves can guide the oil-water interface downward to bottom. Among the four parameters, aspect ratio controls the EOR effect provided the tip length of the bulge is over the threshold.

Lastly, the nanomechanical property of the Pickering emulsions and their transport through the nanopore throat are investigated. It’s found that Pickering emulsions are mechanically robust by the JNP shell and recover from large deformations. Pickering emulsion is able to rupture on the hydrophobic surface due to the strong interactions between the oil core and surface. The larger ϕ of the emulsion, the greater deformation and stress can be withstood at rupture. More importantly, the critical ϕ of the emulsion determines that the JNP shell can form an ordered quasi-solid structure and act the effective shielding, while the size of the emulsion decides the weakening rate of such barrier effect in compression. In addition, Pickering emulsions with varied ϕ share a similar force response in the transport across the hydrophilic pore throat. While for the case of hydrophobic pore throat, the Janus NP shell acts as a protection pad, screening the interaction of the oil core with the residual oil on the surface. Such an effect enables the transport stability of Pickering emulsion with large ϕ across the pore throat. In addition, the smaller opening angle of the pore throat is proved to magnify the Jamin effect and increase the probability of the partial Janus NP shell being captured at the entrance.

Our research not only uncovers the displacement mechanism of trapped oil by nanofluids but also reveals the transport dynamics of NPs stabilized Pickering emulsion in the pores, which is significant for understanding and guiding the nanofluids-enabled EOR applications.