Carboxyl/alkyl composite silica-based amphiphilic nanoparticles enhanced spontaneous imbibition of low permeability sandstone rocks at reservoir conditions

In recent years, hydrophilic silica nanoparticles (HSNPs), as an effective and economic agent, have been demonstrated to exhibit great potential in changing the wettability of oil-wet reservoirs to water-wet. Nevertheless, due to the absence of amphiphilic nature, they hardly reduce the oil-water interfacial tension (IFT), which restricts their application in enhanced oil recovery. Therefore, this paper intended to synthesize carboxyl/alkyl composite silica-based amphiphilic nanoparticles (CSANPs) by grafting triethoxy(octyl)silane and hexanedioic acid on the surface of HSNPs through condensation and esterification reactions under specific conditions, and their structures were characterized with the assistance of advanced techniques. The characterization results indicated that the synthesized CSANPs had one hydrophobic surface composed of carbon chains and one hydrophilic surface composed of carboxyl groups. Then, the nanofluid (CSANF) was formulated by dispersing synthesized CSANPs in formation water to evaluate its stability, interface activity, wettability, and spontaneous imbibition. The evaluation results revealed that the CSANF could remain stable for one month under simulated reservoir conditions without aggregation and precipitation, which was attributed to the enhancement of mechanical barriers (electrostatic repulsion + steric effects) between CSANPs dispersed in the CSANF. The macroscopic contact angle measurement confirmed that the core slice surface treated with the CSANF became water-wet and therefore accelerated CSANF spontaneous imbibition. Because of amphiphilic structure, the CSANPs possessed a strong ability to reduce the oil-nanofluid IFT and improve the oil-nanofluid interfacial film strength, which was conducive to decrease oil discharge resistance during spontaneous imbibition. The highest imbibition recovery factor of the CSANF with a concentration of 0.05 wt% was obtained under the gravity and capillary forces. Finally, the low field nuclear magnetic resonance (NMR) results showed that the CSANF first entered micropores, then mesopores, and finally macropores.