Surface-modified silica nanoparticles for enhanced oil recovery in sandstone cores

• Cost-effective and commercially available SiO 2 nanoparticles (NP) have been explored for EOR. • Simple and one-step nanoparticles functionalisation under relatively mild conditions has been employed. • The use of PEI and TOS agents with SiO 2 NPs has been investigated for EOR. • TOS-NPs enhanced the imbibition recovery rate due to the rock wettability alteration. Enhanced Oil Recovery (EOR) techniques traditionally employ chemical flooding; however, recent integration of nanoparticles (NPs) with water flooding has demonstrated promising enhancements in oil recovery. Silica (SiO 2 ) NPs have been widely used in this regard, however, the primary challenge is their tendency to aggregate in solution due to their ultra-small size and large surface area, which can inhibit their effectiveness in EOR applications. To address this, we modified the surface of commercially sourced SiO 2 NPs using two different agents: polyethylenimine (PEI) for polymer coating and Triethoxy(octyl)silane (TOS) for covalent surface functionalisation, without any binders or preliminary coatings. The modified NPs (TOS-NPs and PEI-NPs) were characterized using techniques such as SEM, TEM, FTIR, TGA, BET and DLS. Their efficacy in EOR was evaluated within sandstone cores through measurements of contact angle, interfacial tension (IFT), and spontaneous imbibition tests. Results indicated that the surface modifications substantially improved the stability of the SiO 2 NPs. Specifically, TOS-NPs effectively altered the wettability of sandstone cores from strongly oil-wet (27⁰) to strongly water-wet (165⁰), in contrast to PEI-NPs (16⁰) and unmodified SiO 2 NPs (94⁰). IFT measurements revealed a 78 % reduction with PEI-NPs, compared to only 5 % with TOS-NPs. In spontaneous imbibition tests, TOS-NPs achieved the highest oil recovery, approximately 64.9 % without noticeable core plugging, compared to 53.0 % for the unmodified NPs and 9.8 % by PEI-NPs. These results, alongside the contact angle measurements, confirm that for the sandstone wettability alteration of the rock could be the major contributing mechanism for EOR using TOS-NPs.