Preparation Of Nanofluid Contain Hydrophobic SiO₂ And Its Impact On Enhanced Oil Recovery

China's ever-growing dependence on foreign crude oil seriously threatens national energy security,the technology development of enhanced oil recovery to reduce the dependence of crude oil on foreign countries has become the key to ensuring national energy security.As most of China's oil reservoirs are in the period of high water cut or extra-high water cut and high recovery,the efficiency of traditional chemical flooding is gradually decreasing,and new high-efficiency,low-cost and environment-friendly oil-driven nanofluids are developed.Studying the performance and mechanism of nanofluids flooding has become an important requirement for the development of tertiary oil recovery technology.Based on the preliminary work of our research group and the micro-emulsion nanofluids theory,this paper uses hydrophobic functional nano-silica to replace the oil phase,forming a"microemulsion"with the surfactant,co-surfactant and water.We established a method for stabilizing hydrophobic silica nanofluids over a long period of time,studied its performance in high-efficiency oil displacement,and investigated the effect of the concentration and the wettability of silica nanoparticles on enhanced oil recovery.The key research contents and results of this paper are as follows:1.Preparation of nanofluids containing hydrophobic silica and its impact on enhanced oil recovery:A series of silica nanofluids were prepared using the hydrophobic nano-silica DNS-118F as a functional oil-displacing particle and adjusting the ratio of sodium dodecyl sulfate and octadecanol.The temperature and salt tolerance of the nanofluids were tested.The flooding performance was studied by using the multi-functional core displacement experimental device.The results show that the silica nanofluids prepared from pure water can be stably dispersed for 12 days at room temperature,and the amount of octadecyl alcohol can be adjusted to improve the temperature resistance of the nanofluids from 60?to 80?for 3 h,and significantly improved the salt tolerance of the nanofluids.The results of multi-core core flooding experiments show that when the concentration of hydrophobic nano-silica is 2000 ppm,the enhanced oil recovery can be increased to 17.46%,which is higher than 7.48% of the recovery rate of SDS-Octadecanol system?no nano-silica?used in oil reservoir flooding,showing a good recovery performance.2.Evaluation of the oil displacement performance of silica?DNS-1?with different dosages of surface modifier and its mechanism of enhanced oil recovery:The surface modification of nano-silica with different modification dosages was prepared using the 16000 L reactor in Henan University pilot base.As a functional additive,a series of aqueous dispersions was prepared and their effects on oil recovery were investigated.The results show that the DNS-118F aqueous dispersion with a nano-SiO₂ and modifier content ratio of 50:9 can improve the recovery factor by17.46%,resulting in a total recovery of crude oil of 26.06%.The mechanism of enhanced oil recovery of DNS-118F hydrophobic nano-silica was further studied.The effects of nanoparticle concentration on surface wettability,complete wetting time of liquid?water droplets?and capillary wettability were investigated.The results show that the silica nanofluids can change the surface wettability from water wet to oil wet,and the water phase capillary force is converted from resistance to impetus.At the same time,the addition of silica can reduce the interfacial tension between oil and water from 3.9 mN/m to 1.2 mN/m.The synergistic effect of surface wettability change and interfacial tension reduction increases the number of water flooding tubes and improves microscopic oil displacement efficiency.In addition,the increase of the injection pressure of the nanofluids during the displacement process increases the macroscopic volume of the displacement fluid;the addition of nano-silica can increase the micro-displacement efficiency and the macroscopic sweep coefficient,thereby significantly enhancing the oil recovery.3.Preparation and characterization of different hydrophilic/hydrophobic nano-silica:A series of H series nano-silicas with different hydrophilic and hydrophobic properties were prepared by adjusting the amount of silane coupling agent hexamethyldisilazane as surface modifier.The surface modification of different hydrophilic/hydrophobic nano-silicas was characterized by TEM,FT-IR,TG,XRD,laser particle size analysis and fully automatic contact angle measurement.The results showed that with the increase of modifier addition,the water contact angle?WCA?increased gradually,and the hydrophobicity of nano-silica increased accordingly.The FT-IR and TG spectra showed that the target group of silane coupling agent was effectively grafted to the surface of silica.The XRD pattern showed that the different hydrophilic/hydrophobic nano-silica prepared was amorphous;TEM results showed that the primary particle size of silica was about 15 nm,and it existed in the form of aggregation with a size of about 4?m in the aqueous solution.The effects of different hydrophilic/hydrophobic nano-silica on oil recovery were investigated.The highest recovery factor of H-141F was 17.28%,and the mechanism of oil displacement was analyzed.Combined with the preliminary work,in the nano-silica-based micro-emulsion system,when the contact angle of silica water is 139°-152°,the enhanced oil recovery is best.The oil film peeling test proves that the alkylation modification of nano-SiO₂ after alkane modification invades the oil phase,and interacts with the oil phase,and the peeling force increases.With the wettability reversal and the expansion of the displacement volume of the displacement fluid,the oil recovery is enhanced.