Mechanism of supercritical CO2-nano-polysilicon composite systems to enhance oil recovery of oil reservoir by water flooding

In view of the low recovery of tight low-permeability oil reservoirs by water flooding in a certain block of Changqing Oilfield， a supercritical CO2-nano-polysilicon composite displacement fluid was developed to explore its oil displacement mechanism and the effect of enhanced oil recovery. Poly （2-acrylamido-2-methylpropane sulfonic acid） （PAMPS） was used for in-situ surface modification of SiO2 to prepare nano-polysilicon particles with strong adsorption capacity and interfacial activity. Through core displacement experiments， the oil displacement performance of composite systems of nano-polysilicon with different mass fractions （1%-5%） and supercritical CO2 was compared under miscible and immiscible conditions. The recovery change was monitored in the experiments， and the effects of nano-polysilicon on rock wettability， interfacial tension， and flow characteristics were analyzed. Under miscible conditions， supercritical CO2 dominated the oil displacement process. The recovery of water flooding was only 32.7%， while the composite system increased it to over 70.0%. Among them， low-dose nano-polysilicon （1%-3%） significantly expanded the swept area of the oil phase and improved micro-scale oil displacement efficiency by enhancing the viscoelasticity and interfacial activity of the displacement fluid. However， high-dose nano-polysilicon （4%-5%） was prone to blocking flow channels due to particle accumulation， leading to a sharp increase in displacement pressure and a slowdown in recovery growth. Under immiscible conditions， nano-polysilicon played a more critical role. It reduced the interfacial tension between supercritical CO2 and crude oil， increased fluid viscosity， and synergistically prolonged the shut-in time， promoting more nano-polysilicon to adsorb on the oil-bearing rock surface， transforming the oil-wet surface into a water-wet or neutral-wet state， and stripping residual oil phases. Under these conditions， the recovery increased from 31.6% of water flooding to 50%-60%. The surface modification of nano-polysilicon gave it amphiphilicity， enabling it to form a stable adsorption layer at the rock， oil， and water interface and weaken the adhesion between crude oil and rock. Supercritical CO2 formed miscible displacement by mixing with crude oil. The synergistic effect of the two significantly improved oil washing efficiency. Supercritical CO2 and nano-polysilicon complemented each other in displacement. CO2 dominated macro-scale oil displacement under miscible conditions， while nano-polysilicon strengthens micro-scale oil washing under immiscible conditions， jointly achieving recovery improvement.