An alkali-free composite system, denoted as CHS-x (where x represents the nano-SiO? concentration), was constructed comprising anionic heavy alkyl benzene sulfonate (HABS), zwitterionic cocamidopropyl hydroxysulfobetaine (CAPH), and nano-SiO?, tailored for the low-permeability reservoir characteristics of a specific block in the Changqing Oilfield. The CHS-x systems were characterized via FTIR, particle size analysis, and SEM, followed by evaluations of interfacial tension (IFT), wettability alteration, emulsification performance, and adsorption behavior on reservoir rocks. Experimental results demonstrated that the CHS-0.15% formulation maintained an IFT in the order of 10?3 mN/m after 60 days of static aging at 45°C and a formation water salinity of 11,000 mg/L, exhibited a water separation rate of only 26% after 30 days, and effectively altered oil-wet core slices to weakly water-wet conditions (reducing the contact angle from 105.2° to 61.2°), with nano-SiO? significantly enhancing resistance to adsorption losses. Core flooding simulations indicated that injecting 0.7 PV of the CHS-0.15% system (composed of 0.15 wt% nano-SiO? and 0.3 wt% surfactants with an HABS:CAPH mass ratio of 7:3) increased oil recovery by 28.63% compared to water flooding, showing enhanced recovery across permeability contrasts of 2 and 5; these findings provide targeted technical insights for addressing development challenges in low-temperature, low-permeability reservoirs analogous to those in the Changqing Oilfield.