1. Surface free energy of the two phases which may be solid–liquid, liquid–liquid, or gas–liquid can be significantly decreased by surfactant mainly due to the existence of the hydrophilic group an...

In this paper, to remove the water lock effect in tight gas reservoirs, amphipathic polymer-modified titanium quantum dots (PTQs) were synthesized via in situ polymerization, showing a hyper-branched structure and an excellent synergistic effect with the nonionic fluorocarbon surfactant to break the water lock. The molecular structure, fluorescent property, and micromorphology of the PTQs were obtained. The surface activity and wettability alteration of rock are discussed. Results show that PTQs have zwitterionic hydrophilic groups and the hydrophobic structure of long-chain groups on their molecular structure. PTQ fluid, with a median particle size of 3.6 nm, showed strong green fluorescence and had excellent dispersibility in 50,000 mg/L of standard saline fluid at 120 °C. Additionally, the surface tension decreased to 18.6 mN/m at a PTQ concentration of 0.08%. At a 0.1% concentration, PTQ fluid altered the water wettability of tight sandstone to 67.2°, which resulted in lower capillary resistance. Furthermore, the surfactant (PHPE) had a good synergistic effect with the PTQs to decrease surface tension and alter the wettability of the sandstone surface, leading to lower surface tension and significant amphiphobicity. The strong surface activity of PTQs results from their specific molecular structure, which enables electrostatic attraction, quantum size effects, hydrogen bonding, and van der Waals forces between the inter-polar molecules of PTQs and the surface of sandstone to forcefully eliminate the water lock effect. This study offers key guidance for the development of a high-performance water-lock-breaking agent and application of titanium quantum dots in tight gas reservoirs.