Objective The pore structure of tight sandstone reservoirs is complex and highly heterogeneous. After blockages are removed by conventional acidizing, a "dominant channel" is formed, which presents an overall problem of "crossflow" and continuous increase in water content, seriously restricting the development effect of oil fields. In this paper, using cationic Gemini surfactants to change the wettability of rocks and control water through interface modification.

Method A dual head cationic quaternary ammonium salt surfactant DWIO with low toxicity, low irritation, and easy biodegradability was synthesized by introducing ester groups and olefins through esterification and quaternization reactions using malic acid, (2-hydroxyethyl) dimethylamine, and 1,4-dibromo-2-butene as raw materials. The wetting reversal, adsorption, water control and sand stabilization, and corrosion inhibition properties of DWIO were evaluated, and it has the characteristics of "one agent, multiple uses". A composite retarded acid (HCl+fluoride salt+organic phosphonic acid) and synergist were used to form an unblocking and water control system.

Result  Indoor experiments showed that the oil phase permeability increased by 2-3 times after displacement by the unblocking system, while the water phase permeability did not increase and the water production rate decreased by 9.71%, while the oil production increased by 10.5%. After the application of this technology at the Shan X well in the Nanliang block of the tight oil reservoir in Changqing Oilfield, the water content decreased by 12.4%, and the maximum daily oil production reached 2.1 t.

Conclusion The cationic Gemini surfactant unblocking and water control system effectively solves the problems of increased water content, short effective period, and low production increase after acidizing and unblocking old wells in tight sandstone reservoirs.