Development and Application of a Chelating Acid for Water Injection Wells of Chang 6 Reservoir of An-Sai Oilfield

Abstract Ansai oilfield is a typical low permeability, low pressure, low yield and serious heterogeneity oilfield. Chang 6 play of Triassic YanChang Formation is a typical ultra-low permeability reservoir. The average porosity of the reservoir is 10 % ~ 14 %, and the average permeability is (0.2 ~ 4.0) × 10 −3 μm 2 . The absolute content of clay minerals in the reservoir is relatively high, with an average content of 18.61 %. The main components are kaolinite, chlorite, illite and illite/montmorillonite interlayer. Due to the reservoir blockage caused by scaling, water sensitivity and formation particle migration in the process of water injection, high injection pressure and under injection wells is increasing dramatically. In view of the above challenges, a fit-for-purpose acid, which contains organic acids and chelating agents, was developed for injection pressure reduction and injection enhancement. It has the characteristics of strong dissolution, good retarding and corrosion inhibition performance. Corrosion rate is 0.5632 g / (m 2 ·h) for J55 steel in 24h, and the core permeability increases by 74.57 % after acid flooding. It has been applied in field test of 3 wells in Chang 6 layer of Ansai oilfield since March 2024. The average daily injection volume increased by 37.87 %, and the validity period was more than 180 days.