Optimization Design and Evaluation of Improved Miscible Assistants for CO2 Flooding and the Application in Pilot in L Reservoir
Authors: Zhou Daiyu , Lian Liming , Wu Zangyuan , Yan Gengping , Zhou Wei , Shao Guangqiang , Sun Haihang , Wang Kaiyu , Bian Wanjiang , Zhang Guoliang , Kuang Xiyu , Li Shaohua , Fan Jin , Li Yang , Zhang Chao 
Published: February 2022

Abstract
This paper provide several improved miscible assistants, trying to mitigate the problem that CO2 miscible flooding is difficult to achieve in reservoirs because of the high miscible pressures, which leads to a lower recovery up to expectations. These miscible assistants could be easily mixed with crude oil by adding into CO2 and reduce the interfacial tension to drive down the minimum miscible pressure (MMP) in order to enhance sweep efficiency.
Some efforts have been made to improve this situation. The effective method is to draw the experiences from structure characteristics of surfactants applied in micro emulsion of CO2-water systems. Amphiphilic organic assistants were designed to synthetize with fluoro-alkane chains and non-fluoro-OAc chains as the CO2-philic ends, as well as alkane structure as the lipophilic ends. The minimization of MMP of CO2 miscible processing assistants has been analyzed and optimized by surface tension testing in CO2-kerosene system and CO2-crude oil system. The crude oil was obtained from field test pilot in L Reservoir.
The results of interfacial tension tests show that per-acetylated glucose dodecyl ester molecules have the ability to lower the interfacial tension most in these five kinds of new miscible processing assistants in two categories. Citric acid triisopropyl ester molecules take the second place, and others almost make no difference. The probable reason is that kerosene is mainly composed of C12 and lack of heavy components, which cause a weak interaction between independent hydroxyl of citric acid and tartaric acid assistant and hydrogen bond of kerosene. The results of interfacial tension tests show that all these miscible assistants possess good effects on minimizing the interfacial tension of the CO2-crudes system, and could also definitely reduce the MMP. Among these, per-acetylated glucose dodecyl ester molecules and citric acid isopropyl ester molecules perform most excellent, and could decrease the MMP of CO2 flooding by 27.5%. The assistants have been implemented in the CO2 flooding plan of L Reservoir with 4 gas injectors and 15 producers. After 15 years’ development simulation, cumulated oil production will reach 3.4 MMbbl with recovery increasing from 41.6% (only CO2 flooding) to 46.6%. Injection test shows that 1400 tons CO2 has been injected with 3800 bbl oil produced.
The improved miscible assistants provided perform as well as other existing assistants in reducing interfacial tension and enhancing sweep efficiency in CO2 flooding. Compared with assistants of light hydrocarbon, these assistants require a little quantity to improve the miscible flooding, which could break the economic limits. Compared with the traditional fluoride assistants, these assistants are quite different in molecular structure and could cause little pollution and have been applied in field test.