| Alkali-surfactant-polymer (ASP) flooding is one of the main methods to improve oil production further in late high water cut stage in Daqing oil field. However, the high concentration of alkali leads to not only an ultra low interfacial tension, but also serious scaling in near well bore and on lift equipments. According to the information from pilots, scaling has affected normal production greatly. And it has been the biggest obstacle for industrial application of ASP flooding. So it is urgent to study the scaling mechanism and its inhibition methods. This project has been a subproject of Study on ASP Flooding Industrial Application in Daqing Oilfield.In this paper, the scale forming mechanism was studied based on the analysis for produced fluid and scale from the bottom of producer, and the reaction mechanism between ASP system and mineral. On the basis of indoor experiments, scale remover and inhibitor with satisfactory results were developed, as well as the operational process. Now perfect results have been obtained in field application. This technology plays an important role in ASP flood, and lays stable foundation for its industrial popularization.Field test and scale formed properties in production wells of ASP flooding in Daqing Oilfield Field test of ASP flooding in Daqing Oilfield The two ASP flooded pilot tests have shown more than 20%OOIP incremented recovery by ASP over water flooding. Performance analysis for composition of scale All of the ASP flooded wells have shown scaling with different degree. It mainly appears in near well bore and on lift equipments, especially on central producer. By discussion of the scaling situation in these two pilots, the following rules can be obtained:a. The appearance, composition of scale and scaling mechanism are different obviously in different displacement stage. There are mainly three stages: initial stage, which is period of maximum severity of scaling, and the scale is loose sheet with dust color, and it is comprised of clay and clastic mineral; medium stage, scale is white and more compact, mainly comprised of calcspar and chalcedony; late stage, scale is gray and silt-liked.b. Concentration of ions in produced fluid varies greatly while scale appears. Ca2+,Mg2+, HCO3- decline nearly to zero, and Si increases with the higher pH.Properties of scale Mineralogy and chemical composition of scale were analyzed by using advanced analyzing instruments. Main minerals are non-crystalline silicon dioxide, calcspar and barium sulphate, etc. Chemical composition includes organic, CaO(33.02%), SiO2(15.50%), MgO, Al2O3, FeO(FeO+ Fe2O3), P2O5, MnO, TiO2, K2O, Na2O.Study on scaling mechanism Based on the former research, the following cognition are obtained:a.Main types of the scale are non-crystalline silicon dioxide, silicate and carbonate, especially, silicate scale, which is affected greatly by concentration of silicon, temperature, and pressure.b. Formation water is type of sodium bicarbonate in ASP flooding area. By injected with ASP system, HCO3- turns into CO32- and react with Ca2+, Mg2+ to form carbonate scale.c The chemical reaction among alkaline in ASP solution ,the water and clay minerals in reservoir results the amount of Ca,Si and CO3 ion increasing. When the concentration of these ions is over dissolving capacity by change in temperature and pressure etc, they will deposit and form into scale.Study on scale removal and inhibition for ASP flooded wells 4.1 Scale remover Based on the former research and a system of indoor experiments, scale remover was developed. It consists of retarded acid, latent acid, surfactant and corrosion inhibitor, etc. For silicate scale, scale conversion agent is proposed. Static scale removing rate is above 80%. On-site application process of scale removal has properties of safe operation and without well pulling job.4.2 Scale inhibitorConventional scale inhibitor can't prevent scaling effectively due to the high pH and high content of silicon ion in produc...
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