Search

The basic properties of alkali–surfactant–polymer (ASP) flooding produced water and effect of oil displacing agent on the stability of flooding produced water were obtained by measuring and analyzing the water quality of Zhong-106, etc., flooding water from Daqing Oilfield and the relationship between oil content-settling time and oil displacing agent-oil drop size. The addition of ternary oil displacement agent to ASP flooding water greatly increases the difficulty of oil–water separation, higher concentration, and worse separation effect after standing. Surfactant alkali and polymer have effect of promoting the reduction of oil droplet size distribution and are all affected by the initial oil content and pH. When pH is between 10.5 and 11.75, the average particle size of oil drop in ASP produced water changes the most significantly, and as the alkali content increases, the pH of the produced water rises faster. Alkali reacts with the active substances contained in the crude oil in the produced water, such as organic acids, thereby increasing the pH of the produced water. The pH of the produced water changes smoothly when alkali content is 5000 mg/L, but all are less than alkaline flooding water, which shows that alkali can promote the partial hydrolysis of polymer in the produced water and the process can reach equilibrium. The ASP flooding produced water after 48 h of settling formed a trace amount of nano-oil droplets, also accompanied by the accumulation and separation of a part of the oil droplets.

The basic properties of produced water and the effect of oil displacement agent for its stability were obtained, through measuring and analyzing water quality characteristics, relationship between critical micelle concentration-oil displacement agent, temperature, pH and content of inorganic salt, particle size distribution and morphology of Nan4-8 and Sanyuan-6 alkali-surfactant-polymer (ASP) produced water of Daqing Oilfield. The critical micelle concentration of ASP flooding was less than its own concentration, and the double bilayer was formed in system. Critical micelle concentration of ASP produced water was less affected by the system temperature and greatly affected by pH. Polymer increases viscosity of produced water and has the properties of surfactant, which partially hydrolyzes under the action of alkali to form polyelectrolyte and promotes the formation of double electric layer. Therefore, the coalescence of oil droplets in ASP produced water is difficult, and the stability is greatly enhanced. ASP produced water contains a large amount of inorganic salts, which can shield the electrostatic repulsion between the surfactant ion head groups, compresses the thickness of the double electric layer, and promotes the formation of surfactant micelles. When it settles after a certain period of time, part of ASP produced water has a particle size distribution less than 100 nm, the micelles are generated in the system, which enhances the stability of ASP produced water.

The concentration and size of suspended solids (SS) in the treated produced water beyond the criteria of injection water in Daqing oilfield have raised great concerns in recent years. The SS in produced water from water, polymer and alkali–surfactant–polymer (ASP) flooding were successfully separated and characterized using some analytical techniques in this study. X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and heating method reveal that some organic compounds besides crude oil were present in the SS samples, and polyacrylamide was found in the SS from polymer and ASP flooding. X-ray diffraction (XRD) shows some crystal inorganic substances such as SiO2, Fe2O3, Fe3O4, and BaSO4 in the SS samples, and XPS analysis indicated that several iron compounds with different valences were present in the three SS samples. The mean diameters of three SS samples were 22.89, 11.28 and 17.61 μm, respectively. Most importantly, the aggregates formed by the SS and oil droplets as well as the small SS adsorbed on the surface of oil droplets were observed using a microscope, indicating that the SS can be removed with crude oil, and crude oil also contributes to the determination of the SS values.

Produced water from polymer flooding was much more difficult to be treated than that from water flooding in many oilfields. The simulated experimental results showed that the small initial sizes of oil droplets were the main reason that caused the difficulty in treating the produced water from the polymer flooding process. Polymer hydrolyzed polyacrylamide (HPAM) increased oil–water separation when its concentration was less than 790 mg l−1. The flocculating effect by the polymer was found to play a dominant role at low HPAM concentration, even though it increased the wastewater viscosity and water film strength. Enhancing the coalescence of oil droplets and shortening rising time of oil droplets were the critical factors affecting the treatment of produced water. A novel crossflow oil–water separator with coalescence and separation sections was exploited. The coalescence section accelerated the coalescence of the small oil droplets and the separation section shortened the rising time of oil droplets. The in situ experiments showed that the separator successfully treated the produced water from the polymer flooding. When the polymer concentration was below 410 mg l−1, the quality of the treated water after the separator can meet the water requirement for the subsequent filtration processes.

Three ASP (Alkaline, surfactant and polymer) flooding projects and one multicomponent foam flooding project have been carried out in Daqing Oilfield. The increases of oil recovery rates are all above 20% (OOIP). One of the major troubles that happened during the three ASP flooding projects and the multicomponent foam flooding project was silicate scale deposition in the tubings of production wells which caused frequent downhole operation failure. To prevent scale deposition in production wells, a scale inhibitor, which works well in brines of pH values as high as 12 was developed. The scale inhibitor is effective both on carbonate scale and on the mixed scale of carbonate and silicate. The scale inhibitor was tested on 6 production wells in two ASP flooding project zones. The pull-out period of the tested production wells were elongated from 1 month to 9 month during the test.