Study On The Emulsion Stability And Treatment Of The Polymer-flooding Produced Water

Polymer flooding technology is an important technology in tertiary oil recovery, which plays a significant role in maintaining oil production in old oilfield of China such as Shengli Oilfield. Compared with the produced water from water flooding, the produced water from polymer flooding contains more oily substances and degradation products of hydrolyzed polyacrylamide (HPAM), which makes it more difficult to treat. At present, although flocculation is widely used in treating polymer flooding produced water, the composition and property of the oily substances in polymer flooding produced water have not been completely elucidated, and there is still not a standard way to choose appropriate flocculants effectively and efficiently. This paper is intended to carry out the basic study on separation, analysis and emulsifying activity of sub-fractions of oily substances in polymer-flooding produced water, and then to intensively investigate the interaction between sub-fractions and flocculants as well as the effect of other components on the process, aiming at developing a useful way to screen out effective flocculants and efficient ways for the treatment of the oily water produced by polymer flooding.The oily substances in the produced water from a polymer flooded reservoir of Gudao, Shengli, were extracted by petroleum ether and trichloromethane successively. Then, the two fractions were separated into six sub-fractions based on polarity by using solvent deposition and extraction. The results of composition, structure and emulsifying activity of these sub-fractions showed that high heteroatom content (25.3% for SF42), high molecular mass(Mm) (4533 for SF42), high metal content (Ca 2357 mg·kg^-1 and Fe 455.8mg·kg^-1 for SF42), low H/C ratio, and strong emulsifying capacity are observed for solid sub-fractions SF41 and SF42 at low concentrations. SF31 and SF1, which contributed to the stability of polymer- flooding produced water, had higher acid values (23.9mgKOH·g^-1 for SF31) and stronger emulsifying activity than SF2 which had weaker emulsifying activity. The interfacial tension for these sub-fractions is arranged as follows:SF42≤SF41 < SF31≈SF1 < SF32≈SF2.The acid substances in polymer-flooding produced water of Gudao2# were dominated by petroleum acid and degradation products of HPAM, the percentage of degradated HPAM was about 30%. The light petroleum acids in water produced by polymer flooding in Gudao2# were mainly composed of C16 and C18 aliphatic acids, C14^-18 naphthenic acids and C18²1 tetracyclic naphthenic acids, and the amount of naphthenic acids were more compared with the aliphatic acids. The hydrolytic degree of HPAM in water produced by polymer-flooding in Gudao oilfield was 30.1% and its Mm was 2.26×106.The contents of HPAM and polar materials extracted by CHCl3 and its sub-fraction SF31 which acid value was the highest among six sub-fractions in produced water of Gudao2# were high than those in produced water of Gudao5#. The strong interaction between HPAM and polar sub-fractions such as SF31 and SF42 was observed, and the emulsion was extremely stabile and the oil content in water was high when HPAM concentration surpassed 200mg·L^-1 and the content ratio of SF31 and SF42 was among 3¹0. HPAM, polar sub-fractions and inorganic particles were the important components of the suspended solids in water produced by polymer flooding, and the emulsion was extremely stabile when the concentration of the inorganic particles was 0⁵0mg·L^-1.The emulsion was more stable and Zeta-potential became positive as NaCl concentration up to 7000mg·L^-1 in the present of HPAM, and especially the interaction between HPAM and salt was a key factor to impact the efficiency of the treatment of the polymer-flooding produced water. Because of the strong synergistic flocculation between cationic polyacrylamide (CPAM^-1, as flocculant) and HPAM in the produced water, CPAM^-1 had a good performance of treating polymer flooding produced water in Gudao oilfield than other flocculants such as PAC and PDMDAA. The strong interaction between HPAM and high Mm, strong polarity sub-fractions SF42 and high acid number SF31 caused the emulsions were more difficult to depose than emulsion composed of HPAM and weak polarity sub-fraction SF2.The efficient mixing between produced liquid and CPAM-1 can increase collision frequency that leads to adhesion of oil droplets and increasing flocculation efficiency. The floating oil also increases flocculation efficiency because the destabilized oil droplets can adhere to it. To improve water treatment efficiency and reduce waste oil creation, the water treating agent should be injected at well-drain.Two water treating agents FX-02 (for Gudao2#) and FX-05 (for Gudao5#) which main component was CPAM^-1 were developed. The oil content of the treated water decreased from 2700mg·L^-1and 950mg·L^-1 to 180mg·L^-1and 144mg·L^-1 in Gudao2# and Gudao5#.