Research On Sulfide Control Of Oilfield Wastewater And The Stability Of Polymer Solution

In order to reduce the sulfide,one of the main factors decreasing the viscosity of polymer solution in oilfield wastewater,produced by sulfate reducing bacteria?SRB?,this thesis conducts its works basing on the wastewater of one oil production plant which concludes four aspects:?1?To find out the main factors influencing the polyacrylamide solution,the compositions of wastewater were analyzed containing cations,anions,sulfide,SRB,nitrate reducing bacteria?NRB?etc;?2?Several factors(Na⁺,K⁺,Mg²⁺,Ca²⁺,Ba²⁺,Fe²⁺,S^2-and pH)were selected basing on the results of water quality analysis to study their influence to polyacrylamide solution by single-factor experiments;?3?To decrease the sulfide concentration of wastewater,two methods were studied.One method was adding nitrate to activate NRB and the other was adding bactericide to kill the microorganism.After adding regents,the viscosity of polyacrylamide solution,sulfide concentration,SBR concentration,NRB concentration and corrosion rate were monitored continually;?4?Field experiments were conducted basing on the results of laboratory experiments.Before and after adding nitrate into wastewater,the sulfide concentration and the viscosity of polyacrylamide solution were continuously monitored.When the concentration of sulfide showed a dramatic decrease and remained stable,the functions and structures of microbial community were analyzed which helped to understand the influence of nitrate.The results show that:?1?The wastewater of this oil production plant contains large amount of SRB and NRB and has a high salinity with 6407mg/L.As the extension of the transportation time,the sulfide concentration in this wastewater shows an upward trend;?2?Monovalent cations Na⁺,K⁺with the same molar concentration show a similar influence on polyacrylamide solution viscosity.Bivalent cations show a greater influence on polyacrylamide solution viscosity compared with monovalent cations.Among tested bivalent cations,Fe²⁺shows a greatest influence on polyacrylamide solution viscosity.The viscosity of polyacrylamide solution can be decreased to 40mPa·s from 1320mPa·s when it is dissolved by 0.001mol/L Fe²⁺solution.Besides,the sulfide in wastewater is another important reason for the instability of viscosity.When the salinity of solution is 5000mg/L,the viscosity of polyacrylamide solution?2000mg/L?can be decreased to 3mPa·s when it is dissolved by 8mg/L S^2-solution;?3?To treat the wastewater of this oil production plant,the best dosages of nitrate and bactericide are 90 mg/L and 80 mg/L respectively,the concentrations of sulfide in wastewater are controlled at around 0.5mg/L and 0.7mg/L during 24 days,the viscosity of polymer solution prepared by the wastewater is still over 30mPa·s and 25mPa·s after 30 days.After comparing two methods,adding biological inhibitor is regarded the better one for this plant.?4?When the dosage of nitrate is 90mg/L in the field experiments,the sulfide concentration remained under0.5mg/L after the 6^th day.Before adding nitrate,the average viscosities of polymer solution in mixing station and injection well were 40mPa·s and 33mPa·s respectively.After adding nitrate,the average viscosity of polyacrylamide solution showed a considerable increase which jumped to 63mPa·s and 53mPa·s.The functional genes of SRB dropped significantly after adding nitrate,from 3.43×10⁴⁴.00×10⁴copies/mL to 3.93×10³copies/mL.By contract,the functional genes of NRB rose considerably after activation,from 5.03×10³⁵.16×10³copies/mL to 7.76×10³copies/mL.The microbial diversity of the re-injected wastewater decreased after adding nitrate.Not every NRB can be activated,species such as Calditerrivibrio nitroreducens show a dramatic decrease.And whether a species can become dominant is also restricted by other environmental factors.