Inhibiting Function Of Denitrification In Reducing Hydrogen Sulfide In The Crude Oil Refilling And Transportation Process

H2S is a toxic gas, which can cause serious harm to human body. As far as the oil industry is concerned, H2S is widespread and it’s harmful for the lives of occupationally exposed workers. Changqing oilfield have37square kilometers exploration area, and its registration Territories shares14%of the China National Petroleum Corporation total registered area, ranking the second in China’s oil. With the expanding of exploration, exploitation and gathering in changing oilfield, its pressing matter of the moment to survey, prevent and manage H2S and other toxic and harmful gases.Based on the controlling of H2S and the demand of security environment and its protection and governance of corporation, the present paper has researched the inhibiting function of denitrification in reducing the activities of sulfate bacteria in the crude oil refilling and transportation process. In our research, we have prepared two sets of UASB (up-flow anaerobic sludge blanket) reactors (UASB-1and UASB-2). The first reactor is to simulate the current environment of sulfate-reducing bacteria in the crude oil refilling and transportation process in order to promote the breeding of such bacteria for producing hydrogen sulfide, whereas the second reactor, the subsequent reactor, is mainly used to form the denitrification condition to let the oil liquid in so as to control the sulfate-reducing bacteria in producing hydrogen sulfide. Through adjusting the ratio of w(SO42-)/w(NO2-), DNB could be made to be the superior population in UASB-2, which helps to control the hydrogen sulfide. The results of our experiments indicate that the optimal ORP (Oxidation-Reduction Potential) value with sulfate-reducing bacteria can be made to increase from-370mV to-300mV when-150mV to-50mV are just needed for denitrifying bacteria. Adding sodium nitrite to UASB-2is not only helpful to improve ORP value but also able to provide sufficient nitrogen sources. In our experiments, we have succeeded in achieving the best inhibitory effect on the decrease of hydrogen sulfide by10%in the ratio of w(SO42-)/w(NO2-) is8:1.2. We have also studied the change taking place in the microbial communities by using16S rDNA gene based method and denaturing gradient gel electrophoresis (DGGE) analysis. The results prove that the sulfate-reducing bacteria could be successfully inhibited by the denitrifying bacteria, with the three typical SRB (Desulfovibrio, Desulfotomaculum, Desulfomonas) gradually vanishing. At the same time, both the genera and the amount of the denitrifying bacteria were found increased remarkably. The analysis of the microorganism growth reveals that the different microenvironments are likely to sift microorganisms in which different species may strive for their survival through competition. In other words, the species vary with the changing environment. The process of our experiments helps to find that there may exists three mechanisms, i.e. the substrate competition between DNB and SRB, the inhibition of the middle product of denitrification and the oxidation of the autotrophic nitrate-reducing bacteria.Subsequently, on the basis of a series of laboratory experiments, aiming at the problem of high concentration of hydrogen sulfide in crude oil gathering and transferring systems of Ai jiawan section in Chang qing Oil field, the inhibiting function of denitrification in reducing the activities of sulfate bacteria in the process was used. The field experiment result shows that when we used the biological inhibition technique in front of the crude oil gathering and transferring systems, the concentration of H2S in sedimentation tank and sewage tank sharply decreased from268mg/m3to less than10mg/m3which meet the demand of "Occupational Exposure Limit for Hazardous Agents in the Workplace"(GBZ2-2002). The number of denitrifying bacteria increased from initial3000cfu/100mL to600000cfu/100mL as the time goes on. The ORP (Oxidation-Reduction Potential) was made to increase from-350to-300mV when-100to-50mV are just needed for denitrifying bacteria. Adding sodium nitrite to the oil well is not only helpful to improve ORP but also able to provide sufficient nitrogen. In our experiments, we have succeeded in achieving the best inhibitory effect on the decrease of hydrogen sulfide. We have also studied the change taking place in the microbial communities by using16S rDNA gene based method. The result prove that the number of microbial communities and DNB increased effectively, it also indicate that three kind of DNB named Bacillus Halodurans, Pseudomonas alcaligenes and Neisseria played the most important role to restrain the activity of SRB in the system. The study developed the biological inhibition technique of H2S produced by SRB, and confirmed the feasibility of treatment technology and process application.