Development And Application Of Lowpressure Backwashing Filter In Oilfield Wastewater Treatment

At present, most oilfields in China have entered into the third oil exploitation stage.The content of water co-produced with crude oil reached to 70%,some oilfields even reached to 90%.Large amount of oily wastewater was produced from primary oil/water separation process. If the oily wastewater without treatment was discharged into the aquatic environment, it would damage the environment, put hazardous risk to public health,and cause economic losses.As the development of polymer flooding technique, the content of polymer in the oilfield wastewater is increasing, which brings more difficulties to wastewater treatment.So, development of some new equipment and wasterwater reuse technics is a large project which can solve the difficult problem of polymer-bearing wasterwater treatment.In common oilfield wasterwater treatment process, filtration is very important.The mechanical structure and performance of filter can determine the quality of filtered water.In this paper, the causes of filtration problems which occur in treating polymer-bearing wasterwater were analyzed.Because of the polymer increasing in the oilfield wastewater, the adsorbability between the filtration media and suspended solids(SS) is increasing too, the relative density of filtration media which adhere impurites will lessen.When filter backwashing at the same velocity, the expansion rate of the impurity-coated filtration media will enlarge, which will cause the backwashing pressure to increase and the backwashing flow rate to decrease, the filtration media can not be regenerated well, so the filtration efficency is lower.To solve this problem, the low-pressure backwashing filter has been invented. The low-pressure backwashing filter can control the media expansion rate and backwashing flow rate, which ensure the media regeneration completely during backwashing and the filtration system running perfect.Furthermore, due to the low backwashing pressure, the backwashing can be done without pump forced water only of filtered water in storage tank.Efficient backwashing is the foundation of filtration process. The optimum backwashing parameter is determined by calculating and field trial.The backwashing velocity of walnut shell filter is 5~8m/s, the pressure is 0.03~0.08MPa; The backwashing velocity of sand filter is 14~20m/s, the pressure is 0.12~0.20MPa.At this velocity, the expanssion rate of walnut shell filter is 180%, sand filter is 160%, which is lower than common filter.Under this parameter, the effect of filtration media regenation is very well.Pilot test using low pressure backwashing filter has been done at Daqing oilfield wastewater treatment plant.A new filtration process that takes out the third stage filtration and adds micro-flocculation to the system has been tested.The performance of new process which only two stage filtration is perfect. The removal rate of oil and SS can reach to 50% after the first stage filtration that exceeds the common filter, and after the second stage filtration, the content of oil in treated water is lower than 1mg/L, the removal rate reach to 99%; the content of SS in treated water is lower than 3mg/L, removal rate can reach to 95%.The water quality can meet the low permeable reservoirs reinjection water standard.Base on the results of pilot test, several wasterwater treatment plants in Daqing oilfield are all using low-pressure backwashing filter.The filtration effect of the plants is improved distinctly.The filtered water from the first and the second stage filtration meets the corresponding standard respectively.The economic benefit of Beiershen wastewater treatment plant has been analyzed.Using low-pressure backwashing filter and new filtration process as a new technique for oilfield wastewater treatment, it not only makes the treated water meet the low permeable reservoirs reinjection water standard but also brings about enormous economic benefit and social effect.Almost all oilfield wasterwater filtration facilities in China have the problem that the filtered water does not meet the reinjection water stardard. The reason for the problem is the deficient structure of filter and inefficient backwashing .The new technique can solve this problem, which has widest appilication in oilfield wasterwater treatment.