Abstract: In the process of development of oil fields, when the reservoir pressure reduces a lot, then you can use water to maintain reservoir pressure. However, the injected water causes not only damage to the environment, but also a serious drain on water resources. So, Recycle oil wastewater should be a focal subject, in aim to reduce environmental damage, to ensure sustainable development of oil fields, and to enhance the economic benefits of oil production.

1.Technology of thin oil industrial wastewater treatment and recycle as irrigation water

1.1 Technical principles and characteristics

Oneimportant way after deep treatment of wastewater is recycle in industrial production, agricultural irrigation and even daily living, it can also effectively alleviate the water resource crisis. Currently,Besides Recycle steam injection ion exchange technology, effective treatment methods include also freezing, distillation, oil film and so on.

Inthe oil field wastewater advanced treatment methods through oil film include: ultrafiltration, microfiltration, electrodialysis, reverse osmosis and nanofiltration. Distillation can generally be divided into gas pressure distillation, multi-effect distillation and multi-level evaporation, which are widely adopted in Netherlands, Middle East countries, Germany and other countries for realizing the wastewater Recycle by one step. Freezing refers to carrying out desalination process by applying the feature of higher freezing point of frozen brine to that of pure water, firstly drop the water temperature below 0 ℃,then, it will form ice on the surface, and then, when the environment temperature is higher than 0 ℃,the ice will melt into water ready for use. Normallyoilfield will employ natural freezing method.

1.2Case Study

In DagangOilfield Group Company wastewater treatment and Recycle project, In order to solve the water problem for three construction projects, i.e.,the thermal power plant, calcined coke and polypropylene, after treatment  wastewater meet concerned standards then mix with living wastewater, afterwards in sequence passing through process of hydrolysis, BAF, coagulation, sedimentation filtration, comes to last step of "double membrane" wastewater deep treatment process, then output water can be used as feed water for power plant boiler, calcined coke and polypropylene project.

Through a technology combining constructed wetlandswith reverse osmosis method some foreign companies have treated high salt content reusing water in oilfield. Its general process is as following: mined water passing through filtration made of polyethylene,ion exchange softening, then through filtration equipment with a polyethylene membrane as 0.45μm, to complete the reaction in reverse osmosis processing apparatus, combing with constructed wetlands, then output water ready for use. After such a process wastewater treatment, the toxicity of the water body decreased dramatically, the salt content reduced96%, the conductivity decreased 98%, basically it not only meet related standards of emission and irrigation, but also provides a feasible way of oilfield wastewater treatment.

2. Technology of heavy oil waterRecycle for steam injection process

2.1 Feed water conditions for steam injection boiler

Waterfor steam injection has to comply with "heavy oil field produced water for the steam generator water treatment design specifications" requirement SY / T0097-2000 of. In the oil industry, the steam generator is also known as steam injection steel furnaces, different with boilers for other usage, its produced steam has a lower dryness, usually around 80% withvapor pressure of about 30MPa. To verify whether it is suitable or not to lower down the silicon content standard of feeding water for injection to boiler, there have been a number of industrial-scale tests taken domestically and around the world, the basic conclusion is as this: When the water hardness in the iron concentration is low, salt accumulation will not occur on the high-sulfur and high-quality monitoring equipment. However, so far, it has not been proven on industrial production level.

2.2 Technicalcharacteristics andprinciples

Based oncareful consideration of heavy oil waste water damage on boiler, considering water quality standards for oilfield steam equipment, different treatmentapproaches apply to different pollutants, including: prior strengthen method and stage intensifying method. Prior strengthen method refers to degreasing treatment at former stage and then filtration treatment in the latter stage. Degreasing treatment applies to swash plate grease traps, regulation pool and flotation tank, by adding some processing agents to remove a large amount of suspended solids, oil, oxygen demanding chemicals, as well as some part of ferrous sulfide. stage intensifying method is based on degreasing treatment at former stage, further removing of oil, suspended solids and total iron, in addition, as a result of weak treatment performance of ion resin exchange on Si02, before the resin ion exchanging the concentration of Si02 should reduce to 45mg / L. So, before start of the ion resin exchanging, it should make sure the iron concentration, suspended solids, oil, and so on meet concerned standards for equipments.

In recent years, for treatment of oilfield waste water on high land mine, multi-effect evaporation method are applied in large scale. It was employed for the first time in Shengli Oilfield south coast station with this multi-effect evaporation treatment, it turned out that the treated water quality indicators were in line with feeding water standard for heat recovery boiler, in addition, also in line with quality indicators of industrial water and liquor configuration water, however, for reason that its trailing exhaust steam can not be recycled, resulting in extra consumption of energy, and in turn higher capital investment.

2.3Case Study

Currentlythere are 7 projects of Liaohe Oilfield wastewater treatment for Recycle by boiler, the total design capacity is 8.1 × 10⁴ m³ / d, wastewater Recycle heat recovery boiler 160 units, wastewater Recycle heat recovery boiler steam injection Total 4.5x10⁴ m³ / d. Liaohe Oilfield based on the characteristics of their heavy oil water has determined the typical wastewater treatment processes.

Because of the high cost of silicon removal by chemicals, plus that it easily leads to scaling, so in Liaohe Oilfield it starts to try the technology of the wastewater Recycle without silicon treatment. Firstly through boiler control technology it ensures the stability of the boiler pressure, temperature and dryness, so as to ensure the smooth operation of the boiler, and then through water quality control technology, replacing the two macroporous weak acid resin with new resin, so as to remove trace divalent / trivalent calcium, magnesium, iron ions scaling, in the end to control effluent concentrations at below 20ppb. Under the premise of the safe operation of the boiler it can endure high silica concentration in reusing wastewater for boiler, even without removal of silicon. From August 1, 2011, it stopped silicon removal operation in wastewater treatment station. On the basis of regular cleaning of the boiler, the process is operating normally, saved cost of chemicals and silicon removal treatment.

In addition, large-scale application of membrane technology brings prospects for water treatment industry, with "a combination of ultrafiltration and reverse osmosis method" as the center of "dual-mode" deal, replacing the previous ion exchange model. Shengli Oilfield processing rate is at 2500m / d, with total investment of over 4.2 million and running costs 2.3 yuan / m. after treatment heavy oil wastewater being Recycled by boiler as feed water, thus with water recycling realized.
After treatment oil wastewater meet technical requirements and then recycle for use, this not only canavoid unnecessary irrigation to groundwater and formation, in result reduces environmental pollution, but also can be used in sewage heat boiler to reduce energy consumption and reduce water consumption, helping to slow the local water shortage problems.