As effective reinjection of produced water can improve the quality of oilfield development, the treatment technology of oilfield produced water is an important part of oilfield sustainable development. This article mainly analyzes and studies the treatment technology of oilfield produced water, the status quo and existing problems of oilfield produced water treatment and puts forward some reasonable suggestions to improve the treatment technology.
In China, water injection is used in many oilfields in a bid to achieve efficient mining. With the continuous development of oilfields, the water content of the water production fluid of the oil well is continuously increasing as well. Thus it is of positive significance for solving the oilfield sewage to realize effective treatment and reinjection of the produced water.
Additionally, this kind of treatment is more economical and practical. In some water-deficient oil areas, oil-bearing produced water has become the main source of water injection for oil fields. To improve the efficient production of oilfields, the treatment technology of oil field produced water must be advanced so as to continuously achieve the high quality of the injected water and to ensure the production quality of oilfields.
1. Overview of Oilfield Produced Water
In the heavy oil area of oilfields, the viscosity of crude oil can be reduced through the injection of high-pressure steam and it facilitates the exploitation of heavy oil. The water-bearing crude oil is extracted from the well, then the water is separated via processing and it is the right produced water. In other words, surfactants, etc. are added after the water is injected into the oil deposit, which can change the water tension and the hydrophilicity of the oil so that the oil and water will gather up, and it can be successfully extracted.
The produced water of oilfields will contain some organic pollutants and besides the ion concentration is high. The water will be injected into the ground again after being treated with a large amount of injection. If not treated properly, the groundwater will be polluted.
The produced water being reinjected after treatment has the following advantages: firstly, the produced water contains surface-active substances with relatively high temperature and good oil washing efficiency; the produced water whose salinity is close to the one extracted from the bottom layer has the maximum oil removal efficiency. Secondly, the water quality of the produced water is quite stable and not easy to mix with the oil layer to produce precipitation.
2. The Status Quo and Existing Problems of Oilfield Produced Water Treatment
2.1 The Status Quo of Oilfield Produced Water Treatment
The oilfield produced water has a complex composition and is often mixed with other sewage, so the effect of the use of a single piece of treatment equipment is often unsatisfying. Therefore, in the actual treatment process, two or three pieces of treatment equipment are usually used to ensure that the quality of the produced water meets the reinjection standard.
In addition, as oilfield types and actual conditions differ, requirements on environmental protection and uses of produced water purification vary, and thus there are significant differences in the treatment technology of produced water.
Generally, a combination of pre-processing and deep processing is adopted in the treatment of oilfield produced water. Pre-processing involves primary treatment and secondary treatment. As to the primary treatment, it refers to gravitational separation and flotation, chiefly aimed to remove the oil slick and granular solids; the secondary treatment mainly conducts demulsification and disperse oil removal through filtration and chemical treatment, etc. Deep processing includes activated carbon adsorption and biochemical treatment, etc., which can achieve the good purpose of removing dissolved oil.
2.2 Existing Problems of Oilfield Produced Water Treatment
The corrosion and scaling constitute the first problem, which is a prominent problem in the treatment of oilfield produced water. However, some advanced technologies, such as electrochemical pre-oxidation technology and electromagnetic anti-scaling technology, can solve the problem. However, due to the limitations of investment in these technologies and high operation costs, and lack of scientific post-support management, it remains difficult to use the said technologies in practice.
The second problem is secondary pollution in the treatment of oilfield produced water. After filtration treatment by sewage treatment stations, the sewage still contains some components that can be oxidized to form precipitates, which will deteriorate the water quality. If the anti-corrosion measures of the water injection pipeline are inadequate, the sulfuric acid-reducing bacteria contained in the sewage will chemically react and precipitate, causing secondary pollution to the water quality.
The high operating cost is the 3rd problem. The treatment process needs large reagent doses and relatively high expenses, and it is difficult to achieve the cost reduction of oilfield exploitation, in conflict with the realization of good economic benefit, so it affects the treatment of water quality. Therefore, efficient and economical treatment agents are an important part of in future.
Fourthly, the limitations of polymer production technology are also problems that need to be solved at the moment. In the process of oil field exploitation, polymers are injected into the strata to improve the fluidity ratio of water and oil in a bid to enhance the oil recovery ratio. However, this method will increase the viscosity in the produced water and it will be difficult to handle. At this stage, the processing technology in this regard is not mature, and it is difficult to make the external drainage water quality reach the standard.
3. Common Oilfield Produced Water Treatment Technology
3.1 Biological Treatment Techniques
The biological method mainly includes measures such as activated sludge method, anaerobic biological treatment, and biofilm method; it mainly utilizes the activity characteristics of microorganisms to decompose and utilize organic and inorganic substances in produced water to remove substances such as sulfides, allowing the produced water to be purified for standard discharge.
3.2 Application of Reinjection Treatment Technology
The reinjection treatment technology is widely used in oilfield produced water, especially in the oil production of some oilfields with high water content. The reinjection treatment method also needs to carry out some purification treatment on the produced water: the particulate matter therein is filtered, and the oil mixed in the water is also separated before the reinjection can be conducted.
The advantage of this treatment technique is that the oil separated in the produced water is sent back to the ground, the water is injected into the same well underground, the power consumption of the treated sewage is reduced, and the good energy-saving and environmental protection effect is gained.
3.3 Ultrafiltration Membrane Treatment Technology
The ultrafiltration membrane, a sort of porous membrane, can be used for the separation of the produced water via the sieve pores. Under the action of the push, the solution containing two types of solute will flow through the surface of the ultrafiltration membrane, and the small molecular solute and solvent will pass through the membrane and be collected as penetrate matters, that is, the filtrate.
The solute of the macromolecule cannot pass through and is trapped as a concentrated solution. This treatment technique can effectively remove suspended solids, bacteria, etc. in the produced water so that the related water quality can reach the relevant discharge standard. In addition, using this method as a pretreatment can well achieve the important goal of reducing oilfield mining damage and abating environmental pollution.
4. Treatment Agent in Oilfield Produced Water Treatment
The produced water contains emulsified oil and colloidal particles, etc., and corrosion and scaling will remain meanwhile. Therefore, the addition of a treatment chemical can improve the water quality of the produced water. Common water treatment chemicals are flocculants, scale inhibitors, and fungicides, etc. Among them, the flocculant mainly relies on the electric neutralization and bridging action to make various colloidal particles and oil droplets destabilize and coalesce into large flocs that are easy to settle, thereby achieving the purpose of purifying the water quality.
However, although this kind of water treatment chemical has a good effect, there is a certain difficulty in promoting the application due to its strong pertinence. Only improving the adaptability and profitability of flocculants can further promotion be made.
The flotation agent is also a common water treatment chemical that not only has a good demulsification effect but also separates the oil and water. At the same time, it can purify the water by floating the oil droplets and the suspended matter. However, there are relatively few types of such treatment agents on the market in China, and they are expensive and difficult to meet the needs of oilfield produced water.
Corrosion inhibitors and scale inhibitors have a good treatment effect as well. They are mainly used to ensure the smooth treatment of the produced water against the corrosion and scaling problems that are easy to occur in the produced water treatment system and can achieve good corrosion inhibition and scale inhibition.
The chemicals used in oilfield produced water treatment are very numerous and rich in types. It is necessary to pay attention to ensuring good operability, high efficiency and versatility for research and development. Further, have the high treatment efficiency and quality of produced water ensured from the aspects of improving the produced water treatment system and purifying water quality.
Improper treatment of oilfield produced water will cause environmental pollution, so relevant treatment technologies and chemicals should be used according to actual conditions. And further improvement remains for these technical methods and the efficacy of chemicals should also be enhanced through research to ensure efficient treatment of the produced water.