Keywords oilfield wastewater; water injection system; treatment technology
To improve the oilfield water injection system must have a systematic and comprehensive consideration. For example, the choice of water sources, the development and improvement of water treatment system technology, a good monitoring program for the system, taking into account the various types of chemicals to match each other and so on. Chemical treatment of oilfield water is only one aspect of the treatment technology, sometimes it also needs the cooperation of other technologies in order to achieve better anti-corrosion, anti-scaling, anti-bacteria and other effects. That is, sometimes only rely on chemical treatment can not completely solve the problem, and the need to use other technologies or methods to assist, such as the use of corrosion of metal materials or non-metallic materials. Especially need to take into account the following factors:
(1) in order to protect the whole process, anticorrosion, anti-scaling, sterilization and other agents generally have to be added to the water treatment station. However, such an addition of chemicals will pass through the filter, and filter also has a strong retention capacity, generally about 10 micron particles, including solid or non-water liquid will be retained to affect the effectiveness of the chemicals. Therefore, if a part of the corrosion-resistant materials, then on this basis, corrosion inhibitors and so on can be considered after the filter to add.
(2) water treatment station in some equipment and parts of the water flow rate is very low, then only counter-use of corrosion inhibitors and its protective effect is not ideal, some of the water treatment station in the standby structures using corrosion inhibitors corrosion effect is also limited.
(3) in the water treatment station using part of the corrosive materials, can add less or no corrosion inhibitor, or the requirements of the corrosion inhibitor and dosage can be reduced. Because of the above reasons, to improve the water injection system, there must be a systematic point of view and comprehensive consideration of the problem.
One, the choice of water source
Selection of water sources should be considered from the two aspects of water quantity and quality. First of all, the water source must provide sufficient water to achieve the design of the maximum required injection. Issues to be considered in relation to the selection of water sources are summarized as follows:
1) water corrosion and scaling trends. Where possible, should try to determine the corrosiveness of the water source used in advance, even if it can not be accurately determined, but also in the case of understanding the water PH value, dissolved gases and salt content, a rough estimate of the presumption of the relative corrosiveness of the water, and it is best to use the fresh water in the field for determination.
2) water mixability. If two or more types of water must be mixed, the scaling calculations should be made with the miscibility test. Generally, the miscibility of the injected water may not create much of a problem in terms of water injection because of the limited exposure, but problems in this regard can arise when the injected water suddenly enters the production well, so it is also necessary to conduct a miscibility test of the injected water and the formation water in order to determine what problems will occur when the water is seen in the production well.
3) Suspended solids and oil content. The concentration of solids in the water, the size of the particles and their distribution, the nature of the solids and their fractions all have a significant effect on water clogging, and if filtration operations are carried out, these parameters also have a significant effect on the choice of filter. In addition, any extracted water that may be selected for water injection must have its oil content determined, because oil in the water will generally lead to a reduction in the injection capacity, and in the formation may form an "emulsified plug section", and the crude oil is a very good binder for certain suspensions, such as iron sulfide, etc., which can cause the filter to fail quickly or reduce the efficiency. As for the determination of oil content in water can be taken with a clean solvent from the water samples to extract the oil, crude oil can make the solvent coloring, its oil content can be used in a specific crude oil preparation of standard samples to determine the comparison, because the color of the color shades of oil extracted from the water is directly proportional to the amount of oil.
Second, the type of treatment system
Oil field water treatment system is generally divided into two types, namely, closed system and open system, in the above water treatment technology, should take into account the different characteristics of these two types of systems. Separate treatment of surface water and groundwater when the open and closed two systems are illustrated as follows:
1) surface water open system
water source → removal of solids → tank → injection pump → well
2) surface water closed system
water source → removal of solids → deoxygenation → tank → injection pump → well
3) groundwater open system
Water source→aeration→solid removal→storage tank→injection pump→well
4)Groundwater closed system
Water source→solid removal→storage tank→injection pump→well
Shown above, it can be seen that each of the two systems has different characteristics:
Closed system is a design requirement for the complete isolation of the system of oxygen, because oxygen is often caused by corrosion and other obstacles Since oxygen is often one of the main causes of corrosion and other obstacles, the closed system is an ideal method if the economy permits. It is customarily used only for water systems that do not originally contain air, because the more components that are added to the system from outside the system, the more difficult it is to isolate oxygen from the system, so it is often economically uneconomical to exclude atmosphere from a saturated system. But for seawater injection system is an exception, untreated seawater is generally saturated with oxygen, and corrosive serious, practice has proved that the dissolved oxygen excluded from the control of corrosion is one of the effective and cost-effective method. Therefore, in recent years, the design of large-scale seawater injection systems abroad is becoming more and more common, in the offshore oil fields and onshore oil fields near the sea are mostly used this method.
Open system is generally not isolated from the system and oxygen, so when the original will be oxygen-saturated groundwater as the injection source, open system can be selected. In addition, when the need to ventilate to remove H2S or CO2, the use of open systems is also appropriate. Because open system corrosion will generally be increased, therefore, in addition to the use of chemical treatment in open systems, most cases need to use coatings, non-metallic materials, etc. to help control corrosion problems.
Three, the monitoring of water quality
In the system is completed and began to inject water, should begin to establish a system of monitoring programs to observe the actual effect of water treatment, so that in the discovery of the problem can be analyzed and take measures to correct the cause in a timely manner. Check and analyze the water samples, preferably along the water treatment process from the water source, through the entire water treatment system stages up to the injection wells, sampling and measurement of selected sampling points, and obtain the following relevant data: charts (omitted)
1) iron content - indicates the degree of corrosion; 2) calcium content -- indicates the tendency to form scale; 3) SO4?2 -- if SO4?2 -- is found to be reduced in the water, there may be deposits of BaSO4, etc.; 4) H2S If there is an increase in the H2S content of the treated water system, there may be the presence of sulfate-reducing bacteria.5) Corrosion rate. An increase in corrosion rate in a closed system may mean that there is oxygen entering the system.
Four, matching agents
Since the entire water treatment system, corrosion inhibitors, scale inhibitors, biocides and purification agents and other agents are almost simultaneous addition of the use of a variety of agents, so much attention should be paid to the matching of the agents with each other. According to relevant practical experience, in the selection of chemicals should consider the following principles:
1) pay attention to the water solubility of chemicals and chemicals between the mutual solubility. First of all, should be added to the various types of chemical agents of water solubility, so that the chemical agents used can be soluble in water. Some chemicals in concentrated salt water will produce precipitation or "salt precipitation" phenomenon should be avoided as far as possible. In addition to the use of biocides and corrosion inhibitors, anti-scaling agents, etc. It is best to use mutually soluble, and do not produce precipitation and degradation and other favorable effects.
2) pay attention to drug resistance. This issue must be considered in the selection of bactericides. Bacteria have a strong ability to adapt, a certain fungicide is used for a period of time, bacteria will be resistant to it. Therefore, it is best to choose two kinds of fungicides alternately, when the bacteria began to produce resistance to the first fungicide, switch to the second fungicide, in order to avoid and solve the problem of drug resistance.
3) Pay attention to the toxicity and economy of the agent. Although the oilfield requirements on the surrounding environment is not as densely populated city on the factory discharge water requirements as strict, but if the use of the agent is too toxic, on the health of workers and the surrounding environment will eventually have a negative impact. Therefore, in the selection of agents should be used as far as possible on the low-toxicity, non-polluting agents, but some introduced from abroad in the fungicide and highly toxic organotin compounds, etc., from the point of view of environmental protection is not desirable. In addition, the cost and price of the agent directly affects the recurrent operating costs, so from the economic considerations, the cost of the agent should be minimized.
References
[1]Xu Baojiu. Water supply treatment. China Construction Industry Press, (1979)
[2]Tang Hongxiao. Fundamentals of water and wastewater chemistry. China Construction Industry Press, (1980)