Measures to prevent secondary pollution of reinjected sewage

The water quality index after filtration represents the effect of sewage treatment, which is the most fundamental guarantee for the water quality of the subsequent process to meet the standard, and the water quality after filtration continues to be stable and qualified.

There will be no secondary pollution to the water injection trunk line, filter tank and wellbore, so the stability of sewage treatment is the source of water injection quality. However, there are many factors affecting water quality, and the effect of sewage treatment should be improved through technological progress and process optimization.

To ensure the continuous stability of the water quality after filtration, and to provide a qualified water source for the subsequent process.

Reasons for secondary pollution of water quality

1.1 Influence of pipe network material on water quality

Since the water injection well was put into production, the water injection pipe network has been made of steel frame polyethylene composite pipe, high-pressure glass fiber pipe, continuous reinforced plastic composite pipe, seamless steel pipe and other materials. The longest service time is seamless steel pipe, which has been in service for fifteen years. Pipes of other materials will serve for 1-12 years.

According to different pipe materials, the wellhead suspended solids and the injection room were compared and analyzed to prevent residual pollution of sewage. Four injection rooms with fresh water were selected for sampling. The content of suspended solids in the injection room was within 0.6mg/L. The content of the substance is within 0.9mg/L, and the difference is within 0.3mg/L.

It can be shown that the pipelines and suspended solids of different materials have different degrees of improvement, but it is not obvious and cannot constitute the main factor of polluting the water quality.

1.2 The impact of sudden changes in flow on water quality

In the process of pipeline transportation, there are two situations. One is the scouring effect of the water body on the pipe wall, which is a process of increasing the turbidity of the water body.

When the flow rate is large, the scouring effect is greater than the precipitation effect, and the turbidity increases; when the flow rate is small, the sedimentation effect is greater than the scouring effect, and the turbidity decreases, and various substances in the water are easy to deposit and pollute the water quality.

According to this feature, the flushing branch line is flushed by increasing the flow rate. Before flushing, the suspended solids content is 9.44mg/L. After 15 minutes, the flow rate increases to 0.45m3/min, and the water quality turns black.

The suspended solids content rose to 42.17mg/L. After 30 minutes, the water suspended solids content decreased. After 2 hours, the suspended solids content decreased to 3.0 mg/L. It can be seen that this method can effectively flush the pipeline and reduce the suspended solids content.

1.3 Impact of backflow pollution on water quality

(1) The pump pressure is low and reverse flow.

When the oil pressure of the water injection well is higher than the pressure of the manifold, the phenomenon of backflow of the injected water will occur. After the backflow, the entire manifold will be polluted, and the water quality of the water injection wells under the jurisdiction of the injection room will be deteriorated.

If the backflow time is too long, the oil content of the backflowed sewage exceeds the standard of 8mg/L, which will cause the blockage of the flowmeter.

(2) let the overflow.

Draining overflow is a commonly used method to relieve pressure and flush water nozzles. It mainly relies on high-pressure water at the bottom of the well to flush water nozzles and solve the situation of water nozzle blockage.

Take and place the overflow water sample of the overflow well. The suspended solids content is as high as 13.12mg/L, and the oil content is as high as 60.46mg/L. Water with excess oil content is injected into the formation.

1.4 Dwell time

For positions such as dead ends and tees that are not emptied in time, there is stagnant water. With the prolongation of retention time, the water quality deteriorates sharply and the transparency decreases.

Measures to prevent secondary pollution of injected water

2.1 Regularly flush the dry line to effectively reduce the content of suspended solids

In 2013, the air pressure pulse method was used to flush the main line. The air pressure pulse flushing the pipeline is to use the compressibility of the air to make the high-pressure gas enter the pipeline at a certain frequency.

An intermittent water flow is formed in the pipeline. With the compression and expansion of the air, the frequency of the water flow in the pipeline changes, and the flakes growing on the inner wall of the pipeline are washed down and discharged with the high-speed air and water flow, and suspended after flushing. The substance content can be lower than 2.5, and the flushing effect is obvious.

2.2 Flush the branch line to further prevent suspended solids content

The water injection branch line is planned to be flushed twice a year, and the wells are flushed periodically, and the branch lines are flushed together to prevent the pipeline from polluting the water quality after the well cleaning is qualified.

The flushing branch is flushed by increasing the flow rate. The high-pressure pump truck is used to connect the emptying process of the injection room, and the high-flow clean water is injected into the water injection manifold of the injection room. For which well is flushed, the impeller inside the flowmeter of the well is removed. , to prevent the flow from being reduced, the normal water injection process is introduced into the dispensing room, the main production gate is closed at the wellhead, the test valve is opened as the outlet, and the outlet enters the sewage tanker.

During flushing, there will be a maximum value of suspended solids. It can be seen that the change of water flow rate can flush out the impurities inside the pipeline. After flushing, the suspended solids content enters the “smoothing period”, and the suspended solids content further decreases.

It is necessary to use a large amount of clean water for squeezing, and the effect is not obvious after squeezing. The main reason is that the imported water source is polluted, and the suspended solids content exceeds the standard, resulting in excessive suspended solids content at the outlet. The current practice is to start the water injection pump after flushing, and replace the pipeline with the normal water injection process. Internally polluted water.

2.3 The filter tank in the dispensing room can effectively reduce the content of suspended solids

The filter tank in the injection room adopts a maintenance-free filter tank. After the water from the water injection station, it enters the inlet of the plunger pump after passing through the filter tank, and is injected into each water injection well after pressurization.

According to the working requirements of the plunger pump, the inlet pressure should not be lower than 0.05MPa. According to the daily dispensing volume, a personalized flushing plan for the filter tank washing cycle was explored. The filter tank can effectively reduce the suspended solids content.

2.4 Wellhead filter can reduce suspended solids and oil content

In order to reduce the content of suspended solids, a filter is installed at the wellhead, which is installed on the inflow pipeline of the wellhead. After filtering, it is injected into the oil layer. Water samples were taken from the wellhead, and one sample was taken before and after filtration.

Laboratory analysis shows that the content of suspended solids dropped from 10.49mg/L to 5.4mg/L, and the oil content dropped from 1.93mg/L to 0.44mg/L. The effect is obvious, which can effectively reduce the content of suspended solids and oil, but not completely. Change the water quality situation, so that the water quality standards.

Conclusion

(1) The material and condition of the filter screen of the water injection pipe, the change of the flow rate, the backflow at the bottom of the well, and the residence time have a great influence on the injected water quality.

(2) Both the wellhead filter and the injection room filter can effectively reduce the suspended solids content and oil content, but cannot completely change the water quality.

(3) The time for flushing the main line does not need to be too long. When the suspended solids content reaches 5.0mg/L, it can be washed with water to avoid excessive suspended solids content in the water tanker and unqualified water quality.

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