Keywords: sewage reinjection; Layered testing; Layered water injection; solution
China Library Classification Number: U664 Document Identification Number: A Document Number:
An oilfield has entered the stage of extra-high water cut, with comprehensive water cut above 95%, and the water cut of produced fluid is far greater than the oil production. When the separated water is injected into the reservoir after being treated by the system, the problem of high oil production cost is greatly solved, the oil production speed is improved and environmental pollution is avoided. It is a widely used oil displacement method in major oil fields at present. However, although the water demand is large, it is still necessary to take water on the spot to reduce production costs. Construction of water drive fine development demonstration plant, the reservoir has changed from a single general water injection mode to the current fine layered water injection. According to the actual water absorption capacity of different intervals, the water injection rate is adjusted by layers to achieve the purpose of flooding oil with water. However, due to serious problems such as excessive sewage quality, some wells can't distribute water normally after sewage is re-injected into the formation, which increases the difficulty of testing. In the process of testing, it is often encountered that the pipe string scaling testing instrument can not go down, which increases the workload, reduces the qualified rate of water injection interval and affects the development effect.
First, the source, composition and harm of sewage
1. 1 underground produced fluid
That is, oily sewage produced from the oil layer to the ground together with crude oil. It is characterized by high salinity of aqueous solution, crude oil, partially dissolved gas and bacteria.
(1) salinity. Is the total amount of inorganic salts dissolved in aqueous solution. Its total amount directly affects the corrosiveness of water. When the actual solubility of these salts in water is greater than the solubility when the solution reaches equilibrium, solid salt crystals will precipitate from excess and deposit on the metal surface to form scale. This part of the scale is very corrosive. At present, the most common scales are CaSO4, CaCO3 and BaSO4. Because the concentration of CaCO3 increases with the concentration of CO2 gas dissolved in the aqueous solution. Therefore, after a series of chemical reactions, a large number of CaCO3 solids will precipitate and form scale.
(2) contains crude oil. The fluid produced underground contains a certain amount of crude oil. A mine can produce one ton of oil on average 10 ton of water. The sewage before reinjection needs careful oil-water separation, and the comprehensive index should meet the reinjection water quality standard, so as to reduce the phenomena of formation plugging, uneven water flooding, large permeability difference and fingering during injection.
(3) Dissolved gas. Sewage mainly contains O2, CO2 and H2S gases, which accelerate the process of metal corrosion and scaling. Gas-liquid separation before reinjection can reduce oxygen content and slow down the corrosion and scaling of sewage treatment system and metals during reinjection. Due to pressure factors, when the formation pressure is lower than the saturation pressure when extracting liquid from underground, a part of gas will be precipitated. However, this process itself cannot completely remove harmful gases from water. Carbon dioxide is a kind of strong acid gas, which generates carbonic acid when it meets water. After the decomposition of carbonic acid, the amount of H+ in water increases, the PH value decreases and the acidity increases. The aqueous solution is acidic, which enhances the corrosiveness of metals.
(4) A variety of bacteria. Sulfate reducing bacteria, iron bacteria and slime bacteria are the main bacteria in sewage. These bacteria accelerate the corrosion rate of pipelines and downhole tools, corrosive substances further block the formation, downhole equipment is seriously perforated, the demand for water injection from water injection wells to corresponding oil wells is insufficient, and the oil well production decreases. Its content in water should be strictly controlled.
1.2 well washing
Another kind of sewage is well washing, because the effect of well washing is not good and the water quality standard cannot be reached. The suspended solids in the water are easy to block the filter screen, which increases the pipeline pressure and reduces the water quantity of the injection well. It is difficult to achieve the qualified rate of water injection, which increases the difficulty of detection.
Second, the water purification index and sewage treatment process of the joint station
(1) Purified water index. Oily sewage produced by oilfield development is collected by oil production wells, and reaches the joint station through transfer stations for oil-water separation and advanced sewage treatment. After advanced treatment, the sewage becomes purified water, which is dispersed and transported to the oil injection layer of the water distribution room under its jurisdiction. It is required that the median particle size of solid suspended solids in the reinjection sewage is ≤2? M, oil content ≤ 8 ㎎/L, suspended matter ≤ 3 ㎎/L. ..
(2) Process flow. See figure 1.
Figure 1 Flowchart of Sewage Treatment System
Third, the influence of sewage reinjection on layered test
(1) test instrument. The sewage in the reinjection layer has high salt content due to unclean impurity treatment, which is easy to corrode the pipe network and block the underground pipe string. At the same time, the sand production well accumulates fine sand in the working barrel of the water injection well, blocking the working barrel, making it difficult for the testing instrument to run into the bottom of the well and affecting the normal debugging work.
(2) blocking the water nozzle. The quality of reinjection sewage is poor, and there are many solid suspended solids, which are easy to block and scale the nozzle blocked by the eccentric water distribution pipe column water distributor. The oil pressure rises rapidly, the water injection quantity decreases, and the qualified rate of water injection decreases, so it is difficult to meet the test qualified rate standard.
(3) blocking the water meter. During the test, due to the formation blockage, the water quantity and pressure fluctuate greatly, and the reading accuracy of the surface water meter is low, which is quite different from that of the underground flowmeter. Sometimes even the blockage is serious, which makes the water meter stop running, affects the final test results and increases the re-examination workload.
(4) take time. Due to the difference of topography and the influence of many impurities in sewage in water injection pipeline, it is easy to corrode and scale on the inner wall of pipeline, which increases the flow resistance of sewage. The flow and pressure are unstable, and the pump pressure measured from the water separator in the water distribution room is too low, which is not convenient for debugging and affects the test process. Various reasons affecting the layered testing process of 20 10-20 1 1 year: 640 wells cannot be run with testing instruments; Plug the water nozzle, 930 well times; The water meter is broken, 190 well; Pump pressure, 123 well times.
Fourth, the solution
(1) purification treatment. Strengthen the monitoring and purification of injected water quality to reduce the chance of scaling of downhole tubing and tools. When the shaft wall is fouled, acidizing can be used to clean the well, or coating can be applied to the inner wall of tubing to isolate gas and avoid tubing corrosion.
(2) Solid suspended matter. Reducing the content of suspended solids can increase the flocculant. Make the small particle size suspended matter into large particle size suspended matter, increase its own gravity settlement and remove solid impurities. Preventing the water nozzle from blocking will reduce the water absorption capacity of the oil layer and often pull it out for cleaning.
(3) impurities such as crude oil. Reduce the content of oily sewage and solid suspended solids, and prevent the formation from being polluted and blocked. Keep the water pressure stable, clean and check the water meter regularly, and replace it with a new one if possible.
(4) Flow. Because the velocity of fluid determines the scaling speed of mineral salt, the content of mineral salt dissolved in water is related to pressure. When the pressure decreases, the solubility decreases. So that the aqueous solution is in a supersaturated state, and crystals are precipitated and deposited on the wall of the metal tube. Therefore, the water injection station to the water distribution room should do a good job of stabilizing the pressure and pump water continuously and evenly as required. The normal test can only be carried out when the pump pressure is kept above 14.0MPa, and the water injection pipeline should be washed frequently.
(5) Other methods. In addition, the joint station of our mine should strictly implement the water quality treatment standard of sewage reinjection. The sewage station should do well in impurity treatment, oil-water separation and oil-gas separation. Control the solubility of oxygen in water to prevent impurities in water from reacting with gas to generate a large number of substances such as rust. Corrosion and scaling block the formation of water injection wells, water injection pipelines and related equipment. At the same time, the contents of CO2 and H2S should not be ignored, and their solubility in water should be reduced as much as possible. In addition, the joint station should carefully check the sewage treatment system regularly and deal with the factors that affect the water quality in each link one by one. It can be solved in the following aspects: (1) The impurities stored, deposited and floated in the sedimentation tank all the year round should be cleaned frequently to reduce the accumulation of impurities and form repeated pollution; Chemicals are mixed according to actual needs, but if the water quality is still difficult to reach the standard, the dosage can be increased. The drugs used should not expire and deteriorate; Operators should be familiar with the flow of the system from upstream to downstream. Strengthen skill training, control the water quantity steadily, make the water injection system stable, and the water injection wells can distribute the water injection quantity well.
Verb (abbreviation of verb) conclusion
(1) An Oilfield has entered the stage of secondary and tertiary oil recovery, and the remaining oil potential is prominent. Secondary oil recovery refers to water flooding, with water as the oil displacement medium. Recharge pure water can effectively replenish formation energy and keep formation water pressure stable. Layered test can grasp the downhole water injection situation at any time, and adjust the injection allocation plan in time according to the actual test data to meet the water demand of each interval.
(2) Due to the large water demand and high water quality standard of water flooding, the oilfield developed by water flooding has been flooded for a long time, and the cooperation between the joint station and the testing team is prone to problems, which brings many difficulties to the testing work. The capacity and effect of sewage treatment system in the joint station directly affect the qualified rate of layered test and layered water injection, and improve the degree of oil layer production. Therefore, only through communication, cooperation and timely treatment can we continuously improve the working environment, improve the quality and effect of water injection, and finally improve oil recovery. The joint station should do a good job in the treatment of produced water and the connection with the test team to ensure the high standard of reinjection water quality. It provides a strong guarantee for the successful completion of fine layered testing, layered injection allocation and timely mastery of accurate data.
(3) The success or failure of water drive development depends on layered testing and layered water injection, and the quality of injected water cannot be ignored.
References:
[1] Feng Shuchu, Guo Yichang. Mining Oil and Gas Gathering and Processing [M]. Dongying: China Youshi University Press, 2006.
Zou genbao Petroleum production engineering [M]. Beijing: Petroleum Industry Press, 1998.
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