(China Petroleum Exploration and Development Research Institute, Beijing 100083)
Most oil fields at home and abroad have entered the high water cut development period, and the comprehensive water cut of crude oil is as high as 90%, which leads to the overload operation of the original surface system, and the transformation investment, energy consumption and operating expenses increase sharply. In order to solve this problem, some oil fields began to implement pre-water separation in the transfer station of gathering and transportation system, and the separated sewage was treated on the spot and reinjected into the formation after reaching the standard. In this paper, the pre-water separation technology widely used at home and abroad is summarized from the aspects of technical principle, advantages and disadvantages, and its future development is prospected.
Keywords: Pre-water-cut prospect in high water-cut stage
Pre-dehydration technology of crude oil
High water cut oilfield
Hu Changchao, Dang Wei
(Sinopec Exploration and Production Research Institute,
Beijing, China 100083)
Most oil fields at home and abroad have entered the high water cut stage, and the comprehensive water cut of crude oil is as high as 90%, which leads to the overload operation of the existing surface system, and the transformation investment, energy consumption and operating expenses increase sharply. In order to solve this problem, some oil fields began to pre-dehydrate in the block stations of the gathering and transportation system, and the separated sewage was treated on the spot and then reinjected to the ground after reaching the water quality standard. The common pre-dewatering technologies at home and abroad are summarized from the aspects of technical principle, advantages and disadvantages, and their development prospects are prospected.
Keywords pre-dehydration; High water cut stage; prospect
Oil field development at home and abroad has experienced three stages: oil production rising stage, oil production reaching peak and stable production stage, oil well water breakthrough and oil production decreasing [1]. At present, most major oil fields in eastern China have entered a period of high water cut or extra-high water cut, and the comprehensive water cut of crude oil has exceeded 90%, and some oil fields even reach 98%. Oilfield development has changed from "oil production" to "water production". In the high water cut stage, a small increase in water content will lead to a large increase in liquid volume. Taking Shengli Oilfield as an example, when the comprehensive water cut of the whole oilfield is 9 1% ~ 92%, every time the water cut increases by 0. 1%, the liquid volume will increase by about 375× 104 t every year, with an increase rate of 1.25%. Because the ground treatment system uses the production facilities in the middle and low water cut period, it can't adapt to the sharp increase of liquid production and the demand of water-based treatment. The main problems are as follows:
1) The treatment capacity of the sewage treatment system of gathering and transportation is obviously insufficient, and it is overloaded and the treatment efficiency is low.
2) The original facilities need to be continuously expanded, the amount of renovation work and investment cost are too large, and the renovation of the original process is also very difficult.
3) Energy consumption and cost increase. Most of the crude oil dehydration stations built in the development stage of low water cut in oil fields adopt two-stage dehydration process. After the crude oil with high water content is collected and transported to the centralized treatment station, it all enters the heating furnace for heating, and most of the heat energy is consumed in heating sewage. In a combined station with liquid inlet of 1700× 104 m3/a and comprehensive water content of 95%, the fuel consumption of heating furnace will reach 1.45× 104t/a only once, in which the heat energy absorbed by sewage accounts for about 97%, which is huge. The discharged sewage needs to be transported back to the water injection station, which increases the sewage transportation cost, reduces the pump energy consumption by back pressure and the operation management and maintenance cost. In addition, with the increase of water content, the temperature of liquid discharged from oil wells is getting lower and lower, and the consumption of heat and chemical additives is further increased, resulting in a sharp increase in the treatment cost of tons of liquid and tons of oil.
4) The circulation of a large amount of sewage accelerates the corrosion of pipelines and equipment and shortens the service life of equipment.
Implementing pre-water separation, separating sewage as soon as possible and reducing sewage flow links can effectively solve the above problems, greatly reduce energy consumption, cost and transformation investment, and improve economic benefits. Therefore, on the one hand, domestic and foreign oil fields have stepped up the research on pre-water separation technology to meet the needs of oil field production in high water cut stage, and successfully developed high-efficiency pre-water separation devices such as terminal phase separation tube and hydrocyclone; On the other hand, the original process is reformed and the pre-water separation link is added. All produced fluids are heated and dehydrated at the joint station, but low-temperature pre-separation is carried out at various well sites, partial pressure pumping stations and transfer stations. The separated sewage is re-injected into the formation after reaching the standard, and the remaining low-water crude oil is sent to the joint station for centralized heating treatment. At present, the commonly used pre-water separation technologies at home and abroad mainly include three-phase separation technology, cyclone separation technology, terminal phase separation technology, inclined tube pre-water separation technology and low-temperature demulsification technology.
1 three-phase separation technology
The technical principle of the three-phase separator is that the oil-water mixture enters the equipment through the equipment inlet, enters the water washing chamber after pre-degassing through the inlet gas distribution bag, and the collision and friction of the oil-water mixture in the water washing chamber can reduce the washing process of the interface membrane, and most of the free water can be separated. The undivided mixed liquid enters the sedimentation chamber after being distributed by the distributor and rectified by corrugated plates, and the final oil-water separation is carried out in the sedimentation chamber to achieve the purpose of dehydration (Figure 1). Three-phase separator has been widely used in oil fields at home and abroad, especially in China, by applying many technologies, such as pre-degassing of incoming liquid, liquid preparation in shallow pool, demulsification by water washing, high-efficiency aggregation rectification and oil-water interface control.
Figure 1 Schematic diagram of high-efficiency three-phase separator
Most onshore oilfields in China have transformed three-phase separators into pre-separators for pre-water separation. According to the physical and chemical characteristics of crude oil in high water cut stage, Henan Oilfield Planning and Design Institute has developed HNS three-phase separator, with the overall dimension of φ 3000 mm×10608 mm×10 mm, which is divided into pre-degassing chamber, steady flow chamber, water washing chamber, settling separation chamber, oil chamber, water chamber, gas space and gas bag. This type of three-phase separator adopts gas pre-separation, secondary fog catching technology and active water washing to enhance demulsification technology, which improves the oil-water separation efficiency; The oil-water interface control is realized by using the pressure balance principle of U-shaped pipe with double baffle structure. The combination of reasonable equipment configuration and process control improves the automation level. The HNS three-phase separator is transformed into a pre-separator, and its processing capacity is 4 ~ 8 times that of the traditional equipment of the same specification. The light crude oil with the density of 0.85g/cm3 in Henan Oilfield has a water content of less than 0.4% and a sewage oil content of less than 500 mg/L [4].
During the 11th Five-Year Plan period, 33 joint stations in Shengli Oilfield popularized and applied 52 sets of high-efficiency three-phase separators104m3/d, and treated incoming liquid 67.55× 104 m3/d, and the water content of crude oil decreased from 85% ~ 90% to 50% ~ 60%, saving about 900t of heating fuel every day, which achieved good energy-saving and consumption-reducing effects. Taking Tuo No.3 Station as an example, the inlet liquid volume is 3.5× 104m3/d, the water content of separator oil is reduced from 94% to 15%, the heating liquid volume is reduced by 90%, and the fuel oil 1068t is saved annually. For remote small fault-block oilfields, Shengli Oilfield changed the original high-water-cut all-liquid transportation to a long-distance joint station, and the water injection source was adjusted to local pre-water reinjection and low-water-cut crude oil transportation. 32 three-phase separators are used in 15 transfer station, and the separated water is 6.98×104m3/d. After local reinjection of sewage, sewage replaces clean water by 0.6× 104m3/d, reducing the sewage transportation volume by 3.6× 104m3 every day, saving transportation power consumption by 3.
Using three-phase separator as pre-separator has the characteristics of large capacity, high separation efficiency, stable operating conditions, convenient management and high degree of automation. After primary treatment, the water-bearing crude oil meets the requirements of purifying crude oil. However, the three-phase separator is designed to make the oil reach a certain water content. The effective space for sewage separation and purification is insufficient and the oil removal efficiency is low. The oil content in the separated water is generally controlled below 1 1,000 mg/L, and the oil content in the water is between 500 ~ 1 1,000 mg/L in actual operation. The follow-up sewage treatment system needs to adopt two-stage oil removal and filtration treatment process, which has high investment, land occupation and operation cost.
2 cyclone separation technology
Fig. 2 Schematic diagram of hydrocyclone
The working principle of hydrocyclone is that under the condition of poor oil-water density, oily sewage enters hydrocyclone from tangential direction under the action of water pump or other external pressure, and then rotates at high speed. Under the action of centrifugal force, the water moves towards the wall of the hydrocyclone, forming a downward external vortex, which flows out through the outlet at the bottom of the hydrocyclone (underflow). The oil moves towards the axis of the cyclone, forming a spiral inner swirl oil core, which overflows (overflows) from the upper end, and finally realizes oil-water separation, as shown in Figure 2 [5, 6].
Cyclone separation technology is an effective technical means to save energy and reduce consumption in high water cut stage of oilfield. Hydrocyclone can remove free water from high water cut crude oil without heating, saving a lot of fuel. Offshore oil fields in Europe and America are widely used as pre-separators, while onshore oil fields are basically not used alone. At present, the development direction is mainly as front-end pretreatment combined with other technologies. Cyclone separation technology is still in the research and development stage in China and has not been widely used. The experiment of cyclone separation technology was carried out in Shengli Oilfield, and the experimental equipment combining cyclone and sedimentation was developed. Its working principle is that the oil-gas-water mixture enters the cyclone, and more than 90% of the associated gas is removed by centrifugal rotation separation and gravity. After secondary liquid removal, the gas and a small amount of gas in the water separator enter the gas system under pressure control, and the oil-water mixed liquid evenly enters the separation zone through the distribution pipe, and then enters the settlement zone through buffer rectification by the rectifying labyrinth plate. In the settling area, the activity of demulsifier is further stimulated by the heater, so that the emulsion is demulsified and separated, and the oil droplets coalesce and float. The dehydrated crude oil enters the oil chamber through the partition plate, and then flows out of the oil-water separator through the liquid level control. The key technologies of the test equipment are as follows: (1) A pre-separation cyclone is designed at the inlet end of the water separator, and more than 95% of the gas in the mixed liquid is pre-separated by the pre-separation technology; (2) The design of the distribution pipe and the rectifying labyrinth plate stabilizes the flow field in the high-efficiency water separator, which is beneficial to oil-water separation; (3) A heater is arranged in the water separator, which can not only stimulate the activity of demulsifier, but also avoid heating the bottom sewage; (4) The designed water level regulator can automatically adjust the oil-water interface in the separator, and the oil content of the treated sewage is basically around 500 mg/L. Jianghan Oilfield has carried out research on two-stage cyclone separation technology. Two cyclone separators are used in series, the first stage is used for pre-water separation and the second stage is used for oil removal. Through field test, the sewage separated by the primary cyclone in Ma56 Station of Mawangmiao Oilfield accounts for more than 50% of the total liquid volume, and the oil content of the sewage after deoiling by the secondary cyclone is below 100mg/L [7].
As a pre-separation equipment, hydrocyclone has the advantages of light weight, small floor space, large capacity per unit volume, high separation efficiency, fast separation speed, small investment, simple structure, no moving parts and convenient installation and maintenance. However, there are also many shortcomings, such as easy wear of cyclone tube, gas affecting separation effect, difficulty in separating crude oil emulsion due to lifting and cyclone, unstable effluent quality, high power consumption, effective separation of free water but basically separation of emulsified water, etc.
Three-terminal phase separation technology
The terminal phase separation pipe is a terminal gathering pipeline with an enlarged diameter, with a length of about 45m (depending on the characteristics of crude oil and the effect of pre-water separation) and a diameter of 1020 ~ 1220 mm. Both ends are blocked with spherical caps, which is mainly used for pre-water separation of crude oil and sewage purification in high water-cut oilfields. The terminal phase separation tube completes five processes of oil-gas-water separation in the tube (fluid hydraulic stirring, mass exchange, diffusion, gravity settlement and water drop aggregation in the coalescer), and has various device functions (I-stage separation device, pre-water separation device and pre-water purification device), which has been widely used in the former Soviet Union. Two terminal phase separation pipes with a diameter of 1020mm and a length of 250m are installed in the cluster well site or booster pump station of Tashkinovo Oilfield in western Siberia. The liquid treatment capacity is 30,000 ~ 32,000 m3/d, and 7800~9000m3 of free water can be separated every day, with the free water yield of 60%, while the water content of crude oil at the outlet is only 9.3% ~ 60.
The terminal phase separation tube can replace the expensive and numerous I-stage separation devices and dehydration devices in the oilfield supporting process, which can greatly reduce the investment (the total investment can be reduced by 25% ~ 40%). It has the characteristics of simple manufacturing, control and operation and large liquid handling capacity, and can be used as a pre-separator in small and remote oil fields, but its disadvantages are low separation efficiency and high oil content in the separated water.
4 Inclined tube pre-water separation technology
The working principle of inclined tube pre-separator is natural precipitation combined with shallow pool separation, which is mainly used to separate free water. In Europe and America, it is called elevation free dehydrator. It combines horizontal and vertical free water separators and adopts vertical design, which overcomes the shortcomings of small coverage area of oil-water interface of vertical container, short distance between oil-water interface of horizontal container and water outlet and insufficient separation time. The liquid inlet is located at the upstream end of the tubular container, and the oil beads in the water can gather and climb to the top oil outlet, while the water sinks to the bottom water outlet for discharge.
The inclined tube pre-separator has the advantages of simple structure, low cost and small floor space. It is mainly used for in-situ mixing of separated sewage from water-bearing oil fields with low oil content in separated water, so as to reduce the energy consumption of mixed water and pipeline investment of gathering and transportation system, and reduce the operating load of the joint station. Inclined tube pre-separator (diameter 1220mm, inclination angle about 45, liquid handling capacity 10000 ~ 15000 m3/d) is widely used in gathering and transporting Russian crude oil with high water cut and extra-high water cut, for removing 80% of free water. European and American countries have also developed and popularized this equipment, but the elevation angle of the inclined tube is designed at a lower angle of12 [8]. At present, the inclined tube pre-separator has not been widely used in China. Only used in Henan Oilfield 1 metering station, the quality of separated water cannot be controlled. The oil content of effluent is generally above 1000mg/L, and the separation efficiency is low.
5 low temperature demulsification technology
It is economical to use low temperature demulsification technology to pre-divide water. The acoustic demulsification equipment for crude oil developed in Canada can be installed on the oil gathering pipeline with the diameter less than 4 inches in high water-cut oil wells, so that the water content of the treated heavy oil can be reduced to 65438 0% at the minimum, and the amount of chemical agents can be saved by 50%. MST modular skid-mounted equipment for microwave demulsification in the United States has also been successfully tested in the field, with remarkable results [8].
In recent years, with the application of tertiary oil recovery technology such as polymer injection, the physical and chemical properties of produced fluid have changed greatly, and the emulsifying phenomenon is very serious, which makes the separation of pre-water more difficult. In order to make up for the shortage of mechanical methods, oil fields generally begin to pay attention to the comprehensive application of high-efficiency equipment and chemical additives, that is, on the basis of the original pre-water separation process, pre-dehydrating agent is added, so that a large amount of sewage can be effectively separated at low temperature and a small amount of chemical agents in the high water cut stage. H 1 crude oil has high viscosity, high oil content in sewage and serious emulsification, which is not convenient for mechanical pre-dehydration. By selecting an efficient pre-dehydrating agent, more than 80% of the sewage in the produced liquid can be pre-separated at the inlet temperature, and the oil content of the separated sewage is about 100mg/L, which can directly enter the sewage treatment system, saving a lot of natural gas and demulsifier, with little process change. Through a large number of laboratory tests, Liaohe Oilfield has developed a pre-dehydrating agent, optimized the process flow based on the original equipment, separated the free water without heating when entering the station, and then carried out subsequent treatment, thus canceling the first heating and saving a large number of demulsifiers, with obvious economic benefits. After the promotion of the whole company, it can save 40-50 million yuan in operating expenses every year.
With the introduction of chemicals, the cost of early water separation increases, and the difficulty of subsequent sewage treatment increases. How to seek advantages and avoid disadvantages needs further study.
6 Development direction of pre-water separation technology
At present, the pre-water separation technologies used in various oilfields have achieved the effect of pre-water separation to a certain extent, but the main control index of these technologies is the water content in crude oil, and there are few restrictions on the oil content in the separated water, resulting in the oil content in the separated sewage as high as1000 mg/L. Therefore, the sewage treatment system needs a complicated treatment process of removing oil once, and the secondary sedimentation and filtration make the sewage water quality meet the standard. Moreover, the sewage system occupies a lot of land, and the facility investment and operation costs are high. In the future, the pre-water separation technology will mainly develop in the following directions:
1) Accelerate the research and development and popularization of efficient oil-water separation equipment and separation technology.
2) When developing high-efficiency pre-separation equipment, more attention should be paid to the study of reducing the oil content index in separated sewage.
3) Develop in the direction of integration, integration, miniaturization, low investment and low cost of various technologies, such as the optimal integration of cyclone, air flotation, sedimentation and coalescence, and the comprehensive application of physical, chemical and biological methods, give full play to the advantages of different technologies and means, broaden the application scope of pre-water separation technology, and improve the stability and treatment effect of pre-water separation equipment.
Based on this, the author has developed a new integrated pre-separation oil removal technology. The technologies of cyclone, air flotation, coagulation and three-phase separation are comprehensively applied to organically combine the functions of pre-separated water and sewage oil removal to form an integrated device. At the same time of efficient pre-separation, the oil removal function of sewage is strengthened, the quality of effluent is improved, the oil content of effluent is reduced to below 15mg/L, the post-treatment process is simplified, and the investment and operation cost are reduced. At present, the research is progressing smoothly, the indoor test has achieved the expected effect, the field test is going on as planned, and the patent results are being declared.
refer to
[1] Hu Shijie, Li Shaowen, Yang Haiyan. Present situation and development trend of oilfield surface engineering in high water cut stage [J]. Pipeline technology and equipment, 20 1 1, 5: 5 1 ~ 53.
Hou Guihua. Application of energy-saving and consumption-reducing technology in crude oil dehydration station [J]. Petroleum Planning and Design, 2008, 19 (4): 4 1 ~ 43.
Niu Bin. Study on oil and gas gathering and transportation processing technology in high water cut period [J]. Journal of Shengli College of Youshi University, China, 2008,22 (4): 8 ~12.
Tang qingbo, money, HNS Efficient Three-phase Separator Technology [J]. Oil and Gas Field Surface Engineering, 2007,26 (6):16 ~17.
Zhang Jinsong, Feng Shuchu, Li Yuxing, et al. Study on flow mechanism and application of hydrocyclone in oil-water separation [J]. Filtration separation, 200 1,1(3):15 ~18.
Chen Jianling Optimal design of hydrocyclone control scheme for sewage treatment platform [J]. China Ship, 20 10/0,51(Supplement 2): 138 ~ 144.
Shicheng iron and steel company. Experimental study on application of two-stage cyclone separation process [J]. journal of jianghan petroleum university of staff and workers, 2005, 18 (2): 33 ~ 34.
Yang Shibang, Ye. Present situation and development trend of oil and gas field surface engineering technology [M]. Beijing: Petroleum Industry Press, 20 1 1: 43 ~ 44.
Zhang, Huang Yaoda, Ma Qiang. Study on application of pre-dehydration technology [J]. Inner Mongolia Petrochemical Company, 2008,18:10 ~11.