(Daqing Oilfield Li Shen Pump Co., Ltd. Heilongjiang Daqing 1633 1 1)

About the author: Dong Zhengang, male, born in June 1967, graduated from Daqing Petroleum Institute 199 1, and obtained a master's degree in engineering from Harbin Engineering University in 2002. Now he works in Li Shen Pump Co., Ltd. of Daqing Oilfield, mainly engaged in the design, development and application research of electric submersible pump, and is a senior engineer. E-mail: dongzhg @ powerlift.dq.cnpc.com.cn.

Based on the current situation of coalbed methane development in China, this paper analyzes the technical characteristics and advantages of submersible pump drainage gas production, puts forward several measures to improve the adaptability of submersible pump to coalbed methane exploitation, and introduces the application of submersible pump in coalbed methane exploitation in Jincheng, Shanxi. Based on the domestic technical status, the future research direction of submersible pump applied to coalbed methane mining is put forward.

Electric submersible pump; coal bed gas

Study on adaptability of electric submersible pump in coalbed methane development

Dong Zhengang, Deng Hui and Wang Huixian

(Daqing Oilfield Lisheng Pump Industry Co., Ltd., Daqing 1633 1 1)

Abstract: According to the present situation of coalbed methane development in China, this paper analyzes the technical characteristics and advantages of electric submersible pump in drainage and gas production, puts forward some measures to improve the adaptability of electric submersible pump in coalbed methane development, and introduces the application of electric submersible pump in coalbed methane development in Jincheng. Finally, the future improvement direction of submersible pump which has been well applied in CBP production practice is pointed out.

Keywords: electric submersible pump; Continental Ballistic Missile (continental ballistic missile)

introduce

As a kind of clean energy, coalbed methane has become an important energy source in China in 2 1 century. How to mine coalbed methane in a more reasonable and economical way is a common concern of people in the industry.

Submersible pump technology is a mature oil recovery technology in oil fields. After half a century's development, it has become an important oil production equipment in the petroleum industry. Especially through the continuous improvement of its own technology and the application of modern technology, its adaptability has been significantly improved, including technologies suitable for heavy oil, high gas content, high temperature, corrosion and sand control, which have made outstanding contributions to oilfield development.

The application of submersible pump in coalbed methane mining has a certain development history, especially in the United States for nearly 20 years. In the exploitation of coalbed methane, the determination and use of its working state and the control of oilfield production are very different due to different purposes. Simple technical grafting can't solve the technical problems in practical application. For example, submersible pumps suitable for coalbed methane are generally small displacement centrifugal pumps, which have poor adaptability to gas. When a large amount of free gas enters the centrifugal pump, cavitation or air lock will occur, which will lead to the pump not working normally or even being damaged. Only by effectively solving the problem of air cavity or air lock can it be solved.

Working principle of 1 submersible pump and its technical characteristics in coalbed methane mining

Submersible pump is a kind of multistage centrifugal pump, and its working principle is the same as that of ordinary centrifugal pump on the ground. Compared with pumping unit and screw pump equipment, it has the characteristics of large displacement, and can be changed in a larger displacement range through frequency conversion. Figure 1 shows the characteristic curve of centrifugal pump. This curve reflects the relationship between centrifugal pump displacement and head, pump efficiency and shaft power.

Figure 1 centrifugal pump characteristic curve

Under the condition of frequency conversion, centrifugal pump has the following characteristics:

Technical progress in exploration, development and utilization of coalbed methane in China: Proceedings of the 2006 symposium on coalbed methane.

Where: Q 1, Q2-pump displacement at different frequencies; F 1, F2- frequency.

Technical progress in exploration, development and utilization of coalbed methane in China: Proceedings of the 2006 symposium on coalbed methane.

Where: H 1, H2- pump head at different frequencies.

Technical progress in exploration, development and utilization of coalbed methane in China: Proceedings of the 2006 symposium on coalbed methane.

Where: N 1, N2-pump head at different frequencies.

η2=η 1 (4)

Where: η 1 and η 2-pump efficiency at different frequencies.

Using the inherent characteristics of centrifugal pump, the process of water drainage and gas production can be flexibly controlled. For example, in the early stage of water drainage and gas production, a large amount of well fluid needs to be discharged from the well to facilitate gas production. At this time, a suitable centrifugal pump can be matched according to the displacement demand, and the pump can be pumped quickly by using the characteristics of large displacement of centrifugal pump, and the liquid production can be adjusted by frequency conversion of frequency converter. At this time, the centrifugal pump mainly works in area B. When the well fluid reaches a certain level of dynamic fluid, the casing forms a certain casing pressure to meet the production requirements. At this time, it is necessary to maintain a stable dynamic balance, and the liquid output of the pump drops greatly, generally from 0 to 65438+1100m3/d, which belongs to the normal production period of coalbed methane. Due to the small displacement during normal production, the centrifugal pump actually works in the A area of the characteristic curve, and its lift reaches the highest. During the normal production of coalbed methane, according to the needs of production scheduling, the height of dynamic liquid level can be adjusted by frequency conversion to achieve a new dynamic balance.

Although submersible pump, pumping unit and screw pump can all be used for drainage and gas production, in contrast, submersible pump has incomparable characteristics compared with pumping unit and screw pump:

(1) Through frequency conversion, large-displacement drainage can be realized, with quick effect and no harm to the pump itself; Pumping units and screw pumps have limited pumping capacity, and their reliability will be greatly affected if they are set too high.

(2) It can automatically maintain the height of dynamic liquid level to prevent evacuation. In other words, the set dynamic liquid level height is the critical point to ensure that the pump reaches the lifting capacity limit at this critical point. When the height of the dynamic liquid level is higher than the critical point, the pump will automatically increase the liquid production, improve the pumping capacity and reduce the dynamic liquid level; When the dynamic liquid level approaches the critical point, the lifting capacity of the pump reaches the maximum, and the liquid level will not drop further. As both the oil well pump and the screw pump are positive displacement pumps, they have no automatic adjustment function and can only be realized by manually adjusting parameters. Once evacuation occurs, the unit will be damaged quickly.

(3) Due to its own characteristics, the centrifugal pump is very convenient to manage. According to the data of the well and the characteristic curve of the pump, the parameters can be directly adjusted by theoretical calculation, and the remote control system can realize direct control without intermediate debugging and testing.

(4) Because the centrifugal pump is driven by submersible motor and has no transmission rod, it can be applied to drainage and gas production in inclined and horizontal wells. Pumping units and screw pumps have great limitations in deviated wells and horizontal wells because they are rod-driven.

2 several measures to improve the adaptability of submersible pump to coalbed methane mining

Submersible pumps for coalbed methane are generally small displacement centrifugal pumps, which have poor adaptability to free gas. Therefore, solving the adaptability of centrifugal pump to gas is the key to ensure the application of submersible pump.

(1) Deepen the pump hanging, and lower the centrifugal pump below the gas producing layer to reduce the influence of gas on the centrifugal pump, as shown in Figure 2. Due to the different specific gravity of gas and liquid, the gas-liquid mixture from the gas-producing layer moves upward and enters the annulus of casing; The liquid enters the centrifugal pump downwards, and the gas quantity entering the centrifugal pump is reduced by using the specific gravity relationship. Deepening pump hanging technology is a mature oil recovery technology in oil fields, which has been widely used in gas-bearing oil wells and achieved obvious application results. For pocket wells, deepening the pump hanger is not conducive to motor heat dissipation. If the radial space of the casing is enough, adding a guide cover can solve the heat dissipation problem of the motor. The deepening pump hanging technology is generally not suitable for submersible pumps operating in highly deviated wells or horizontal wells. The main reason is that there is not enough height difference to ensure the effective separation of gas-liquid mixture.

Fig. 2 Schematic diagram of deepening pump hanging of the unit

(2) Adopt high-efficiency gas-liquid separator. Gas-liquid separator, called oil-gas separator in petroleum industry, is a device designed for gas-bearing oil wells and installed at the inlet end of centrifugal pump. In the production process, the gas-liquid mixture first enters the oil-gas separator, and the separated gas is discharged into the annular space of the shell through the gas-liquid separation of the oil-gas separator, and the separated liquid is transported to the pump inlet to reduce the influence of free gas on the centrifugal pump. The data show that the gas-liquid separator can separate 35% of the total volume of gas-liquid two phases, and the separation efficiency can reach over 90%. For gas wells with high water cut, because gas is the main phase, the gas-liquid separator can't completely separate gas, and the distance between the suction port and the discharge port of the gas-liquid separator is close, generally less than 1m, which may not achieve obvious results. The pilot test of coalbed methane in Panhe, Jincheng, Shanxi Province also verified this point. If a guide cover is added and the distance between the suction port and the discharge port of the gas-liquid separator is increased, the effect will be obvious. High efficiency gas-liquid separator is generally not suitable for submersible pumps operating in highly deviated wells or horizontal wells. The main reason is that there is not enough height difference between the suction port and the discharge port of the separator, which is easy to form the self-circulation of free gas, as shown in Figure 3.

(3) Combination pump matching mode is adopted. Different types of centrifugal pumps have different adaptability to gas. Pure centrifugal pump has poor adaptability to gas and is easy to form air lock. Mixed-flow centrifugal pump has good adaptability to air and is not easy to form air lock. Generally speaking, the impeller of a pump with a small displacement (such as below 250m3/d) is a centrifugal impeller; For large displacement pumps, the impeller is a mixed-flow impeller. The matching method of combined pump is to combine centrifugal impeller and mixed-flow impeller, and make use of the strong adaptability of mixed-flow impeller to air to ensure that the whole pump will not produce air lock and keep working continuously. We applied this technology in the pilot test area of coalbed methane in Panhe, Jincheng, Shanxi, and achieved obvious application results.

Fig. 3 Free gas self-circulation process

(4) Adopt special gas processor. Gas processor is a new technology developed in recent years. It is installed between the gas-liquid separator and the centrifugal pump, and can also be used alone instead of the gas-liquid separator. The installed impeller is an axial impeller with strong cavitation resistance. When a large amount of gas enters the gas processor, it will not form air pockets or air locks, and strongly push the gas-liquid mixture to continue to run upwards and enter the centrifugal pump. Although the gas-liquid mixture may form air pockets or air locks after entering the centrifugal pump, the air lock section will be strongly pushed up by the gas processor until the centrifugal pump is pushed out, thus ensuring the normal operation of the whole machine.

Application of submersible pump in coalbed methane mining in Jincheng, Shanxi Province

Since June 2005 10, four electric submersible pumps have been put into operation in the pilot test area of Panhe coalbed methane in Jincheng, Shanxi Province. See table 1 for specific applications.

Table 1 Usage of ESP in Shanxi Jincheng Coalbed Methane Pilot Test Area

Note: The deadline for statistics is June 26th, 2006.

From the application situation, the effect of simply using high-efficiency gas-liquid separator is not as good as adding combined pump. Take Well PH46-02 as an example. When the well was first started, the unit operated well and produced normal fluid. After running for a period of time or the liquid level drops to about 240 meters, no more liquid is produced. At this time, the unit is still running, the measured three DC resistances and voltages are completely normal, and the current is basically the same as when starting. At the same time, the liquid level began to rise, and the casing pressure gradually decreased, and the casing pressure decreased to 0. 1 MPa within1hour. If the machine is stopped and restarted, the above actions will be repeated. This symptom shows that although the gas-liquid separator is used, the residual gas after separation still causes the gas lock on the pump, while the combined pump technology is used in wells PH 1-009 and PH 1-008, so there is no gas lock problem.

4 Future research direction

In view of the particularity of coalbed methane drainage, the application of submersible pump is still in the primary stage of understanding, and improving the reliability and economy of submersible pump application is an important topic for future development.

(1) For gas wells with shallow coal seams and good drainage and production conditions, simplified design should be given priority to. Taking the coalbed methane in Jincheng, Shanxi Province as an example, the well depth is generally about 400m and the well temperature is about 20℃. The whole set of technical equipment using electric submersible pump is expensive and the structure is too complicated.

A. The motor power is low, generally around 15kW, and the protector used can meet the use requirements of 60kW, so the motor protection device can be simplified, and even the integrated design of the motor and the protection device can be realized, which not only meets the use, but also reduces the cost;

B. Reduce the temperature resistance grade of the unit and save the material cost. Because the well depth of oil field is generally above 1000m and the formation temperature is high, the designed motor can meet the requirements of well temperature above 120℃, and the material cost is relatively high. For low-temperature wells of coalbed methane, the temperature resistance grade of the unit can still meet the requirements when it is reduced to 60℃;

C. research suitable low-cost submersible cables. At present, submersible cable is suitable for high temperature, high pressure and high insulation performance, and it has high cost and poor economy when used for shallow coalbed methane mining.

(2) For gas wells with deep coal seam and poor downhole conditions, the design should focus on improving the performance of the unit. Taking the exploitation of coalbed methane in Liaohe as an example, the well depth 1800m or more, the well temperature is high, the normal liquid production rate 1 ~ 3m3/d, and the drainage condition is worse than that of oil field, so there is no good solution at present.

(3) Carry out research on gas production by drainage in deviated wells and horizontal wells of ESP. Because the deepening pump hanging technology and gas-liquid separator technology can not fully adapt to drainage and gas production in inclined wells and horizontal wells, optimizing the combination of other technologies, such as submersible electric screw pump technology, is the future research direction.

(4) Research on the application technology of submersible pump drainage gas production. Different geological conditions have different requirements for equipment, such as sand content and corrosion degree, which will affect the service life of equipment to varying degrees. Therefore, according to different geological conditions and drainage technologies, the maximum benefit can be achieved by reasonable matching.

refer to

Mei Sijie, Shao Yongshi, Liu Jun, Shi Shigang. 2004. ESP Technology, Beijing: Petroleum Industry Press, 60, 1 18.