Electrical submersible pumps
The design of a submersible pumping unit, under most conditions, is not a difficult task, especially if reliable data are available. Although, if the information, especially that related to the well’s capacity, is poor, the design will usually be important. Bad data often result in a misapplied pump and costly operation. A misapplied pump may operate outside the recommended range, overload or underload the motor, or ruin the well at a rapid rate, which may result in formation damage. On the other extreme, the pump may not be large enough to provide the desired production rate.
Too often data from other wells in the same field or in a nearby area are used, assuming that wells from the same producing horizon will have similar characteristics. Unfortunately for the engineer sizing the submersible installations, oil wells are much like fingerprints, that is no two are quite alike.
The actual selection procedure can vary significantly depending on the well fluid properties. The three major types of ESP applications are:
- High water cut wells producing fresh water or brine;
- Wells with multi-phase flow;
- Wells producing highly viscous fluids.
The performance of a centrifugal pump is considerably affected by the gas. As long as the gas remains in solution, the pump behaves normally as if pumping a liquid of low density. However, the pump starts producing lower than the normal as the gas-to-liquid ratio (at pumping conditions) increases beyond a certain “critical” value (usually about 10-15%). It is mainly due to the separation of the liquid and gas phases and a slippage between the two phases.
This phenomenon has not been well studied and there is no general correlation describing the effect of free gas on pump performance. A submersible pump is usually selected by assuming no slippage between the two phases.
Ideally, a well would be produced with a submergence pressure above the bubble point pressure to keep any gases in solution at the pump intake. This is typically not possible, so the gases must be separated from other fluids prior to pump intake to achieve maximum system efficiency.
TEXT 2
FLUID INJECTION
The practice of injecting gas and water underground has become an important part of oil-producing operations. The injection of gas into formation increases recovery of oil, and sometimes results in a saving of natural gas. Water is injected into underground formations for two reasons. One is to dispose salt water that is produced with oil. The other is to increase the amount of oil recovered by injecting water into a producing formation.
Injection of gas requires the use of compressors to raise the pressure of the gas so that it will go into the wells, which take the gas to the producing formation. Water can sometimes be injected into an underground formation by gravity, but in many cases a pump is necessary in order to inject water at the rate desired. Pumping is usually required where water is injected into a producing formation for a water-flood or pressure-maintenance program.
For water flood projects water is injected into certain wells to flush oil from the reservoir rock. In some cases, oil recovery may exceed by a large factor that realized during original production.
Thermal processes include the concept of burning some of the oil in a reservoir.
Wasteful at first glance, the process actually uses some of the oil that could not be produced by any known method as an energy source to increase the total amount of oil recovered. Other thermal methods use either heated water or steam.
EXERCISE 10 Answer questions about the texts.
1. In what cases the design of a submersible pump can be important?
2. What does a misapplied pump result in?
3. What is the most common mistake made when reviewing well data?
4. What does the actual pump selection procedure depend upon?
5. What are the three major types of ESP applications?
6. What does considerably affect ESP performance?
7. What is the ideal condition for a well production?
8. What are the reasons of water injection into underground?
9. What are compressors used for?
10.What do other thermal processes include?
EXERCISE 11 Give the Russian equivalents to the following word combinations
1. Bubble point
2. Fluid
3. Submersible
4. Injection
5. Maintenance
6. Water injection
7. Pressure
8. Thermal processes
9. Oil-producing operations
10. Producing formation
11. Water-flood
12. Energy source
EXERCISE 12 Are the following statements true or false? Correct the false ones with the right information and discuss your answers with a partner.
1. The design of a submersible pumping unit, under most conditions, is not a difficult task, especially if reliable data are available.
2. Too often data from other wells in the same field aren’t used, assuming that wells from the same producing horizon will have similar characteristics.
3. As long as the gas remains in solution, the pump behaves normally as if pumping a liquid of low density.
4. The injection of gas into formation reduces recovery of oil, and results in a saving of natural gas.
6. Injection of gas requires the use of compressors to raise the pressure of the gas so that it will go into the wells, which take the gas to the producing formation.
EXERCISE 13 Make up sentences out of two parts from the table:
| 1. A misapplied pump may operate outside the recommended range, overload or | a) the gas-to-liquid ratio increases beyond a certain “critical” value. |
| 2. Unfortunately for the engineer sizing the submersible installations, oil wells are | b) a producing formation for a water-flood or pressure-maintenance program. |
| 3. The pump starts producing lower than the normal as | c) to dispose salt water that is produced with oil. |
| 4. Water is injected into underground formations for two reasons. The first reason is | d) underload the motor, or ruin the well at a rapid rate, which may result in formation damage. |
| 5. Water can sometimes be injected into | e) some of the oil that could not be produced by any known method as an energy source to increase the total amount of oil recovered. |
| 6. Pumping is usually required where water is injected into | f) much like fingerprints, that is no two are quite alike. |
| 7. Wasteful is the process actually uses | g) an underground formation by gravity, but in many cases a pump is necessary in order to inject water at the rate desired. |
EXERCISE 14 Translate into English in written form.
1. Закачка газа в пласт увеличивает добычу нефти, а иногда приводит к экономии природного газа.
2. Закачка газа требует использования компрессоров для повышения давления газа таким образом, чтобы он вошел в скважины, которые принимают газ в продуктивный пласт.
3. Вода закачивается в подземные формации по двум причинам: расположение соленой воды, которая производится с нефтью, увеличение количества нефти, извлекаемой при впрыскивании воды в продуктивный пласт.
4. Вода может иногда быть введена в подземную формацию под действием силы тяжести, но во многих случаях насос необходим для того, чтобы впрыскивать воду в желаемом количестве.
5. Практика закачки газа и воды под землю стало важной частью нефтедобывающих операций.
CHAPTER VIII