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Application and Practice of Medium-voltage VFD in Water Injection System of Oilfield
Introduction
Injecting water into oil beds through water injection wells is an important developing measure widely used for ensuring the oil bed pressure and increasing oil-extracting speed and extraction rate of oil storage. In the water injection works of oilfields, water-injecting pumps are the source equipment for water injection of oilfield and ensuring oil bed pressure. To ensure meeting the water injection need of oil bed for relative long time period, the selected capacity of water injection pumps in design is rather high usually. In practical production, the daily-injected water quantity is a parameter with rather high fluctuation. Especially in high-pressure water injection system, there is rather high pressure-difference between the pumping pressure of water injection pumps and the pressure of water injection pipelines. The water injection pressure of water injection pipeline network must be ensured through controlling the outlet valves. This leads to a large quantity of electric energy is wasted and it is also disadvantageous to the operation of pumps for rather high pressure. The subject we study always is how to improve the efficiency of water injection and reduce the energy losses with the prerequisite to ensure distributive water injection of oilfield. In recent years, the breaking out in medium-voltage high power VFD technology in the country has created a condition for applying AC VFD to high pressure water injection systems. The VFD parameters are set according to the pressure needed for water injection system and regulate the injected water quantity. It saves a large quantity of electric energy and reduces the losses of the pumps. This has very great influence on decreasing the oilfield operating and production costs.

Water Injection Technology for Oilfield
There are five water injection stations in Shenyang Oilfield with a total of 16 high-pressure water injection pumps installed and a total capacity of 23,400kW. At present, 4-5 water injection pumps operate with an operating capacity of 7,900kW. To adapt the water quantity variation and realize water injecting reasonably, the operation mode of the water injection pumps need to be adjusted continuously. Under condition of no speed regulators, the flow can only be controlled through switching on/off the pumps or adjusting the valves manually. This must lead to increase the energy losses of the water injection system. Take the second oil-extracting job zone as an example to perform analysis:

The water injection work for the second oil-extracting job zone is undertaken by the Shenyang Second Water Injection Station and Shenyang Forth Water Injection Station commonly, which are connected in network to operate. The length of the main pipeline of the network is about 8km. The total injected water quantity varies from 8,200 m3/day to 9,700 m3/day, with the amplitude of fluctuation ranging from +15% to - 20%. Two high-pressure water injection pumps (1,800 kW) operate in this pipeline network normally and the injected water quantity is 250m3/h (Shenyang Second Water Injection Station) and 200m3/h respectively (Shenyang Forth Water Injection Station). The average pressure at the main line of the pipeline network is 16.9mPa and the pump pressure is 18.9mPa, thus there is a pressure difference of 2mPa. The injected water quantity varies rather widely for day and night and the original equipment can only operate at constant speed, so injected water quantity can only be regulated with the valve opening. The resistance of the pipeline network is changed artificially and the pipe losses are increased, leading to quite a lot energy wasting on the valves and leads to high water consumption. Economic operation is impossible and the worker's task increases. Adjusting artificially will make the pressure of the pipeline network too high or too low, affecting production technology and the safe operation of equipment. Therefore, if VFD are added to the water injection pump sets, the energy losses caused by the pressure difference between pumps and pipelines will be reduced considerably, even will be eliminated. In addition, with the VFD, through the optimizing and simulating, the water injection pumps may operate according to an optimized principal and the flow and pressure of the water injection pipeline network may be regulated according to the optimized principal. The throttling loses between the water distributors and at the well opening, thus making the whole water injection system operating in an area of high efficiency and energy-saving.

Reform Scheme of Water Injection System
1. Energy Saving Principle for Water Pump Operating with VFD
When water pumps operate with constant speed at industry frequency, the operation of the pumps can only be adjusted with varying the outlet valves in order to ensure the pressure and flow in the pipeline network as well as the motors not overloaded simultaneously. When the water pumps are adjusted by closing the valves, it operates under the working condition of small flow and high pumping level (comparing with the original working condition), the pressure difference between the pump and the pipeline will be increased, so will the energy losses be done, as shown in Figure 1. Provided that when the water pump operates at working point B, the outlet valve opens fully, the out flow is QB and the pumping level is HB. If the injected water quantity is to be reduced, decreasing to QA. When no regulating device is added, the out water flow can only be kept to QA by closing the valve and the power losses QA (HA -HB) will generate at the outlet valve. Thus, not only a large quantity of electric energy is wasted, but also the outlet valve is hit by the flow for long time, leading to failure increasing and life shortened.

Figure 2
Q-H curve
Q-∑h curve
Measured Q-∑h curve of adjusting mechanism
Figure 3
Q-H curve
Q-∑h curve
Measured Q-∑h curve of adjusting mechanism
The typical operating curve of the pump is shown in Figure 2. AB is the characteristic curve of the pump and it is matched to the pressure curve QB. The normal flow under the normal pressure may be obtained at point B, at which the pump has the highest
efficiency. Closing the valve may control the flow. When the flow decreases, the pump operates at point P, Q, R and S respectively and the pump will operate under very high pressure-difference. Therefore, the output energy of the pump is much higher than the energy actually needed by the system. The surplus energy is consumed at the valve in the form of heat and brought out by the water flow. The efficiency of the pump divided by the efficiency of the pump is the total input energy of pump. Controlling and reducing the flow with adjusting the valve makes the energy losses rather high. With VFD, the pump can operate under a different speed, and the characteristic curve of the pump is matched to the need of the system under condition of any flow. The flow is proportional to the motor speed and the pressure difference is also proportional to the square of speed, thus a set of curves may be obtained, as shown in Figure 3. Adopting continuous VFD regulation, countless characteristic curves of the pump AB~CD may be obtained. Any cross point in the shadow area formed by the pipe characteristic curve and ABCD may become the working condition point, adapting to variation of the water network flow and the flow corresponding to P, Q, R and S may be obtained with lower pumping level, the energy loss is very low and effect of energy-saving is obvious.
2. Equipment Reform & Operation
After repeatedly demonstrating, based on operation of water injection system of the second job zone and the practical condition of the site, it is decided to use the medium-voltage VFD of Model HARSVERT-A06/220 made by Beijing Leader & Harvest for the driving motor for the water injecting pump of Shenyang Forth Water Injection Station.

The HARSVERT-A06/220 medium-voltage VFD began to install on July 24, 2003 and was put into formal operation on August 1. At the beginning, it operated in open-loop mode, i.e. through observing the variation of the injected water quantity, the frequency for the motor was adjusted to modify the motor speed. Different flow and pressure varying point were collected and operation experience was accumulated. On August 27, 2003, it was turned to closed-loop operation. Through collecting the pressure signals of the main line of the pipeline network, tracking the variation of pressure of the pipeline network and adjusting the motor speed, automatic control is performed. From the operation for two months, it is known that the medium-voltage VFD of Model HARSVERT-A06/220 has a broad variety of protective functions, such as overload/under-load of DC bus, failures of driving power units, transformer temperature alarming, breakdown of UPS, illegal opening of cabinet door, especially, the alarming function for upper/lower limits of pressure and flow, which can prevent the gas corrosion of the pump from too low flow to ensure the safe operation of the equipment. Through the interface of the control computer of the equipment, the waveforms of the input/output voltage/current may be observed directly, at the same time, the instant variation and the accumulated data of the water injecting flow, the electricity consumption of the water pump and the actual pressure of the pipeline network may be collected at real time for the working personnel at site performing analysis and management of energy saving easily.

Analysis for Energy Saving & Economic Effects
We have made a statistics for the main parameters, electricity consumption and injected water quantity of the daily operation of the water injection pump and the motor of 1,800kW of the Shenyang Forth Water Injection Station in August and September 2003 and then made a comparison and analysis. The conclusion was as follows:
1. Direct Economic Effects
1) The electricity consumption for each cubic meter of injected water is decreased from 7.87kWh/m3 of the same period of last year to 6.33 kWh/m3 at present Figure 4. Comparison of Electricity Consumption for Each Cubic Meter of Injected Water for September 2002 – 2003, Shenyang Forth Water Injection Stati

Unit consumption
Date
Electricity consumption for each cubic meter of injected water for September 2002, Shenyang Forth Water Injection Station
Electricity consumption for each cubic meter of injected water for September 2003, Shenyang Forth Water Injection Station
On September 23, 3-hour system downtime for replacing operating system
2) Calculating according to annual water injection quantity of 1,500,000 m3, electricity price 0.46 rmb/kWh, annual operation time 700 hours, the annual saved electricity is 2,310,000kWh and the saved electricity costs 1,062,600 rmb
2. Indirect Economic Effects
1) When direct starting with industry frequency of 50Hz, the shock to the electric network and the mechanical shock of the motor are rather high and the sound is high too, it is estimated that the loss is Ws=0.5J(1+r1/r2)Tm/(Tm-TL). The square moment characteristics of the load of the centrifugal pump is partly similar to the mechanical characteristic curve when the asynchronous motor starts, and it may be calculated at Tm/(Tm-TL)= 1. For soft starting with VF, the loss is very low, just 1/10 of the above Ws. The energy saving for starting for every year is also considerable.
2)When using VF speed regulation, the power factor approaches to 1 at 50Hz full load. The operating current is much lower than the normal current of the motor because the inside filtering capacitor of the VFD has the function to improve the power factor and may save the capacity of the electric network.
3)After using VF speed regulation, the vibration caused from the pump operation deviates the normal operation zone too much for using valve controlling. The vibration may lead to breaking of the extended axle of the pump for serious condition. Since slow starting and speed reducing, the life of many parts and components, such as seals and bearings, is extended correspondingly, especially the mechanical damage of the starting torque to the motor is decreased, extending the operating life of the motor efficiently. The costs for maintenance are saved considerably.

Conclusions
1. From the operation of medium-voltage VFD in the water injection system of oilfields and the actual energy saving effects, the reform for the water injection pumps with VFD has achieved the predicated effects. The purposes for reducing electricity consumption for each cubic meter of injected water and improving the technology have been achieved. It is proved that the reform is successful.
2. Through operating practice, it is found that the energy-saving effects of the medium-voltage VFD in the system are directly related to the pressure in the pipeline network and the opening of the valve of the pump with fixed speed of the Shenyang Second Water Injection Station. It is necessary to try to find out the operation rule, looking for a optimal working condition point to bring the performance of the medium-voltage VFD into full play and develop more potential for energy saving.
3. If the pump with fixed speed of the Shenyang Second Water Injection Station is reformed to a pump with varying speed and the speed of the two pumps are both regulated, the comprehensive energy-saving effects will be better.
4. Through the analysis for the electricity consumption for each cubic meter of injected water for the present operation, all the investment for the reform with medium-voltage high-power VFD may be returned in two years.

Brief Introduction of the Author:
Cai Guoqing, born in September 1965, Engineer of Shenyang Oil Production Plant of CNPC's Liaohe Oilfield, engaged in the management of electric power system and development of energy saving equipment mainly.

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