Progressive cavity pumps or PCPs are commonly used either installed downhole or on the surface to pump highly viscous mixtures. This could either be lifting heavy crude laden with sand, dewatering of the coal bed methane, or pumping potash slurry from the potash mine. The ability to handle solids and highly viscous fluids makes it an ideal method of pumping with very low capital and operating costs and high efficiencies in the range of 25% to 95%.
The progressive cavity pump consists of a rotor and stator, the rotor rotates while the stator remains stationary. PCP is best described as a gear pump with a single helical rotor which rotates inside an internal double-helical elastomer lined stator.
One of the important things about the efficiency of the PCP is that it is dependent on the initial interference fit of the rotor and stator, temperature, rotational speed, and the properties of the formation fluid. For instance, water cut (EC), gas-to-oil ratio (GOR), and fluid density. The elastomer that lines the stator is the heart of a PCP and it influences the run life and performance.
Progressive cavity pumps are tested on a test bench to determine the efficiency of the pump before it is commissioned. The elastomer in the stator will swell when it comes in contact with the formation fluid, the amount of that swell is based on the properties of the formation fluid so it is critical to determine the “non-swell efficiency” of the pump, in doing so the pump must be tested at the temperature it will encounter while in service which is same as the temperature downhole.
The test setup is shown in figure 1. The test setup operates in a closed loop where an electric motor is connected to the view gearbox to a shaft which in turn is connected with the rotor of the PCP. A load cell water tank is used to measure the volume of water pumped by the PCP. The volume of water pumped for each test cycle will be used to determine the efficiency of the PCP. The main body of the test setup houses the 600-litre reservoir where the water is heated by the immersion heater.
A charge pump is installed at the back side of the test setup. This feeds water from the main reservoir through a 3-inch hose into the PCP. The fluid pumped by the PCP is forced through a choke valve which will restrict the flow and set the required differential pressure at the pump. This is a simulation of the head or the pressure that is required to pump the fluid to the surface.
The main body houses a 600-litre tank in which a 12 KW immersion heater is installed to heat up the test fluid to the test temperature. The immersion heater is installed on the base of the tank and is a 3-phase heater with 480 V supply and insertion length of 36 inches. Assuming room temperature of 20 °C and final Test temperature of 40 °C.
Power to heat 600 litres of water = P =4.2 X L X T
L= Amount of test fluid which is water = 600 litres
T=Difference between final and initial temperature= 20°C
P= 14 KW
Heater element rating = 12 KW
Dividing P with the heater element rating will give us the time it will take to heat the water i.e. 1.16 hrs.
Which presents the opportunity to optimize the size of heating elements to reduce the time to heat the test fluid.
As mentioned earlier, PCP efficiency is affected by the changes in the temperature whether on the surface or downhole. To quantify the change two PCPs of the same lift and capacity were tested on the same bench using the same immersion heater. The tests were performed at 100 RPM and 200 RPM. Figure 2 shows the pump efficiency comparison of the two pumps, Pump A and Pump B at 200 RPM. Pump A was tested at 30 ͦC while pump B was tested at 40 ͦC. Both Pump A and B are equal pumps with the same capacity.
The efficiency increases from 21% at 13500 kPa to 30% at the same pressure when temperature increases from 30 ͦC to 40 ͦC which quantifies that downhole temperature variation of 10 ͦC will increase the efficiency of the pump by 10% as well. The efficiency of the pump physically manifests in terms of the capacity of the pump or the production that can be achieved at the desired temperature.
One implication of this quantification is for the Steam assisted gravity drainage process (SAGD) where steam is injected in the formation to reduce the viscosity of the fluid (heavy oil and tar sands). By quantifying the effect of temperature on the PCP one can increase the efficiency of the pump by injecting steam.
The immersion heater provides the necessary heat transfer to the test fluid to simulate the downhole temperature that the PCP will encounter. Thus, accurate logging of the downhole temperature through temperature gauges or wireline logging tools will provide temperature that PCP will encounter in service and that can determine the required “non-swell efficiency” of the system. The “non-swell efficiency” will play a crucial part in the run time and performance of the PCP.
However, temperature is not the only factor that can increase the efficiency of the system, Other factors include increasing the speed of rotation of the rotor but both factors increasing speed and increasing temperature have effects on the run time of the PCP. PCPs are limited in terms of maximum temperature that elastomers can handle before it starts degrading. The immersion heater should be sized based on the maximum temperature that elastomer in the PCP stator is rated for and the size of the reservoir in the main body of the test bench.
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