Pumps & Systems, April 2008
As oil prices and gasoline usage have increased, the demands on U.S. oil and gas wells have grown dramatically. In the past, there was a low cost supply from the Middle East, and U.S. oil wells were not used as frequently due to low reservoirs and the energy cost associated with extracting the oil. A large number of U.S. wells were shut down in reaction to high electric costs, low oil prices, the need for monitoring the oil reservoir and machine maintenance. As of 2004, there were just over 800,000 onshore oil wells in the world, with about two-thirds of them in the United States.
With the increase in oil prices, companies are now reactivating wells and finding new technology is helping monitor and control their use and productivity.
The first advancement in technology is the variable frequency drive (VFD). A VFD is a solid-state device used to control the speed of a motor or pump. Since most U.S. oil wells are low pressure, oil must be mechanically pumped out of the ground with a beam pump. When there are abundant oil reserves in the ground, the pumps run almost 100 percent of the time. However, when wells have a low reservoir, pumps can only run a portion of that time, producing a limited amount of oil.
A VFD helps create an uninterrupted flow of oil, reduces downtime and lowers costs. The drive creates smooth starts and stops, which reduces energy costs compared to using a direct online (DOL) starter. In addition, the VFD reduces mechanical stress associated with DOL starting, creating more reliable pump operation and fewer equipment failures. The pump motor can also run at a reduced speed, extracting 100 percent oil on each stroke.
If a DOL system was used on a well that was not producing enough oil to match the pump's speed, timers would be added to control how long the pump would need to run in order to obtain 100 percent efficiency, a process called pump-off control. Turning the pump on and off can be avoided by reducing the speed of the pump with a VFD.
The problem with using timers or running a VFD at a fixed speed is matching the settings to the pump reservoir. If the pump runs too much or too fast, it wastes energy and incurs excessive mechanical stress. Conversely, if the pump runs too little or too slowly, not enough oil will be pumped. Keeping the pump running at an optimal speed would necessitate constant monitoring, which is not only expensive, but also difficult to do since the conditions of the well change over time.
In the ideal situation, the electrical system monitors the oil extraction and then matches the pump's speed via a load cell mounted on the well rod. Today's pump-off controller monitors the amount of oil being extracted, and then uses those measurements to match the speed of the motor/pump to the reservoir. One approach is to use a programmable logic controller (PLC) card that mounts directly onto the VFD and provides this information. The card not only provides the control needed but also reduces the enclosure space required.
A second advancement in beam pump technology is the elimination of dynamic braking resistors or an active/regenitive front end on the VFD. Each time the pump changes direction, it actually creates energy instead of consuming electricity to drive the load. The concept is similar to a hoist on a crane. It requires energy to push the load up, but on the way down the load creates energy and needs to be slowed down. For pumps, the energy that is created can either be dissipated in resistors as lost heat, or go back onto the AC line through an active front end on the VFD. Both of these solutions add cost and associated maintenance to the system.
Another solution includes using a VFD with an Energy Adaptation System (ENA). Some VFDs offer ENA as a standard programming feature. The setting allows the motor/pump to overspeed as it changes directions. An overspeed ensures that the VFD does not trip while maintaining a constant speed on each machine cycle.