PUMP-OFF CONTROL AS A MATURE TECHNOLOGY, By: Alfred Majek
The level of technology offered by Pump-Off Controllers has reached a point of maturity that justifies application in most areas implementing artificial lift with beam pumps. The Cost savings offered by POCs is thoroughly proven, and economic analysis usually indicates system pay out within one to two years. In addition, equipment has been operating in a number of environments worldwide for over a decade.
Pump-off Controllers are micro-processor-based devices capable of autonomous operation, and are primarily applied in conjunction with sucker rod pumps. Controllers monitor for pump down conditions, defined as the state when fluid in a reservoir is insufficient to warrant continued pumping. When fluid falls below a certain level, pumps are shut down until those levels are restored. Allowing pumps to run only when sufficient fluid is available for lift provides clear advantages, including efficient use of Energy, minimal maintenance costs and optimum production.
The methods used by pump-off controller to attain those advantages vary, and most units use one or a combination of a number of system variables. Units can be configured to monitor:
- Production flow;
- Motor amperage draw;
- Well load only, which is detected by using strain gauges; and
- Load versus position, which combines strain gauge sensors with beam position monitoring devices to allow traditional dynagraph card creation.
Figure A illustrates typical techniques used to monitor these variables. Advanced POCs support provisions for telemetry to a master computer station, enabling all well parameters and control functions to be handled remotely. Furthermore, a single controller can be employed to control one or a number of wells at any one time.
One major oil company has been involved with computerized pump off control work since the late 1970's. Employing a variety of methods in the Permian Basin in West Texas through the years, fields have realized noticeable success utilizing a system based solely on load and position. Both single and multiple-well devices using a combination of radio and hard-wired cable communications have been installed. Figure B depicts a composite field application.
Load MonitoringLoad can be derived by two types of strain gauge transducers (or Load Cells) mounted either on beams or polish rods. Load Cells mounted on walking beam indirectly determine load through beam stress, while polish rod version measure the weight of the rod strings and fluid columns. Beam-mounted cells suffer minimum damage from well workover crews, since cable can be routed out of the way and cells do not have to be removed.
The constant movement of the beam has little or no effect. On the downside, temperature effects on the walking beam can be very pronounced, placing the burden of adjustment on the electronic package. Also, because the fundamental technique is indirect, it is inherently less accurate than a direct approach.
Polish Rod-mounted cells, on one hand, have to be removed during workover, require running cables somewhat obtrusively from the frame, and are under the constant grinding, sometimes pounding, action of the girdle. On the other hand, superior accuracy permits eventual processing by sophisticated computer analysis programs. Either type of cell functions suitably for simple pump-off control activity.
The position of the beam can be derived by using a proximity switch to monitor counterweights, a resistive pot mounted underneath the I beam, or an inclinometer mounted directly to the walking beam. Proximity switches are sufficient for low cost controllers, although POC systems based on proximity switches tend to assume that upstrokes and downstrokes are of equal duration. Of course, that is rarely the case.
Resistive pots have limited lifetimes, frequently rated at 1,000,000 rotations. For a rod pump running at six strokes a minute, resistive pot failure can occur in as little as four months. Inclinometers, while costing slightly more, have a virtually unlimited lifetime, which generally justifies the added expense.