A substantial cost for operating most oil properties is the monthly electricity charge. Analyzing the motor current in a beam pumping system is a difficult task in that the current is normally changing from consumption to generation during each pump stroke. A gear "slap" will occur during a pump cycle when the motor changes from driving the pumping system to being driven by the counterbalance and the unit's inertial forces. Conventional clamp-on current meters indicate the magnitude of the current flow in the wire but do not indicate whether the motor is consuming or generating electricity.
A new computerized technique for acquiring motor current using a clamp-on probe permits the acquisition of apparent current and the calculation of real (or active) current. The apparent current is acquired at a one kilohertz rate. These data are processed to obtain real current. A plot of the active current (showing consumption and generation) along with the apparent current is presented to the user. This plot is extremely beneficial for determining electricity costs, pumping unit balance, cost per barrel of oil lifted and other power consumption factors.
The effort to reduce lifting costs in beam pumping operations has focussed the attention of production personnel to reduction of electrical power consumption. A recent study shows that properly operated beam pumping systems provide very good efficiencies in the order of 1.1 to 1.4 kw-hr/bbl when lifting fluid from 4300 to 6700 ft respectively. Equivalent submersible pumping systems exhibited efficiencies of the order of 1.8 to 2.5 kw-hr/bl. This study shows that when properly designed and operated, the beam pumping systems exhibit overall efficiencies of the order of 57%, where overall efficiency was defined as the ratio of the power out of the system (based on net depth of lift, fluid gravity and production rate) to the power into the system as measured kw-hr.
Unfortunately a large number of pumping systems are not properly designed nor properly operated to achieve such a high efficiency. One of the reasons stems from the fact that it is not customary to routinely take measurements of electrical power consumption of individual beam pumping units. The one electrical measurement that is commonly made in the oil field is motor current, using a standard clamp-on amp probe, with the objective of adjusting the unit's counterbalance. Normally current data is displayed at a rate of one reading per second and corresponds to the average of the apparent motor current over the last second.
The majority of operators do not have readily available means of determining the true electrical power consumption of a given beam pump and thus cannot establish whether it is operating efficiently or the motor is oversized or even if the unit is properly balanced.
The principal objective of the system described in this paper is to provide a simple, inexpensive and acceptably accurate means to determine the electrical performance of a beam pumping system based only on the measurement of motor current using the conventional clamp-on probe which is part of the standard equipment provided with the Well Analyzer system. The Well Analyzer is a PC-based data acquisition and analysis package that allows automatic measurement of annular fluid level, calculation of producing bottom hole pressure, measurement of polished rod dynamometer data and calculation of downhole pump dynamometers and includes the measurement of motor current and power.
The cyclic nature of the pumping system and the variable loading which is dependent on the mechanics and efficiency of the down-hole pump and sucker rod string, result in a continuously variable current flow over a pumping cycle. This translates in the fact that averaged values of apparent current are not indicative of the actual power usage and requirements. Moreover to reduce torque requirements, counterbalancing of the rod load plus one half of the fluid load is commonly used and most installations exhibit torque reversals during the pumping cycle even if properly balanced. This means that during portions of a pump stroke the prime mover drives the gear box and that during other portions the gearbox drives the motor. In the first case the motor is using electrical power, in the second case it is generating electricity. The most common indication that this reversal in current flow is taking place is the widely observed "gearbox backlash".