Modifications improve beam-pump performance

An experimental program, which made electrical and mechanical modifications to beam-type oil pumping units, increased the efficiency and performance of the units.

This U.S. Department of Energy (DOE) motor challenge showcase demonstration project involved four organizations:

OXY USA Inc. hosted the project by providing five oil wells at its Bemis oil field in Ellis County, Kansas.
DynCorp Corp. was responsible for data analysis.
Midwest Energy Inc., the local electric utility, contributed electrical metering, personnel, and some funding.
Center for Energy Studies at Wichita State University provided technical consultation during the project and co-authored a report with DynCorp-An Investigation of Methods for Reducing the Cost of Pumping Oil in Kansas. The study covers overall oil production in Kansas and includes recommendations on how to reduce pumping costs.

The Kansas Corporation Commission (KCC), the state's utility regulatory body, funded the study with DOE providing some additional funding.

Project scope

According to the study, each beam-type pumping unit (Fig. 1 [73,437 bytes]) included an electric motor (Fig. 2 [12,304 bytes]), belt drive, gear reducer, crank arm with counter weight, walking beam, horsehead, sucker rod, and underground pump. The units normally operate at a fixed speed 24 hr/day, 365 days/year and are usually only shutdown for maintenance or because of operational problems.

Shut-off valves in the discharge line from the wellhead are the only controls on these pumping units.

Total average demand (average apparent power) and annual energy consumption of the five wells were 765,624 kva and 445,884 kw-hr respectively, for a total annual operating cost of $28,165.

To identify modifications that could improve the operational efficiencies, the team ran a series of tests.

OXY USA performed dynamometer tests on all five wells, measured pump speed, fluid level, stroke length at both the surface and pump, and pump size.

On each well, OXY USA kept daily logs of total liquid flow rate, measured with totalizing flow meters.

In addition, Midwest Energy took short-term electrical power measurements using power analyzers and loggers, and long-term electrical power measurements using utility power meters.

OXY USA supplied oil fraction and pumping speed statistics. Several months worth of data establish the motor system's capability with respect to the well requirements.

Electrical modifications included:

Checking the service conductor sizing and losses
Replacing one well's motor with a smaller-sized unit to better match the motor system's capability with the well requirements
Adding capacitors to correct the power factor.

Mechanical modifications included:

Inspecting and lubricating gearboxes and bearings and replacing worn parts
Dynamic balancing of the unit
Inspecting and tightening belts and replacing worn belts
Inspecting and adjusting the seal of the packing head
Adjusting stroke length.

Project implementation

After analyzing the five wells, the team made the following electrical and mechanical modifications:

Well B4-Lubricated gearbox, inspected bearings, balanced beam pump, installed and tensioned new matched set of drive belts, adjusted pump stroke, greased and serviced beam pumping unit, and installed secondary capacitors
Well B15-Replaced the oversized 30 hp, 480 v, three-phase NEMA D motor with a similar 10-hp unit, reduced pumping speed from 10.6 to 8.75 spm, greased and serviced beam pumping unit, and installed secondary capacitors
Well B19-Repaired high-resistance connection in one phase of the motor-control center and installed secondary capacitors
Well B20-Installed secondary capacitors
Well B21-Balanced beam pump, installed and tensioned new matched set of drive belts, adjusted pump stroke, greased and serviced beam pumping unit, and inspected bearings.

Results

The report indicates that because the operating point of each well changed during the evaluation period, the results obtained were normalized to provide a comparison between cases where the well output was the same. Also, one common observation was that all five of these wells exhibited a steady deterioration in well performance over time.

According to the study, because Wells B19 and B20 did not undergo any physical modifications at the well head, they were suitable for quantifying the magnitude of this long-term decrease in performance. Thus, the impact of the mechanical and electrical modifications was determined after the team factored out the effect of well deterioration over time.

Based on data taken over several months, the report noted that the mechanical modifications yielded varying degrees of improvement from well to well.

The greatest decrease in energy consumption occurred in Well B15, where the installation of a new, smaller motor and other mechanical modifications resulted in a 21% decrease in electricity usage. In all cases, the addition of secondary capacitors significantly improved the power factor, increasing it from an average of 0.58 to 0.76. This decreased demand represented more than half of the cost savings realized.

The study indicates that after using the adjusted measurements, the five wells showed decreases in energy demand ranging from 24 to 40%. In addition, the wells that underwent modifications beyond the installation of secondary capacitors realized a drop in energy consumption ranging from 13 to 21%.

The project's total annual cost savings of $5,362 were derived from reduced demand charges, which fell 32%, and reduced energy costs, which fell 12%.

The report indicates that the oil industry considers simple paybacks of 12-18 months for small companies and 2-3 years for large companies economically viable. Therefore, the project's simple payback of 6.5 months demonstrates that the well modifications are economically worthwhile.

Because OXY USA has a program to perform routine maintenance, the report estimated that the oil wells studied are more efficient than the average well. Thus, implementing a program of this nature to the general oil well population can be expected to yield much greater savings than in this project.

The report concluded that for mature wells, where about 1% of the fluid pumped is oil and the rest is brackish water, this represented a savings of 8 kw-hr (about $0.32)/bbl of oil pumped. The total savings in both energy and demand charges amounted to about $0.77/bbl of oil pumped.

In addition to the electrical cost savings of this project, according to the report, the project provided several other benefits for Oxy USA such as helping to prevent potential equipment failure by detecting problems before they were serious enough to cause downtime.

For example, the high-resistance connection in Well B19 was discovered during these tests. Furthermore, the varying energy measurements of specific wells provided OXY USA with data it can use to examine the potential causes of low efficiency found in some wells.

This information can help determine if a change in well operation is mandated or if a well has merely reached the end of its useful life.

Lessons learned

The report indicates that the project provided several practical lessons that can be applied to future analyses at OXY's Bemis oil field and elsewhere.

First, using the smallest motor that enables the pump to operate can significantly improve oil well efficiency.

Second, the total cost savings are a function of both energy saved and reduced demand. The value associated with each is a function of a utility's rate structure.

In this study, a large part of the savings was due to a reduction in demand charges. For situations where the power factor penalty is large, additional attempts to correct the power factor even closer to 1.0 may be economical.

Finally, the report concluded that measuring liquid level in the well, along with power and flow, ensures that the correct elevation level is used when calculating the minimum energy required for pumping. Further analysis using this parameter can help determine if specific pumps are over or underpumping.

Table 1 [31,787 bytes] summarizes the performance improvement seen during the project.