BEAM PUMPS SURPASS ESP EFFICIENCY

May 18, 1992
James F. Lea Amoco Production Research Co. Tulsa J. D. Minissale Amoco Production Co. Houston A study showed that beam pumps, for the range of depths and rates studied, were always more energy efficient than electric submersible pumps (ESPs). In fact, the ESPs had lower efficiency and the beam pumps were more efficient than expected. The data are from a West Texas field. The study's object was to find a production rate above which EPSs are more energy efficient and below which beam pumps
James F. Lea
Amoco Production Research Co.
Tulsa

J. D. Minissale
Amoco Production Co.
Houston

A study showed that beam pumps, for the range of depths and rates studied, were always more energy efficient than electric submersible pumps (ESPs).

In fact, the ESPs had lower efficiency and the beam pumps were more efficient than expected.

The data are from a West Texas field. The study's object was to find a production rate above which EPSs are more energy efficient and below which beam pumps are more efficient.

Many types of formulas are in use with varying degrees of sophistication or approximation.

This study reviewed several equations for which the literature contained measured data for calculating the efficiency of artificial lift installations.

To assure good comparisons of artificial lift installations, the user should be sure that the data are for an efficiency with the same definition.

Relatively low efficiencies, in the mid 30s%, were measured in this study for installations that were believed to be operating properly.

EFFICIENCY

Efficiency of an artificial lift installation is the useful power transmitted to the produced fluid, divided by the power of the electrical energy being supplied to the prime mover. This definition applies to electric motor prime movers.

However, the output can be taken at the pump output, as it frequently is by electric submersible pump manufacturers, or at the surface after power is lost due to friction in the tubing.

Both definitions are nearly the same when losses from the pump to the surface are low. However, when comparing types of lift by efficiency comparisons, a good practice is to ensure that the calculated values use the same efficiency formula.

It is suggested that all artificial lift installations have frequent collection of the few data pieces necessary to calculate the installation's efficiency.

This efficiency number might be used to trouble shoot the well, compare the efficiency to other wells using different types of lift, or compare wells using similar types of lift to see what efficiencies are possible and what might be done to tune wells to near the possible peak efficiency.

LITERATURE DEFINITIONS

Many excellent papers refer to system efficiency of an artificial lift installation. However, as will be seen, there are many definitions, each differing somewhat in the actual definition or in the simplifications used to arrive at the final form of the expression.

Efficiency from measured data is the ratio of useful system output, Ho divided by the input to the motor times 100.

The input horsepower to the motor (or to the cable and motor) will be taken as the input kilowatts (kw) divided by 0.746, or kw/0.746.

The differences in expressions for efficiency will then depend on what is selected as the definition of the useful system output of the artificial lift installation.

See book for equation (1)

Another definition, from Moncrief,1 calculates the system output to the fluid as:

See book for equation (2)

(In Reference 1, Equation 2 is a misprint, depth should be removed.)

where:

Q = Flow rate, b/d

Note: Q should be the volumetric flow rate through the pump although most references use the production, or the production times a formation volume factor.

Ho is referred to as hydraulic hp, hhp.

DELTA P = Pressure increase across the pump, psi

Equation 2 is incorrect due to a misprint and should read:

See book for equation (3)

or in terms of lift:

See book for equation (4)

where:

Lift = Net lift, ft.

Lift is sometimes approximated by depth.

In Equation 4, lift does not include a height to account for friction, then Ho would be at the surface and not at the pump discharge.

Reference 2 presents the following for the output of the system where Ho is called hhp.

See book for equation (5)

where:

SG Specific gravity of fluid

FG Fluid gradient, psi/ft (flowing or static?)

Equation 5 is approximately correct, but has unnecessary terms in it. Ho at pump discharge is just Q x DELTA P times a constant to obtain the appropriate units.

The DELTA P across the pump is determined by the type of fluids flowing, and no additional specific gravities or gradients need be in the formula. Essentially, Equation 5 should reduce to Equation 3.

Gault 3 uses the following expression for Ho referred to as hhp.

See book for equation (6)

where:

Net lift Depth, ft, as used by Gault, which becomes Equation 4.

The net lift used in Gault's paper does not include the tubing friction. In other words, friction has been subtracted out before using the formula.

Because friction is not included before calculating Ho, Equation 6 is finding Ho at the surface and not at the pump discharge.

Again, if the friction is low, this does not matter.

Also, although Gault used the depth as the net lift for calculations, it is usually reduced by the fluid in the annulus. Gault did his calculations at near pumped off conditions.

Equation 6 also is for a fluid with a specific gravity, SG = 1.

Clegg 4 gives the following expression for Ho, referred to as hhp:

See book for equation (7)

where:

L = Net lift (surface to fluid level), ft.

In Equation 7, the lift is modified by a specific gravity, so that a water gradient is not implied. However, the fluid in the annulus usually has a different specific gravity than the tubing.

Therefore, there is an approximation if only one SG is used.

Also, the formula by Clegg does not include friction, so this becomes an equation for the output at the surface and not at the pump discharge. However, approximations in the equation are probably better than the usual quality of the data entered into the efficiency equation.

Butlin 5 gives the formula for Ho (called hhp) as follows:

See book for equation (8)

Equation 8 is the same as Equation 3 presented by Gault, therefore, the comments are the same.

Kilgore and Tripp 6 give the following expression for Ho (hhp):

See book for equation (9)

where:

H = Height of the fluid. In this article it was taken at pump conditions so that no height in the annulus was needed, ft.

Q = Flow through the pump accounted for by the authors, gpm.

Because the flow rate Q is in gpm and not b/d, then this looks different than previous equations. But if Q is in b/d, then it is the same as Equation 7.

Because Equation 9 is the same as Equation 7, then the discussion is the same. However, there is a question from reading Reference 6 because it says that the Ho was derived from gradients in the tubing.

This implies that the gradient included friction. However, an SG and height H imply that friction is not included in the final expression. The previous discussion shows that the efficiency (from measured data) of artificial lift takes many forms in the literature.

Some of the forms were developed for specific situations, such as when the well is completely pumped off. Other forms differ only since they have quantities input in different units, or are in terms of different variables such as heights or pressures with/without a specific gravity, etc. Other differences occur in how the pressure in the annulus is treated.

PREFERRED EFFICIENCY

Reference 7, page 90, lists the pump power out as:

See book for equation (10)

where:

Po = Pump power output, hp. Ho is used as Po in preceding discussion.

Q = Flow rate through the ump, gpm.

If one uses Equation 10 as a starting point of the definition of the output of an artificial lift installation, then the pump discharge would be chosen as of the efficiency definition where the H,, includes the horsepower necessary to overcome friction effects in tubing, etc.

This has been expanded in oil field units in Fig. 1 to show how to calculate the efficiency of a pumping unit, as per Equation 10.

However, two wells on artificial lift devices (ESP, Beam pump, Moyno, etc.) could have equal efficiency even if one well had correctly sized tubing for the flow rate such that the friction pressure drop is low, and the other well had a too small tubing diameter that causes a large friction drop.

Both wells would have the same efficiency at the pump discharge because the ratio of power out to power in would be the same.

However after reaching the surface, the well with the small tubing would have losses greater than the well with the correctly sized tubing.

Therefore, the entire system, including the tubing, would have a lower efficiency if there were substantial tubing losses. This is essentially what one obtains from calculating a discharge pressure with a height and a specific gravity instead of a flowing gradient.

The example of subtracting out the tubing friction is also shown in Fig. 1.

For many cases in the oil industry, the efficiencies as calculated in Fig. 1 are nearly equal because the friction is low. However if friction is high in the tubing, then the calculation at the surface will be noticeably lower.

When comparing different artificial lift, after calculating the efficiency at the pump discharge, one should examine the effects of friction separately to be sure that there are no excess friction drops.

Another way is to calculate both efficiencies and if they are close, then the friction is low.

Regardless, when comparing artificial lift efficiencies, especially from one method of lift to another, it is wise to be sure that one is using the same efficiency definition before the comparisons are made.

MEASURING EFFICIENCIES

In a study on a West Texas property, the initial intent was to determine approximately at what producing rates efficiencies for ESPs become better than the efficiencies for beam pump installations.

Beam pumping and ESP system efficiencies were calculated by determining the operating conditions where the installation of an ESP would be more economical than a beam pumping unit.

Based on measured electrical power usage over the production ranges considered (500-900 b/d), a beam pumping system in this study always used less electric power per barrel of fluid produced than an ESP.

Electrical power usage for ESPs and beam pumping units were compared on a per barrel of fluid produced basis. This comparison was made using measured electric power usage from six wells (three ESPs and three beam pumping units) on two different properties (Property A and Property B), for a total of 12 wells.

All of the beam-pumped wells were equipped with pump-off controllers even though two of the wells ran 24 hr/day. Electric power measurements were collected for the six beam pumping units over a time period that ranged from 3 to 7 days using kw-hr meters. ESP power usage was collected over a time period that ranged from 4 hr to 1 week.

Power readings were converted to kw-hr/hr terms.

Existing test facilities were used to measure oil and water production rates. Gas production from the test wells was low, generally ranging from 5 to 30 Mcfd, and was not recorded.

Well fluid levels taken during the test period were averaged. From the pump set depth and average fluid level, the net producing depth was determined.

The average overall beam pumping unit efficiency was about 57%, while the average overall ESP efficiency was about 34%.

Efficiencies were determined by dividing the power out of the system by the power into the system.

The power out of the system is the horsepower over time calculated by the net depth, fluid specific gravity, and production rate.

The power into the system is the electric power usage (measured kw-hr) divided by time.

An effort was made to select test well locations where the lift equipment was properly applied. Details of the operating conditions of the wells in Property A and Property B are listed in Tables 1 and 2.

Based on pump volumetric efficiencies, the rod pumps appear to be in good condition. ESP pump test data available indicated that the ESPs were operating near the published performance specifications.

From the field measurements collected in this particular study, a beam pumping unit will use approximately 40% less electric power per barrel of fluid lifted than an ESP.

Table 3 contains electrical operating expense projections for ESPs and beam pumping units based on these measurements. Using a cost for electric power of $0.03/kw-hr and a net pumping depth of 4,500 ft, electrical power savings of approximately $5,000/year can be achieved by using a beam pumping unit instead of an ESP at a 600 b/d producing rate. Electric power measurements for beam pumping units and ESPs were also available from a previous field study for wells with producing depths of 5,000 and 6,700 ft (Properties C and D).

Power consumption, in terms of kw-hr/bbl of fluid produced, for each net producing depth (Properties A, B, C, and D) were plotted and the relationship between net depth and kwhr/bbl of fluid produced was established. This relationship is show in Fig. 2.

The relationship between output power (100% efficiency) and depth is also shown in Fig. 2. As can be seen from this graph, beam pumping system power usage is considerably lower than ESP system power usage at all producing depths.

In Fig. 2, the efficiency is calculated at the pump discharge. Also, based on the slope of the graphs, system efficiencies remain relatively constant with changes in producing depth.

The ESP installations showed efficiencies in the mid 30% range and the beam pump installations were in the high 50%. The beam units were in the high end of ranges reported by Reference 6 although Reference 4 indicates a beam efficiency of 61% for a "typical good installation."

However, Reference 4 reports a good installation for ESPs to be in the high 40%.

REFERENCES

1. Moncrief, J.B., "Efforts to Conserve Energy in Field Operations," SPE Paper No. 6878, 52nd Annual Fall Conference and Exhibition, Denver, Oct. 9-12, 1977.

2. Neely, A.B., Gipson, F., Clegg, J., Capps, W., and Wilson, P., "Selection of Artificial Lift Method," SPE Paper No. 10337, 58th Annual Fall Technical Conference and Exhibition, San Antonio, Oct. 5-7, 1981.

3. Gault, R.H., "Designing an Energy Efficient Sucker Rod Pumping System," 32nd Southwestern Petroleum Short Course, Lubbock, Tex., Apr. 23-25, 1985.

4. Clegg, J.D., "Rod Pumping Selection and Design," proceedings of the 38th Southwestern Petroleum Short Course, Lubbock, Tex., Apr. 17-18, 1991.

5. Butlin, D.M., "A Comparison of Beam and Submersible Pumps in Small Cased Wells," SPE Paper No. 21692, Production Operations Symposium, Oklahoma City, Apr. 7-9, 1991.

6. Kilgore, J.J., and Tripp, H. A., "Walking Beam Pumping Unit System Efficiency Measurements," SPE Paper No. 77788, 68th Annual Technical Conference and Exhibition, Dallas, Oct. 6-9, 1991.

7. Hydraulic Institute Standards for Centrifugal, Rotary and Reciprocating Pumps, Hydraulics Institute, 14th Ed., Cleveland, Ohio, 1983.

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