Tests on wire-rope pumping units in two oil fields in China showed that these units eliminated sucker-connection problems, decreased labor requirements, increased effective plunger displacement and pump efficiency, lengthened well repair cycle, and decreased loads on the hanger.
The units have widespread potential because China has about 90,000 oil wells and 9,000 wells are completed annually. Of these, 90% need artificial lift for oil recovery.
According to test data from the Qinghai and Zhongyuan oil fields, four wells received a Yuan 27.83 million (about $3.3 million) direct economic benefit from the wire-rope installation.
History
A joint program between E.I. du Pont de Nemours & Co. and Bethlehem Steel Corp. to develop wire-rope pumping units began in the 1960s.1 In one test, the companies installed a 0.75-in. OD strand made of 37, 0.08-in. wires in a 2,800-ft deep well in 1965. A nylon coating protected the wire from corrosion.
Another report on wire rope replacing sucker rods was from Russia.2 In this case, toward the end of 1970s, a closed-type wire rope was used with an appropriate three-series differential deep-well pump, developed for the application. It was reported that one installation worked effectively for more than 2 months in a well with the pump setting depth at 1,018 m.
In another application, the Russia Academy of Petroleum Scientific Research tested three galvanized wire ropes with different structures and a polymer coating during a 9-month test.
In China, the fundamental research on wire rope replacing sucker rods started at the University of Petroleum in the early 1990s. After 1996, in cooperation with the Qinghai and Zhongyuan oil fields and several iron and steel works, research started on using a full-locked coil rope in the oil field.
In 1997, the wire rope was successfully run to a setting depth of 1,450 m in Well No. 16-6, Gasikule oil field. The well is still in operation and its output and pump efficiency have increased.
In Well No. Pu 1-221, Zhongyuan oil field, the wire rope setting depth was 2,250 m. The well has worked for over 18 months of operation, and the wire rope is still functioning.
To date, flexible wire rope has been installed in 20 wells in six Chinese oil fields.
Operating results
Data from two wells in the Qinghai and Zhongyuan oil fields indicate the potential benefits of wire rope in pumping wells (Table 1).
Pump depths in the 20 installations have been 1,215-2,300 m, with two wells having greater than 11/2 years of run time. Pump diameters are 57 mm.
Compared to sucker-rod installation in the same well, the wire rope with equivalent diameters has offered the following benefits:
- Increased output and pump efficiency.
- Decreased maximum load on landing.
- Decreased crank torque and power consumption.
- Increased system efficiency.
At the same time, problems with sucker rod connections and rod parts were eliminated, and wear between wire rope and tubing was less than with sucker rods. There was almost no wear on the surface of the wire rope pulled out of the well.
Furthermore, the wire rope reduced liquid flow resistance and paraffin deposition.
Average pump efficiency with the wire rope in Well No. 16-6 was 96.5%, a 31.45% increase over sucker rods.
For Well No. 17-27, wire rope initially had a 52.7% average efficiency. But after the well was operated, average pump efficiency increased to 92.17%.
The pump efficiency also increased in Well No. Pu 1-221. It went from 47.7% before wire rope to 81% after installation of wire rope. Likewise, pump efficiency in Well No. Pu 51-99 increased from 48% to 71.4%.
Fluid production also increased. In Well No. 16-6, it increased to 65.37 tonnes/day from 37.88 tonnes/day, while in Well 17-27 fluid production increased to 65.37 tonnes/day from 37.88 tonnes/day.
Another benefit was the reduced power consumption. In Well No. 17-27, before the wire rope, the three-phase current during the up and down stroke was 57.6-39.5 amp, 59.3-42.1 amp, and 56.2-39.2 amp. The maximum average current was 57.7 amps.
After installation of the wire rope, the current readings on the up and down stroke were 51-39 amp, 52-40.6 amp, and 50-37 amp. The maximum average current was 51 amp, and the maximum input current of the well pumping unit motors decreased 6.7 amp.
For Well No. Pu 1-221, before wire rope, the input motor current at the up and down stroke was 34 amp and 32 amp. After the wire rope was installed, the current decreased to 24 amp and 22 amp. The maximum input current of the well pumping unit motors decreased to 10 amp.
For a well in the Qinghai oil field, the pump is usually repaired twice per year. But in Well No. 16-6, since wire rope was installed in September 1997, the pump lasted 30 months until March 2000. The repair cycle was greatly lengthened and repair costs were less.
Cost savings
In a well, sucker rods commonly part once per year. Repair operations cost about Yuan 15,000/event. Because wire rope reduced repairs by 2.5 times, costs were reduced by Yuan 37,500/year.
In Zhongyuan oil field, typical well repairs are need 1.5 times/year, with each repair costing about Yuan 28,000 each. Wire ropes, therefore, can save about Yuan 56,000/year.
Wire rope has no connector sub that can cause a piston effect. Also because the specific gravity is less, the wire rope load on landing can be effectively reduced, and motor input current is decreased.
Assuming a decrease of 6.7 amp, one can reduce power consumption by 3.75 kw. If electricity cost Yuan 0.35/kw-hr, a well can save Yuan 11,497/year. With four wells, this is a savings of Yuan 45,900/year.
Another cost savings is the trip time to make a repair. Normal trip time with wire rope is only 40-70 min, thus reducing labor costs.
Because the wire rope rod carries almost no paraffin, one does not need steam equipment for removing the wax in wells with this problem.
Incremental oil production in wells with wire rope also has been greater. To the end of August 1999, Well No. 16-6 production was 27.49 tonnes/day more than before installation of wire rope. Over 24 months, the increased incremental production is about 20,068 tonnes.
Likewise, Well No. 17-27 has increased its production by 18.45 tonnes/day after the installation of the wire rope. Total incremental production for 74 days is 1,365 tonnes.
In Zhongyuan oil field to the end of August 1999, incremental production from the two wells was 600 tonnes. Assuming a crude oil price is Yuan 1,257/tonne, the economic benefit is Yuan 27.69 million.
With added electric and operating cost saving to the incremental oil gained, the total benefit of wire rope for the two wells is Yuan 27.83 million.
Analysis of benefits
The wire rope's density is 18.5% less than a steel sucker rod with the same diameter. As a result, the maximum load on landing can be reduced by about 18.5%. Measured values in the oil field indicate a 20-30% reduction.
Because the difference between the maximum and the minimum load is less, crank torque is decreased, resulting in lower power costs for the pumping unit.
The wire rope's elasticity modulus is 28% less than for a steel sucker rod, 150 gigapascals vs. 210 gigapascals.
As sucker rods travel, they vibrate and their acceleration changes. This will displace the pump plunger travel on the up and down stroke. For example at the top dead center, the sucker rod will continue to go upwards because of inertia. Likewise, at the lower dead center, the sucker rod will continue to go downward because of inertia.
This causes the sucker rods to travel further and even result in an overstroke phenomena. The actual distance a sucker rod can increase is 5% under static loading and 15% under dynamic loading.
As shown in Well No. Pu 1-221, Zhongyuan Oil field, the wire rope stroke is longer than that for steel sucker rods, resulting in a greater stroke efficiency. This is another reason why wire-rope pumping units have more output and higher pump efficiency than steel sucker rod units.
Wire rope has a smooth surface and uses few connections. This reduces the piston-effect phenomenon and paraffin problems on the rope surface. Therefore, there is far less fluid resistance, resulting in a reduced maximum load on the landing, decreased torque, and less power consumption.
Wire rope is flexible and avoids hard collisions and hard contact. Therefore, its resistance to motion is far less than that of steel sucker rods. This also reduces the maximum load on landing, decreases torque, and lessens power consumption.
Main problem
Although wire rope is starting to be used in oil fields, some problems persist:
- Wire-rope connections sometimes loosen.
- Improved stroke efficiency is difficult to attain because of the lack of a swab parameter for the wire rope.
- Wire-rope protective coatings can fail because of deficiencies in the adhesion strength between the coating and wire rope. With a failed coating, a prolong exposure in oil can result in fractures in the wire rope.
References
- Joy, R.F., and Coleman, G.H., "Flexible Sucker Rod in the Works," OGJ, July 15, 1968, pp. 138-40.
- Ivanovski, V.N., "Possible applications of cables used for continuous pumping rods," Machines and Oil Equipment, (in Russian) 1981.
The authors
Wang Kuisheng is an associate professor and dean of the department of mechanical and electronic engineering at the University of Petroleum, Beijing. He has a PhD from the University of Petroleum in China.
Zhang Siwei is a professor of mechanical engineering and tribology at the University of Petroleum in Beijing. His major interests include oil field equipment, friction, and tribology of polymers. Zhang Siwei is a graduate of the Beijing Petroleum Institute. He is also the vice-president of the China Petroleum & Petrochemical Engineering Industry Association and the vice-president of the Beijing Society of Petroleum.
