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Home / Issue Archive / 2008 / October #10 / Submersible Pump Motor Drives Get New Piston Seal Sections

№ 10 (October 2008)

Submersible Pump Motor Drives Get New Piston Seal Sections

Today oil companies most often use electric submersible pumps like ESP and ESPCP for production purposes.

By Ivan Pyatov, Vitaly Nikolaev

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Operational capacity of the pumps’ submersible electric motors (ESP motors) largely depends on the operation of the seal section. The seal section’s functions include the following: equalizing the pressure between formation fluid and electric motor cage, protecting motor oil from external contamination, and compensating for thermal variations of the oil volume. Today, the open-type single-casing seal sections with elastomer bags, which are hydraulically connected with formation fluid, are used most frequently.
However, the amount of wells with complicated production conditions is growing each year. These problems include high gas-oil ratio (over 50 percent by volume), high temperature of the formation fluid (over 120 C), as well as the increasing unit suspension depth (exceeding 2,000 meters). Also, to produce viscous oil, technologies for thermal treatment of the productive beds, with SEM ambient temperature exceeding 200 C, are employed more and more often. Under these conditions, elastomer bags often prove to be the most important element in the seal section design.
With regard to bag seal section, gas permeability of elastomer bags is one of the most topical problems.In this connection, manufacturers of submersible equipment started considering alternative seal section designs (without bag): labyrinth, metal-bellows, and piston types. The piston-type seal sections appear to be most efficient for the purposes of ESP motors thermal stress reduction since they make it possible to effectively use the protector casing surface to chill down motor oil.
However, the earlier designs of seal sections with piston elements for pressure compensation have certain drawbacks, which prevent their wider application. Mostly, it refers to the mechanical damage of piston seals caused by abrasive wear resulting from penetration of the formation fluid, which contains solids, into the sealing area. Corrosion or scale deposition on the working surfaces of leading cylinders also may be spoken of as one of the major drawbacks.
Another problem encountered in application of these piston systems is a possible piston jamming caused by the seal section casing deformation in cases of too small radius of the well curvature.
Provided the mentioned drawbacks are eliminated, the piston system of the seal section may be considered the most efficient due to:
– possibility to better chill down the ESP motors oil due to direct contact of oil and the seal section casing, which is chilled down by the formation fluid;
– immunity to gas-oil ratio;
– possibility to carry out operations in directional and horizontal wells;
– reasonable price;
– serviceability.
Oil&Gas Eurasia and Eurasia Offshore (March 2007 Issue, p. 6-8, and August 2007 Issue 48-50, respectively) have already published articles on the latest achievements of REAM-RTI, which is the leading Russian manufacturer of elastomer diaphragms.
Its product line includes heat-resistant diaphragms (capable to resist heat up to 230 ). The company has developed a new design for the seal section piston module to enable ESP motors serviceability at temperatures up to 300 . The new design was developed to conform to Tomskneft-VNK’s specifications and subsequently tested in Tomskneft-VNK’s wells at Strezhevoi. Borets Company assisted in testing.
The new design’s main features include:
– efficient protection of the working surfaces, along which piston seals are moving, against the influence of formation fluid, salt deposition and solids;
– higher sensitivity of the internal pressure control system;
– possibility to operate the piston system at the motor oil temperatures exceeding 230 ;
– guaranteed operating time of the seal section module not less than 1,000 days;
– serviceability in case of the equipment unscheduled pull-out.
Testing of the new piston module was enabled by new technologies used in Borets-manufactured PBP92 seal section. The PBP92 piston seal section is based on the successful PB92 diaphragm seal section design, but the diaphragm has been replaced with the piston module designed and patented by REAM-RTI (Fig. 1).
Following the REAM-RTI order, the Borets plant manufactured a batch of PBP92 piston seal sections rated for oil temperatures up to 170 . This batch of PBP92 seal sections with piston modules successfully underwent check operating tests in conformity with Tomskneft-VNK program.
The seal sections were used in the wells of the Vakhsky, Katylginsky, Prigranichny, and Lomovoi fields. All fields are distinguished by complicated operating conditions (Table 1), including: large amount of suspended particles (up to 600 mg/l), deep unit suspension (to 2,500 meters). The gas-oil ratio in the wells selected for the check operation reached 93 cu. m/cu. m, and temperature in the suspension zone was as high as 90 . The wells were deliberately selected by Tomskneft-VNK specialists to test the piston seal section operation.
Having analyzed the results of operation of the units with PBP92 seal section, Borets and Tomskneft-VNK specialists could confirm the success of the check operation (Fig. 2).
Upon inspection of PBP92 seal section of the pulled-out ESP, the following characteristics of the said section have been confirmed:
– the high degree of the piston module protection against contaminating factors, such as solids, scale, and corrosion of inner cylinder surfaces;
– simple and efficient control of the operating capacity of the tested seal sections, which enables their reuse without the need to re-assemble the in-hole equipment;
– lesser loss of motor oil leaking through the ESP motors shaft seals.
That is why some of the PBP92 piston seal sections were used twice or even three times in operating wells as parts of ESP, without the need to replace their components and mechanical rubber seals. Due to introduction of piston seal sections, the operating time increased from 1.5 to 2.7 for some wells.
For some Russian ESP manufacturers REAM-RTI started manufacturing seal section piston modules designed  specifically for operation in severe conditions, including high formation well fluid temperature (up to 200 C) and presence of free gas in the pumped-out fluid (over 50 percent by volume). The piston modules design allows using them with new  equipment. It also enables their use for the purposes of upgrade and repair of outdated seal sections of the ESP motors. The scope of application of the new modules has been extended to include technologies involving overheated steam supply.

Author`s Bio:
Ivan Pyatov, Ph.D, and Vitaly Nikolaev are REAM-RTI specialists.

 

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