This invention relates to a protective device for oil field type submersible electric motor and pump assemblies used for many years in the production of oil and water from reservoirs and well bores. Such a cylindrically-shaped downhole assembly normally includes a fluid-filled motor on the bottom, a pump disposed above the motor and a fluid-filled protective device for the motor in between which has internal fluid communication with the motor. The three components are usually assembled at the well site. The motor and its protective device are filled with a high dielectric strength fluid, usually a high-grade mineral oil. The assembly is positioned at the desired depth in the well bore and operated to produce well fluids from the well bore and reservoir.
Premature failures of submersible pump motors frequently occur due to the inability of the protective devices to perform the required functions for an extended period of time. Intrusive well fluids, often containing over 95 percent salt water, migrate downwardly through the protection devices and into the interior of motors to cause electric short circuits and burned windings. It is well recognized that protective devices for submersible motors are crucial to the efficient and economical operation of downhole electric pump assemblies.
Protective devices for fluid-filled submersible electric motors have been employed since the inception of submersible pump assemblies. Such protective devices are commonly referred to as "protectors", or "seal sections". The basic functions required of such a device include the means to
(a) transmit torsional power between the motor and pump, PA1 (b) restrict intrusive well fluid from the interior of the motor, PA1 (c) provide a reservoir of high dielectric strength motor fluid that communicates with the motor to enhance electrical insulation, provide lubrication for bearings, and dissipate heat generated by the motor when running, PA1 (d) provide for the expansion and contraction of the motor fluid due to changes in pressure and temperature and substantially equalize the internal pressure of the motor to that of the well bore, and PA1 (e) isolate the motor from axial thrust that may be imposed by the pump.
The thermal expansion and contraction of fluid in the motor and protective device assembly, responding to ambient thermal conditions in the well bore and subsequent changes when the motor is run and stopped, is an important phenomenon that strongly influences the design of such devices.
After a motor and its protective device are assembled, their interconnected interior chambers are filled with motor fluid before being run into a well bore. As the pump assembly is lowered into the well bore, the motor fluid simultaneously experiences thermal expansion in response to temperature increases within the well bore.
The fluid expands with some being expelled from the device into the well bore through a vent near its upper end. Internal heat is generated when the motor is running and the fluid in the motor and protective device expands again with the expanded volume expelled into the well bore. When the motor is stopped, the temperature of the assembly diminishes and the motor fluid contracts with a volume of well fluid being drawn into the upper portion of the protective device. A proportional volume of motor fluid is drawn from the protective device back into the motor. The cycle of thermal expansion and contraction of the motor fluid is repeated each time the motor is run and stopped. Motor fluid in the upper chamber of the protective devices now in use can become progressively more contaminated after each cycle and its dielectric strength significantly reduced. Well fluids having a higher specific gravity than the motor fluid mitigate downwardly through the motor fluid causing contamination. Eventually, the contaminated motor fluid can migrate downwardly through the device and into the motor to cause the ultimate electrical failure.
There are two types of protective device designs that have been employed in field service for a many years. Each type satisfies the basic requirements briefly described above but differ in the method of creating the motor fluid chamber and effectiveness to maintain the motor fluid free from contamination by well bore fluid for an extended period of time.
One type of protective device is normally referred to as a "labyrinth-type" as described in U.S. Pat. Nos. 1,701,468, 3,153,160, 3,502,919, 3,671,786, for example. It employs one or more vertical labyrinth paths disposed in a plurality of serially connected cavities having fixed volumes initially filled with motor fluid. The function of the labyrinth is to retard the detrimental downward migration of intrusive well fluid through the device and into the motor. The cavities are not sealed from the well fluid and a vent near the upper end of the device provides direct communication between the well bore and the upper cavity. The well fluid and motor fluid interface with each other in that cavity and the effectiveness of the device is strongly dependent upon the immiscibility of the two fluids and their difference in specific gravity. Fluid is expelled from that cavity through the vent, and well fluid is drawn into the cavity in like manner, in response to the expansion and contraction in the volume of motor fluid in the device and motor due to changes in temperature. Since the vent is in the upper reaches of the protective device, labyrinth means in the cavity cause well fluids having higher and lower specific gravities than the motor fluid to be trapped within the very device designed to exclude them. Subsequently, retained well fluid can migrate downwardly traversing the labyrinth path entering the motor to cause a failure.
The second type of protective device is referred to as a "bladder-type", or "bag type" as described in U.S. Pat. Nos. 3,947,709, 3,571,636, 4,992,689, 3,072,810, for example. It has a sealed chamber for the motor fluid and generally comprises a plurality of serially connected cavities wherein at least one rubber-like tubular bladder is radially disposed around the axis and provides a variable capacity chamber to accommodate the expansion and contraction of motor fluid inside the device. The bladder provides a positive barrier between the well fluid and motor fluid. These devices describe a vent through the housing near the upper end of the device above the bladder which communicates with the well bore to facilitate the entry of well fluid into, and expulsion of fluid from, the cavity surrounding the exterior of the bladder in response to expansion and contraction. The bladder breathes out and in as the volume of motor fluid changes responsive to changes in temperature. Motor fluid, in excess of the capacity of the expanded bladder, is expelled into the well bore to maintain substantial equalization of pressure between the interior of the motor and the well bore. One or more labyrinth path means are generally disposed in the cavities to retard the detrimental downward migration of intrusive well fluids through the chamber and entry into the motor. The labyrinths may be disposed above and/or below the bladder. In most prior art devices the motor fluid in the bladder directly communicates with a passageway toward the motor. Thus, bladder failure creates a short path for contaminated fluid to access the motor. The "bladder-type" design configuration is technically superior to the "labyrinth-type" because well fluid is not in direct communication with the motor fluid when the bladder is in good condition. However, as commonly experienced, bladders develop structural failures and well fluid migrates through the failure point and enters the sealed chamber. Subsequently, the device then performs like the "labyrinth-type" and contaminated motor fluid can migrate downwardly traversing the labyrinth path to enter the motor and cause a failure.
U.S. Pat. No. 4,421,999 is an example of prior art which includes both labyrinth protection and bladder protection, even though the variable capacity bladder is located below the motor.
A third type of protection device is described in U.S. Pat. No. 4,487,299, which has mechanical internal parts biased to urge the motor fluid toward the motor.
Protective devices normally employ a rotatable seal about the shaft to preclude the migration of well fluid along the shaft and prevent entry into an upper cavity filled with motor fluid at the joint between the protector and the pump. Such seals also perform like a check valve to restrict fluid flow from one side, normally above, and bypass fluid within a selected pressure range, say 10-15 psi, from the other side, normally below. The physical failure of these seals provide another means by which intrusive well fluid may circumvent a labyrinth and be introduced directly into a lower chamber that communicates with the motor. Present protective devices are vented near their upper end and have serially connected cavities that are filled with motor fluid. Intrusive well fluid flows downwardly from the vent toward the motor. All of the cavities have communication with the motor connected below the device. Further, as can be seen from the patents mentioned above, the housing of these protective devices or seal sections are highly segmented involving complicated construction and assembly requirements. Many devices have been developed in the attempt to improve performance and ease construction, but much is lacking with respect to longevity of operation and simplicity of construction.
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