Patent Publication Number: US-10787873-B2

Title: Recirculation isolator for artificial lift and method of use

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-in-Part application of and claims the benefit of priority to U.S. patent application Ser. No. 16/047,983, entitled “ARTIFICIAL LIFT,” filed Jul. 27, 2018, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to artificial lift systems, for example, including recirculation isolators. 
     BACKGROUND 
     Artificial lift equipment, such as electric submersible pumps, compressors, and blowers, can be used in downhole applications to increase fluid flow within a well, thereby extending the life of the well. Such equipment often requires complex tools to sufficiently position the artificial lift equipment in place for operation. Additionally, complex tools can be costly to implement, add weight to the overall system, and introduce additional points of possible failure, among other negative effects. 
     SUMMARY 
     This disclosure describes deployable and retrievable recirculation isolators of an artificial lift system disposed in a downhole environment. 
     In some aspects, a system for use in a completion string of a well, where the completion string includes an electrical stator, includes a retrievable string and a recirculation isolator. The retrievable string is to be disposed in the completion string of the well, the retrievable string including a rotor for receipt within the electrical stator and to rotate in response to electromagnetic fields generated by the stator, an impeller coupled to the rotor and positioned above the rotor relative to the position of the impeller in the well, a housing to surround the impeller, and the recirculation isolator coupled to the housing. The recirculation isolator includes a sealing element and a locking tool, the sealing element to sealingly engage with the completion string, and the locking tool to position the retrievable string in the completion string and detachably couple the retrievable string to the completion string. The locking tool includes a rotational locking feature to engage an indexing receptacle of the completion string and rotationally lock the retrievable string in the completion string, an axial positioning feature to axially position the recirculation isolator in the completion string, and an anchoring feature to axially lock the recirculation isolator to the completion string. 
     This, and other aspects, can include one or more of the following features. The retrievable string can include a rotational lock between the housing and the rotor, the rotational lock to selectively rotationally lock the rotor to the housing. The rotational locking feature can include a set of energized keys on the recirculation isolator, the energized keys to selectively extend and engage the indexing receptacle, and the indexing receptacle can include matching vertical slots in the completion string. The recirculation isolator can include a packer, the packer including the sealing element to radially expand to engage and seal against the completion string. The recirculation isolator can include a fluid channel through the recirculation isolator, the fluid channel to guide fluid flow from the impeller through the recirculation isolator. The recirculation isolator can include a check valve in the fluid channel of the recirculation isolator to allow fluid flow in a first direction through the fluid channel and to restrict fluid flow in a second direction opposite the first direction through the fluid channel. The check valve can be a passive, one-way check valve. The recirculation isolator can include a running feature connection at a longitudinal end of the recirculation isolator, the running feature connection to selectively couple to a running tool. The recirculation isolator can include a plug proximate an uphole longitudinal end of the recirculation isolator, the plug to at least partially seal against the completion string and urge the recirculation isolator along the completion string in response to an applied pressure against the plug. The locking tool can include a first shoulder of the recirculation isolator, the first shoulder to land on and engage a second shoulder of the completion string at a predetermined depth of the completion string. The axial positioning feature of the recirculation isolator can include a magnetic sensor to generate a voltage signal in response to aligning with a magnetic component of the completion string. The locking tool can include a retractable key to selectively expand and retract in a radial direction, the retractable key to engage a locking profile of the completion string. The locking tool can include a slip assembly including a slip plate to expand radially outward to engage a surface of the completion string. The slip plate can include at least one of horizontal teeth or vertical teeth, the horizontal teeth to dig into the surface of the completion string to axially secure the slip plate to the completion string, and the vertical teeth to dig into the surface of the completion string to rotationally secure the slip plate to the completion string. The slip assembly can include a radial guide to guide movement of the slip plate a radial direction toward the surface of the completion string. The recirculation isolator can include a second locking tool, where the first-mentioned locking tool is positioned at a first longitudinal side of the rotor and the second locking tool is positioned at a second longitudinal side of the rotor opposite the first longitudinal side. The system can include a stator configured to attach to a tubing of the completion string, where the stator is to drive the rotor in response to receiving power. The retrievable string can include a motor permanent magnet, and the system can include an electromagnetic coil to, in response to the electromagnetic coil receiving power, generate a first magnetic field to engage the motor permanent magnet and cause the rotor to rotate. The retrievable string can include a rotating portion and a non-rotating portion, where the rotating portion comprises the rotor and the impeller, and the non-rotating portion comprises the housing and the recirculation isolator. 
     Some aspects of the disclosure encompass a method for locking a retrievable string in a completion string. The method includes positioning, with a locking tool of a recirculation isolator of a retrievable string, the retrievable string in a completion string. The completion string includes an electrical stator, and the retrievable string includes a rotor for receipt within the electrical stator and to rotate in response to electromagnetic fields generated by the stator, an impeller coupled to the rotor, a housing to surround the impeller, and the recirculation isolator coupled to the housing. The method also includes anchoring, with the locking tool, the retrievable string to the completion string, where anchoring the retrievable string to the completion string includes engaging, with a rotational locking feature of the locking tool, an indexing receptacle of the completion string to rotationally lock the retrievable string to the completion string, engaging, with an axial positioning feature of the locking tool, the completion string to axially position the recirculation isolator in the completion string, and engaging, with an anchoring feature of the locking tool, the completion string to axially lock the recirculation isolator to the completion string. The method also includes sealing, with a sealing element of the recirculation isolator, the retrievable string to the completion string. 
     This, and other aspects, can include one or more of the following features. Positioning the retrievable string in the completion string can include landing a first shoulder of the locking tool of the recirculation isolator on a second shoulder of the completion string at a predetermined depth of the completion string. Engaging, with a rotational locking feature, an indexing receptacle can include selectively engaging, with a set of energized keys on the recirculation isolator, matching vertical slots of the indexing receptacle. Anchoring the retrievable string to the completion string can further include axially positioning the retrievable string in the completion string with an axial positioning feature of the recirculation isolator, and axially locking the retrievable string to the completion string with an anchoring feature of the locking tool. Anchoring the retrievable string to the completion string can include engaging a slip plate of a slip assembly of the locking tool with a surface of the completion string. Engaging a slip plate of a slip assembly with a surface of the completion string can include digging horizontal teeth of the slip plate into the surface of the completion string. Engaging a slip plate of a slip assembly with a surface of the completion string can include digging vertical teeth of the slip plate into the surface of the completion string. Sealing the retrievable string to the completion string can include setting a packer of the recirculation isolator to radially expand and engage the completion string, the packer including the sealing element. The method can further include unsealing the sealing element of the recirculation isolator with the completion string, and unsetting the locking tool from anchoring engagement with the completion string. The method can further include engaging, with a pulling tool carried on a cable, the recirculation isolator, and retrieving the retrievable string from the completion string. 
     Certain aspects of the disclosure include a system for use in a well completion. The system includes a production tubing string to be set in a wellbore to form a well completion, the production tubing string including a landing sub and an electrical stator disposed in the well completion, the landing sub comprising an indexing receptacle, and a retrievable string to be disposed in the production tubing string in the wellbore. The retrievable string includes a rotor for receipt within the electrical stator and to rotate in response to electromagnetic fields generated by the stator, an impeller coupled to the rotor and positioned above the rotor relative to the position of the impeller in the production tubing string, and a recirculation isolator including a sealing element and a locking tool. The sealing element sealingly engages with the well completion, and the locking tool detachably couples to the landing sub to position, axially lock, and rotationally lock the retrievable string to the well completion. The locking tool includes a rotational locking feature to engage the indexing receptacle of the landing sub and rotationally lock the retrievable string to the landing sub, an axial positioning feature to axially position the recirculation isolator at the landing sub, and an anchoring feature to axially lock the recirculation isolator to the landing sub. 
     This, and other aspects, can include one or more of the following features. The axial positioning feature of the recirculation isolator can include a magnetic sensor configured to generate a voltage signal in response to aligning with a magnetic component of the production tubing string. The locking tool can include a first shoulder of the recirculation isolator, and the landing sub can include a second shoulder having a matching profile to the first shoulder, the first shoulder to land on and engage the second shoulder at a predetermined depth of the well completion. The locking tool can include a slip assembly including a slip plate to expand radially outward to engage a surface of the landing sub. The rotational locking feature can include a set of energized keys on the recirculation isolator, and the indexing receptacle can include matching vertical slots in the completion string, where the set of energized keys selectively extend and engage one or more of the matching vertical slots of the indexing receptacle. 
     The details of one or more implementations of the subject matter described in this disclosure are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic partial cross-sectional side view of an example well. 
         FIG. 2  is a schematic partial cross-sectional side view of an example system within the well of  FIG. 1 . 
         FIG. 3  is a schematic diagram of an example subsystem in the well of the system of  FIG. 2 . 
         FIG. 4A  is a schematic partial cross-sectional side view of an example retrievable string of the system of  FIG. 2 . 
         FIG. 4B  is a schematic partial cross-sectional side view of an example retrievable string of the system of  FIG. 2 . 
         FIG. 5A  is a half cross-sectional side view of an example running connection tool including a fishing tool and fishing neck. 
         FIG. 5B  is a cross-sectional side view of an example locking tool including a retractable latch key. 
         FIG. 5C  is a half cross-sectional side view of an example locking tool that can be incorporated into the retrievable string of  FIG. 4A  or  FIG. 4B . 
         FIG. 5D  is a partial cross-sectional side view of an example locking tool with a slip plate that can be incorporated into the example locking tool of  FIG. 5C . 
         FIG. 6  is a flow chart of an example method applicable to a system including a stator and a retrievable string. 
         FIGS. 7A, 7B, 7C, and 7D  are schematic partial cross-sectional diagrams of example systems within the well of  FIG. 1 . 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     This disclosure describes artificial lift systems, including deployable and retrievable recirculation isolators, in a downhole environment. A retrievable string including an artificial lift system and a recirculation isolator lands on and engages with a landing sub, production tubing, or other structure of a well completion in a wellbore to provide assisted fluid flow to the wellbore. For example, a slickline with a running tool connects to running features on an uphole end of the recirculation isolator to lower the retrievable string into the wellbore. As the retrievable string reaches a desired, predetermined depth, sealing features, locking features, and/or indexing features on the recirculation isolator engage with elements of the well completion to axially index (i.e., position), rotationally lock, anchor, and seal the recirculation isolator to the well completion. The recirculation isolator includes a plurality of features to axially index, axially lock, rotationally lock, and seal the recirculation isolator, such that the deployable and retrievable string with the recirculation isolator can operate as a standalone tool that does not require additional indexing tools, additional locking tools, or other additional downhole tools to operate. 
     Reliably indexing (e.g., axial positioning) downhole-type equipment in a hostile downhole environment is sometimes difficult due to the presence of caustic fluids, pressures, temperatures, and a relative distance between the downhole equipment and any supporting equipment (e.g., surface equipment) that cannot be repackaged to fit in a small diameter tube. For example, many conventional downhole systems include multiple tools connected (directly or indirectly) together at different downhole depths in order to provide uphole axial indexing, downhole axial indexing, and rotational indexing. However, conventional use of multiple indexing tools can be complex, difficult to activate and deactivate, and difficult to operate. Also, the artificial lift systems described herein can be more reliable than comparable artificial lift systems, resulting in lower total capital costs over the life of a well. The improved reliability can also reduce the frequency of workover procedures, thereby reducing periods of lost production and maintenance costs. The modular characteristic of the artificial systems described herein allows for variability in design and customization to cater to a wide range of operating conditions. The artificial lift systems described herein include a deployable and retrievable string, which can be removed from the well simply and quickly. A replacement retrievable string can then be installed quickly to minimize lost production, thereby reducing replacement costs and reducing lost production over the life of a well. 
     While issues and risks exist for downhole operations, the potential benefit of well intervention with production-enhancing tools, such as artificial lift tools and other downhole-type tools, is often worth the risk because of the enhanced production it can offer, among other benefits. While these benefits have been demonstrated, reliability, robustness, and operability of equipment in this harsh and remote environment is not close to conventional topside mounted equipment. The concepts described herein improve reliability and ease of use of downhole-type tools and equipment, for example, by providing the downhole tool with an arrangement of positioning and indexing features to reliably position and secure the tool. The concepts described herein regard the local positioning, indexing, and locking of a deployable and retrievable downhole tool, such as a retrievable string with a recirculation isolator, relative to a production tubing or other well completion in a wellbore. 
       FIG. 1  is a schematic partial cross-sectional side view of an example well  100  constructed in accordance with the concepts herein. The well  100  extends from the surface  106  through the Earth  108  to one or more subterranean zones of interest  110  (one shown). The well  100  enables access to the subterranean zones of interest  110  to allow recovery (that is, production) of fluids to the surface  106  (represented by flow arrows in  FIG. 1 ) and, in some implementations, additionally or alternatively allows fluids to be placed in the Earth  108 . In some implementations, the subterranean zone  110  is a formation within the Earth  108  defining a reservoir, but in other instances, the zone  110  can be multiple formations or a portion of a formation. The subterranean zone can include, for example, a formation, a portion of a formation, or multiple formations in a hydrocarbon-bearing reservoir from which recovery operations can be practiced to recover trapped hydrocarbons. In some implementations, the subterranean zone includes an underground formation of naturally fractured or porous rock containing hydrocarbons (for example, oil, gas, or both). In some implementations, the well can intersect other suitable types of formations, including reservoirs that are not naturally fractured in any significant amount. For simplicity&#39;s sake, the well  100  is shown as a vertical well, but in other instances, the well  100  can be a deviated well with a wellbore deviated from vertical (for example, horizontal or slanted) and/or the well  100  can include multiple bores, forming a multilateral well (that is, a well having multiple lateral wells branching off another well or wells). 
     In some implementations, the well  100  is a gas well that is used in producing natural gas from the subterranean zones of interest  110  to the surface  106 . While termed a “gas well,” the well need not produce only dry gas, and may incidentally or in much smaller quantities, produce liquid including oil and/or water. In some implementations, the well  100  is an oil well that is used in producing crude oil from the subterranean zones of interest  110  to the surface  106 . While termed an “oil well,”: the well not need produce only crude oil, and may incidentally or in much smaller quantities, produce gas and/or water. In some implementations, the production from the well  100  can be multiphase in any ratio, and/or can produce mostly or entirely liquid at certain times and mostly or entirely gas at other times. For example, in certain types of wells it is common to produce water for a period of time to gain access to the gas in the subterranean zone. The concepts herein, though, are not limited in applicability to gas wells, oil wells, or even production wells, and could be used in wells for producing other gas or liquid resources, and/or could be used in injection wells, disposal wells, or other types of wells used in placing fluids into the Earth. 
     The wellbore of the well  100  is typically, although not necessarily, cylindrical. All or a portion of the wellbore is lined with a tubing, such as casing  112 . The casing  112  connects with a wellhead at the surface  106  and extends downhole into the wellbore. The casing  112  operates to isolate the bore of the well  100 , defined in the cased portion of the well  100  by the inner bore  116  of the casing  112 , from the surrounding Earth  108 . The casing  112  can be formed of a single continuous tubing or multiple lengths of tubing joined (for example, threadedly and/or otherwise) end-to-end of the same size or of different sizes. In  FIG. 1 , the casing  112  is perforated in the subterranean zone of interest  110  to allow fluid communication between the subterranean zone of interest  110  and the bore  116  of the casing  112 . In some implementations, the casing  112  is omitted or ceases in the region of the subterranean zone of interest  110 . This portion of the well  100  without casing is often referred to as “open hole.” 
     The wellhead defines an attachment point for other equipment to be attached to the well  100 . For example,  FIG. 1  shows well  100  being produced with a Christmas tree attached the wellhead. The Christmas tree includes valves used to regulate flow into or out of the well  100 . The well  100  also includes an artificial lift system  200  residing in the wellbore, for example, at a depth that is nearer to subterranean zone  110  than the surface  106 . The system  200 , being of a type configured in size and robust construction for installation within a well  100 , can include any type of rotating equipment that can assist production of fluids to the surface  106  and out of the well  100  by creating an additional pressure differential within the well  100 . For example, the system  200  can include a pump, compressor, blower, or multiphase fluid flow aid. 
     In particular, casing  112  is commercially produced in a number of common sizes specified by the American Petroleum Institute (the “API), including 4½, 5, 5½, 6, 6⅝, 7, 7⅝, 16/8, 9⅝, 10¾, 11¾, 13⅜, 16, 116/8 and 20 inches, and the API specifies internal diameters for each casing size. The system  200  can be configured to fit in, and (as discussed in more detail below) in certain instances, seal to the inner diameter of one of the specified API casing sizes. Of course, the system  200  can be made to fit in and, in certain instances, seal to other sizes of casing or tubing or otherwise seal to a wall of the well  100 . 
     Additionally, the construction of the components of the system  200  are configured to withstand the impacts, scraping, and other physical challenges the system  200  will encounter while being passed hundreds of feet/meters or even multiple miles/kilometers into and out of the well  100 . For example, the system  200  can be disposed in the well  100  at a depth of up to 20,000 feet (6,096 meters). Beyond just a rugged exterior, this encompasses having certain portions of any electrical components being ruggedized to be shock resistant and remain fluid tight during such physical challenges and during operation. Additionally, the system  200  is configured to withstand and operate for extended periods of time (e.g., multiple weeks, months or years) at the pressures and temperatures experienced in the well  100 , which temperatures can exceed 400° F./205° C. and pressures over 2,000 pounds per square inch, and while submerged in the well fluids (gas, water, or oil as examples). Finally, the system  200  can be configured to interface with one or more of the common deployment systems, such as jointed tubing (that is, lengths of tubing joined end-to-end, threadedly and/or otherwise), sucker rod, coiled tubing (that is, not-jointed tubing, but rather a continuous, unbroken and flexible tubing formed as a single piece of material), slickline (that is, a single stranded wire), or wireline with an electrical conductor (that is, a monofilament or multifilament wire rope with one or more electrical conductors, sometimes called e-line) and thus have a corresponding connector (for example, a jointed tubing connector, coiled tubing connector, or wireline connector). Some components of the system  200  (such as non-rotating parts and electrical systems, assemblies, and components) can be part of or attached to the production tubing  128  to form a portion of the permanent completion or well completion, while other components (such as rotating parts) can be deployed within the production tubing  128 . 
     A seal system  126  integrated into or provided separately with a downhole system, as shown with the system  200 , divides the well  100  into an uphole zone  130  above the seal system  126  and a downhole zone  132  below the seal system  126 .  FIG. 1  shows the system  200  positioned in the open volume of the bore  116  of the casing  112 , and connected to a production string of tubing (also referred as production tubing  128 ) in the well  100 . The wall of the well  100  includes the interior wall of the casing  112  in portions of the wellbore having the casing  112 , and includes the open hole wellbore wall in uncased portions of the well  100 . Thus, the seal system  126  is configured to seal against the wall of the wellbore, for example, against the interior wall of the casing  112  in the cased portions of the well  100  or against the interior wall of the wellbore in the uncased, open hole portions of the well  100 . In certain instances, the seal system  126  can form a gas-tight and liquid-tight seal at the pressure differential the system  200  creates in the well  100 . For example, the seal system  126  can be configured to at least partially seal against an interior wall of the wellbore to separate (completely or substantially) a pressure in the well  100  downhole of the seal system  126  from a pressure in the well  100  uphole of the seal system  126 . For example, the seal system  126  includes a production packer. Although not shown in  FIG. 1 , additional components, such as a surface compressor, can be used in conjunction with the system  200  to boost pressure in the well  100 . 
     In some implementations, the system  200  can be implemented to alter characteristics of a wellbore by a mechanical intervention at the source. Alternatively, or in addition to any of the other implementations described in this specification, the system  200  can be implemented as a high flow, low pressure rotary device for gas flow in sub-atmospheric wells. Alternatively, or in addition to any of the other implementations described in this specification, the system  200  can be implemented in a direct well-casing deployment for production through the wellbore. Other implementations of the system  200  as a pump, compressor, or multiphase combination of these can be utilized in the well bore to effect increased well production. 
     The system  200  locally alters the pressure, temperature, and/or flow rate conditions of the fluid in the well  100  proximate the system  200 . In certain instances, the alteration performed by the system  200  can optimize or help in optimizing fluid flow through the well  100 . As described previously, the system  200  creates a pressure differential within the well  100 , for example, particularly within the locale in which the system  200  resides. In some instances, a pressure at the base of the well  100  is a low pressure (for example, sub-atmospheric); so unassisted fluid flow in the wellbore can be slow or stagnant. In these and other instances, the system  200  introduced to the well  100  adjacent the perforations can reduce the pressure in the well  100  near the perforations to induce greater fluid flow from the subterranean zone  110 , increase a temperature of the fluid entering the system  200  to reduce condensation from limiting production, and/or increase a pressure in the well  100  uphole of the system  200  to increase fluid flow to the surface  106 . 
     The system  200  moves the fluid at a first pressure downhole of the system  200  to a second, higher pressure uphole of the system  200 . The system  200  can operate at and maintain a pressure ratio across the system  200  between the second, higher uphole pressure and the first, downhole pressure in the wellbore. The pressure ratio of the second pressure to the first pressure can also vary, for example, based on an operating speed of the system  200 . 
     The system  200  can operate in a variety of downhole conditions of the well  100 . For example, the initial pressure within the well  100  can vary based on the type of well, depth of the well  100 , production flow from the perforations into the well  100 , and/or other factors. In some examples, the pressure in the well  100  proximate a bottomhole location is sub-atmospheric, where the pressure in the well  100  is at or below about 14.7 pounds per square inch absolute (psia), or about 101.3 kiloPascal (kPa). The system  200  can operate in sub-atmospheric well pressures, for example, at well pressure between 2 psia (13.8 kPa) and 14.7 psia (101.3 kPa). In some examples, the pressure in the well  100  proximate a bottomhole location is much higher than atmospheric, where the pressure in the well  100  is above about 14.7 pounds per square inch absolute (psia), or about 101.3 kiloPascal (kPa). The system  200  can operate in above atmospheric well pressures, for example, at well pressure between 14.7 psia (101.3 kPa) and 5,000 psia (34,474 kPa). 
       FIG. 2  is a schematic partial cross-sectional side view of the example system  200  in the well  100  of  FIG. 1 . The example system  200  includes a subsystem  300  and a retrievable string  400 . The subsystem  300  is installed as a portion of a completion string of the well  100 . In some instances, the subsystem  300  is referred as the well completion in this disclosure. In some implementations, the subsystem  300  (in part or in whole) is part of the casing and can be cemented in place within the well  100 . The subsystem  300  can be connected to the seal system  126  (for example, a production packer) to form a part of the completion string of the well  100 . Similarly, the subsystem  300  can be connected to and/or include part of the production tubing  128  to form part of the completion string of the well  100 . The retrievable string  400  can be configured to interface with one or more of the common deployment systems described previously (for example, slickline), such that the retrievable string  400  can be deployed downhole into the well  100  and retrieved from the well  100 . For example, a cable  202  in the form of a slickline is shown as connected to the retrievable string  400 . However, the cable  202  can take a variety of other forms, as described previously, such as a slickline, wireline, e-line, coil tubing, sucker rod, a combination of these, or other deployment cable. At least a portion of the retrievable string  400  can be positioned within the subsystem  300 . In some implementations, the entire retrievable string  400  can be positioned within the subsystem  300 . 
     The subsystem  300  and the retrievable string  400  each include corresponding coupling parts that are cooperatively configured to couple the retrievable string  400  and the subsystem  300  to each other. For example, the subsystem  300  includes a landing sub  304  formed in the completion string, for example, as part of the production tubing  128 , and the retrievable string  400  includes a recirculation isolator  404  (described later) configured to at least partially engage with the landing sub  304 . Coupling the corresponding landing sub  304  and the recirculation isolator  404  can secure the relative positions of the subsystem  300  and the retrievable string  400  to each other. The subsystem  300  and the retrievable string  400  are detachably coupled to each other via the landing sub  304  and the recirculation isolator  404 —that is, the subsystem  300  and the retrievable string  400  can subsequently be decoupled and detached from each other. The landing sub  304  and recirculation isolator  404  can include corresponding indexing features and locking features, for example, to axially index (i.e., axially position) and rotationally lock the retrievable string  400  to the subsystem  300 . The landing sub  304  and recirculation isolator  404  can each take a variety of forms, and are described in greater detail later in this disclosure. 
     In some implementations, the subsystem  300  includes a stator  302  (described later), which can attach to a tubing of the completion string (such as the production tubing  128 ). The retrievable string  400  includes a rotor  402  (described later). While the retrievable string  400  is coupled to the subsystem  300 , the stator  302  is configured to drive the rotor  402  in response to receiving power. In some implementations, the electrical components are part of the stator  302  of the subsystem  300 , while the retrievable string  400  is free of electrical components. In some implementations, the subsystem  300  is free of rotating components. The concepts herein likewise apply to a generator, where the rotor  402  is spun and generates electricity in the coils of the stator  302 . 
     Referring to  FIG. 3 , the subsystem  300  can include an electrical connection  306 , a seal  326 , and an electromagnetic coil  350 . Although described as separate components, a conglomerate of various components of the subsystem  300  can be referred as the stator  302 . For example, the stator  302  is sometimes referenced in this disclosure as including the seal  326  and the electromagnetic coil  350 . The stator  302  has an inner surface defined by an inner diameter, and the stator  302  can define a chamber  340  formed on the inner surface. The chamber  340  can house the electromagnetic coil  350 . The stator  302  can include a protective sleeve  390  that is configured to attach to the production tubing  128 . The protective sleeve  390  can be configured to isolate the chamber  340  from production fluid (that is, fluid produced from the subterranean zone  110 ). The protective sleeve  390  can be metallic or non-metallic. The protective sleeve  390  can be made of a material suitable for the environment and operating conditions (for example, downhole conditions). For example, the protective sleeve  390  can be made of carbon fiber or Inconel. The protective sleeve  390  can serve a similar purpose as the production tubing  128 , that is, isolating the casing from production fluid, while also allowing magnetic flux to penetrate from the stator  302 , through the sleeve  390 , and into the inner space of the production tubing  128 . The protective sleeve  390  can be a part of (that is, integral to) the production tubing  128  or can be attached to the production tubing  128 . 
     The electrical connection  306  is connected to the electromagnetic coil  350 . The electrical connection  306  can include a cable positioned in an annulus, such as the inner bore  116  between the casing  112  and the production tubing  128 . The annulus can be filled with completion fluid, and the completion fluid can include a corrosion inhibitor in order to provide protection against corrosion of the electrical connection  306 . The electrical connection  306  can be connected to a power source located within the well  100  or at the surface  106  via the cable to supply power to the electromagnetic coil  350 . The electrical connection  306  can be connected to the chamber  340  and can be configured to prevent fluid from entering and exiting the chamber  340  through the electrical connection  306 . The electrical connection  306  can be used to supply power and/or transfer information. Although shown as having one electrical connection  306 , the subsystem  300  can include additional electrical connections. The electrical connection  306  can also be configured to use an inductive coupler (not shown) to provide electrical power to the stator coil(s), where the inductive coupler can connect to the stator  302  outward of the production tube  128  or protective sleeve of the stator  302 , and connect to the cable  306  from the surface within the production tube  128  or protective sleeve of the stator  302 . This connection of the inductive coupler minimizes the cable in the subsystem  300 , putting the electrical connection cable  306  within the retrievable string  400 , which makes the electrical connection cable  306  more easily replaceable. 
     The seal  326  can be positioned at a downhole end of the subsystem  300 . The seal  326  can be configured to directly or indirectly connect to a production packer disposed in the well downhole of the stator  302  (such as the production packer  126  disposed in the well  100 ), in order to isolate an annulus between the stator  302  and the well  100  (such as the inner bore  116  between the casing  112  and the stator  302 ) from a producing portion of the well  100  downhole of the annulus (for example, the downhole zone  132 ). In some implementations, the seal  326  is a seal stack that is configured to connect to (for example, stab into) a polished bore receptacle connected to the production packer  126  in order to form a pressure-tight barrier. 
     In some implementations, the subsystem  300  includes additional components, such as a thrust bearing actuator, a radial bearing actuator, and/or a cooling circuit, and the chamber  340  can house the additional components. In some implementations, the stator  302  defines one or more additional chambers (separate from the chamber  340 ) which can house any additional components. In some implementations, the subsystem  300  includes one or more sensors, which can be configured to measure one or more properties (such as a property of the well  100 , a property of the stator  302 , or a property of the retrievable string  400 ). Some non-limiting examples of properties that can be measured by the one or more sensors are pressure (such as downhole pressure), temperature (such as downhole temperature or temperature of the stator  302 ), fluid flow (such as production fluid flow), fluid properties (such as viscosity), fluid composition, a mechanical load (such as an axial load or a radial load), and a position of a component (such as an axial position or a radial position of the rotor  402 ). 
     In some implementations, the subsystem  300  includes additional components or duplicate components (such as multiple stators  302 ) that can act together or independently to provide higher output or redundancy to enhance long-term operation. In some implementations, the subsystem  300  is duplicated one or more times to act together with other subsystems to provide higher output or independently for redundancy. The presence of multiple subsystems  300  can enhance long-term operation. In some implementations (for example, where multiple subsystems  300  operate in conjunction to provide higher well output), each additional or duplicate subsystem  300  can operate with different retrievable strings. In some implementations (for example, where multiple subsystems  300  operate independently for redundancy), each additional or duplicate subsystem  300  can operate with a single retrievable string (such as the retrievable string  400 ), which can be relocated within the well depending on whichever subsystem the retrievable string is operating with to provide well output. 
       FIG. 4A  is a schematic partial cross-sectional side view of the retrievable string  400 . Referring to  FIG. 4A , the retrievable string  400  includes a rotating portion  410  and a non-rotating portion  420 . The rotating portion  410  includes the rotor  402 , and the non-rotating portion  420  includes the recirculation isolator  404 . In response to receiving power, the electromagnetic coil  350  of the subsystem  300  can be configured to generate a magnetic field to engage a motor permanent magnet  450  of the retrievable string  400  and cause the rotor  402  to rotate. The electromagnetic coil  350  and the motor permanent magnet  450  interact magnetically. The electromagnetic coil  350  and the motor permanent magnet  450  each generate magnetic fields, which attract or repel each other. The attraction or repulsion imparts forces that cause the rotor  402  to rotate. The subsystem  300  and the retrievable string  400  can be designed such that corresponding components are located near each other when the retrievable string  400  is positioned in the subsystem  300 . For example, when the retrievable string  400  is positioned in the subsystem  300 , the electromagnetic coil  350  is in the vicinity of the motor permanent magnet  450 . As one example, the electromagnetic coil  350  is constructed similar to a permanent magnet motor stator, including laminations with slots filled with coil sets constructed to form three phases with which a produced magnetic field can be sequentially altered to react against a motor permanent magnetic field and impart torque on a motor permanent magnet, thereby causing the rotor  402  to rotate. 
     The retrievable string  400  is configured to be positioned in a well (such as the well  100 ). The rotor  402  of the retrievable string  400  is configured to be positioned in and driven by a stator of a well completion (such as the stator  302 ). The retrievable string  400  includes at least one impeller  432  coupled to the rotor  402 . The non-rotating portion  420  of the retrievable string  400  and the impeller  432  are cooperatively configured to induce fluid flow in the well  100  from an inlet of the string  400  to an outlet of the string  400  in response to the stator  302  driving the rotor  402 . The recirculation isolator  404  is configured to support the retrievable string  400  to position the rotor  402  in (e.g., adjacent to) the stator  302 , and can detachably couple to the corresponding landing sub  304  and/or other structures (e.g., production tubing  128 ) of the well completion (subsystem  300 ). The landing sub  304  of the subsystem  300  and the recirculation isolator  404  of the retrievable string  400  act to position, secure, and seal the retrievable string  400  relative to the subsystem  300  to ensure proper alignment of the rotor  402  with the stator  302 . The recirculation isolator  404  is configured to isolate the output of the non-rotating portion  420  of the retrievable string  400  and the impeller  432  from the inlet, flowing fluid from a downward, or downhole location to an upward, or uphole location. 
     The example retrievable string  400  of  FIG. 4A  includes a motor permanent magnet  450  and in some instances, includes a protective sleeve at least partially surrounding the motor permanent magnet  450 . The protective sleeve can surround the rotor  402  and can be similar to the protective sleeve  390  lining the inner diameter of the stator  302 . The protective sleeve can be metallic or non-metallic. For example, the protective sleeve can be made of carbon fiber or Inconel. 
     The motor permanent magnet  450  is configured to cause the rotor  402  to rotate in response to the magnetic field generated by the electromagnetic coil  350  of the stator  302 . With the rotor  402  configured to rotate, the retrievable string  400  can make up at least part of an electric submersible pump, a compressor, a blower, a combination of these, or another rotational lift tool. For example, the rotating portion  410  includes the impellers  432  and central rotating shaft of an electric submersible pump, while the non-rotating portion  420  includes the diffuser and/or housing  434  of the electric submersible pump. The concepts herein likewise apply to a generator, where the rotor  402  is spun and generates electricity in the stator coils  350  of the stator  302 . For example, unassisted production fluid flow through the retrievable string  400  can drive the impeller(s)  432  to rotate, thereby rotating the rotor  402  and the permanent magnet  450  to generate electricity in the electromagnetic coil(s)  350  of the stator  302 . The retrievable string  400  can be exposed to production fluid from the subterranean zone  110 , and the electric submersible pump can include a fluid inlet and a fluid outlet for flow of production fluid across the pump. In some implementations, the retrievable string  400  includes a protector configured to protect a portion of the rotor  402  against contamination of production fluid. In some implementations, the retrievable string  400  can allow production fluid from the subterranean zone  110  to flow over an outer surface of the rotor  402 . In some implementations, production fluid from the subterranean zone  110  flows through the annulus defined between the outer surface of the rotor  402  and the inner surface of the stator  302  (or the protective sleeve  390 ). In some implementations, production fluid from the subterranean zone  110  can flow through an inner bore of the rotor  402 . 
     In some implementations, the retrievable string  400  includes a rotational lock  418  between the non-rotating portion  420  (e.g., housing  434 , or recirculation isolator  404 ) and the rotatable portion  410  (e.g., the rotor  402 , or impellers  432 ) to temporarily lock rotation of the rotatable portion  410  relative to the non-rotating portion  420 , for example, as the retrievable string  400  is run into the well  100 . The rotational lock  418  is temporary, and selectively rotationally locks the rotor  402  to the housing  434 , for example, until the locking tool of the recirculation isolator  404  engages, where the rotor  402  and stator poles of the stator  302  are aligned prior to releasing the rotational lock between the rotor  402  and the housing  434 . The selective rotational lock  418  can take a variety of forms. For example, the rotational lock  418  can include a retractable key on the rotor  402  or impellers  432  that can selectively retract or extend to disengage or engage, respectively, a slot in the non-rotating portion  420  (e.g., housing  434 ). In some examples, the rotational lock  418  includes a frangible element on the rotor  402  or impellers  432  that temporarily locks rotation of the rotor  402  relative to the housing  434 , or an abradable material on the rotor  402  or impellers  432  that forms a temporary friction lock against the housing  432 . The temporary, or selective, rotational lock  418  can allow for a safer deployment of the retrievable string  400  during deployment of the string  400  by restricting (completely or substantially) rotational movement of the rotor  402  as the retrievable string  400  traverses uphole or downhole in the well completion. The rotational lock  418  also rotationally positions the rotor  402  at a predetermined rotational position, for example, such that the position of the rotor  402  relative to the subsystem  300  is known as the retrievable string  400  is anchored to the subsystem  300 . 
     The recirculation isolator  404  is coupled to the non-rotating housing  434  of the pump. The fluid outlet of the pump is fluidly coupled to a fluid channel  412  of the recirculation isolator  404  to allow fluid flow through the recirculation isolator, for example, in an uphole direction. The fluid channel  412  guides fluid flow from the pump through the recirculation isolator  404  and out of the recirculation isolator  404  at an uphole longitudinal end of the recirculation isolator  404 . The non-rotating housing  434  can include axial and radial bearings, where the non-rotating housing  434  conveys to the recirculation isolator  404 , in addition to the system weight associated with the tool itself, forces generated during operation of the subsystem  300 . The recirculation isolator  404  is configured to support these loads during all operating conditions of the tool by anchoring to the landing sub  304 , thereby transferring loads to the permanent completion structure. In some implementations, the recirculation isolator  404  includes a check valve  414  in the fluid channel  412  of the recirculation isolator  404 , for example, to prevent or reduce backflow of fluids or solids through the fluid channel  412  toward the pump. The check valve  414  can be a passive, one-way check valve with a valve seat, such as a flapper valve, ball valve, diaphragm check valve, tilting disc valve, a lift-check valve, another type of shuttle valve energized by a spring, a combination of these, or another type of one-way valve. 
     The recirculation isolator  404  of the non-rotating portion  420  of the retrievable string  400  enables deployment, indexing, sealing, operation, and retrieval of the retrievable string  400 . The recirculation isolator  404  can selectively connect to the well completion (i.e., subsystem  300 , landing sub  304 , and/or production tubing  128 ) and prevent rotation of the non-rotating portion  420  while the rotating portion  410  rotates. Connecting the recirculation isolator  404  to the well completion can also locate (that is, position) the non-rotating portion  420  relative to the well completion and prevent axial movement of the non-rotating portion  420  relative to the well completion in either an uphole or downhole direction. In particular, the recirculation isolator  404  includes conveyance and retrieval features to convey and retrieve the retrievable string  400  from the well, axial indexing features to position the retrievable string  400  relative to the well completion, anchoring features to axially lock the retrievable string  400  in the well completion, anti-rotation features to rotationally lock the retrievable string  400  in the well completion, and sealing features to seal the annulus between the retrievable string  400  and the well completion. One or more locking tools of the recirculation isolator  404  can include one or more of these conveyance and retrieval features, axial indexing features, anchoring features, anti-rotation features, or sealing features. This locking tool(s) and its respective conveyance and retrieval features, axial indexing features, anchoring features, anti-rotation features, and/or sealing features can take many forms. 
     For example, the recirculation isolator  404  of the example retrievable string  400  of  FIG. 4A  includes a running feature connection  406  positioned at an uphole end of the retrievable string  400 . The running feature connection  406  is configured to connect to and engage a running tool  408  carried on a cable, such as cable  202 , from a location at the surface  106 , allowing the retrievable string  400  to be deployed in the well  100  and, additionally or alternatively, retrieved from the well  100  after the retrievable string  400  (i.e., the recirculation isolator  404 ) has been decoupled from the subsystem  300  and/or other parts of the well completion. The running feature connection  406 , running tool  408 , and cable  202  can make up the conveyance and retrieval features of the recirculation isolator  404 . In  FIG. 4A , the running tool  408  of the cable  202  is shown as being coupled to the running feature connection  406  of the recirculation isolator  404 . In some implementations, the retrievable string  400  includes the running tool  408  and cable  202  (such as a slickline, wireline, or coiled tubing) configured to connect to the running feature connection  406  of the recirculation isolator  404 . The cable can extend to lower the retrievable string  400  into the well  100  and retract to retrieve the retrievable string  400  from the well  100 . In some implementations, once the retrievable string  400  is installed in the well  100 , the running tool  408  of the cable  202  can be disconnected from the running feature connection  406  of the recirculation isolator  404  of the retrievable string  400 , and the cable  202  and running tool  408  are retrieved from the well  100  so that the cable  202  is not hanging within the production tubing  128  while the well  100  is producing. 
     The running feature connection  406  and the running tool  408  can take a variety of forms, and can engage with each other in a variety of ways to connect, interface, and disconnect from each other. For example, the running tool  408  can include a lip or shoulder of a mechanical member (e.g., collets, retainer dogs, keys, lugs, and/or other) to engage with a shoulder in a recess of the running feature connection  406 . The running feature connection  406  includes a profile configured to engage with the running tool  408 , such as the recess with the shoulder to engage with the lip of the mechanical member of the running tool  408 . In some examples, the running feature connection  406  includes a fishing neck, such as an external or internal fishing neck, and the running tool  408  includes a fishing hook configured to engage with the fishing neck of the running feature connection  406 .  FIG. 5A  is a half cross-sectional side view of an example running connection tool  480  including a fishing tool  482  and fishing neck  484 , which can be used in the running tool  408  and running feature connection  406  of  FIG. 4A . The fishing neck  484  includes an inner profile configured to selectively engage with an outer profile of the fishing tool  482 . The fishing tool  482  can selectively expand or retract to engage or disengage the corresponding fishing neck  484 , for example, during deployment and retrieval of the running connection tool  480 . 
     Referring back to  FIG. 4A , the running feature connection  406 , running tool  408 , and cable  202  can function to trigger, activate, and/or deactivate a feature of the recirculation isolator  404 . In some implementations, the running tool  408  can trigger the activation of the recirculation isolator  404 . This triggering can be achieved mechanically (e.g., manipulations of the cable, jar-up and jar-down by cable and jar), electrically (e.g., sending electrical signals via wireline conductor to start the sequence of activation), hydraulically (e.g., changing the downhole pressure to a pre-set value to start a sequence of activation), or acoustically (e.g., sending a certain patterns of frequency or magnitude of acoustic wave via with the column of fluids of the well or the well completions to start a preset sequence of activation). In certain implementations, the running tool  408  can activate the recirculation isolator  404  by mechanical energy (e.g., manipulations of the cable, jar-up and jar-down by cable and jar), electrical energy (e.g., electrical power via wireline conductor or stored electrical power in batteries), hydraulic energy (e.g., increase the downhole pressure to act on an atmospheric chamber), or chemical energy (e.g., ignite propellants to increase gas pressure on one side of a actuation piston against the atmospheric pressure on the other side of the actuation piston). In some implementations, the running tool  408  can disengage the recirculation isolator  404 , for example, once activation is completed. This disengagement between the running tool  408  and the recirculation isolator  404  can happen so that the running tool  408  and cable  202  can be retrieved. This disengagement can be achieved by unlocking (or breaking) the connections (e.g. keyed, or pinned) between a member of the running tool  408  and a member of the recirculation isolator  404  at a final sequence of the activation. 
     For example, the running tool  408  can exert a force on the recirculation isolator  404  by manipulation of the cable  202 , such as by pulling or releasing the cable  202 , and/or jarring up or down on the running tool  408 . The cable manipulation can shear a frangible element (e.g., shear pin) of the recirculation isolator, for example, to activate or deactivate a mechanism of the recirculation isolator  404 . In some examples, cable manipulation can also dislodge a component from gripping engagement with a profile (e.g., drive a spring finger, soft metal/elastomer/frangible ring, or snap ring past a shoulder or profile or pull a ball or pin from a detent) to set (activate) or unset (deactivate) a feature of the retrievable string  400 . In some implementations, the running tool  408  includes a conventional wireline setting tool, such as an explosive setting tool or an electrical setting tool, to break one or more frangible elements (e.g., shear one or more shear pins) or dislodge grippingly engaged components that sets or unsets a mechanism of the recirculation isolator  404 , such as keys, sleeves, locks, slips, wedges, valves, a combination of these, or other features. In certain implementations, the running tool  408  can connect and/or disconnect from the running feature connection  406  with the force from an explosive setting tool or rotational/axial movement from an electrical setting tool, by sending an electrical signal via the cable  202  (e.g., wireline) to activate a mechanism, or by sending an acoustic signal to activate a mechanism of the running feature connection  406  or running tool  408 . 
     The running feature connection  406  and running tool  408  can also act as a pulling feature connection and pulling tool, respectively, to engage and retrieve the retrievable string  400 . The pulling feature connection and pulling tool can take a variety of forms, as described earlier with respect to the running feature connection  406  and running tool  408 . In some instances, the pulling tool (i.e., running tool  408 ) includes a set of slips to engage the pulling feature connection (i.e., running feature connection  406 ), for example, where there is no recess or shoulder on the pulling feature connection of the recirculation isolator  404 . 
     In some implementations, the recirculation isolator  404  of the retrievable string  400  includes a plug (not shown) in addition to or instead of the running feature connection  406 . The plug can be positioned at the uphole end of the retrievable string  400  and can be configured to allow the retrievable string  400  to be pumped down into the well. For example, the plug can include a low pressure seal, and fluidic pressure can be applied on top of the plug in order to push the retrievable string  400  down into the well  100 . The running feature connection  406  can be configured to be connected by an electrical connection, which can be used to transfer signals to and from a location at the surface  106 . For example, one or more sensors of the non-rotating portion  420  can transmit signals to and from a location at the surface  106  through the electrical connection connected to the running feature connection  406 . 
     The axial indexing features of the recirculation isolator  404  position the retrievable string  400  at a desired depth in the well. The axial indexing features can be formed in the housing of the recirculation isolator, and are configured to engage part of the well completion, such as the landing sub  304 . For example, the recirculation isolator  404  of example retrievable string  400  of  FIG. 4A  includes a shoulder  416  that defines a landing surface in the housing of the non-rotating portion  420  of the recirculation isolator  404 . The shoulder  416  defines a radial protrusion from an outer surface of the isolator  404 . As the retrievable string  400  is lowered into the well, the shoulder  416  lands on a corresponding shoulder of the well completion (e.g., corresponding shoulder of landing sub  304 ). The shoulder  416  and the corresponding shoulder of the well completion are positioned such that, when the shoulder  416  lands on the corresponding shoulder of the well completion, the retrievable string  400  is at a predetermined, desired axial position in the well. The predetermined axial position can include a relative position between the retrievable string  400  and the subsystem  300  such that the coils  350  of the stator  302  align with the magnet(s)  450  of the rotor  402 . The corresponding shoulder of the well completion can be formed in the landing sub  304 , the production tubing  128 , or another component of the subsystem  300 . In some examples, the landing sub  304  includes a no-go profile, such as a no-go shoulder, to engage with a corresponding profile, such as shoulder  416 , of the recirculation isolator  404 . The no-go profile prevent the retrievable string  400  from passing beyond the depth of the no-go profile, and can indicate the predetermined depth that the retrievable string  400  is to be positioned at. 
     In some instances, the axial indexing features of the recirculation isolator  404  can also act as a rotationally lock feature. For example, the shoulder  416  of the recirculation isolator  404  and the corresponding shoulder of the well completion can include rotational locking features such that engagement of the shoulders together both axially positions and rotationally locks the retrievable string  400  relative to the subsystem  300 . In some examples, the shoulder  416  and the corresponding shoulder of the well completion can include splines, keys, or other alignment structures to rotationally lock the non-rotating portion  420  of the retrievable string  400  to the subsystem  300 . 
     The axial indexing features can take other forms to axially index the retrievable string  400  in the well. In some implementations, the axial indexing features include a shoulder (e.g., shoulder  416 ) of the recirculation isolator  404  to land on a corresponding shoulder of the well completion, a spring biased key, dog or snap ring to snap into engagement with a corresponding profile of the well completion (or vice versa), a magnetic sensor or magnet to engage with a corresponding magnet or magnetic sensor of the well completion which, in turn, send a signal confirming axial indexing to the operator, a combination of these features, or other axial indexing features. In some examples, the recirculation isolator  404  includes a lock with one or more keys configured to engage a corresponding profile (e.g., recess) in the landing sub  304 , stator  302 , production tubing  128 , or other component of the subsystem  300  or well completion. The keys can be spring-loaded to radially expand and engage the corresponding profile as the keys reach the profile. The keys can also be radially retractable, for example, to disengage the keys from the profile. In certain implementations, the axial indexing features include a selective or non-selective lock and nipple profile, such as a no-go lock and nipple profile. In some examples, the recirculation isolator  404  includes a magnet or magnetic sensor, and the landing sub  304  includes a corresponding magnetic sensor or magnet, such that a magnetic field from the magnet triggers the corresponding magnetic sensor or coils to indicate the predetermined depth when the magnet from the recirculation isolator  404  aligns with the corresponding magnetic sensor from the landing sub  304  (or the magnetic sensor from the recirculation isolator  404  aligns with the magnet from the landing sub  304 ). As the rotor is deployed in the well, a magnetic sensor or magnetic bearing stator can provide positive measurement of rotor axial location, where voltage present on the stator or a signal from the magnetic sensor is used to determine rotor location. In some implementations, a magnetic bearing stator can be energized to hold the rotor in place, and can allow for a soft landing of the rotor and retrievable string  400  onto a step or other feature of the well completion. For example, the recirculation isolator can include the magnetic sensor that can generate a voltage signal in response to aligning with a magnetic component of the completion string, where the axial positioning feature of the locking tool of the recirculation isolator includes the magnetic sensor. In some instances, the axial indexing features include magnetic or electro-magnetic features. For example, the permanent completion can include a permanent magnet positioned at a predetermined depth, and the axial indexing features includes a magnetic switch on the recirculation isolator  404  that can be activated by the permanent magnet on the completion to indicate that the desired depth has been reached. In some examples, an RFID tag in the permanent completion can indicate to an RFID sensor in the recirculation isolator  404  that the desired depth has been reached. In certain examples, the completion string can include a varying thickness of a metallic wall, where the recirculation isolator  404  includes a casing collar locator to sense and identify the change of wall thickness to indicate a desired depth. In some examples, the motor stator windings (e.g., the electromagnetic coils  350  of the stator  302 ) can act to provide positive measurement of rotor axial location. For example, as the rotor  402  is deployed in and lowered in the wellbore, a voltage present on the stator can be used to determine rotor axial position. The stator can be energized to hold the rotor (in cases where the motor portion is deployed separately from the pump portion), which can allow for a soft landing of the rotor into a step or other feature of the well completion. 
     The anti-rotation (rotational locking) features of the recirculation isolator  404  rotationally lock the non-rotating portion  420  of the retrievable string  400  relative to the subsystem  300 . In some instances, the subsystem  300  includes an indexing receptacle (for example, in the landing sub  304 , or elsewhere in the well completion) to engage the rotational locking feature of the locking tool of the recirculation isolator  404 . The rotational locking features ensure that the non-rotating portion  420  of the retrievable string  400  is rotationally fixed relative to the subsystem  300 , for example, during operation of the pump (e.g., rotation of the impeller  432 ). Due to the symmetry of the rotor  402  and the coils  350  of the stator  302 , the recirculation isolator  404  can rotationally lock the retrievable string  400  relative to the subsystem  300  at any radial position. It is not required to radially index the retrievable string at a specific alignment for the rotor  402  and stator coils  350  to operate. In some implementations, the rotational locking features include a set of spline teeth on the recirculation isolator  404  and corresponding spline teeth on the landing sub  304 , a set of energized keys on the recirculation isolator  404  and matching vertical slots on the landing sub  304 , a set of slips with vertical teeth to bite into an inside diameter of the well completion (e.g., landing sub  304  or production tubing  128 ), a combination of these features, or other rotational locking features. In some examples, the rotational locking features includes any shape of protrusions, or alternating protrusions with recesses, corresponding to matching recesses (or imprints, deformation, or cutting into a surface) with interference edges of the landing sub  304 , that can function to stop relative radial or rotational motions between the recirculation isolator  404  and the landing sub  304 . 
     The anchoring features of the recirculation isolator  404  axially lock the retrievable string  400  in the well completion. The anchoring features axially lock the recirculation isolator  404  to substantially prevent uphole or downhole movement of the retrievable string  400  relative to the well completion, and can also rotationally lock the non-rotating portion  420  of the retrievable string  400 . The anchoring features can be formed in the housing of the recirculation isolator, and are configured to engage part of the well completion, such as the landing sub  304 . In some implementations, the anchoring features include a retractable key to engage with a corresponding profile of the well completion, a lock and nipple profile, a tubing stop, anchor, or packer with a set of slips having teeth to bite into the inside diameter surface of the well completion (e.g., landing sub  304 ), a combination of these features, or other anchoring features. In some examples, the recirculation isolator  404  includes a lock with one or more keys configured to engage a corresponding profile (e.g., recess) in the landing sub  304 , stator  302 , production tubing  128 , or other component of the subsystem  300  or well completion. The keys can be spring-loaded to radially expand and engage the corresponding profile as the keys reach the profile. The keys can also be radially retractable, for example, to disengage the keys from the profile. In certain implementations, the axial indexing features include a selective or non-selective lock and nipple profile, such as a no-go lock and nipple profile. In some examples, the anchoring features includes any shape of protrusions, or alternating protrusions with recesses, corresponding to matching recesses (or imprints, deformation, or cutting into a surface) with interference edges of the landing sub  304 , that can function to stop relative axial motions between the recirculation isolator  404  and the landing sub  304 . 
     For example,  FIG. 5B  is a cross-sectional side view of an example locking tool  490  including a retractable latch key  492  engaged with a recess  494  of a well completion. The example locking tool  490  can be used in the recirculation isolator  404  of  FIG. 4A , for example, as an axial anchoring feature and/or rotational locking feature. The latch key  492  is configured to reside in an inner recess  496 , for example during a run in of the locking tool  490  into a well, and activated to radially extend out of the inner recess  496  to a radially outward position, as shown in  FIG. 5B , to engage the recess  494  of the well completion. The retractable latch key  492  can be activated to reside in the inner recess  496  or extend radially outward toward the recess  494  using an activation method described earlier. For example, a latch finger  498  coupled to the latch key  492  can be activated to translate relative to a housing of the locking tool  490  to retract (into the inner recess  496 ) or radially extend (toward the recess  494 ) the latch key. 
     The locking tool of the recirculation isolator  404  can include any combination of example axial positioning features, rotational locking features, and anchoring features described herein. In some instances, the order in which the features of the locking tool are operated can promote a more reliable positioning and operation of the retrievable string  400 . For example, the locking tool can operate to first engage a rotational locking feature of the locking tool with the completion string to rotationally lock the retrievable string  404  to the completion string; for example, the locking tool can engage a radial key of the recirculation isolator with an indexing receptacle of the completion string. The locking tool can then operate to secondly engage an axial positioning feature (e.g., a no-go shoulder, retractable key, magnetic sensor, or other example axial positioning feature) with the completion string to axially position the recirculation isolator  404  in the completion string. The locking tool can then operate to thirdly engage an anchoring feature (e.g., no-go shoulder, slip plates, retractable keys, or other example anchoring feature) with the completion string to axially lock the recirculation isolator  404  to the completion string. In some implementations, the locking tool of the recirculation isolator  404  engages the rotational locking feature first, the axial positioning feature second, and the anchoring feature third. In certain instances, the locking tool of the recirculation isolator  404  can simultaneously engage two of or all of the rotational locking feature, the axial positioning feature, or the anchoring feature. For example, the locking tool can simultaneously axially positions and anchors the recirculation isolator  404 , simultaneously axially positions and rotationally locks the recirculation isolator  404 , simultaneously rotationally locks and anchors the recirculation isolator  404 , or simultaneously axially position, rotationally lock, and anchor the recirculation isolator  404 . 
     In some instances, the rotational locking feature of the locking tool of the recirculation isolator  404  includes a key, key way, or similar structure to position the recirculation isolator  404  in the completion string prior to the axial positioning feature engaging the completion string. The completion string can include an order of corresponding profiles for the rotational locking feature, axial positioning feature, and/or anchoring feature such that as the retrievable string is deployed downhole, the desired (predetermined) order of engagement of the features of the locking tool is implemented. For example, as the retrievable string  400  is deployed and lowered downhole, a key of the rotational locking feature begins to engage a keyway (tapered, straight, or otherwise) of the completion string prior to an axial positioning feature and/or anchoring feature engages corresponding profiles of the completion string. The keyway can extend along a length of the completion string (e.g., landing sub) a sufficient length to engage the key of the rotational locking tool and allow for sufficient axial length to have the axial positioning feature and anchoring feature engage with respective portions of the completion string. 
     The sealing features of the recirculation isolator  404  are configured to create a seal between the non-rotating portion  420  and the subsystem  300  and/or production tubing  128 . By creating the seal between the non-rotating portion  420  and the subsystem  300  and/or production tubing  128 , the recirculation isolator  404  can force produced fluid to flow through the space between the impellers  432  and the non-rotating portion  420  (e.g., housing) of the pump and also prevent discharged fluid from recirculating upstream (in the context of a vertical production well, upstream can be understood to mean downhole) through the annulus between the retrievable string  400  and the subsystem  300  and/or production tubing  128 . The seal prevents (substantially or completely) produced fluids exiting the uphole end of the recirculation isolator  404  from flowing back to the intake of the pump through the annular space between the pump housing and the well completion (i.e., production tubing  128 , subsystem  300 , and/or landing sub  304 ). In some implementations, the sealing features includes one or more sealing elements on the recirculation isolator  404  configured to radially extend and engage the well completion to create the seal between the recirculation isolator  404  and the well completion. The sealing element(s) can be formed of varying materials, such as a rubber polymer like nitrile butadiene rubber (NBR), Viton, or other polymer. The recirculation isolator  404  can include a packer, a bridge plug, a tubing pack-off, or other structure that includes the sealing element. 
     In some instances, the sealing feature of the recirculation isolator  404  includes a labyrinth seal. The housing  434  of the pump (or other housing portion of the recirculation isolator  404 ) can have an outer diameter that approaches the inner diameter of the subsystem  300  or production tubing  128 . This small clearance between the recirculation isolator  404  and the well completion (i.e., subsystem  300 , production tubing  128 , or both) minimizes the flow path for returning fluid, and in some instances, includes abradable sections that can act as seals on the outer diameter of the recirculation isolator  404 . The abradable sections can include an abradable material, frangible material, or other material that can engage with an inner diameter of the production tubing  128  or subsystem  300 , and provide a low pressure seal between the recirculation isolator  404  and the well completion. The labyrinth seal can minimize the pressure differential locally at each or multiple stages of the pump of the retrievable string  400 , and can provide a tortuous return path of fluid from the outlet of the pump to the inlet of the pump to reduce fluid recirculation. In some examples, the abradable sections include a ring of abradable material that can engage the inner diameter of the well completion to provide a seal, but can also at least partially break down and reduce in diameter to allow translation along the well completion and fit within the inner diameter of the well completion, for example, in response to jarring or other movement of the recirculation isolator  404  to degrade or reduce the outer diameter of the abradable material. The abradable material can take many forms and include a variety of materials, such as Fluorosint®, Si—Al graphite, Ni-graphite, Al—Si-polyester, a combination of these, or other materials. The abradable material can act as a sealing feature of the recirculation isolator  404 , and can be a primary, secondary, or redundant sealing feature of the recirculation isolator  404 . In some implementations, the recirculation isolator  404  can include an anti-rotation feature (examples described earlier) to prevent or reduce relative rotation between the pump housing (e.g., housing  434  or recirculation isolator  404 ) and the production tubing  128 , and can include an axial stop to support the pump housing on the well completion. 
     The retrievable string  400  is retrievable, in that the string  400  can be run into the well, set in place, operated to provide assisted fluid flow, unseated, and retrieved from the well. The conveyance and retrieval features, axial indexing features, anchoring features, anti-rotation features, and/or sealing features can be activated (e.g., set, expanded, or otherwise activated) and subsequently deactivated (e.g., unset, retracted, or otherwise deactivated), for example, by manipulation of the cable  202  (e.g., jarring up or down on the running tool  408  with the cable  202 ) or with a setting tool of the running tool  408  or running feature connection  406 . 
       FIG. 4A  shows the recirculation isolator  404  as disposed only uphole of the pump. However, the recirculation isolator  404  can extend elsewhere on the retrievable string  400 . For example,  FIG. 4B  is a schematic partial cross-sectional side view of a retrievable string  400 ′ with a recirculation isolator  404 ′. The retrievable string  400 ′ and recirculation isolator  404 ′ are the same as the retrievable string  400  and recirculation isolator  404  of  FIG. 4A , except the recirculation isolator  404 ′ further extends downhole of the pump and the rotor  402 , for example, to provide axial indexing features, anchoring features, anti-rotation features, and/or sealing features at a location downhole of the pump and rotor  402 . In some implementations, the rotor  402  can be separable such that the motor portion of the rotor  402  (for example, the portion of the rotor  402  with the permanent magnet  450 ) can be removably coupled to the pump portion (for example the portion of the rotor  402  with the impellers  432 ). Similarly, the recirculation isolator  404 ′ can be split such that a first portion of the recirculation isolator  404 ′ couples to the motor portion and a second portion of the recirculation isolator  404 ′ couples to the pump portion. In some examples, the motor portion and the first portion of the recirculation isolator  404 ′ is separately deployable in the well, and can selectively lock to the well completion with a first set of locking features; the pump portion and the second portion of the recirculation isolator  404  can subsequently be deployed in the well, and can selectively lock to the motor portion and the first portion of the recirculation isolator  404 ′, and selectively locks to the well completion with a second set of locking features. The motor portion can have a separate deployment and locking operation, with a subsequent deployment and locking operation of the pump portion that engages the same or different feature of the well completion and connects to the motor portion rotor. 
     In some implementations, the anchoring feature of the recirculation isolator  404  can also act to rotationally lock, axially index, and/or seal the recirculation isolator  404  relative to the well completion. In some examples, the recirculation isolator  404  includes an anchor with mechanical slips that can stab into an inner diameter of the well completion (such as the stator  302  or the production tubing  128 ) to provide rotational and/or axial locking of the recirculation isolator  404  in the well completion. For example,  FIG. 5C  is a half cross-sectional side view of an example locking tool  500  that can be incorporated into the recirculation isolator  404  of  FIG. 4A  and/or the recirculation isolator  404 ′ of  FIG. 4B . The example locking tool  500  can provide rotational locking, axial anchoring, and sealing to a recirculation isolator. The example locking tool  500  includes a housing  502 , a central bore  504  to allow fluid flow through the locking tool  500 , a first anchor slip assembly  506   a , a second anchor slip assembly  506   b , and a seal assembly  508  having a seal element  510  configured to radially expand, for example, to engage and seal to the well completion. The first anchor slip assembly  506   a  and the second anchor slip assembly  506   b  includes a first slip plate  512   a  and a second slip plate  512   b , respectively, configured to expand radially outward to engage a surface of the well completion. The anchor slip assemblies  506   a  and  506   b  include a first radial guide (e.g., channel)  514   a  and second radial guide  514   b , respectively, to guide movement of the slip plates  512   a  and  512   b  in a radial direction. The slip plates  512   a  and  512   b  sit adjacent to and contact wedges  516   a  and  516   b , respectively, such that a translation of the wedges  516   a  and  516   b  relative to the slip plates  512   a  and  512   b  drive the slip plates to extend radially outward along the radial guides  514   a  and  514   b . Each of the slip plates  512   a  and  512   b  include horizontal teeth  518  and vertical teeth  520 , for example, to dig into the surface of the well completion when expanded radially outward to lock the locking tool  500  both vertically (i.e., axially) and horizontally (i.e., rotationally). 
     The locking tool  500  can set the seal element  510  of the seal assembly  508 , the first slip plate  512   a , and the second slip plate  512   b  by a setting force acting on the elements of the locking tool  500  in an axial direction. For example, jarring the locking tool  500  (e.g., using the cable  202  and running tool  408  of  FIGS. 4A and 4B ), can break a frangible lock (e.g., shear pins  522 ) of the locking tool  500  to compress and radially expand the seal element  510 , and outwardly move the first slip plate  512   a , and second slip plate  512   b  to engage the well completion. The seal element  510  provides a seal at the recirculation isolator, and the first slip plate  512   a  and second slip plate  512   b  provide axial anchoring and rotational locking of the recirculation isolator relative to the well completion. 
     In some implementations, slip plates of a locking tool (e.g., slip plates  512   a  and  512   b  of locking tool  500  of  FIG. 5C ) includes a rotational guide structure, for example, to rotationally lock the slip plate while allowing for axial (e.g., longitudinal) movement of the slip plate.  FIG. 5D  is a partial cross-sectional side view of an example locking tool  550  with a slip plate  552  connected to a housing  554  with a rotational guide structure  556 . The rotational guide structure  556  includes a longitudinal (i.e., vertical) channel  558  at a radially inward surface of the slip plate  552  that engages with a protruding key  560  in the housing  554 . The protruding key  560  fits within the vertical channel  558  to substantially prevent lateral (i.e., rotational) movement of the slip plate  552  relative to the housing  554  while allowing longitudinal (i.e., axial) movement of the slip plate  552  relative to the housing  554 . The example locking tool  550  can be used in the locking tool  500  of  FIG. 5C , for example, to rotationally lock the slip plates  512   a  and  512   b.    
     In some implementations, multiple retrievable strings  400  can be deployed to act together or independently to provide higher output or redundancy to enhance long-term operation. 
       FIG. 6  illustrates steps of a method  600  as a flow chart. At step  602 , a retrievable string (such as the retrievable string  400 ) is positioned in a stator (such as the stator  302 ) of a completion string (well completion) installed in a well (such as the well  100 ). The retrievable string  400  can be positioned in the stator  302  such that the various corresponding components are aligned with each other. For example, the electromagnetic coil  350  of the stator  302  is aligned with the motor permanent magnet  450  of the retrievable string  400 . As described previously, the retrievable string  400  includes a rotating portion  410  and a non-rotating portion  420 . The rotating portion  410  includes a rotor (such as the rotor  402 ) and an impeller (such as the impeller  432 ) coupled to the rotor  402 . In some implementations, although the impeller  432  is part of the rotating portion  410  of the retrievable string  400 , the impeller  432  resides within the non-rotating portion  420  of the retrievable string  400 . As described previously, the retrievable string  400  can include at least one of an electric submersible pump, a compressor, or a blower. 
     In some implementations, the stator  302  is installed as part of the completion string in the well  100  before the retrievable string  400  is positioned in the stator  302 . In some implementations, an annulus between the stator  302  and the well  100  (such as the inner bore  116  between the casing  112  and the production tubing  128 ) is filled with a completion fluid, which includes a corrosion inhibitor. 
     In some implementations, positioning the retrievable string  400  in the stator  302  of the well completion includes running the retrievable string  400  into the well using a cable  202  and running tool  408  coupled to a running feature connection  406  of the recirculation isolator  404  of the retrievable string  400 , or applying fluidic pressure on a plug of the recirculation isolator  404  at an uphole end of the retrievable string  400  (this deployment method is sometimes referred to as a “pump down” method). In certain implementations, positioning the retrievable string  400  includes axially indexing the retrievable string  400  in the well completion with axial indexing features of the recirculation isolator  404  engaging a landing sub  304  or other component of the well completion. 
     At step  604 , the recirculation isolator  404  of the retrievable string  400  anchors the retrievable string  400  to the well completion (completion string). The recirculation isolator  404  can include one or more anchoring features or rotational locking features to anchor the retrievable string  400  to the well completion. The stator  302  can then be used to drive the rotor  402  of the retrievable string  400  to rotate the impeller  432 . 
     At step  606 , the recirculation isolator  404  of the retrievable string  400  seals the retrievable string  400  to the well completion. The recirculation isolator  404  can include one or more sealing features (e.g., expandable sealing element, packer, bridge plug, or other) to seal the retrievable string  400  to the well completion. 
     In some implementations, the stator  302  includes an electromagnetic coil (such as the electromagnetic coil  350 ), and the retrievable string  400  includes a motor permanent magnet (such as the motor permanent magnet  450 ) coupled to the rotor  402 . A magnetic field can be generated by the electromagnetic coil  350  of the stator  302  to engage the motor permanent magnet  450  of the retrievable string  400 , causing the rotor  402  (and the impeller  432 ) to rotate. The rotating impeller  432  induces fluid flow within the well  100 . In some implementations, one or more properties (such as a property of the well  100 , a property of the stator  302 , and a property of the retrievable string  400 ) are determined by a sensor of the stator  302 . Various operating parameters can then be adjusted based on the one or more determined properties. For example, the operating speed (rotation speed of the rotor  402 ) can be adjusted. The one or more determined properties can be used to determine shutdown or impending maintenance issues. The one or more determined properties can be used to assess changes in production fluid properties. The one or more determined properties can be used to assess changes in well characteristics over time. 
     The stator  302  can include an actuator (such as a thrust bearing actuator or radial bearing actuator), and the retrievable string  400  can include a bearing target (such as a thrust bearing target or radial bearing target). In some implementations, the bearing target includes a bearing permanent magnet. A mechanical load on the rotor  402  can be counteracted by generating a magnetic field using the actuator to engage the bearing target. In some implementations, the mechanical load on the rotor  402  is an axial (thrust) load on the rotor  402 . In some implementations, the mechanical load on the rotor  402  is a radial load on the rotor  402 . The stator  302  can include additional actuators, and the retrievable string  400  can include additional bearing targets. In some implementations, one or more of the actuators and one or more of the bearing targets are cooperatively configured to counteract axial loads on the rotor  402 , while the remaining actuators and the remaining bearing targets are cooperatively configured to counteract radial loads on the rotor  402 . Each of the actuators can be one of a thrust bearing electromagnetic coil, a radial bearing electromagnetic coil, a thrust bearing permanent magnet, and a radial bearing permanent magnet. 
     In the case that the retrievable string  400  requires maintenance, the retrievable string  400  can be decoupled from the completion string and retrieved from the well  100 . While the retrievable string  400  is decoupled from the completion string and retrieved from the well  100 , the stator  302  can remain in the well  100 . The retrievable string  400  can undergo maintenance and be re-deployed in the well  100 . In some implementations, another retrievable string (the same as or similar to the retrievable string  400 ) can be deployed in the well after the retrievable string  400  is positioned in and coupled to the well completion. 
     In some implementations, during deployment of the retrievable string  400 , cable  202  (e.g., a slickline) with the running tool  408  connects to the running feature connection  406  on the uphole longitudinal end of the recirculation isolator  404  to lower the retrievable string  400  into the well  100 . As the retrievable string  400  is lowered into the production tubing  128  by the cable  202  and reaches the desired, predetermined depth, the indexing features on the recirculation isolator  404  engage with the corresponding indexing features on the landing sub  304  (or other structure) of the well completion. Once the indexing features are fully engaged and the retrievable string  400  is set, the running tool  408  can disengage with the recirculation isolator  404 , for example, by manipulating (e.g., pulling or releasing with the facilitation of jarring up or down to shear a frangible element, e.g. shear pin, and/or activate a mechanism) the cable  202  to retrieve the cable  202  and the running tool  408  back to the surface  106  and leave the retrievable string  400  in the well  100 . Once the running tool  408  is disengaged from the running feature connection  406  of the recirculation isolator  404 , a pulling feature (e.g., the running feature connection  406 ) on the longitudinal uphole end of the recirculation isolator  404  is exposed for engagement of a pulling tool (e.g., running tool  408 ) during retrieval. The retrievable string  400  axially aligns (longitudinally along the production tubing  128 ) with the permanent completion or well completion. In this case, the motor and bearing magnets  450  on the retrievable string  400  also axially align with their corresponding motor and bearing stator coils  350 . 
     In some implementations, when the retrievable string  400  is set, the anchoring features on the recirculation isolator  404  engage with corresponding features on the landing sub  304 . In this case, the anchoring features can prevent either the uphole or downhole axial movements of the retrievable string  400  relative to the permanent completion or well completion by external forces. 
     In some implementations, when the retrievable string  400  is set, the anti-rotation features on the recirculation isolator  404  engage with the corresponding features on the landing sub  304 . In this case, the anti-rotation features can prevent the recirculation isolator  404  and the pump stator from rotating when the motor is rotating the pump rotor so that the recirculation isolator  404  and the pump stator are both axially aligned and radially fixed with the permanent completion or well completion. 
     In some implementations, during normal operation of the retrievable string when the motor is rotating the pump rotor, the recirculation isolator  404  allows the produced fluids to flow from the pump outlet through the check valve  414  of the recirculation isolator  404  into the production tubing  128  uphole of the recirculation isolator  404 . In the meantime, the recirculation isolator  404  includes sealing features with a sealing element on its radially outward diameter to seal against the inside diameter of the corresponding sealing bore of the landing sub  304  (or other feature) of the well completion, to prevent the produced fluids at the top of the recirculation isolator  404  to flow back to the intake of the pump through the clearance between the pump housing and the production tubing  128 . 
     In some implementations, before start-up or during shutdown of the retrievable string  400  when the motor has stopped rotating the pump rotor, the check valve  414  inside the recirculation isolator  404  restricts fluids and solids on the top of the recirculation isolator  404  from flowing back into the pump. In this case, solids will not settle and fill the pump during well shut-in so that the pump can be restarted without problems caused by solids. 
     In some implementations, during retrieval of the retrievable string  400 , the cable  202  with a pulling tool (e.g., running tool  408 ) is lowered into the well  100 . The pulling tool can engage with the pulling features (e.g., running feature connection  406 ) on the longitudinally uphole end of the recirculation isolator  404 . Once pulling features are fully engaged and the retrievable string  400  is ready to be unset, the recirculation isolator  404  can disengage with the landing sub  304  (and/or other features) of the well completion by manipulating (e.g., pulling or releasing with the facilitation of jarring up or down to shear a frangible element, e.g. shear pin, and/or activate a mechanism) the cable  202  to allow the indexing features, anchoring features, anti-rotational features, and sealing features of the recirculation isolator  404  to disengage with their corresponding features on the landing sub  304  and/or other portions of the well completion. The retrievable string  400  can then be retrieved from the well  100 , for example, back to the surface  106 . 
     Referring to  FIG. 7A , the system  700   a  of  FIG. 7A  includes a first subsystem  300   a  and a second subsystem  300   b , separate from each other and positioned at different locations along the production tubing  128 . The first subsystem  300   a  and the second subsystem  300   b  can include any of the components that were previously described with respect to the subsystem  300 . In some implementations, the first subsystem  300   a  and the second subsystem  300   b  are substantially the same (that is, they include the same components). The system  700   a  includes a first retrievable string  400   a  and a second retrievable string  400   b . The first retrievable string  400   a  can be positioned within the first subsystem  300   a , and the second retrievable string  400   b  can be positioned within the second subsystem  300   b . The first retrievable string  400   a  and the second retrievable string  400   b  can include any of the components that were previously described with respect to the retrievable string  400  or  400 ′. In some implementations, the first retrievable string  400   a  and the second retrievable string  400   b  are substantially the same. The first subsystem  300   a  and the first retrievable string  400   a  can be coupled together with a landing sub  304   a  of the first subsystem  300   a  and a corresponding recirculation isolator of the first retrievable string  400   a . The first subsystem  300   a  and the first retrievable string  400   a  can co-operate to induce fluid flow within the well. The second subsystem  300   b  and the second retrievable string  400   b  can be coupled together with a second landing sub  304   b  of the second subsystem  300   b  and a corresponding recirculation isolator of the second retrievable string  400   b . The second subsystem  300   b  and the second retrievable string  400   b  can co-operate to induce fluid flow within the well. 
     The system  700   b  of  FIG. 7B  is substantially similar to the system  700   a . The retrievable string  400  of system  700   b  can co-operate with either the first subsystem  300   a  or the second subsystem  300   b  to induce fluid flow within the well. For example, the retrievable string  400  can be positioned within and coupled to the first subsystem  300   a  with the landing sub  304   a  and the recirculation isolator of the retrievable string  400 . The retrievable string  400  can co-operate with the first subsystem  300   a  to induce fluid flow at a first location within the well (for example, at the location of the first subsystem  300   a ). The retrievable string  400  can be de-coupled from the first subsystem  300   a  and positioned within and coupled to the second subsystem  300   b  with the second landing sub  304   b  and the recirculation isolator of the retrievable string  400 . The retrievable string  400  can co-operate with the second subsystem  300   b  to induce fluid flow at a second location within the well (for example, at the location of the second subsystem  300   b ). 
     The system  700   c  of  FIG. 7C  is substantially similar to the system  700   a , but the first subsystem  300   a  and the second subsystem  300   b  of system  700   c  are connected to each other. The system  700   d  of  FIG. 7D  is substantially similar to the system  700   b , but the first subsystem  300   a  and the second subsystem  300   b  of system  700   d  are connected to each other. In such cases, the first subsystem  300   a  and second subsystem  300   b  together can be considered a single subsystem (for example, the subsystem  300 ). For example, the stator of the first subsystem  300   a  and the stator of the second subsystem  300   b  can each be considered sub-stators making up a single stator. 
     Although systems  700   a  and  700   c  are shown in  FIGS. 7A and 7C  (respectively) as having two subsystems ( 300   a ,  300   b ) and two retrievable strings ( 400   a ,  400   b ), the systems  700   a  and  700   c  can optionally include additional subsystems (for example, the same as or similar to the subsystem  300 ) and additional retrievable strings (for example, the same as or similar to the retrievable string  400 ), each of which can be either connected to each other or positioned at different locations in the well  100 . Although systems  700   b  and  700   d  are shown in  FIGS. 7B and 7D  (respectively) as having two subsystems ( 300   a ,  300   b ) and one retrievable string ( 400 ), the systems  700   b  and  700   d  can optionally include additional subsystems (for example, the same as or similar to the subsystem  300 ) and additional retrievable strings (for example, the same as or similar to the retrievable string  400 ), each of which can be either connected to each other or positioned at different locations in the well  100 . 
     In this disclosure, the terms “a,” “an,” or “the” are used to include one or more than one unless the context clearly dictates otherwise. The term “or” is used to refer to a nonexclusive “or” unless otherwise indicated. The statement “at least one of A or B” has the same meaning as “A, B, or A and B.” In addition, it is to be understood that the phraseology or terminology employed in this disclosure, and not otherwise defined, is for the purpose of description only and not of limitation. Any use of section headings is intended to aid reading of the document and is not to be interpreted as limiting; information that is relevant to a section heading may occur within or outside of that particular section. 
     In this disclosure, “approximately” can mean a deviation or allowance of up to 10 percent (%) and any variation from a mentioned value is within the tolerance limits of any machinery used to manufacture the part. Values expressed in a range format should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a range of “0.1% to about 5%” or “0.1% to 5%” should be interpreted to include about 0.1% to about 5%, as well as the individual values (for example, 1%, 2%, 3%, and 4%) and the sub-ranges (for example, 0.1% to 0.5%, 1.1% to 2.2%, 3.3% to 4.4%) within the indicated range. The statement “X to Y” has the same meaning as “about X to about Y,” unless indicated otherwise. Likewise, the statement “X, Y, or Z” has the same meaning as “about X, about Y, or about Z,” unless indicated otherwise. “About” can allow for a degree of variability in a value or range, for example, within 10%, within 5%, or within 1% of a stated value or of a stated limit of a range. 
     While this disclosure contains many specific implementation details, these should not be construed as limitations on the scope of the subject matter or on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular implementations. Certain features that are described in this disclosure in the context of separate implementations can also be implemented, in combination, in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations, separately, or in any suitable sub-combination. Moreover, although previously described features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. 
     Particular implementations of the subject matter have been described. Other implementations, alterations, and permutations of the described implementations are within the scope of the following claims as will be apparent to those skilled in the art. While operations are depicted in the drawings or claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed (some operations may be considered optional), to achieve desirable results. 
     Accordingly, the previously described example implementations do not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure.