You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD 
       [0001]    This relates primarily to the field of oil and gas exploration and production. More particularly, this relates to harvesting energy with a downhole oilfield tool to perform work downhole. 
       BACKGROUND 
       [0002]    An appreciable fraction of oilfield services are provided by lowering tools down a well to perform particular tasks. Possible tasks include formation evaluation (e.g. logging in open hole and cased wells), opening and closing of valves, analyzing downhole fluids, taking fluid samples, removal of scale build-up (e.g. in producing wells). Some of the downhole oilfield tools are conveyed with cables of appropriate mechanical strength. Additionally, the cables may carry electrical power to the tools as well provide a communication link. Cables that carry power and provide downhole communication are generally called “wireline” cables. 
         [0003]    However, because of cost constraints associated with wireline operations, many downhole applications use more simple cables that do not have electrical capability. These simple cables are typically called “slick line” cables. In slick line applications, the energy required to power the tool once it is down in the well generally comes from batteries that are included with or added to the tool. Nevertheless, the batteries are expensive, occupy a sizable amount of tool space, and are typically not very environmentally friendly. 
       SUMMARY 
       [0004]    The present disclosure addresses weaknesses of the prior art described above and others. Specifically, one embodiment provides an apparatus comprising a downhole oilfield system. The downhole oilfield system comprises a conveyance and a downhole tool attached to the conveyance. The downhole tool comprises a work performing module (e.g. for logging and/or fluid analysis, etc.) and a potential energy harvesting device. The potential energy harvesting device may be capable of converting potential energy (including pressure fluctuations) into kinetic energy, electrical energy, or stored energy for later use. In one embodiment, the potential energy harvesting device is configured to convert and store potential energy as a result of lowering the downhole tool into a well. In one embodiment, the potential energy harvesting device comprises a turbine/generator pair. In one embodiment, the generator is electrically connected to a battery. 
         [0005]    In one embodiment, the potential energy harvesting device comprises a hollow mandrel having an interior portion and at least one side opening in the mandrel leading to the interior portion. In one embodiment, the turbine is arranged in the interior portion. In another embodiment, the potential energy harvesting device comprises at least one external wheel configured to contact and roll along a well wall, and an energy conversion module operatively connected to the at least one external wheel. The energy conversion module may comprise a generator. In one embodiment, an energy storage module is operatively connected to the at least one external wheel. In one embodiment, the energy storage module comprises a flywheel, and the apparatus may further comprise a belt or chain connecting the at least one external wheel to the flywheel. In one embodiment, the energy storage module comprises a generator and a battery. 
         [0006]    In one embodiment, the potential energy harvesting device comprises piezoelectric elements electrically connected to an energy storage apparatus, such as a battery. In one embodiment, the potential energy harvesting device comprises a hollow mandrel having an interior portion, at least one opening in the mandrel leading to the interior portion (the interior portion comprising an inside surface geometry configured to cause pressure fluctuations when fluids pass through the interior portion), and the inside surface comprises the piezoelectric elements. 
         [0007]    In some embodiments of the apparatus, the conveyance comprises a slick line, wireline, or coiled tubing. In one embodiment, the work performing module comprises a logging module or a fluid analysis module. 
         [0008]    One aspect provides a method comprising moving a downhole oilfield tool through a borehole, harvesting energy from the downhole oilfield tool—the harvesting comprising collecting energy from the moving of the downhole tool through a borehole—and storing the energy collected from the moving of the downhole tool through the borehole. One method further comprising performing work downhole with the stored energy. In one aspect, the work comprises one or more of: logging the borehole, opening/closing a valve, analyzing downhole fluids, and removing scale build. 
         [0009]    In one aspect of the method, the harvesting comprises flowing fluids through the downhole oilfield tool, rotating a turbine with the flowing fluids, and driving a generator with the turbine. In one aspect, the flowing comprises one or more of: lowering the downhole oilfield tool through the fluids, and oscillating the downhole oilfield tool through the fluids. In one aspect, the harvesting comprises rolling at least one wheel of the downhole oilfield tool along a wall of the borehole, and converting the rolling motion into a usable, stored energy form. In one aspect, the harvesting comprises rolling a plurality of wheels of the downhole oilfield tool along a cased wall of the borehole. In one aspect, the harvesting comprises rolling at least one wheel of the downhole oilfield tool along a wall of the borehole, and rotating a flywheel with the rolling of the at least one wheel. In one embodiment, the harvesting comprises rolling at least one wheel of the downhole oilfield tool along a wall of the borehole, and rotating a generator with the at least one wheel. In one aspect, the harvesting comprises providing an interior channel in the downhole oilfield tool, flowing fluids through the interior channel, causing flow fluctuations through the interior channel with appropriate surface geometry, generating pressure changes from the flow fluctuations, and converting the pressure changes into electrical energy with an active material. The active material may comprise a piezoelectric material. In one aspect, the flowing comprises lowering the downhole oilfield tool through the fluids and/or oscillating the downhole oilfield tool through the fluids. 
         [0010]    One embodiment provides an apparatus comprising a downhole slick line tool system. The downhole slick line tool system comprises a slick line, a slick line tool attached to the slick line, the slick line tool comprising a work performing module and an energy harvesting device. The energy harvesting device comprises a mandrel having a channel therethrough, a turbine on a rod disposed in the channel, a generator connected to the rod, and electrical circuitry between the generator and the work performing module. In one embodiment, the work performing module comprises a formation evaluation device. 
         [0011]    One aspect provides a method comprising converting potential energy in the form of an oilfield tool mass suspended above a borehole and subject to a gravitational force into one of: stored, reusable kinetic energy or stored electrical energy; and using the stored, reusable kinetic energy or stored electrical energy to perform a task downhole. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The accompanying drawings illustrate certain embodiments and are a part of the specification. 
           [0013]      FIG. 1  illustrates a borehole in cross-section and a downhole oilfield system in partial cross-section. The downhole oilfield system includes an energy harvesting device—in the case of  FIG. 1  a turbine/generator pair. 
           [0014]      FIG. 2A  illustrates the borehole of  FIG. 1  in cross-section and another downhole oilfield system in partial cross-section. The downhole oilfield system of  FIG. 2A  includes a pair of rolling wheels to harvest energy as a downhole tool moves. 
           [0015]      FIG. 2B  illustrates the borehole of  FIG. 1  in cross-section and another downhole oilfield system in partial cross-section. The downhole oilfield system of  FIG. 2B  includes a pair of angled rolling wheels to harvest energy as a downhole tool moves 
           [0016]      FIG. 3  illustrates the borehole of  FIG. 1  in cross-section and another downhole oilfield system in partial cross-section. The downhole oilfield system of  FIG. 2A  includes an active surface for harvesting energy resulting from changes in pressure. 
       
    
    
       [0017]    Throughout the drawings, identical reference numbers indicate similar, but not necessarily identical elements. While the principles described herein are susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention includes all modifications, equivalents and alternatives falling within the scope of the appended claims. 
       DETAILED DESCRIPTION 
       [0018]    Illustrative embodiments and aspects of the invention are described below. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, that will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
         [0019]    Reference throughout the specification to “one embodiment,” “an embodiment,” “some embodiments,” “one aspect,” “an aspect,” or “some aspects” means that a particular feature, structure, method, or characteristic described in connection with the embodiment or aspect is included in at least one embodiment of the present invention. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, methods, or characteristics may be combined in any suitable manner in one or more embodiments. The words “including” and “having” shall have the same meaning as the word “comprising.” 
         [0020]    Moreover, inventive aspects lie in less than all features of a single disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention. 
         [0021]    Turning now to the drawings, and in particular to  FIG. 1 , one embodiment of a downhole oilfield system  100  is disclosed. The downhole oilfield system  100  includes a conveyance such as a slick line  102 . The conveyance may also comprise coiled tubing, a wireline, or other conveyance. As shown in  FIG. 1 , a downhole tool  104  is attached to the slick line  102 . The downhole tool  104  includes a work performing module  106 . The work performing module  106  may include any device for performing work downhole, including, but not limited to a logging device, a fluid analyzer, a descaler, and a mechanical mover (e.g. valve opener). 
         [0022]    In some embodiments, the downhole tool  104  also includes a potential energy harvesting device  108 . The potential energy harvesting device  108  may be capable of converting potential energy (which includes pressure fluctuations) into kinetic energy, electrical energy, or stored energy for later use. In one embodiment, the potential energy harvesting device  108  is configured to convert and store potential energy as a result of lowering the downhole tool  104  into a well or borehole  110 . The potential energy harvesting device  108  may take on any form. In the embodiment of  FIG. 1 , the potential energy harvesting device  108  comprises a turbine/generator pair. The turbine/generator pair includes at least one turbine  112  (or a plurality of turbines as shown in  FIG. 1 ) coupled to a generator  114 . A rod  116  may be common to both the turbine  112  and the generator  114 . Further, in one embodiment, the generator  114  is electrically connected to a battery  118 . The battery  118  may then store energy to perform work (for example by the work performing module  106 ). The battery  118  may therefore be electrically connected to any electrically operated machine. 
         [0023]    As shown in  FIG. 1 , the potential energy harvesting device  108  may include a hollow mandrel  120 . The hollow mandrel  120  has an interior portion  122  and at least one opening  124  providing for fluid communication between the borehole  110  and the interior portion  122 . In the embodiment of  FIG. 1 , there are a plurality of side openings  124  leading into the interior portion  122 , but any other openings may be used. In the embodiment of  FIG. 1 , the turbine  112  is arranged in the interior portion  122 .  FIG. 1  illustrates the turbine  112  centrally located in the interior portion  122 , but it could also be offset or otherwise arranged. 
         [0024]    A mentioned above, there is often a considerable amount of potential energy that is typically lost by conventional downhole tools as they moves from the surface down through a borehole. However, according to principles described herein, methods and apparatus are employed to recover and/or store some of the potential energy associated with movement of the downhole tool  104 . The downhole tool  104  of  FIG. 1  is equipped with the potential energy harvesting device  108  that harvests energy as the tool is moved through the borehole  110 . Movement of the downhole tool  104  may be due to the force of gravity. However, in some aspects, movement is generated by imposing an oscillatory up/down motion from the surface, provided the downhole tool is suspended by a conveyance of appropriate mechanical strength. 
         [0025]    According to the embodiment of  FIG. 1 , harvesting potential energy is accomplished by flowing fluids through the interior portion  122  as the downhole tool  104  traverses the borehole  110 . The openings  124  allow downhole fluids to pass through the interior portion  122  as the downhole tool  104 , and the flowing fluids rotate the turbine  112 . The turbine  112  drives the rod  116 , and the rod  116  drives the generator  114 . The generator may produce electricity that can be used as it is produced or stored by the battery  118 . It will be understood by one or ordinary skill in the art having the benefit of this disclosure that the flowing by the turbine  112  is not necessarily inside the interior portion  112  and can be facilitated simply lowering the downhole tool  104  through the fluids or oscillating the downhole tool  104  through the fluids. The battery  118  may then operate the work performing module  106 , and may eliminate the need for separate battery power or wired power from the surface. Accordingly, the apparatus of  FIG. 1  may especially useful for slick line applications. The work performing module may consume energy from the generator  114  or the battery  118  to log the borehole  110 , cause mechanical movement (for example to open or close a valve), analyze downhole fluids, remove scale build, etc. 
         [0026]    Alternate embodiment are disclosed in  FIGS. 2A and 2B . Similar to the embodiment of  FIG. 1 , the embodiment of  FIGS. 2A and 2B  provide a downhole oilfield system  200 . The downhole oilfield system  200  includes a conveyance such as a slick line  202 . A downhole tool  204  is attached to the slick line  202 . The downhole tool  204  includes a work performing module  206 . The work performing module  206  may include any device for performing work downhole. 
         [0027]    The downhole tool  204  also includes a potential energy harvesting device  208 . The potential energy harvesting device  208  is capable of converting potential energy into kinetic energy, electrical energy, or stored energy for later use. As with the embodiments described above, the potential energy harvesting device  208  of  FIG. 2A  is configured to convert and store potential energy as a result of lowering or moving the downhole tool  204  into (or out of) the well or borehole  110 . In the embodiment of  FIG. 2A , the potential energy harvesting device  208  comprises at least one wheel or other rolling members. For example, as shown in  FIG. 2A , the potential energy harvesting device  208  includes two external wheels  212 ,  213  configured to contact and roll along a well wall  226 , especially a cased wall. The two external wheels  212 ,  213  are operatively connected to an energy conversion and/or storage module  216 . The energy conversion and/or storage module  216  may comprise a generator. However, in the embodiment of  FIG. 2A , the energy conversion and/or storage module comprises first and second flywheels  220 ,  222 . The first external wheel  212  is connected to the first flywheel  220  by a first belt or chain  224 , and the second external wheel  213  is connected to the second flywheel  222  by a second belt or chain  225 . It will be understood by one of ordinary skill in the art having the benefit of this disclosure that any number of external wheels and flywheels may be used, along with any other connection mechanism therebetween. The flywheels  220 ,  222  may store the energy until used mechanically, or they may power a generator or other device. 
         [0028]    Accordingly, in some aspects, the harvesting potential energy comprises rolling at least one wheel  212 ,  213  of the downhole tool  204  along the wall  226  of the borehole  110 , and converting the rolling motion into a usable, stored energy form. In one aspect, converting the rolling motion into a usable, stored energy form includes rolling at least one wheel  212 ,  213  of the downhole tool  204  along the wall  226  of the borehole  110 , and rotating the associated flywheel  220 ,  222  with the rolling of the at least one wheel  212 ,  213 . However, the rolling wheels  212 ,  213 , may also rotate one or more generators. 
         [0029]    Another embodiment is disclosed in  FIG. 3 . Similar to the embodiments of  FIGS. 1-2 , the embodiment of  FIG. 3  provides a downhole oilfield system  300 . The downhole oilfield system  300  includes a conveyance such as a slick line  302 . A downhole tool  304  is attached to the slick line  302 . The downhole tool  304  includes a work performing module  306 . The work performing module  306  may include any device for performing work downhole. 
         [0030]    The downhole tool  304  also includes a potential energy harvesting device  308 . The potential energy harvesting device  308  is capable of converting potential energy in the form of pressure changes into electrical energy for concurrent or later use. As with the embodiments described above, the potential energy harvesting device  308  of  FIG. 3  is configured to convert and store potential energy as a result of lowering or moving the downhole tool  304  into (or out of) the well or borehole  110 . In the embodiment of  FIG. 3 , the potential energy harvesting device  308  comprises an active material such as piezoelectric elements  330  electrically connected to an energy storage apparatus, such as a battery  118  ( FIG. 1 ). As shown in  FIG. 3 , the potential energy harvesting device  308  comprises a hollow mandrel  320  having an interior portion  322 , and at least one opening  324  in the mandrel leading to the interior portion  322 . The interior portion  322  exhibits an inside surface geometry configured to cause pressure fluctuations when fluids pass therethrough, and the inside surface comprises the piezoelectric elements  330 . For example, the inside surface geometry of the interior portion  322  may alternate between increases and decreases in diameter as shown. Changes in internal diameter with a flow therethrough results in pressure fluctuations. The piezoelectric elements convert pressure fluctuations into electrical currents, which can be used immediately to perform work to charge a battery. 
         [0031]    Accordingly, in one aspect, the lowering (or raising/oscillating) the downhole oilfield tool  304  through fluids in the borehole  110  causes pressure fluctuations in the interior portion  322 . Pressure fluctuations may be converted by the piezoelectric elements  330  into electrical currents that charge batteries and/or power work from the work producing module  306 . 
         [0032]    The preceding description has been presented only to illustrate and describe certain embodiments. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. 
         [0033]    The embodiments and aspects were chosen and described in order to best explain the principles of the invention and its practical application. The preceding description is intended to enable others skilled in the art to best utilize the principles in various embodiments and aspects and with various modifications as are suited to the particular use contemplated.

Summary:
Methods and apparatus for harvesting energy while moving a tool through a well are shown and described. The harvested energy can be used by the tool to perform work once it reaches an intended location in the well, or along the way. A considerable amount of potential energy is typically lost by oilfield tools as they move down through a borehole. Methods and apparatus described herein recover and/or store some of the energy during the downward movement of the tool.