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[0001]    This application is the U.S. National Stage under 35 U.S.C. §371 of International Patent Application No. PCT/US2010/048100 filed Sep. 8, 2010, entitled “Downhole Piston Accumulator System.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND 
       [0003]    During the drilling and completion of oil and gas wells, it may be necessary to engage in ancillary operations, such as evaluating the production capabilities of formations intersected by the wellbore. For example, after a well or well interval has been drilled, zones of interest are often tested to determine various formation properties or formation fluid characteristics, or to gather fluid samples. Examples of information obtained include fluid identification, fluid type, fluid quality, formation permeability, formation temperature, formation pressure, bubblepoint and formation pressure gradient. These tests are performed in order to determine whether commercial exploitation of the intersected formations is viable and how to optimize production. The acquisition of accurate data from the wellbore is critical to the optimization of hydrocarbon wells. This wellbore data can be used to determine the location and quality of hydrocarbon reserves, whether the reserves can be produced through the wellbore, and for well control during drilling operations. 
         [0004]    A downhole tool is used to acquire and test a sample of fluid from the formation. Formation testing tools may be used in conjunction with wireline logging operations or as a component of a logging-while-drilling (LWD) or measurement-while-drilling (MWD) package. In wireline logging operations, the drill string is removed from the wellbore and measurement tools are lowered into the wellbore using a heavy cable (wireline) that includes wires for providing power and control from the surface. In LWD and MWD operations, the measurement tools are integrated into the drill string and are ordinarily powered by batteries and controlled by either on-board or remote control systems. In these systems, a probe assembly may be used for engaging the borehole wall and acquiring the formation fluid samples. 
         [0005]    With LWD/MWD testers, the testing equipment is subject to harsh conditions in the wellbore during the drilling process that can damage and degrade the formation testing equipment before and during the testing process. These harsh conditions include vibration and torque from the drill bit, exposure to drilling mud, drilled cuttings, and formation fluids, hydraulic forces of the circulating drilling mud, high downhole temperatures, and scraping of the formation testing equipment against the sides of the wellbore. Sensitive electronics, sensors and even mechanical components must be robust enough to withstand the pressures and temperatures, and especially the extreme vibration and shock conditions of the drilling environment, yet maintain accuracy, repeatability, and reliability. 
         [0006]    As downhole testing equipment gets progressively smaller to accommodate smaller boreholes and increasingly complex tools, the high pressures and temperatures of the downhole environment are pushing the limits of conventional testing apparatus. The embodiments disclosed herein overcome these deficiencies and others in the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    For a detailed description of exemplary embodiments of the invention, reference will now be made to the accompanying drawings in which: 
           [0008]      FIG. 1  is a schematic view, partly in cross-section, of a drilling apparatus with a formation tester; 
           [0009]      FIG. 2  is a schematic view, partly in cross-section, of a formation tester conveyed by wireline; 
           [0010]      FIG. 3  is a schematic view, partly in cross-section, of a formation tester disposed on a wired drill pipe connected to a telemetry network; 
           [0011]      FIG. 4  is a cross-section view of a section of wired drill pipe; 
           [0012]      FIG. 5  is a side view, partly in cross-section, of a drill collar including a formation probe assembly; 
           [0013]      FIG. 6  is a perspective view of an embodiment of a piston of a piston accumulator system; 
           [0014]      FIG. 7  is an end view of the piston of  FIG. 6 ; 
           [0015]      FIG. 8  is a longitudinal cross-section view of the piston of  FIGS. 6 and 7 ; 
           [0016]      FIG. 9  is a longitudinal cross-section view of an embodiment of an assembled piston accumulator system including the piston of  FIGS. 6-8 ; 
           [0017]      FIG. 9A  is an enlarged view of the seal assembly of the piston of  FIG. 9 ; 
           [0018]      FIG. 10  is a cross-section view of an alternative coupling between the piston tube and the end coupler to capture the spacer, in another piston accumulator system; 
           [0019]      FIG. 11  is a flow chart of a method for accumulating formation fluids downhole during a large pressure-temperature cycle using embodiments of the piston accumulator system; and 
           [0020]      FIG. 12  is a flow chart of another method for accumulating formation fluids downhole during a large pressure-temperature cycle using embodiments of the piston accumulator system. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In the drawings and description that follow, like parts are typically marked throughout the specification and drawings with the same reference numerals. The drawing figures are not necessarily to scale. Certain features of the disclosure may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. The present disclosure is susceptible to embodiments of different forms. Specific embodiments are described in detail and are shown in the drawings, with the understanding that the present disclosure is to be considered an exemplification of the principles of the disclosure, and is not intended to limit the disclosure to that illustrated and described herein. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. For example, the piston accumulator embodiments have application in the field of high pressure liquid chromatography. 
         [0022]    In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Unless otherwise specified, any use of any form of the terms “connect”, “engage”, “couple”, “attach”, or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the elements and may also include indirect interaction between the elements described. Reference to up or down will be made for purposes of description with “up”, “upper”, “upwardly” or “upstream” meaning toward the surface of the well and with “down”, “lower”, “downwardly” or “downstream” meaning toward the terminal end of the well, regardless of the well bore orientation. In addition, in the discussion and claims that follow, it may be sometimes stated that certain components or elements are in fluid communication. By this it is meant that the components are constructed and interrelated such that a fluid could be communicated between them, as via a passageway, tube, or conduit. Also, the designation “MWD” or “LWD” are used to mean all generic measurement while drilling or logging while drilling apparatus and systems. The various characteristics mentioned above, as well as other features and characteristics described in more detail below, will be readily apparent to those skilled in the art upon reading the following detailed description of the embodiments, and by referring to the accompanying drawings. 
         [0023]    Referring initially to  FIG. 1 , a drilling apparatus including a formation tester is shown. A formation tester  10  is shown enlarged and schematically as a part of a bottom hole assembly  6  including a sub  13  and a drill bit  7  at its distal most end. The bottom hole assembly  6  is lowered from a drilling platform  2 , such as a ship or other conventional land platform, via a drill string  5 . The drill string  5  is disposed through a riser  3  and a well head  4 . Conventional drilling equipment (not shown) is supported within a derrick  1  and rotates the drill string  5  and the drill bit  7 , causing the bit  7  to form a borehole  8  through formation material  9 . The drill bit  7  may also be rotated using other means, such as a downhole motor. The borehole  8  penetrates subterranean zones or reservoirs, such as reservoir  11 , that are believed to contain hydrocarbons in a commercially viable quantity. An annulus  15  is formed thereby. In addition to the formation tester  10 , the bottom hole assembly  6  contains various conventional apparatus and systems, such as a down hole drill motor, a rotary steerable tool, a mud pulse telemetry system, MWD or LWD sensors and systems, and others known in the art. 
         [0024]    In some embodiments, and with reference to  FIG. 2 , a formation testing tool  60  is disposed on a tool string  50  conveyed into the borehole  8  by a cable  52  and a winch  54 . The testing tool includes a body  62 , a sampling assembly  64 , a backup assembly  66 , analysis modules  68 ,  84  including electronic devices, a flowline  82 , a battery module  65 , and an electronics module  67 . The formation tester  60  is coupled to a surface unit  70  that may include an electrical control system  72  having an electronic storage medium  74  and a control processor  76 . In other embodiments, the tool  60  may alternatively or additionally include an electrical control system, an electronic storage medium and a processor. 
         [0025]    Referring to  FIG. 3 , a telemetry network  100  is shown. A formation tester  120  is coupled to a drill string  101  formed by a series of wired drill pipes  103  connected for communication across junctions using communication elements. It will be appreciated that work string  101  can be other forms of conveyance, such as wired coiled tubing. The downhole drilling and control operations are interfaced with the rest of the world in the network  100  via a top-hole repeater unit  102 , a kelly  104  or top-hole drive (or, a transition sub with two communication elements), a computer  106  in the rig control center, and an uplink  108 . The computer  106  can act as a server, controlling access to network  100  transmissions, sending control and command signals downhole, and receiving and processing information sent up-hole. The software running the server can control access to the network  100  and can communicate this information via dedicated land lines, satellite uplink  108 ), Internet, or other means to a central server accessible from anywhere in the world. The formation tester  120  is shown linked into the network  100  just above the drill bit  110  for communication along its conductor path and along the wired drill string  101 . 
         [0026]    The formation tester  120  may include a plurality of transducers  115  disposed on the formation tester  120  to relay downhole information to the operator at surface or to a remote site. The transducers  115  may include any conventional source/sensor (e.g., pressure, temperature, gravity, etc.) to provide the operator with formation and/or borehole parameters, as well as diagnostics or position indication relating to the tool. The telemetry network  100  may combine multiple signal conveyance formats (e.g., mud pulse, fiber-optics, acoustic, EM hops, etc.). It will also be appreciated that software/firmware may be configured into the formation tester  120  and/or the network  100  (e.g., at surface, downhole, in combination, and/or remotely via wireless links tied to the network). 
         [0027]    Referring briefly to  FIG. 4 , sections of wired drill pipe  103  are enlarged for clarity. The wired drill pipe  103  includes conductors  150  that traverse the entire length of the pipe sections. Communication elements  155  allow the transfer of power and/or data between the pipe sections  103 . A data/power signal may be transmitted along a pipe section of the wired drill string, such as the pipe section with formation tester  120  ( FIG. 3 ), from one end through the conductor(s)  150  to the other end across the communication elements  155 . In some embodiments, the conductor(s)  150  comprise coaxial cables, copper wires, optical fiber cables, triaxial cables, and twisted pairs of wire. The conductor(s)  150  may be disposed through a hole formed in the walls of the outer tubular members of the pipes  103 . The communication elements  155  may comprise inductive couplers, direct electrical contacts, optical couplers, and combinations thereof. Portions of the wired drill pipes  103  may be subs or other connections means. The ends of subs or connections means of the wired subs  103  are configured to communicate within the downhole telemetry network  100 . 
         [0028]    Referring next to  FIG. 5 , an embodiment of an MWD formation probe collar section  200  is shown in detail, which may be used as the tool  10  in  FIG. 1  or the tool  120  in  FIG. 3 . A drill collar  202  houses the formation tester or probe assembly  210 . The probe assembly  210  includes various components for operation of the probe assembly  210  to receive and analyze formation fluids from the earth formation  9  and the reservoir  11 . An extendable probe member  220  is disposed in an aperture  222  in the drill collar  202  and extendable beyond the drill collar  202  outer surface, as shown. The probe member  220  is retractable to a position recessed beneath the drill collar  202  outer surface. The probe assembly  210  may include a recessed outer portion  203  of the drill collar  202  outer surface adjacent the probe member  220 . The probe assembly  210  includes a draw down or piston accumulator assembly  208 , a sensor  206 , a valve assembly  212  having a flow line shutoff valve  214  and equalizer valve  216 , and a drilling fluid flow bore  204 . At one end of the probe collar  200 , generally the lower end when the tool  10  is disposed in the borehole  8 , is an optional stabilizer  230 , and at the other end is an assembly  240  including a hydraulic system  242  and a manifold  244 . 
         [0029]    The piston assembly  208  includes a piston chamber  252  containing a piston  254  and a manifold  256  including various fluid and electrical conduits and control devices. The piston assembly  208 , the probe  220 , the sensor  206  (e.g., a pressure gauge) and the valve assembly  212  communicate with each other and various other components of the probe collar  200 , such as the manifold  244  and hydraulic system  242 , as well as the tool  10  via conduits  224   a ,  224   b ,  224   c  and  224   d . The conduits  224   a ,  224   b ,  224   c ,  224   d  include various fluid flow lines and electrical conduits for operation of the probe assembly  210  and probe collar  200 . 
         [0030]    An embodiment of a piston accumulator assembly or system for use in the various systems described above will now be described. Referring now to  FIG. 6 , a piston  300  includes a first end portion  302 , a second end portion  304 , and an intermediate portion  306  having a seal assembly recess or o-ring groove  308 . The end portion  304  may be configured to receive a hydrocarbon sample (e.g., crude oil). The second end portion  302  may be configured to receive a hydraulic fluid (e.g., water). 
         [0031]    In exemplary embodiments, the piston  300  is nonmetallic. In further embodiments, the piston  300  is made from polytetrafluoroethylene (PTFE), or Teflon, plus fiberglass. In certain embodiments, the piston is made from a composition of Teflon plus fiberglass called Rulon. The above-mentioned materials make the piston  300  relatively “soft” compared to surrounding metallic components, as described more fully below. The Teflon plus fiberglass composite material may be adapted for systems accommodating, for example, 20,000 to 25,000 p.s.i., and a wide temperature range up to about 450° F., as is sometimes present in the downhole environment. Another exemplary operating range of the soft piston  300  is 20,000 p.s.i. and 350° F. The small diameter or low volume of the chamber in which the soft piston moves, and the high pressure application of the soft piston makes conventional systems inappropriate. The wide temperature range also complicates the working environment of the soft piston. 
         [0032]    To further condition the soft piston  300  for operation in the environments described, the outer surface of the soft piston  300  may be polished. In further embodiments, the soft piston  300  may also be heat treated at 100-150° F., or alternatively at 350° F. Heat treating and/or polishing the soft piston  300  creates good tolerance between the soft piston  300  and the metallic cylinder or tube in which it reciprocates during use. Such treatments also optimize the sealing capability between the soft piston and the tube at widely varying temperatures, including low temperatures. In some embodiments, the piston/tube tolerance and sealing capability is customized for a preferred operating range by variously tweaking the composition of the Rulon, adjusting the amount or type of polishing, and/or adjusting the temperature of the heat treatment. Because the soft Rulon piston  300  is a thermoplastic, a desired actuating pressure of the soft piston can be achieved for a given temperature. Thus, the various characteristics of the soft piston  300  just described can be adjusted for a predetermined and/or anticipated operating range of pressure and/or temperature for the soft piston. In extreme examples of low operating temperatures, such as down to −70° F., the soft piston can be customized to include a silicone seal in the seal recess  308 . 
         [0033]    In the embodiments just described, the soft thermoplastic or Rulon piston material is mechanically robust and chemically unreactive. In these embodiments, and in the downhole environment with operating ranges described, the piston is soft relative to the surrounding tube such that damage to the soft piston is avoided, the soft piston does not cold flow, the soft piston includes a low coefficient of friction, and the soft piston includes close tolerances and sealing capabilities. These characteristics are adjustable based on the predetermined or anticipated operating ranges by manipulating the soft piston specifications described above. 
         [0034]    Referring to  FIGS. 7 and 8 , an end view and a longitudinal cross-section view of the soft piston  300  are shown. The soft piston  300  includes a cavity  310  in the end portion  302  for receiving a magnet or other sensor device. The magnet may be secured in the cavity  310  with epoxy. The seal assembly recess  308  may receive a seal assembly that can be custom fitted to get the preferred tolerances and operating range of the piston system, as referenced above and detailed further below. 
         [0035]    In  FIG. 9 , an embodiment of an assembled piston accumulator system  330  is shown in longitudinal cross-section and including the soft piston  300 . The piston accumulator system  330  includes a cylindrical housing or tube  332  captured between two end caps or couplers  334 ,  336 . The cylinder  332  may be high pressure tubing, such as an Autoclave high pressure nipple or cylinder. The inside surface  360  of the cylinder bore may be honed and/or polished. The soft or nonmetallic piston as previously described, and engaged with the honed surface  360  of the cylinder  332 , provides a desirable interaction between the piston and the cylinder. 
         [0036]    A spacer  338  is captured between the end cap  334  and the cylinder  332  and forms a chamber  342  with the soft piston  300  (such as for hydrocarbon samples taken from the formation, e.g., crude oil). A spacer  340  is captured between the end cap  336  and the cylinder  332  and forms a chamber  344  with the soft piston  300  (such as for hydraulic fluid, e.g., water). In some embodiments, the spacers  338 ,  340  are made from polyether ether ketone (PEEK). 
         [0037]    When the end caps  334 ,  336  are coupled with the cylinder  332  ends, such as by the threaded connections  364 ,  366 , the inner tapered surfaces  372 ,  374  engage the outer tapered surfaces  368 ,  370  of the corresponding cylinder ends. This engagement causes a crimping between the cylinder  332  and the end caps  334 ,  336  resulting in undercuts, deformations, projections, or shoulders  380 ,  382  that are discontinuities in the inner cylinder bore  360 . The spacers  338 ,  340  include intermediate projections or ribs  384 ,  386  between outer surfaces  376 ,  378  engaged with the end cap tapered surfaces  372 ,  374  and inner surfaces  388 ,  390  that extend into the cylinder bore  360 . In some embodiments, the spacer projections  384 ,  386  are pre-formed onto the spacers  338 ,  340 . In other embodiments, the spacer projections  384 ,  386  are formed by deformation of the spacer material into the spaces left between the crimping undercuts  380 ,  382  and the end caps  334 ,  336  when the spacers are captured between the cylinder and end caps. The projections  384 ,  386  are then captured between the cylinder  332  and the end caps  334 ,  336  in the crimping spaces. 
         [0038]    The spacers  338 ,  340  also include fluid passages  392 ,  394  fluidicly coupled with and between the axial bore  360  of the cylinder  332  and fluid passages  396 ,  398  in the end caps  334 ,  336 . Hydrocarbon samples and hydraulic fluid can communicate through these fluid passages. When the piston accumulator system  330  and the cylinder  332  are coupled into a formation tester, such as formation testers  10 ,  60 ,  120 ,  200 , these fluid passages communicate with inputs to the cylinder  332  that are connected to a network of one or more pipes and valves that permit fluid to enter and prevent fluid from leaving the cylinder  332 . The network of pipes and valves are part of the formation tester necessary for transporting fluids for analysis. 
         [0039]    The spacers  338 ,  340  are captured by and do not move relative to the cylinder  332  and the end caps  334 ,  336 . The spacers  338 ,  340  provide fitment between the cylinder  332  and the end caps  334 ,  336 . The spacers  338 ,  340  provide tolerance or space filling between the end cap/cylinder coupling and the soft piston  300 , such that the soft piston stroke is between the inner spacer surfaces  388 ,  390  and the soft piston avoids contact with the crimping undercuts  380 ,  382 . 
         [0040]      FIG. 9A  illustrates an enlarged portion of the soft piston  300  including the seal assembly  350  disposed in the piston seal recess  308 . The seal assembly  350  includes a blend of components to achieve sealing between the soft piston  300  and the cylinder bore surface  360  for the desired operating ranges of pressure and temperature. For example, the seal assembly  350  includes upper and lower, or outer, sealing components  352 , intermediate sealing components  354 , and a center sealing component  356 . In exemplary embodiments, the outer sealing components  352  are rigid, nonmetallic members and the inner components  354 ,  356  are more pliable, nonmetallic members. In certain embodiments, the outer sealing components  352  are made from PEEK. The intermediate sealing components  354  may be made from Teflon or comprise Teflon Z-cut rings. The center or primary sealing component  356  may comprise a Viton o-ring or an o-ring made from a fluoroelastomer based on an alternating copolymer of tetrafluoroethylene and propylene (TFE/P), also known as AFLAS® or Fluoraz®. In descending order of rigid to pliable, the aforementioned materials are ordered: PEEK, Teflon, and the group comprising fluoroelastomer, TFE/P, Viton, and AFLAS® or Fluoraz®. The sealing components can be arranged in various combinations to achieve rigid outer components and relatively more pliable inner component(s). 
         [0041]    The seal assembly  350  maintains a dynamic seal for the moveable soft piston  300  throughout wide ranges of pressure (for example, from ambient to 20,000 to 25,000 p.s.i.) and temperature (for example, from ambient to 400 to 450° F.) created in the downhole environment. During the pressure and temperature cycle from ambient to the above-noted pressures and temperatures, and back to ambient, the seal assembly  350  as well as the soft piston  300  maintain operability and seal integrity while also preserving the high pressure formation sample received by the accumulator system. The soft piston materials help to maintain a close tolerance of the piston with the metallic cylinder over the pressure-temperature cycle, while also providing additional functionality such as resistance to heat with continuous service temperature capability of greater than 400° F., resistance to strong acids, bases, and other downhole chemicals, resistance to oil, high electrical resistivity, positive pressure sealing at the piston faces, reduced damage to the inner cylinder surface, and piston “self healing” from embedded solid phase particles. 
         [0042]    During the same pressure-temperature cycle, the seal assembly  350  employs multiple components to ensure seal integrity. The center, most pliable sealing component  356  provides the primary seal between the piston  300  and the inner surface  360  of the cylinder  332 . As pressure and temperature increase, the sealing component  356  tends to deform undesirably. A first set of sealing components  354  is provided adjacent the sealing component  356  to back up the sealing component  356  against deformation. The sealing components, as described above, are more rigid than the sealing component  356  to ensure proper support. As pressure and temperature continue to increase, the sealing components undergo additional undesirable deformation. A second set of backup rings is provided as sealing components  352 , which are more rigid than the sealing component  356  and the sealing components  354  to ensure proper support. Thus, the seal assembly  350  accommodates sealing the piston  300  under increased pressures and temperatures by backing up the center sealing component  356  with the sealing components  354 ,  352  having increasing rigidity and varying component materials. 
         [0043]    In further embodiments, the soft piston  300  and seal assembly  350  are constrained in a small volume accumulator system, such as for formation testers in small diameter tool strings and existing formation tester flow lines. Nonetheless, the soft piston  300  accommodates the large pressure-temperature cycle as described above while the seal assembly  350  maintains sealing integrity with the pliable inner sealing component and at least one set of outer rigid sealing components. 
         [0044]    Turning to  FIG. 10 , an alternative embodiment of an assembled end of a piston accumulator system  430  is shown in longitudinal cross-section. The piston accumulator system  430  includes a cylindrical tube or nipple  432  captured connected to an end cap or coupler  436 . A spacer  440  is captured between the end cap  436  and the cylinder  432  and forms a chamber  444  with the soft piston (not shown). To properly engage the tapered surfaces of the nipple  432 , the coupler  436 , and the spacer  440 , as shown and previously described, a gland and nut system is provided. More specifically, a gland  460  threadably engages a left hand threaded portion  465  of the outer surface of the nipple  432 . A nut  470  threadably engages a right hand threaded portion  475  of the inner surface of the nipple coupler  436 . As the gland and nut system is secured, the inner tapered surfaces of the coupler  436  engaged the outer tapered surfaces of the nipple  432  and the spacer  440  as shown in  FIG. 10 . This engagement causes a crimping between the nipple and the coupler resulting in undercuts, deformations, projections, or shoulders that are discontinuities in the inner cylinder bore. As previously described, the spacer include a portion that fills the undercut or discontinuity. The spacer  440  is captured by and does not move relative to the cylinder  432  and the coupler  436 . The spacer  440  provides fitment between the cylinder and the coupler. The spacer  440  provides tolerance or space filling between the end cap/cylinder coupling and the soft piston, such that the piston stroke is between the inner spacer surfaces and the soft piston avoids contact with the crimping undercuts. 
         [0045]    The piston accumulator embodiments described herein provide a system adapted for high pressure downhole fluids, for optical fluid identification as well as other fluid analyses. The piston accumulator system includes better resistance to harsh and wide operating ranges of pressure and temperature in small diameter and small volume applications, through various combinations of the soft piston design characteristics, the seal assembly design characteristics, the honed and polished cylinder bore, and the spacers in the cylinder. The soft piston member maintains structural and sealing integrity with the surrounding metal cylinder, at least because the material makeup of the soft piston results in close tolerances and sealing capabilities, resistance to cold flow, a low coefficient of friction, reduced damage from and to the metal cylinder, resistance to heat and chemicals, and piston “self healing” from embedded solid phase particles. The soft piston materials also allow sizing down of the piston for use in small diameter or low volume cylinders while also accommodating the described pressure-temperature cycle. A sized down soft piston and accumulator system can be connected into an existing flow line of a formation tester without increasing the inner diameter of the flow line. Additionally, the sealing capabilities of the soft piston are enhanced by the multi-component seal assembly including a primary, pliable sealing member and one or more sets of more rigid backup sealing components. Finally, the adaptabability of the soft piston to varying operating pressures and temperatures is also increased with a piston accumulator system including a honed and polished bore, and spacers that define a stroke that avoids bore undercuts or discontinuities between the cylinder and the end caps. 
         [0046]    Based on these various characteristics, the soft piston member and the piston accumulator embodiments are adaptable for use in wireline, reservoir description tools (RDT), drill stem testing (DST), MWD formation testing, and high pressure liquid chromatography. In very harsh and dynamic environments, the system allows physical pressure-volume-temperature (PVT) analysis downhole. Further, the system allows micro-PVT, i.e., PVT with smaller samples resulting in less waste. Still further, smaller sample volumes leads to smaller tool cross-sections, in turn resulting in accessibility to more formation zones and narrower holes, as well as reduced sticking of the drill or work string. 
         [0047]    A piston accumulator system with one or more of the above characteristics or capabilities may include a cylindrical housing with an axial bore extending between end portions of the housing, a soft piston slidably disposed in the axial bore, an end cap coupled to each end portion of the cylindrical housing to contain the soft piston in the axial bore, and a seal assembly disposed between the soft piston and the axial bore, the seal assembly comprising rigid outer components and a pliable inner component. The soft piston may be nonmetallic, or include PTFE plus fiberglass, Rulon, or a combination thereof. The soft piston is operable during a pressure-temperature cycle including ambient to 25,000 p.s.i. and ambient to 450° F. In some embodiments, the soft piston is captured in a small volume of the capped cylindrical housing such that the system is connectable into an existing flow line of a formation tester. In certain embodiments, the soft piston includes a polish treatment wherein the polish treatment is adjustable based on a predetermined operating pressure or temperature of the soft piston. In further embodiments, the soft piston includes a heat treatment wherein the heat treatment is adjustable based on a predetermined operating pressure or temperature of the soft piston. 
         [0048]    To further enhance the pressure-temperature cycle resistance capabilities of the piston accumulator system, the seal assembly may include a pair of rigid outer sealing components, a pair of pliable intermediate sealing components, and a pliable center sealing component, wherein the pliable intermediate sealing components are more pliable than the rigid outer sealing components, and the pliable center sealing component is more pliable than the rigid outer sealing components and the pliable intermediate sealing components. In some embodiments, the rigid outer sealing components comprise PEEK, the pliable intermediate sealing components comprise Teflon, and the pliable center sealing component comprises at least one of a fluoroelastomer, TFE/P, Viton, AFLAS® and Fluoraz®. 
         [0049]    To reduce discontinuities and ensure a smooth piston stroke in the cylinder bore, the piston accumulator system may include a spacer captured between each end cap and each housing end portion, wherein each end cap includes an inner tapered surface engaged with an outer tapered surface of the housing end portions, and wherein an outer tapered surface of the spacers engage the inner tapered surfaces of the end caps. In some embodiments, the spacers include an outer surface engaged with the end caps, an inner surface, and an intermediate portion including a projection captured between the housing end surface and the end cap to file an undercut formed between housing and the end caps. The spacers may be nonmetallic and include materials disclosed herein to properly accommodate the pressure-temperature cycle. Further, the spacers may include a fluid passage fluidicly coupled between the axial bore of the housing and fluid passages in the end caps, wherein the fluid passages communicate with a network of one or more pipes and valves that permit fluid to enter and prevent fluid from leaving the cylinder bore. 
         [0050]    In one embodiment, the piston accumulator system includes a cylindrical housing with an axial bore extending between end portions of the housing, a soft piston slidably disposed in the axial bore, wherein the soft piston comprises at least one of PTFE plus fiberglass and Rulon, a seal assembly disposed between the soft piston and the axial bore, the seal assembly comprising rigid outer components and a pliable inner component, an end cap coupled to each end portion of the cylindrical housing to contain the soft piston in the axial bore, and a spacer captured between each end cap and each housing end portion. 
         [0051]    Now with reference to  FIG. 11 , a method ( 500 ) for accumulating formation fluids downhole during a large pressure-temperature cycle includes moving a soft piston in an axial bore of a metal cylindrical housing to draw formation fluids into the bore ( 502 ), sealing between the soft piston and the bore of the metal housing with a pliable inner component of a seal assembly ( 504 ), and backing up the pliable inner component with rigid outer components of the seal assembly ( 506 ). In some embodiments of the method, the formation fluids may be high pressure formation fluids ( 508 ), and the method may further include maintaining movability, integrity, and close tolerances of the soft piston within the bore of the metal housing while receiving the high pressure formation fluids ( 510 ). The high pressure formation fluids may include a pressure up to 25,000 p.s.i., and a temperature up to 450° F. In certain embodiments of the method, the backing up of the pliable inner component with the rigid outer components is in response to the high pressure formation fluids ( 512 ). 
         [0052]    Next with reference to  FIG. 12 , another method ( 600 ) for accumulating formation fluids downhole during a large pressure-temperature cycle includes moving a soft piston in an axial bore of a metal cylindrical housing to draw high pressure formation fluids into the bore ( 602 ), sealing between the soft piston and the bore of the metal housing with a pliable inner component of a seal assembly ( 604 ), backing up the pliable inner component with a pair of less pliable intermediate components of the seal assembly in response to the high pressure formation fluids ( 606 ), and backing up the less pliable intermediate components with a pair of rigid outer components of the seal assembly in response to the high pressure formation fluids ( 608 ). The soft piston and the sealing components may include the materials as described herein. The soft piston resists the high pressure formation fluids to maintain movability, integrity, and close tolerances of the soft piston within the bore of the metal housing while receiving the high pressure formation fluids ( 610 ). In additional embodiments, the method includes capturing a nonmetallic spacer between end portions of the cylindrical metal housing and end caps thereon ( 612 ), filling an undercut between each end portion and end cap with a spacer projection ( 614 ), and moving the soft piston between inner surfaces of the nonmetallic spacers ( 616 ). In some embodiments, prior to the previous steps, the method includes creating the soft piston from at least one of PTFE plus fiberglass and Rulon ( 618 ), polishing the soft piston ( 620 ), and heat treating the soft piston ( 622 ). 
         [0053]    The embodiments set forth herein are merely illustrative and do not limit the scope of the disclosure or the details therein. It will be appreciated that many other modifications and improvements to the disclosure herein may be made without departing from the scope of the disclosure or the inventive concepts herein disclosed. Because many varying and different embodiments may be made within the scope of the inventive concept herein taught, including equivalent structures hereafter thought of, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense. For example, the piston accumulator embodiments have application in the field of high pressure liquid chromatography.

Summary:
A downhole piston accumulator system is disclosed, such as for a formation tester. The soft piston of the system is designed to withstand high pressure downhole fluids in small volume cylinders, the fluid being collected for optical fluid identification or other analyses. The temperature range of the fluid may vary widely, which can be accommodated by the soft piston. Sealing components on the soft piston include additional materials for sealing the soft piston and otherwise helping to accommodate the wide ranging pressures and temperatures. The piston container or cylinder is designed to properly capture the piston and accommodate piston movement. The piston accumulator system allows an outer or exterior position sensor to detect piston movement, such as by a magnetic sensor.