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CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a Continuation-In-Part of U.S. patent application Ser. No. 09/257,292 filed Feb. 25, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the art of earth boring and the collection of formation fluid samples from a wellbore. More particularly, the invention relates to methods and apparatus for collecting a deep well formation sample and preserving the in situ constituency of the sample upon surface retrieval. 
     2. Description of Related Art 
     Earth formation fluids in a hydrocarbon producing well typically comprise a mixture of oil, gas, and water. The pressure, temperature and volume of formation fluids control the phase relation of these constituents. In a subsurface formation, high well fluid pressures often entrain gas within the oil above the bubble point pressure. When the pressure is reduced, the entrained or dissolved gaseous compounds separate from the liquid phase sample. The accurate measure of pressure, temperature, and formation fluid composition from a particular well affects the commercial interest in producing fluids available from the well. The data also provides information regarding procedures for maximizing the completion and production of the respective hydrocarbon reservoir. 
     Certain techniques analyze the well fluids downhole in the wellbore. U.S. Pat. No. 5,361,839 to Griffith et al. (1993) disclosed a transducer for generating an output representative of fluid sample characteristics downhole in a wellbore. U.S. Pat. No. 5,329,811 to Schultz et al. (1994) disclosed an apparatus and method for assessing pressure and volume data for a downhole well fluid sample. 
     Other techniques capture a well fluid sample for retrieval to the surface. U.S. Pat. No. 4,583,595 to Czenichow et al. (1986) disclosed a piston actuated mechanism for capturing a well fluid sample. U.S. Pat. No. 4,721,157 to Berzin (1988) disclosed a shifting valve sleeve for capturing a well fluid sample in a chamber. U.S. Pat. No. 4,766,955 to Petermann (1988) disclosed a piston engaged with a control valve for capturing a well fluid sample, and U.S. Pat. No. 4,903,765 to Zunkel (1990) disclosed a time delayed well fluid sampler. U.S. Pat. No. 5,009,100 to Gruber et al. (1991) disclosed a wireline sampler for collecting a well fluid sample from a selected wellbore depth, U.S. Pat. No. 5,240,072 to Schultz et al. (1993) disclosed a multiple sample annulus pressure responsive sampler for permitting well fluid sample collection at different time and depth intervals, and U.S. Pat. No. 5,322,120 to Be et al. (1994) disclosed an electrically actuated hydraulic system for collecting well fluid samples deep in a wellbore. 
     Temperature downhole in a deep wellbore often exceed 300 degrees F. When a hot formation fluid sample is retrieved to the surface at 70 degrees F., the resulting drop in temperature causes the formation fluid sample to contract. If the volume of the sample is unchanged, such contraction substantially reduces the sample pressure. A pressure drop changes in the situ formation fluid parameters, and can permit phase separation between liquids and gases entrained within the formation fluid sample. Phase separation significantly changes the formation fluid characteristics, and reduces the ability to evaluate the actual properties of the formation fluid. 
     To overcome this limitation, various techniques have been developed to maintain pressure of the formation fluid sample. U.S. Pat. No. 5,337,822 to Massie et al. (1994) pressurized a formation fluid sample with a hydraulically driven piston powered by a high pressure gas. Similarly, U.S. Pat. No. 5,662,166 to Shammai (1997) used a pressurized gas to charge the formation fluid sample. U.S. Pat. Nos. 5,303,775 (1994) and 5,377,755 (1995) to Michaels et al. disclosed a bi-directional, positive displacement pump for increasing the formation fluid sample pressure above the bubble point so that subsequent cooling did not reduce the fluid pressure below the bubble point. 
     Existing techniques for maintaining the sample formation pressure are limited by many factors. Pretension or compression springs are not suitable because the required compression forces require extremely large springs. Shear mechanisms are inflexible and do not easily permit multiple sample gathering at different locations within the wellbore. Gas charges can lead to explosive decompression of seals and sample contamination. Gas pressurization systems require complicated systems including tanks, valves and regulators which are expensive, occupy space in the narrow confines of a wellbore, and require maintenance and repair. Electrical or hydraulic pumps require surface control and have similar limitations. 
     Accordingly, there is a need for an improved system capable of compensating for hydrostatic wellbore pressure loss so that a formation fluid sample can be retrieved to the well surface at substantially the original formation pressure. The system should be reliable and should be capable of collecting the samples from the different locations within a wellbore. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus and method for controlling the pressure of a pressurized wellbore fluid sample collected downhole in an earth boring. The apparatus comprises a housing having a hollow interior. A compound piston within the housing interior defines a fluid sample chamber wherein the piston is moveable within the housing interior to selectively change the fluid sample chamber volume. The compound piston comprises an outer sleeve and an inner sleeve moveable relative to the outer sleeve. However, movement of the inner sleeve relative to the outer sleeve is unidirectional. An external pump extracts formation fluid for delivery under pressure into the fluid sample chamber. A positioned opened valve permits pressurized wellbore fluid to move said piston for pressurizing the fluid sample within the fluid sample chamber so that the fluid sample remains pressurized when the fluid sample is moved to the well surface. 
     The method of the invention is practiced by lowering a housing into a wellbore. The compound piston is displaced within the sample chamber by formation fluid delivered by the external pump. When the sample chamber has filled, a valve is opened to introduce wellbore fluid at hydrostatic wellbore pressure against the piston to move the piston for pressurizing the well fluid sample within the fluid sample chamber. By means of piston area differential, force on a inner sleeve of the compound piston is unbalanced to compress the fluid sample by a volumetric reduction. The reduced volume is secured by mechanically securing the relative positions of the compound piston against the sample chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages and further aspects of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic earth section illustrating the invention operating environment; 
     FIG. 2 is a schematic of the invention in operative assembly with cooperatively supporting tools; 
     FIG. 3 is a schematic of a representative formation fluid extraction and delivery system; 
     FIG. 4 is an isometric view of a sampling tank magazine; 
     FIG. 5 is an isometric view of the present invention; 
     FIG. 6 is an axially sectioned isometric view of the invention; 
     FIG. 7 is a sectioned detail of the sample inlet end of the invention; 
     FIG. 8 is a sectioned detail of the sample chamber portion of the invention assembly; 
     FIG. 9 is a sectioned detail of the hydrostatic wellbore pressure end of the compound piston; 
     FIG. 10 is an axially sectioned isometric view of the invention in the course of receiving a sample of formation fluid; 
     FIG. 11 is a sectioned detail of the compound piston position for wellbore fluid entry; 
     FIG. 12 is a sectioned detail of relative axial displacement between the elements of the compound piston; 
     FIG. 13 is an axially sectioned view of the invention in the course of sample extraction; and, 
     FIG. 14 is an orthographic axial section of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 schematically represents a cross-section of earth  10  along the length of a wellbore penetration  11 . Usually, the wellbore will be at least partially filled with a mixture of liquids including water, drilling fluid, and formation fluids that are indigenous to the earth formations penetrated by the wellbore. Hereinafter, such fluid mixtures are referred to as “wellbore fluids”. The term “formation fluid” hereinafter refers to a specific formation fluid exclusive of any substantial mixture or contamination by fluids not naturally present in the specific formation. 
     Suspended within the wellbore  11  at the bottom end of a wireline  12  is a formation fluid sampling tool  20 . The wireline  12  is often carried over a pulley  13  supported by a derrick  14 . Wireline deployment and retrieval is performed by a powered winch carried by a service truck  15 , for example. 
     Pursuant to the present invention, a preferred embodiment of a sampling tool  20  is schematically illustrated by FIG.  2 . Preferably, such sampling tools are a serial assembly of several tool segments that are joined end-to-end by the threaded sleeves of mutual compression unions  23 . An assembly of tool segments appropriate for the present invention may include a hydraulic power unit  21  and a formation fluid extractor  22 . Below the extractor  22 , a large displacement volume motor/pump unit  24  is provided for line purging. Below the large volume pump is a similar motor/pump unit  25  having a smaller displacement volume that is quantitatively monitored as described more expansively with respect to FIG.  3 . Ordinarily, one or more tank magazine sections  26  are assembled below the small volume pump. Each magazine section  26  may have three or more fluid sample tanks  30 . 
     The formation fluid extractor  22  comprises an extensible suction probe  27  that is opposed by borewall feet  28 . Both, the suction probe  27  and the opposing feet  28  are hydraulically extensible to firmly engage the wellbore walls. Construction and operational details of the fluid extraction tool  22  are more expansively described by U.S. Pat. No. 5,303,775, the specification of which is incorporated herewith. 
     Operation of the tool is fundamentally powered by electricity delivered from the service truck  15  along the wireline  12  to the hydraulic power supply unit  21 . With respect to FIG. 3, the constituency of the hydraulic power supply unit  21  comprises an A.C. motor  32  coupled to drive a positive displacement, hydraulic power pump  34 . The hydraulic power pump energizes a closed loop hydraulic circuit  36 . The hydraulic circuit is controlled, by a solenoid actuated 4-way valve  47 , for example, to drive the motor section  42  of an integrated, positive displacement, pump/motor unit  25 . The pump portion  44  of the pump/motor unit  25  is monitored by means such as a rod position sensor  46 , for example, to report the pump displacement volume. Formation fluid drawn through the suction probe  27 , is directed by a solenoid controlled valve  48  to alternate chambers of the pump  44  and to a tank distributor  49 . By this route, sample volumes of selected formation fluid is extracted directly from respective in situ formations and delivered to designated sample chambers among the several sample tank tools  30 . 
     As sub-steps in the formation fluid extraction procedure of the present invention, the large volume motor/pump unit  24  is employed to purge the formation fluid flow lines between the suction probe  27  and the small volume pump  25 . Since these sub-steps do not require accurate volumetric data, measurement of the pump displacement volume is not required. Otherwise, the motor/pump unit  24  may be substantially the same as motor/pump unit  25  except for the preference that the pump of unit  24  has a greater displacement volume capacity. 
     A representative magazine section  26  is illustrated by FIG. 4 to include a fluted cylinder  50 . Preferably, the cylinder  50  is fabricated to accommodate three or four tanks  30 . Each tank  30  is operatively loaded into a respective alcove  52  with a bayonet-stab fit. Two or more cylinders  50  are joined by an internally threaded sleeve  23  that is axially secured to one end of one cylinder but freely rotatable about the cylinder axis. The sleeve  23  is turned upon the external threads of a mating joint boss  52  to draw the boss into a compression sealed juncture therebetween whereby the fluid flow conduits  54  drilled into the end of each boss  52  are continuously sealed across the joint. 
     FIGS. 5,  6  and  7  illustrate each tank  30  as comprising a cylindrical pressure housing  60  that is delineated at opposite ends by cylinder headwalls. The bottom-end headwall comprises a valve sub-assembly  62  having a socket boss  63  and a fluid conduit nipple  66  projecting axially therefrom. A conduit  68  within the nipple  66  is selectively connected by a respective conduit  54  to the tank distributor  49  and, ultimately, to the suction probe  22  of the formation fluid extractor  27 . Fluid flow within the conduit  68  is rectified by a check valve  69 . Within the valve sub-assembly  62  is a formation fluid flow path  74  between the conduit  68  and a formation fluid reservoir internally of the pressure housing  60 . A solenoid actuated shut-off valve  76  is disposed to selectively open and close the channel of flow path  74 . As best seen from the isometric detail of FIG. 7, a bleed valve  78  selectively closes a shunt conduit  79  that junctions with the flow path  74 . 
     Referring again to the axial half-section of FIG. 6, the pressure housing top-end headwall comprises a sub  64  having a fluid inlet conduit  70  that connects the interior bore  80  of the pressure housing  60  with a threaded tubing nipple socket  72 . The conduit  70  is a normally open fluid flow path between the interior bore  80  and the in situ wellbore environment. Within the interior bore  80  of the pressure housing  60  is a traveling trap sub-assembly  82  that comprises the coaxial assembly of an inner traveling/locking sleeve  86  within an outer traveling sleeve  84  as shown by FIG. 8 . Unitized with the outer traveling sleeve  84  by a retaining bolt  88  as shown by FIG. 9, is a locking piston rod  90 . A fluid channel  92  along the length of the rod  90  openly communicates the inner face  96  of a floating piston  94  with the open well bore conduit  70 . The floating piston  94  is axially confined within the inner bore of the inner traveling/locking sleeve  86  by a retaining ring  98 . A mixing ball  99  is placed within the sample (formation fluid) receiving chamber  95  that is geometrically defined as that variable volume within the interior bore  80  of pressure housing  60  between the valve sub-assembly  62  and the end area of the traveling trap sub-assembly  82 . 
     A body lock ring  100  having internal barb rings  102  and external barb rings  104  selectively connects the rod  90  to the inner traveling/locking sleeve  86 . The selective connection of the barbed lock ring  100  permits the sleeve  86  to move coaxially along the rod  90  from the piston  84  but prohibits any reversal of that movement. 
     Another construction detail of the inner traveling/locking sleeve  86  is the sealed partition  122  between the opposite ends of the sleeve  86 . The chamber  124  created between the partition  122  and the piston head  106  of the rod  90  is sealed to the atmospheric pressure present in the chamber at the time of assembly. 
     The body lock ring  100  between the locking piston rod  90  and the inner bore wall of the inner traveling/locking sleeve  86  above the partition  122  does not provide a fluid pressure barrier. Consequently, the chamber  126  between the partition  122  and the body lock ring  100  functions at the same fluid pressure as the wellbore fluid flood chamber  120  when the flood valve  110  is opened. 
     Still with respect to FIG.9, the base of the floating piston/sleeve  84  includes a flood valve  110  having a pintle  112  biased by a spring  114  against a seal seat  116 . The pintle includes a stem  118  that projects beyond the end plane of the floating piston/sleeve  84 . When the end plane of the floating piston/sleeve  84  is pressed against the inner face of the top sub  64  (FIG.  11 ), the pintle  112  is displaced from engagement with the seal seat  116  to admit wellbore fluid into the flood chamber  120  as is illustrated by FIGS. 11 and 12. The flood chamber  120  is geometrically defined as the variable volume bounded by the annular space between the outer perimeter of the rod  90  and the inner bore  85  of the outer traveling sleeve  84 . 
     OPERATION 
     Preparation of the sample tanks  30  prior to downhole deployment includes the closure of bleed valve  78  and the opening of shut-off valve  76 . Under the power and control of instrumentation carried by the service truck  15 , the sampling tool is located downhole at the desired sample acquisition location. When located, the hydraulic power unit  21  is engaged by remote control from the service truck  15 . Hydraulic power from the unit  21  is directed to the formation fluid extractor unit  22  for borewall engagement of the formation fluid suction probe  27  and the borewall feet  28 . The suction probe  27  provides an isolated, direct fluid flow channel for substantially pure formation fluid. Such formation fluid flow into the suction probe  27  is first induced by the suction of large volume pump  24  which is driven by the hydraulic power unit  21 . The large volume pump  24  is operated for a predetermined period of time to flush the sample distribution conduits of contaminated wellbore fluids with formation fluid drawn through suction probe  27 . When the predetermined line flushing interval has concluded, hydraulic power is switched from the large volume pump  24  to the small volume piston pump  25 . Referring to FIG. 3, formation fluid drawn from the suction probe  27  by the pump  25  is shuttled by 4-way valve  48  into successively opposite chambers  44 . Simultaneously, the valve  48  directs discharge from the chambers to a multiple port rotary valve  49 , for example, which further directs the formation fluid on to the desired sample tank  30 . 
     Formation fluid enters the tank  30  through the nipple conduit  68  and is routed past the check valve  69  and along the flow path  74  into the sample receiving chamber  95 . The tank shut-off valve  76  was opened before the tank was lowered into the wellbore. Pressure of the pumped formation fluid in the receiving chamber  95  displaces both, the outer traveling sleeve  84  and the inner traveling/locking sleeve  86 , against the standing wellbore pressure in the interior bore  80  of pressure housing  60  as shown by FIG.  10 . When the pressure of the formation fluid sample within the formation fluid sample chamber  95  reaches the boost pressure limit of pump  25 , high pressure check valve closes to trap the sample of formation fluid within the sample chamber  30  and passage  32 . 
     Also, when the sample receiving chamber  95  is full, the base plane of the outer traveling sleeve  84  will engage the inside face of the top sub  64 . Thereby, the stem  118  is axially displaced to open the flood valve  110 . Internal conduits within the outer traveling sleeve  84  direct wellbore fluid into the flood chamber  120 . The wellbore pressure in the flood chamber  120  bears against the inneR traveling/locking sleeve  84  over the cross-sectional area of the flood chamber  120  annulus. 
     Opposing the flood chamber force on the traveling/locking sleeve  86  are two pressure sources. One source is the formation fluid pressure in the sample chamber  95  bearing on the annular end section of the traveling/locking sleeve  86  as was provided by the small volume pump unit  25 . The other pressure opposing the flood chamber pressure is the closed atmosphere chamber  124  acting on the area of the annular partition  122 . Initially, the force balance on the traveling/locking sleeve  86  favors the flood chamber side to press the annular end of the sleeve  86  into the sample chamber  95 . Since the liquid formation fluid is substantially incompressible, intrusion of the solid structure of the sleeve  86  annulus into the sample chamber volume exponentially increases the pressure in the sample chamber until a final force equilibrium is achieved. Nevertheless, at the pressures of this environment, measurable liquid compression may be achieved. 
     This axial movement of the inner traveling/locking sleeve  86  relative to the outer sleeve  84  also translates to the piston rod  90  which is secured to the outer sleeve  84  via the retaining bolt  88 . Consequently, the sleeve  86  partition  122  is displaced toward the piston head  106  to compress the gaseous atmosphere of chamber  124  thereby adding to the equilibrium forces. 
     Due to the internal and external barb rings  102  and  104  respective to the body lock ring  100 , movement of the piston  90  relative to the inner traveling sleeve  86  is rectified to maintain this volumetric invasion of the structure  86  into the sample chamber volume. 
     By compressing the volume of the formation fluid sample, the fluid sample pressure is greatly increased above the wellbore pressure. Although this greatly increased in situ pressure declines when the confined formation sample is removed from the wellbore, the operative components may be designed so that when the collected formation sample is removed from the well, the sample pressure does not decline below the bubble point of entrained or dissolved gas. Movement of the inner traveling/locking sleeve  86  further compresses the collected formation fluid sample above the boost capability of the pump  25 . Such compression continues until the desired boost ratio is accomplished. 
     For example, a down hole fluid sample can have a hydrostatic wellbore pressure of 10,000 psi. The typical compressibility for such a fluid is 5×10 −6  so that a volume decrease of only eight percent would raise the fluid sample pressure by 16,000 psi to 26,000 psi, for a boost ratio os 2.6 to 1.0. When the magazine section  26  and the collected formation fluid sample is raised to the surface of wellbore  11 , the formation fluid sample temperature will cool, thereby returning the formation fluid sample pressure toward the original pressure of 10,000 psi. If the downhole fluid temperature is 270° F. and the wellbore  11  surface temperature is 70° F., the resulting 200° drop in temperature will lower the fluid sample pressure by approximately 15,300 psi in a fixed volume, thereby resulting in a surface fluid sample pressure of approximately 10,700 psi. 
     To hold the volume of fluid sample chamber  95  constant as the magazine  26  is removed from the wellbore  11 , inner traveling/locking sleeve  86  is fixed relative to outer traveling sleeve  84  during retrieval of the magazine  26 . The invention accomplishes the fixed relationship by means of the body lock ring  100 . This mechanism permits additional boost to be added to the formation fluid sample pressure within the sample chamber  95  as a proportionality of the in situ wellbore pressure. For example, the magazine section  26  may subsequently be lowered to additional depths within a wellbore  11  where the hydrostatic pressure is greater than a prior sample extraction. The hydrostatic wellbore pressure increase is transmitted through flood valve  112  into flood chamber  120  to further move the inner traveling/locking sleeve  86  and to further compress the formation fluid sample within the sample chamber  95  to a greater pressure. Such pressure boost may be accomplished quickly and magazine  26  removed to the surface of wellbore  11  before a significant amount of heat from the additional wellbore depth is transferred to the previously collected formation fluid sample. 
     At the surface of wellbore  11 , tank shut-off valve  76  is closed to trap the formation fluid sample. Thereafter, bleed valve  78  may be opened to relieve the fluid pressure in the flow passage between tank shut-off valve  76  and the high pressure check valve  69 . This pressure release provides a positive indication of fluid pressure and facilitates removal of a tank  30  from a magazine  26 . 
     FIG. 13 illustrates one technique for removing the formation fluid sample under pressure from within fluid sample chamber  95 . Tank  30  is connected to a pressure source  130  engaged with aperture  132  through top sub  64 . Pressure from the pressure source  130  is introduced until the inverse of the boost ratio times the expected pressure within fluid sample chamber  95  is reached. For a fluid sample pressure of 10,000 psi, the extraction pressure required would be: 
     
       
         1/2.6×10,000=3,850 psi 
       
     
     After the inverse boost ratio is reached, shut-off valve  76  is cracked open and the formation fluid sample is permitted to pass through passage  74  into an attached receiver line  140 . The reverse boost pressure can be increased to displace the collected formation fluid sample until the sleeve edge of the inner traveling/locking sleeve  86  bottoms out against the valve sub  62 . Continued extraction fluid from the pressure source  130  displaces the outer traveling sleeve  84  relative to the inner sleeve  86 . Hence, the piston head  106  engages the floating piston  94  to sweep most of the formation fluid sample from the chamber  95 . The only volume within the chamber  95  not removed by the extraction pressure is found in an annular space between the outer traveling sleeve  84  and the valve sub  62 . The components of tank  30  can be dissembled and reset for another use. 
     In summary, the invention permits multiple tanks  30  to be lowered in the same operation so that different zones within wellbore  11  can be sampled. Each tank can be selectively operated to collect different samples at different pressures and to compress each sample to different rates exceeding the bubble point for gas within the sample. Operating costs are significantly reduced because less rig time is required to sample multiple zones. The invention prevents the pressure within each fluid sample from being reduced below the bubble point therefore delivering each fluid sample to the wellbore surface in substantially the same pressure state as the downhole sampling state. The invention accomplishes this function without requiring expanding gases, large springs and complicated mechanical systems. The fluid sample is collected under pressure and additional pressure is added with a force exerted by the downhole hydrostatic pressure. 
     Although the invention has been described in terms of certain preferred embodiments, it will become apparent to those of ordinary skill in the art that modifications and improvements can be made to the inventive concepts herein without departing form the scope of the invention. The embodiments shown herein are merely illustrative of the inventive concepts and should not be interpreted as limiting the scope of the invention.

Summary:
An apparatus and method for maintaining the pressure of a well fluid sample as the sample is transported to the well surface from a downhole wellbore location. The invention collects a formation fluid sample under pressure. The fluid sample is further pressurized with a traveling piston powered by the hydrostatic wellbore pressure. The pressurized formation fluid sample is contained under high pressure within a fixed volume chamber for retrieval to the well surface. Multiple collection tanks can be lowered into the wellbore during the same run to sample different zones with minimal rig time. The tanks can be emptied at the well surface with an evacuation pressure so that the fluid sample pressure is maintained above a selected pressure at all times.