Patent Publication Number: US-6702017-B1

Title: Apparatus and method for well fluid sampling

Description:
FIELD OF THE INVENTION 
     This invention relates to a well fluid sampling tool and to a well fluid sampling method. 
     The invention particularly, though not exclusively, relates to a so-called single phase or monophasic sampling tool, and related method. 
     BACKGROUND OF THE INVENTION 
     There are many circumstances where it is desirable to sample a fluid material, whether as a gas, a liquid, or a mixture of the two, and determine its nature, for example, its physical and chemical composition, to determine information about the body of fluid from which the sample was taken. On some such occasions the sample may be obtained under one set of ambient conditions—of pressure and temperature, say—and thereafter removed to a quite different set for analysis such that, if unprotected, the sample&#39;s state—e.g. its physical and chemical form—may change during this removal until it is no longer sufficiently representative of the original fluid. One typical example of this situation occurs when sampling fluids issuing from geological formations into which a well, such as an oil/gas well, has been drilled. At the bottom of the well, which may be several miles deep, pressure and temperature are high—possibly several hundred atmospheres, and in the low hundreds of degrees Celsius. Whilst the formation fluid may under these ambient conditions be a single phase fluid, nevertheless a sample of this fluid transported to quite different ambient conditions of the surface (specifically of pressure and temperature—often referred to as NAP, Normal Atmospheric Pressure, or as NTP, Normal Temperature and Pressure), where it is to be analysed to reveal useful information about the well, may easily separate into two or more distinct phases—for example, a liquid phase, a gas phase (originally dissolved in the liquid), and a solid phase (originally suspended or in solution in the liquid). 
     As such, the separated sample is no longer truly representative of the original fluid—or, at least, not in an easily-understood way—and so has lost much of its value. Indeed in some circumstances it may be impractical to reconstitute the original fluid sampled. 
     Single phase sampling tools are known. For example, WO 91/12411 (OILPHASE SAMPLING SERVICES) discloses a well is fluid sampling tool and method for retrieving single-phase hydrocarbon samples from deep wells. In that document the sampling tool is lowered to the required depth, an internal sample chamber is opened to admit well fluid at a controlled rate, and the sample chamber is then automatically sealed. The well fluid sample is subjected to a high pressure to keep the sample in its original single-phase form until it can be analysed. The sample is pressurised by a hydraulically-driven floating piston powered by high-pressure gas acting on another floating piston. Once sampling is initiated e.g. by an internal clock, the entire sequence is automatic. 
     GB 2 252 296A (EXAL SAMPLING SERVICES) discloses an arrangement which is pressure compensated, so that as the container is lifted to the surface, and the ambient pressure and temperature drop, firstly the sample itself is sealed off to prevent it expanding (and separating) under the reduced pressure, and secondly the original ambient pressure is positively maintained despite any temperature change seeking to cause a corresponding pressure change (so that temperature-induced pressure drop and phase separation is avoided). This end is attained by a sampler wherein the sample chamber, in which the sample itself is received and stored, is sealingly closed at one end by a moveable partition to the other side of which is applied either directly or indirectly (via a buffer fluid) a source of suitably pressured gas. 
     The aforementioned sampling tools essentially use compensation techniques, i.e. the pressurised gases act on the sample to compensate for pressure drop in the sample due to temperature drop. These sampling tools, therefore, require the provision of a gas reservoir and complicated mechanisms to apply pressure to the sample to compensate for temperature reduction induced pressure changes. 
     SU 368 390 (MAMUNA et al) discloses a device for withdrawing samples of formation oil, including a body, a receiving chamber with a piston, and an inlet valve, wherein the receiving chamber is fitted with an electric heater connected to a thermometer mounted in the piston, with the aim of preserving the properties of the formation oil in the sample withdrawn. 
     WO96/12088 (OILPHASE SAMPLING SERVICES) discloses a well fluid sampling tool and method for retrieving reservoir fluid samples from deep wells. In this document the sampling tool is lowered to the required depth, an internal sample chamber is opened to admit well fluid at a controlled rate, and the sample chamber is then automatically sealed. The temperature of the sampled well fluid is maintained at or near initial as sampled temperature to avoid the volumetric shrinkage otherwise induced by temperature reduction, mitigate precipitation of compounds from the sample, and/or maintain the initial single phase condition of the sample. The sample chamber is thermally insulated, provided with a storage heater, electrically heated, given a high heat capacity, and/or pre-heated to sample temperature. 
     A problem with prior art single phase sampling tools is that the tool must be lowered, in use, down within a is drillstring. The tool must, therefore, be of less than a predetermined outer diameter. However, the tool should also be as short as possible, for example, to seek to avoid the tool becoming stuck or “hanging-up” within the drillstring. 
     It is an object of at least one aspect of the present invention to obviate or mitigate one or more of the aforementioned problems in the prior art. 
     It is a further object of at least one aspect of the present invention to seek to provide an optimum sized sample chamber within a tool of particular outer dimensions (outer diameter and length). 
     SUMMARY OF THE INVENTION 
     These objects are addressed by the general solution of providing a well fluid sampling tool with an evacuated chamber surrounding at least part of a sample chamber, an outer wall of the evacuated chamber being adjacent to or preferably forming an outer wall of the tool. 
     According to a first aspect of the present invention there is provided a well fluid sampling tool having, at least in use, a sample chamber at least partly contained within an at least partially evacuated jacket, an outermost wall of the jacket being adjacent to or forming an outermost wall of the tool. 
     In such a tool the evacuated jacket acts to maintain the sample as originally retrieved, e.g. in single phase form (at original temperature). 
     Advantageously the sample chamber is substantially contained within the evacuated jacket. 
     Preferably, the evacuated jacket comprises first and second tubular bodies, the first tubular body comprising the outermost wall of the jacket and the second tubular body being provided within the first tubular body, an evacuated chamber being provided between the two bodies. 
     Advantageously, the evacuated chamber is formed by a longitudinal annular space between the bodies. 
     The pressure in the annular space may be approximately between 10 −7  PSI and 10 −11  PSI and typically around 10 −8  PSI. 
     Preferably, the first and second bodies are formed in one piece, being joined at least one end. 
     Preferably also, the sample chamber is provided with a third tubular body which is at least partly provided within the second tubular body. 
     Advantageously, sample temperature maintenance means are provided, preferably between the second and third tubular bodies. 
     Preferably, the temperature maintenance means include a plurality of heaters spaced longitudinally between the second and third tubular bodies. 
     Advantageously the heaters are sized to seek to compensate for heat loss at their respective locations. 
     Advantageously first and second heaters provided at first and second ends of the third tubular body are more powerful than heaters provided distal from the first and second ends. This arrangement is particularly advantageous so as to seek to compensate for heat loss from the ends of the sample chamber. Preferably the second heater is more powerful than the first heater. 
     Preferably the temperature maintenance means further comprises at least one temperature sensor for detecting the temperature of the fluid sample. 
     Preferably the at least one temperature sensor measures the temperature of an outer wall of the third tubular body. 
     Preferably the tool further comprises means for controlling admission of a sample into the sample chamber. 
     The admission control means may comprise a floating piston controllably moveable longitudinally within the sample chamber. 
     The admission control means may further comprise means for controllably moving the floating piston. 
     The controllable movement means may comprise a further fluid and means for controllably reducing pressure of the further fluid. 
     Preferably the piston is mounted on and moveable along a piston rod. 
     The piston rod may have a piston stop at one end adapted to limit travel of the piston at that one end of the piston rod. 
     The piston rod may further carry a plug at another end. Advantageously ends of the sample chamber are defined by he piston stop and the plug. 
     The tool may be provided with one or more sample inlet ports. 
     The tool may also be provided with one or more sample utlet ports, which outlet ports may be distinct from the inlet ports. 
     The tool may also provide means for removing a sample from the sample chamber. 
     The sample removal means may include first and second ports which communicate with first and second outer ends of the sample chamber. Thus, in use, a pump may be connected across the first and second ports so as to apply a differential pressure across the first and second ends of the sample chamber, thereby effecting movement of the sample chamber within the tool towards one or more sample outlet ports. 
     In use, a sample transfer vessel may be connected to the one or more sample outlet ports via one or more valves so as to allow controllable transfer of the sample from the sample chamber to the transfer vessel. 
     Advantageously the transfer vessel may include a further floating piston provided within a transfer chamber. 
     Preferably the transfer chamber is of substantially the same volume as the sample chamber. 
     According to a second aspect of the present invention there is provided a well fluid sampling method comprising the steps of: 
     providing a well fluid sampling tool having a sample chamber at least partly contained within an evacuated jacket, an outermost wall of the jacket being adjacent to or forming an outermost wall of the tool; 
     lowering the tool down a wellbore to a location where well fluid is to be sampled; 
     admitting a sample into the sample chamber by means of controllable admission means; 
     sealing the sample chamber; 
     retrieving the sample to surface while substantially maintaining the temperature of the sample; 
     removing the sample from the sample chamber into a chamber of a sample transfer vessel. 
     By such a method it is sought to maintain the sample as originally sampled, e.g. in single phase form (and at substantially original temperature). 
     This may be achieved as the sample chamber has a predetermined volume; thus by seeking to maintain the temperature of the sample the pressure of the sample is also maintained. 
     Advantageously on admitting the sample into the sample chamber temperature and pressure outside the tool are measured and stored by suitable measurement means and storage means. 
     According to a third aspect of the present invention there is provided a well fluid sampling tool including a sample chamber and an at least partially evacuated jacket surrounding at least part of the sample chamber, the evacuated jacket comprising first and second tubular bodies having an at least partially evacuated annular space therebetween, the first and second bodies being integrally formed with one another. 
     Preferably the first and second bodies are integrally connected to one another at least at or near first adjacent ends of each body. 
     Preferably such integral connection may be formed by welding, and advantageously e-beam welding. 
     Preferably also, the first and second bodies are connected to one another at or near second adjacent ends of each body. 
     Advantageously a centraliser may be provided between the first and second bodies, which centraliser may preferably be made at least partly from titanium. 
     According to a fourth aspect of the present invention there is provided a method of operating a well fluid sampling tool, the tool comprising a sample chamber, heater means in thermal communication with the sample chamber and means for controlling the heater means including means for measuring temperature external of the tool, the method comprising: 
     storing a preset temperature on the control means; 
     lowering the tool down a borehole; 
     continually monitoring the temperature external the tool at predetermined intervals; 
     comparing the measured external temperature to the preset temperature and if the measured external temperature is greater than the preset temperature then causing the heater means to heat at least part of the sample chamber to the measured external temperature. 
     Advantageously, as the tool is lowered if the external temperature is greater than the preset temperature then the external temperature is stored as the preset temperature. 
     Advantageously as the tool is lowered the pressure external the tool is also continually monitored, and preferably the highest external pressure monitored is stored on the control means. 
     In a preferred embodiment the tool includes an electronic clock circuit and a memory logger circuit. 
     According to a fifth aspect of the present invention there is provided a well fluid sampling tool including a sample chamber and pressure relief means communicating between the sample chamber and external the tool such that, in use, if pressure in the chamber exceeds a predetermined level the pressure is relieved via the pressure relieve means. 
     The pressure relieve means may comprise a pressure relief valve or a breakable disc. The tool may include sample temperature maintenance means. 
     Provision of the pressure relief means seeks to avoid excessive pressure build-up within the sample chamber, e.g. due to thermal runaway of the temperature maintenance means. 
     A tool according to any of the first, third or fifth aspects hereinbefore mentioned may be inserted into a borehole by wireline and may be coupled together with similar tools or with other tools, for example, memory pressure gauges, togging tools, spinners or the like, by threaded cross-overs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, which are: 
     FIGS.  1 (A)-(E) a series of cross-sectional side views of a well fluid sampling tool according to an embodiment of the present invention in a first position; 
     FIGS.  2 (A)-(E) a series of cross-sectional side views of the well fluid sampling tool of FIGS.  1 (A)-(E) in a second position; 
     FIGS.  3 (A)-(E) a series of cross-sectional side views of the well fluid sampling tool of FIGS.  1 (A)-(E) in a third position; 
     FIG. 4 a sectional view along line A—A of FIG.  2 (B) 
     FIG. 5 a sectional view along line B—B of FIG.  2 (B); 
     FIG. 6 a sectional view along line C—C of FIG.  2 (B); 
     FIG. 7 a cross-sectional side view of a choke holder forming part of the tool of FIGS.  1 (A)-(E). 
     FIG. 8 a sectional view along line D—D of FIG. 7; 
     FIG. 9 a sectional view along line E—E of FIG. 7; 
     FIG. 10 a sectional view along line F—F of FIG.  3 (E); 
     FIG.  11 (A) a schematic perspective view from one side to one end and above of a plurality of heaters provided on a sample chamber comprising part of the tool of FIGS.  1 (A)-(E); 
     FIG.  11 (B) a schematic perspective view from one side to one end and also to an enlarged scale of one of the heaters of FIG.  11 (A) provided on the sample chamber comprising part of the tool of FIGS.  1 (A)-(E); 
     FIG. 12 a schematic diagram of electronic circuitry associated with the tool of FIGS.  1 (A)-(E); 
     FIGS.  13 (A)-(C) a series of detailed circuit diagram of a clock board comprising part of the electronic circuitry of FIG. 12; 
     FIGS.  14 (A)-(C) a series of detailed circuit diagram of a logger board comprising part of the electronic circuitry of FIG. 12; 
     FIG. 15 a detailed circuit diagram of a heater electronics board comprising part of the electronic circuitry of FIG.  12 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIGS.  1 (A)-(E) there is illustrated a well fluid sampling tool, generally designated  5 , according to an embodiment of the present invention. The tool  5  has a first end  10 , which end is normally the uppermost end when the tool  5  is conveyed down a borehole of a well, and a second end  15 , which end is normally the lowermost end when the tool  5  is conveyed down the borehole. 
     The preferred maximum outer diameter of the tool  5  is approximately 2″ so as to facilitate ease of transit of the tool  5  through an innerbore of a standard 2¼ test valve (not shown) up and down. 
     The tool  5  comprises a connector in the form of a top cross-over  20  by means of which the tool  5  can be connected to wireline, slickline, electric line or the like so as to be conveyed down or up a borehole of a well. Indeed the tool  5  may be coupled together with similar tools or with other downhole tools as is known in the art, e.g. by threaded cross-overs. 
     An end of the top cross-over  20  is threadably connected to and sealably engaged with a first end of a battery housing  25 , which housing  25  provides a battery chamber holding a battery  30 . In this embodiment the battery  30  is a lithium battery. The battery  30  powers all electrical/electronic components of the tool  5  hereinafter described. 
     A second end of the battery housing  25  is threadably connected to and sealably engaged with a first end of a clock board housing  35 . The clock board housing  35  provides a clock board chamber  40 , which chamber  40  holds a clock board  45  and a solenoid valve  50  which is controlled by the clock board  45 . 
     A second end of the clock board housing  35  is threadably connected to and sealably engaged with a first end of a solenoid nipple  55 . 
     A second end of the solenoid nipple  55  is threadably connected to and sealably engaged with a first end of a buffer chamber housing  60 . The buffer chamber housing  60  provides a buffer chamber  65  which when the tool  5  is initially run downhole, prior to sampling, is filled with air. Further an input port  70  of the solenoid nipple  55  at the second end of the solenoid nipple  55  which communicates with the solenoid valve  50  via line  56  through the nipple  55  is connected to a first end of a tubing piece  75 . The tubing piece  75  is filled with an hydraulic fluid, e.g. a mineral oil. 
     A second end of the buffer chamber housing  60  is threadably connected to and sealably engaged with a first end of a buffer chamber bleed-off nipple housing/prime port sub  80 . The buffer chamber bleed-off nipple housing/prime port sub  80  provides a first output port  85  which is connected to a second end of the tubing piece  75 , a first input port  90  at a second end of the buffer chamber bleed-off nipple housing  80  which communicates with the first output port  85  via a choke  86  including a pressure multiplier  91  which multiplier  91  divides (reduces) fluid pressure seen at the first input port  90  by, for example, X15 to provide a lower pressure at the first output port  85 . Thus if fluid pressure at the first inlet port  90  is 15,000 PSI, fluid pressure at the first outlet port  85  would be 1,000 PSI. The choke  86  further provides a pressure activated valve/flow regulator  101 . 
     In this way the pressure of fluid across the first inlet port  90  to the first outlet port  85  is divided by the multiplier  91 , while the flow rate of fluid flowing from the first inlet port  90  to the first outlet port  85  is controlled. This control is important in controlling the timing of sample acquisition as will hereinafter become apparent. It is, for example, important not to sample too quickly thereby causing phase separation. 
     The housing/sub  80  also houses a pressure and temperature transducer  81  which measures the ambient downhole pressure and temperature before, at, and after the time of sampling and sends such information to a logger board  114  or alternatively the clock board  45  or a heater electronics board  115 . 
     The second end of the housing/sub  80  is threadably connected to and sealably engaged with a first end of a heater board housing  105 . The heater board housing  105  provides an air filled chamber  110  which contains the logger board  114  and a heater electronics board  115 . 
     A second end of the heater board housing  105  is threadably connected to and sealably engaged with a first end of a connector piece  120 . The first end of the connector piece  120  provides a first output port  125  which is connected to the first input port  90  of the housing/sub  80  via a first pipe piece  130 . 
     A second end of the connector piece  120  is provided with a first inlet port  140  which communicates with the first outlet port  125 . 
     A second end of the connector piece  120  is rigidly connected to a first end of a first tubular body  160 . The first tubular body  160  comprises an outermost wall of the tool  5 . The first tubular body  160  is integrally formed at or near a second end thereof with a second tubular body  165  such that the first and second tubular bodies  160 ,  165  are substantially concentric and an annular space  170  is formed between the two bodies  160 ,  165 . The annular space  170  is at least partially evacuated, e.g. to a pressure of around between 10 −7  PSI and 10 −11  PSI, and typically around 10 −8  PSI. 
     The annular space  170  is sealed at or near the first end of the first tubular body  160  by a portion  161  of connector piece  120 , which portion  161  may be welded to the first tubular body  160 , e.g. by e-beam welding. Further a centraliser  175  is provided between the first and second tubular bodies  160 , 165 . 
     The first and second tubular bodies  160 ,  165  and the evacuated annular space  170 , therefore, form an evacuated jacket, wherein an outermost wall of the jacket comprises an outermost wall of the tool  5 . 
     Contained substantially concentrically within the second tubular body  165  is a third tubular body  180 . The third tubular body  180  is sealed at a first end by an end plug  185  which has a through flow orifice  190  allowing communication between an hydraulic chamber  195  of the third tubular body  180  and the first input port  140 . The hydraulic chamber  195  is initially filled with hydraulic fluid, e.g. mineral oil. 
     As can be seen from FIGS.  1 (D) and  1 (E) a further annular space  200  is provided between the second and third tubular bodies  165 ,  180 . A plurality of heaters  205  are provided in the annular space  200 . Referring to FIGS.  11 (A) and (B) there is illustrated in more detail the heaters  205  provided upon an outer surface of the third tubular body  180 . As can be seen, in this embodiment eight heaters are provided along the length of the third tubular body  180 . The heaters  205  provided at each end of the third tubular body  180  are more powerful—i.e. capable of dissipating a larger amount of heat—than the other heaters. This is because heat loss can be expected to be greater from the ends of the third tubular body  180 , in use. 
     As can further be seen from FIG. 11 a plurality of pressure/temperature transducers (PRT&#39;s)  210  are provided on the outer surface of the third tubular body  180 . In use, the PRT&#39;s  210  detect the pressure and/or temperature of a sample contained within the third tubular body  180 . The measured pressure/temperature is compared to the originally sampled pressure/temperature stored by the heater electronics board  105 , and if the measured pressure/temperature is below the originally sampled pressure/temperature the board  105  switches on the heaters  205  until the originally sampled pressure/temperature is regained. 
     A second end of the first tubular body  160  is threadably connected to and sealably engaged with a portion of the third tubular body  180  adjacent a second end thereof. The second end of the third tubular body provides a plurality of sample ports  211  through a side wall thereof. In this embodiment there are four such sample ports  211 . In use, two sample ports  211  are used for retrieving a sample into the tool  5 , while the other two sample ports  211  are used for retrieving the sample out of the tool  5 . Thus when retrieving the sample into the tool  5  the first two sample ports  211  are open and the second two sample ports  211  are plugged by appropriate means, while when retrieving the sample out of the tool  5  the first two sample ports  211  are appropriately plugged, while the second two sample ports are unplugged. This arrangement seeks to ensure that foreign matter such as dirt is not entrained into the sample. 
     The second end of the third tubular body  180  is threadably connected to and sealably engaged with a dog housing  215 . The dog housing  215  includes a tapered recess  220  for reception of spring-loaded dogs  225  carried by a sampling assembly  230  moveable longitudinally within the third tubular body  180  and dog housing  215 . 
     The sampling assembly  230  comprises a floating piston  235 , a first surface of which is exposed to the pressurised hydraulic fluid. The piston  235  is mounted for longitudinal movement upon a piston rod  240 . The piston rod  240  provides a piston stop  245  at a first end thereof. Further the sampling assembly provides at a second end of the piston rod  240  an end valve plug  244  which carries an end valve body  250 . The end valve body  250  carries the spring-loaded dogs  225 . It is noted that the floating piston  235 , the end valve plug  245  and the end valve body  250  all carry on their outer surfaces one or more seals so as to provide sealing engagement with an internal surface of the third tubular body  180  and/or an internal surface of the dog housing  215  as the sampling assembly  230  is held within and moves within the third tubular body  180  and the dog housing  215 . 
     The recess  220  communicates with an outer surface of the dog housing  215  via through-apertures  254  each containing a grub screw  255  and filter screen  260 . In use, a tool (not shown) can be applied to the dogs  225  via the apertures  254  to effect collapse of the dogs  225 , as will be described hereinafter. 
     The valve end body  250  further provides a pressure relief means  265  (which may preferably be in the form of a burst disc or alternatively a pressure relief valve) and nipple  270  protruding from an end thereof. The pressure relief means  265  may be designed so as to relieve pressure of a sample within the tool  5  if the pressure exceeds a predetermined value. 
     For retrieval of a sample into the tool  5 , a second end of the dog housing  215  is threadably connected to and sealably engaged with a first end of a nose cone  275  or cross-over to another tool. The nose cone  275  includes a plurality of inlet ports  280  (in this embodiment four) at a second end thereof. 
     Protruding from the second end of the dog housing  215  and carried thereby is a front inlet plug  285  having a through flow orifice  290  capable of receiving the nipple  270 . The nipple  270  carries one or more seals  295  such that the nipple  270  may be sealably engaged in the orifice  290 . 
     For retrieval of a sample from the tool  5  the nose cone  275  is replaced by a transfer head  300 . The dog housing  215  is threadably connected to and sealably engaged with a first end of the transfer head  300 . A second end of the transfer head  300  provides a pump connection port  305 . As can be seen from FIG. 3C the housing/sub  80  provides a further pump connection port  310 . As will be described hereinafter, in use, a pump (not shown) may be connected across the pump connection ports  305 ,  310  to effect removal of a sample. Alternatively the housing/sub  80  may be removed while maintaining pressure of the sample. 
     As will be appreciated from the foregoing, in use, a sample chamber  315  is formed by a second face of the floating piston  235 , inner wall of the third tubular body  180 , and an end of the end valve plug  245 . In this embodiment the volume of the sample chamber  315  is approximately 300 cc. However, it is envisaged that in alternative embodiments the chamber  315  volume may be in the range 300 cc-600 cc and preferably 350 cc-500 cc. 
     Regarding material selection, the first and second tubular bodies  160 ,  165  may each be made from stainless steel. In this embodiment the first tubular body  160  is designed to withstand a pressure of approximately 20,000 PSI from outwith. Further the third tubular body  180  may be made from stainless incanel, and designed to withstand a pressure of approximately 15,000 to 20,000 PSI from within. 
     Referring now to FIG. 12 there is shown a schematic diagram of electronic circuitry associated with the tool  5 . The electronic circuitry comprises the battery  30  which powers the clock board  45 , logger board  114  and heater electronics board  115 . As can be seen from FIG. 12 the clock board  45  is connected to and controls solenoid valve  50 . Further the clock board  45  is connected to the logger board  114  such that at a predetermined (programmable) time a clock on the clock board  45  activates the solenoid valve  50 , causing the pressure and temperature transducer  81  to instantaneously measure the downhole pressure and temperature and log these measurements to the clock board  45 . The clock board  45  is further connected to the heater electronics board  115  such that the measured value of temperature and pressure at time of sampling stored in a memory on the clock board  45  can be compared to the measured values of temperature measured by the temperature transducers  210  while the tool  5  is retrieved to surface, and indeed thereafter until the sample is removed from the tool  5 , in order that the heater electronics board  115  can thereby seek to maintain the original sampled conditions within the sample chamber  315  by means of the heaters  205 . 
     Referring now to FIGS.  13 (A) through  15 , which show circuit diagrams for various parts of the electronic circuitry of FIG.  12 . It will be appreciated that each of these FIGS.  13 (A)- 15 , includes traditional circuit diagram numbering and symbols in addition to the specific reference numerals referenced in this specification. Such symbols and numbering are known in the art. However, in general, the symbols and numbering may be identified as follows: the reference symbols R# (where # is a number) refer to resistors; the reference symbols C# (where # is a number) refer to capacitors, the reference symbols U 3 * (where * is a letter) refer to inverters; the reference symbols U 4 * (where * is a letter) refer to NAND gates, the reference symbols Q# (where # is a number) refer to transistors, and the reference symbols D# (where # is a number) refer to diodes. 
     Referring to FIGS.  13 (A)-(C), the clock board  45  comprises a regulator  320  for powering the clock board  45 , an analog-to-digital convertor  325 , a memory  330 , a microprocessor  335  and a solenoid control circuit  340 . The clock board  45  includes a communications line Rx 1  which allows communication to and from a computer before and after sampling, solenoid control lines S 1  and S 2  and communications line SWC to logger board  114 . 
     Referring to FIGS.  14 (A)-(C), the logger board  114  comprises a regulator  345 , a communications receive/decode circuit  350 , an analog-to-digital convertor  355 , a microprocessor  360 , a sampling pressure/temperature memory  365 , addressing latches  370 , and a flash memory for data storage  375 . The logger board  114  also provides temperature input lines T 4 , T 5  and pressure input lines T 6 , T 7 , T 8 , and T 9  from the temperature/pressure transducer  81 , as well as communication output line T 12  which may be connected to a computer after retrieval of the tool  5  from downhole. 
     Referring now to FIG. 15 there is show circuitry of the heater electronics board  115  which comprises a heater control circuit  380  having an output T 14 , CH 4 , T 15 , CH 5 , T 16 , CH 6  to each of the heaters  205 , an input VBATT from the battery  30  and inputs Q 3 , Q 5 , and Q 7  from the latches  370  of the logger board  114 . 
     The heater electronics board  115  also provides input circuit  385  comprising inputs T 1 , T 2 , and T 3  from the temperature transducers  210  and outputs CH 0 , CH 1 , and CH 2  to the analog-to-digital converter  355  of the logger board  114 . 
     In use, prior to the tool  5  being lowered down a borehole the clock on the clock board  45  is set to activate the solenoid valve  50  after a predetermined time. 
     The tool  5  is then lowered down within a borehole, e.g. by wireline, in a first position as illustrated in FIGS.  1 (A)-(E). In this first position pressurised hydraulic fluid, e.g. mineral oil, is contained within the hydraulic chamber  195 . The pressurised fluid holds the floating piston  235  at the second end of the piston rod  240  against the end valve plug  245 . In this position a first two of the sample ports  211  are appropriately plugged, while a second two of the sample ports  211  are left opened. However, well fluid cannot enter into the tool  5  via those ports  211  as the force of the pressurised hydraulic fluid acting on the piston  235  exceeds the force of the well fluid seeking to enter the tool  5 . 
     It should be noted that the heaters  205  may be used to heat the hydraulic fluid within the third tubular body  80 . Such heating may occur on surface, while the tool  5  is lowered down the borehole, and/or when the tool  5  is lowered to a required position. In this way the third tubular body  180  may be pre-heated to close to an expected sample temperature, thereby seeking to avoid cooling of a sample when it enters the sample chamber  315 . 
     After the predetermined time the clock activates the solenoid valve  50 . This causes a flow path to open between the tubing piece  75  and buffer chamber  65  thereby allowing mineral oil to bleed into the buffer chamber  65 . This causes hydraulic fluid, i.e. mineral oil, to exit the hydraulic chamber  195  and bleed into the buffer chamber  65  via first pipe piece  130 , choke  86  and tubing piece  75 . Thus the pressure of the hydraulic fluid is eventually caused to fall below the ambient downhole pressure. At this point the piston  235  begins to move towards the piston stop  245  thereby admitting sample into the sample chamber  315 . 
     As sample enters the sample chamber  315  the piston  235  moves towards and ultimately strikes the piston stop  245 . It is noted that a first end of the nipple  270  is attached to an end of the end valve plug  244 . Thus the effective area of the first (top) end of the end valve plug  244  is greater than the effective area of the second (bottom) end of the end valve plug  244 . That is to say the effective well fluid pressure seen at the first end is less than that seen at the second end. Thus, a pressure imbalance exists causing the sampling assembly  230  to move towards the first end of the third tubular body  180 . Such movement causes the sample chamber  315  to be sealed from the ports  211 . Continued movement causes the dogs  225  to engage in recess  220 . In this way a well fluid sample is retrieved into the sample chamber  315 . The tool  5  is then in the position shown in FIGS.  2 (A)-(E). 
     The tool  5  may then be retrieved to the surface, and the sample retrieved out of the tool  5  as hereinafter described. However, before the sample is retrieved out of the tool the temperature and pressure of the sample within the fixed volume sample chamber  315  is monitored by temperature transducers  210 , compared to the original values detected by transducer  310  stored on the clock board  45 , and if the temperature of the sample falls below the originally sampled values the logger board  114  circuitry causes the heater controller circuit  380  to controllably turn on the heaters  205  until the original values are regained. In this way the. tool  5  seeks to maintain the sample in its original state. The evacuated jacket forming an outer wall of the tool  5  assists in maintaining the sample in its original state by seeking to reduce heat loss therefrom. 
     Referring finally to FIGS.  3 (A)-(E) once the tool  5  is retrieved the sample may be retrieved from the tool  5  by the following procedure, either on-shore e.g. in a laboratory, or alternatively off-shore, if facilities permit. Firstly, the nose cone  275  is replaced by a transfer head  300 . Secondly, the first two sample ports  211  are plugged, and the second two sample ports  211  unplugged and connected to a transfer vessel via an on-off valve. Thirdly, the clock board  45  is interrogated to deduce the as-sampled temperature and pressure values. Fourthly, a pump (not shown) is connected across the pump connection ports  305 ,  310  and the pressure thereacross equalised with the pressure of the sample. Fifthly, a tool (not shown) may be applied to collapse the dogs  225 . The sample  315  is then free to move within the tool  5 . 
     Next a pressure imbalance is provided between the pump connection ports  305 ,  310  thereby causing the sample and the sampling assembly  230  to move towards the second two sample ports  211 . Samples can then communicate with these ports  211 . Finally, the on-off valve is opened and sample transferred into the transfer vessel by manipulation of the pressure imbalance while carefully maintaining the volume of the sample at all times, and also seeking to maintain the temperature and pressure of the sample as originally taken from the well. 
     It will be appreciated that the embodiment of the invention hereinbefore described is given by way of example only, and is not meant to limit the scope of the invention in any way.