Patent Publication Number: US-7707878-B2

Title: Circulation pump for circulating downhole fluids, and characterization apparatus of downhole fluids

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is related to co-pending and commonly owned U.S. patent application Ser. No. 11/203,932, filed Aug. 15, 2005, entitled “Methods and Apparatus of Downhole Fluid Analysis”, the entire contents of which are incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to the field of analysis of downhole fluids of a geological formation for evaluating and testing the formation for purposes of exploration and development of hydrocarbon-producing wells, such as oil or gas wells. More particularly, the present invention is directed to a circulation pump for circulating downhole fluids, and a characterization apparatus of downhole fluids including the circulation pump. 
   BACKGROUND 
   Downhole fluid analysis is an important and efficient investigative technique typically used to ascertain characteristics and nature of geological formations having hydrocarbon deposits. In this, typical oilfield exploration and development includes downhole fluid analysis for determining petrophysical, mineralogical, and fluid properties of hydrocarbon reservoirs. Fluid characterization is integral to an accurate evaluation of the economic viability of a hydrocarbon reservoir formation. 
   Typically, a complex mixture of fluids, such as oil, gas, and water, is found downhole in reservoir formations. The downhole fluids, which are also referred to as formation fluids, have characteristics, including pressure, temperature, volume, among other fluid properties, that determine phase behavior of the various constituent elements of the fluids. In order to evaluate underground formations surrounding a borehole, it is often desirable to obtain samples of formation fluids in the borehole for purposes of characterizing the fluids, including composition analysis, fluid properties and phase behavior. Wireline formation testing tools are disclosed, for example, in U.S. Pat. Nos. 3,780,575 and 3,859,851, and the Reservoir Formation Tester (RET) and Modular Formation Dynamics Tester (MDT) of Schlumberger are examples of sampling tools for extracting samples of formation fluids from a borehole for surface analysis. 
   Formation fluids under downhole conditions of composition, pressure and temperature typically are different from the fluids at surface conditions. For example, downhole temperatures in a well could range from 300° F. When samples of downhole fluids are transported to the surface, change in temperature of the fluids tends to occur, with attendant changes in volume and pressure. The changes in the fluids as a result of transportation to the surface cause phase separation between gaseous and liquid phases in the samples, and changes in compositional characteristics of the formation fluids. 
   Techniques also are known to maintain pressure and temperature of samples extracted from a well so as to obtain samples at the surface that are representative of downhole formation fluids. In conventional systems, samples taken downhole are stored in a special chamber of the formation tester tool, and the samples are transported to the surface for laboratory analysis. During sample transfer from below surface to a surface laboratory, samples often are conveyed from one sample bottle or container to another bottle or container, such as a transportation tank. In this, samples may be damaged during the transfer from one vessel to another. 
   Furthermore, sample pressure and temperature frequently change during conveyance of the samples from a wellsite to a remote laboratory despite the techniques used for maintaining the samples at downhole conditions. The sample transfer and transportation procedures currently in use are known to damage or spoil formation fluid samples by bubble formation, solid precipitation in the sample, among other difficulties associated with the handling of formation fluids for surface analysis of downhole fluid characteristics. 
   In addition, laboratory analysis at a remote site is time consuming. Delivery of sample analysis data takes anywhere from a couple of weeks to months for a comprehensive sample analysis. This hinders the ability to satisfy users&#39; demand for real-time results and answers (i.e., answer products). Typically, the time frame for answer products relating to surface analysis of formation fluids is a few months after a sample has been sent to a remote laboratory. 
   As a consequence of the shortcomings in surface analysis of formation fluids, recent developments in downhole fluid analysis include techniques for characterizing formation fluids downhole in a wellbore or borehole. In this, the MDT may include one or more fluid analysis modules, such as the composition fluid analyzer (CFA) and live fluid analyzer (LFA) of Schlumberger, for example, to analyze downhole fluids sampled by the tool while the fluids are still located downhole. 
   In downhole fluid analysis modules of the type described above, formation fluids that are to be analyzed downhole flow past a sensor module associated with the fluid analysis module, such as a spectrometer module, which analyzes the flowing fluids by infrared absorption spectroscopy, for example. In this, an optical fluid analyzer (OFA), which may be located in the fluid analysis module, may identify fluids in the flow stream and quantify the oil and water content. U.S. Pat. No. 4,994,671 (incorporated herein by reference in its entirety) describes a borehole apparatus having a testing chamber, a light source, a spectral detector, a database, and a processor. Fluids drawn from the formation into the testing chamber are analyzed by directing the light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information (based on information in the database relating to different spectra), in order to characterize the formation fluids. 
   In addition, U.S. Pat. Nos. 5,167,149 and 5,201,220 (both incorporated herein by reference in their entirety) describe apparatus for estimating the quantity of gas present in a fluid stream. A prism is attached to a window in the fluid stream and light is directed through the prism to the window. Light reflected from the window/fluid flow interface at certain specific angles is detected and analyzed to indicate the presence of gas in the fluid flow. 
   As set forth in U.S. Pat. No. 5,266,800 (incorporated herein by reference in its entirety), monitoring optical absorption spectrum of fluid samples obtained over time may allow one to determine when formation fluids, rather than mud filtrates, are flowing into the fluid analysis module. Further, as described in U.S. Pat. No. 5,331,156 (incorporated herein by reference in its entirety), by making optical density (OD) measurements of the fluid stream at certain predetermined energies, oil and water fractions of a two-phase fluid stream may be quantified. 
   On the other hand, samples extracted from downhole are analyzed at a surface laboratory by utilizing a pressure and volume control unit (PVCU) that is operated at ambient temperature and heating the fluid samples to formation conditions. However, a PVCU that is able to operate with precision at high downhole temperature conditions is not currently available. Conventional apparatuses for changing the volume of fluid samples under downhole conditions use hydraulic pressure with one attendant shortcoming that it is difficult to precisely control the stroke and speed of the piston under the downhole conditions due to oil expansion and viscosity changes that are caused by the extreme downhole temperatures. Furthermore, oil leakages at O-ring seals are experienced under the high downhole pressures requiring excessive maintenance of the apparatus. 
   Conventionally, a linear stroke piston type pump has been used for the described application. However, this kind of pump has several disadvantages when used for the downhole fluids. The linear stroke piston pump is big and requires a very powerful motor with ball pumping screw and valves. The dead volume of the linear stroke piston type pump is very big, and it requires a dynamic pressure seal on the pistons. Further, the pump of this type contributes to volume changes in the pumped fluids. In addition, when this pump stops, the fluid is prevented from passing through. In other words, unless the pump functions, the fluid sample cannot be introduced into the looped flowline. Further, if the pump does not function, it takes a long time to change a first sample of a first measurement point to a second sample of another measurement point by purging the first sample out from the looped flowline. As a result, two samples are mixed, and measurement error may occur when the purging time is not sufficient. 
   Further, a gear pump may be used for the above application. However, the size of the gear pump is big, and the dead volume is also big because of the size of the gears. If a small amount of sand is present in the fluid, the sand sticks between the gears and damages them or stops their rotation. Similarly, to the linear stroke piston pump, the fluid cannot flow through the gear pump when it is not operational. 
   A PCP (progressive cavity pump) is also known in the art. This pump is used as a downhole production pump. This pump may not stick due to sand contamination. PCP is a robust and reliable pump in oil field operations that does not get clogged by sand. However, a PCP stator is made with elastic material (typically rubber). This is not suitable for use in quick pressure change circuits such as bubble point detectors. This has high reverse flow impedance. To get large flow rate, a large rotator is required. 
     FIG. 15  shows an example of the structure of a centrifuge magnetic coupling pump. The centrifuge magnetic coupling pump  300  includes a housing  301 , an impeller  304 , a shaft  306 , an inside magnet  308 , an outside magnet  310  and a motor  312 . The housing  301  includes an inlet  302  from which fluids  314  are introduced and an outlet  303  from which the fluids  314  are discharged. The impeller  304 , the shaft  306  and the inside magnet  308  are provided in the housing  301 . The impeller  304  is provided at one end of the shaft  306  and the inside magnet  308  is provided around the shaft such that inside magnet  308  and the impeller  304  rotate with the shaft  306 . The outside magnet  310  is provided outside the housing  301  to face the inside magnet  308 . The outside magnet  310  is connected to the motor  312  to be rotated by the motor  312 . When the outside magnet  310  is rotated by the motor  312 , the inside magnet  308  follows the outside magnet  310  to rotate the shaft  306  and the impeller  304  therewith. With this function, the fluid  314  is introduced from the inlet  302  and discharged from the outlet  303 . This pump has capability of large flow rate, but the pump itself requires dead fluid volume. Further, reverse flow impedance is dependent on the gap between the impeller  304  and the housing. The housing section around the impeller  304  has to have a much larger diameter than the intake line diameter because this pump uses centrifuge force. Therefore, housing thickness has to be increased. As described above, conventionally, there have been problems in finding a proper circulation pump to be used for circulating downhole fluids 
   SUMMARY OF THE INVENTION 
   In consequence of the background discussed above, and other factors that are known in the field of downhole fluid analysis, applicants discovered methods and apparatus for downhole analysis of formation fluids by isolating the fluids from the formation and/or borehole in a flowline of a fluid analysis module. In preferred embodiments of the invention, the fluids are isolated with a pressure and volume control unit (PVCU) that is integrated with the flowline and characteristics of the isolated fluids are determined utilizing, in part, the PVCU. 
   The applicants further discovered that when the isolated fluid sample is circulated in a closed loop line, accuracy of phase behavior measurements can be improved. Therefore, in order to circulate the sample in a closed loop line, a circulation pump is provided in the flowline of the apparatus. 
   According to one aspect of the present invention, there is provided a circulation pump for circulating downhole fluids, including a cylindrical pump housing through which the fluids flow in a longitudinal direction thereof; a shaft which is fixed in the pump housing to extend in the longitudinal direction of the cylindrical pump housing; an impeller having a through hole at its center through which the shaft is inserted and capable of rotating around the shaft in the pump housing; a cylindrical magnetic coupler having a through hole at its center through which the pump housing is inserted and capable of rotating around the pump housing, the cylindrical magnetic coupler including a magnet; and a motor provided outside of the pump housing and connected to the magnetic coupler to rotate the magnetic coupler around the pump housing, wherein the impeller is provided with a magnetic piece which is capable of being magnetically connected with the magnet of the cylindrical magnetic coupler to have the impeller rotate around the shaft by rotating the cylindrical magnetic coupler around the pump housing. 
   This structure can minimize the size of the circulation pump. Furthermore, even when the circulation pump is not operated, the fluids can pass through the flowline. In other words, even when the pump does not function, the fluid sample can be introduced into the looped flowline. Thus, two samples are not mixed when a first sample of a first measurement point is changed to a second sample of another second measurement point by purging the first sample out from the looped flowline. Therefore, the problem that happens when the samples are to be changed as described for the linear stroke piston type pump can be prevented. Further, the circulation pump (both inside and outside of the flowline) can be cleaned and maintained easily. 
   In addition, the circulation pump of the present invention is an axis flow type pump. As for the axis flow type pump, reverse flow impedance becomes smaller than that of the centrifuge magnetic coupling pump. With the reverse flow, fluids are easily and effectively filled in the housing. 
   Additional advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading the materials herein or practicing the invention. The advantages of the invention may be achieved through the means recited in the attached claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate preferred embodiments of the present invention and are a part of the specification. Together with the following description, the drawings demonstrate and explain principles of the present invention. 
       FIG. 1  is a schematic representation in cross-section of an exemplary operating environment of the present invention. 
       FIG. 2  is a schematic representation of one embodiment of a system for downhole analysis of formation fluids according to the present invention with an exemplary tool string deployed in a wellbore. 
       FIG. 3  shows schematically one preferred embodiment of a tool string according to the present invention with a fluid analysis module having a pressure and volume control unit (PVCU) for downhole analysis of formation fluids. 
       FIG. 4  schematically represents an example of a fluid analysis module with a pressure and volume control unit (PVCU) apparatus according to one embodiment for downhole characterization of fluids by isolating the formation fluids. 
       FIG. 5  is a schematic depiction of a PVCU apparatus with an array of sensors in a fluid analysis module according to one embodiment of the present invention. 
       FIG. 6  is a schematic representation of a scattering detector system of the PVCU apparatus according to one embodiment of the present invention. 
       FIG. 7  schematically shows the structure of the fluid analysis module with the PVCU apparatus according to another embodiment in a simplified manner. 
       FIG. 8  shows the structure of the circulation pump according to one embodiment of the present invention. 
       FIG. 9  shows the structure of an impeller assembly of the circulation pump for one embodiment of the present invention. 
       FIG. 10  is a schematic depiction of the structure of the impeller assembly of the circulation pump. 
       FIG. 11  schematically shows a cross sectional view of the circulation pump showing the pump housing, the impeller, the shaft, and the magnetic coupler. 
       FIG. 12  shows a relation between the flow speed that is generated by the circulation pump and the viscosity of the sample. 
       FIG. 13  shows the structure of the circulation pump for another embodiment of the present invention. 
       FIG. 14  schematically represents yet another embodiment of a fluid analysis module according to the present invention. 
       FIG. 15  shows an example of the structure of a conventional centrifuge magnetic coupling pump. 
   

   Throughout the drawings, identical reference numbers indicate similar, but not necessarily identical elements. While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION 
   Illustrative embodiments and aspects of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in the specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, that will vary from one implementation to another. Moreover, it will be appreciated that such development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having benefit of the disclosure herein. 
   The present invention is applicable to oilfield exploration and development in areas such as down hole fluid analysis using one or more fluid analysis modules in Schlumberger&#39;s Modular Formation Dynamics Tester (MDT), for example. 
     FIG. 1  is a schematic representation in cross-section of an exemplary operating environment of the present invention wherein a service vehicle  10  is situated at a wellsite having a borehole or wellbore  12  with a borehole tool  20  suspended therein at the end of a wireline  22 .  FIG. 1  depicts one possible setting for utilization of the present invention and other operating environments also are contemplated by the present invention. Typically, the borehole  12  contains a combination of fluids such as water, mud filtrate, formation fluids, etc. The borehole tool  20  and wireline  22  typically are structured and arranged with respect to the service vehicle  10  as shown schematically in  FIG. 1 , in an exemplary arrangement. 
     FIG. 2  is an exemplary embodiment of a system  14  for downhole analysis and sampling of formation fluids according to the preferred embodiments of the present invention, for example, while the service vehicle  10  is situated at a wellsite (note  FIG. 1 ). In  FIG. 2 , a borehole system  14  includes a borehole tool  20 , which may be used for testing earth formations and analyzing the composition of fluids from a formation. The borehole tool  20  typically is suspended in the borehole  12  (note also  FIG. 1 ) from the lower end of a multiconductor logging cable or wireline  22  spooled on a winch  16  (note again  FIG. 1 ) at the formation surface. The logging cable  22  typically is electrically coupled to a surface electrical control system  24  having appropriate electronics and processing systems for the borehole tool  20 . 
   Referring also to  FIG. 3 , the borehole tool  20  includes an elongated body  26  encasing a variety of electronic components and modules, which are schematically represented in  FIGS. 2  and  3 , for providing necessary and desirable functionality to the borehole tool  20 . A selectively extendible fluid admitting assembly  28  and a selectively extendible tool-anchoring member  30  (note  FIG. 2 ) are respectively arranged on opposite sides of the elongated body  26 . Fluid admitting assembly  28  is operable for selectively sealing off or isolating selected portions of a borehole wall  12  such that pressure or fluid communication with adjacent earth formation is established. The fluid admitting assembly  28  may be a single probe module  29  (depicted in  FIG. 3 ) and/or a packer module  31  (also schematically represented in  FIG. 3 ). Examples of borehole tools are disclosed in the aforementioned U.S. Pat. Nos. 3,780,575 and 3,859,851, and in U.S. Pat. No. 4,860,581, the contents of which are incorporated herein by reference in their entirety. 
   One or more fluid analysis modules  32  are provided in the tool body  26 . Fluids obtained from a formation and/or borehole flow through a flowline  33 , via the fluid analysis module or modules  32 , and then may be discharged through a port of a pumpout module  38  (note  FIG. 3 ). Alternatively, formation fluids in the flowline  33  may be directed to one or more fluid collecting chambers  34  and  36 , such as 1, 2¾, or 6 gallon sample chambers and/or six 450 cc multi-sample modules, for receiving and retaining the fluids obtained from the formation for transportation to the surface. Examples of the fluid analysis modules  32  are disclosed in U.S. Patent Application Publications Nos. 2006/0243047A1 and 2006/0243033A1, both incorporated herein by reference in their entirety. 
   The fluid admitting assemblies, one or more fluid analysis modules, the flow path and the collecting chambers, and other operational elements of the borehole tool  20 , are controlled by electrical control systems, such as the surface electrical control system  24  (note  FIG. 2 ). Preferably, the electrical control system  24 , and other control systems situated in the tool body  26 , for example, include processor capability for characterization of formation fluids in the tool  20 , as described in more detail below. 
   The system  14  of the present invention, in its various embodiments, preferably includes a control processor  40  operatively connected with the borehole tool  20 . The control processor  40  is depicted in  FIG. 2  as an element of the electrical control system  24 . Preferably, the methods of the present invention are embodied in a computer program that runs in the processor  40  located, for example, in the control system  24 . In operation, the program is coupled to receive data, for example, from the fluid analysis module  32 , via the wireline cable  22 , and to transmit control signals to operative elements of the borehole tool  20 . 
   The computer program may be stored on a computer usable storage medium  42  associated with the processor  40 , or may be stored on an external computer usable storage medium  44  and electronically coupled to processor  40  for use as needed. The storage medium  44  may be any one or more of presently known storage media, such as a magnetic disk fitting into a disk drive, or an optically readable CD-ROM, or a readable device of any other kind, including a remote storage device coupled over a switched telecommunication link, or future storage media suitable for the purposes and objectives described herein. 
   In some embodiments of the present invention, the methods and apparatus disclosed herein may be embodied in one or more fluid analysis modules of Schlumberger&#39;s formation tester tool, the Modular Formation Dynamics Tester (MDT). The present invention advantageously provides a formation tester tool, such as the MDT, with enhanced functionality for the downhole characterization of formation fluids and the collection of formation fluid samples. In this, the formation tester tool may advantageously be used for sampling formation fluids in conjunction with downhole characterization of the formation fluids. 
     FIG. 4  schematically represents an example of a fluid analysis module  32  with a pressure and volume control unit (PVCU) apparatus  70  according to the present embodiment for downhole characterization of fluids by isolating the formation fluids (note  FIG. 3 ). 
   In preferred embodiments, the PVCU apparatus  70  may be integrated with the flowline  33  of the module  32 . The apparatus  70  includes a bypass flowline  35  and a circulation flowline  37  in fluid communication, via main flowline  33 , with a formation surrounding a borehole. In one preferred embodiment, the apparatus  70  includes two seal valves  53  and  55  operatively associated with the bypass flowline  35 . The valves  53  and  55  are situated so as to control the flow of formation fluids in the bypass flowline segment  35  of the main flowline  33  and to isolate formation fluids in the bypass flowline  35  between the two valves  53  and  55 . A valve  59  may be situated on the main flowline  33  to control fluid flow in the main flowline  33 . For example, each of the seal valves  53  and  55  may have an electrically operated DC brushless motor or stepping motor with an associated piston arrangement for opening and closing the valve. The seal valves  53  and  55  may be replaced with any suitable flow control device, such as a pump, valve, or other mechanical and/or electrical device, for starting and stopping flow of fluids in the bypass flowline  35 . Moreover, combinations of devices may be utilized as necessary or desirable for the practice of the present invention. 
   One or more optical sensors, such as a 36-channels optical spectrometer  56 , connected by an optical fiber bundle  57  with an optical cell or refractometer  60 , and/or a fluorescence/refraction detector  58 , may be arranged on the bypass flowline  35 , to be situated between the valves  53  and  55 . The optical sensors may advantageously be used to characterize fluids flowing through or retained in the bypass flowline  35 . U.S. Pat. Nos. 5,331,156 and 6,476,384, and U.S. Patent Application Publication No. 2004/0000636A1 (all incorporated herein by reference in their entirety) disclose methods of characterizing formation fluids. 
   A pressure/temperature gauge  64  and/or a resistance sensor  74  also may be provided on the bypass flowline  35  to acquire fluid electrical resistance, pressure and/or temperature measurements of fluids in the bypass flowline  35  between seal valves  53  and  55 . A chemical sensor  69  may be provided to measure characteristics of the fluids, such as CO2, H2S, pH, among other chemical properties. An ultra sonic transducer  66  and/or a density and viscosity sensor (vibrating rod)  68  also may be provided to measure characteristics of formation fluids flowing through or captured in the bypass flowline  35  between the valves  53  and  55 . U.S. Pat. No. 4,860,581, incorporated herein by reference in its entirety, discloses apparatus for fluid analysis by downhole fluid pressure and/or electrical resistance measurements. U.S. Pat. No. 6,758,090 and Patent Application Publication No. 2002/0194906A1 (both incorporated herein by reference in their entirety) disclose methods and apparatus of detecting bubble point pressure and MEMS based fluid sensors, respectively. 
   A pump unit  71 , such as a syringe-pump unit, may be arranged with respect to the bypass flowline  35  to control volume and pressure of formation fluids retained in the bypass flowline  35  between the valves  53  and  55 . 
     FIG. 5  shows the structure of the pump unit  71 . The sensors such as the spectrometer  56 , the chemical sensor  69 , the density and viscosity sensor  68 , and the like are simply shown as the numeral  11 . 
   The pump unit  71  has an electrical DC stepping/pulse motor with a gear to decrease the effect of backlash; ball screw  79 ; piston and sleeve arrangement  80  with an O-ring (not shown); a linear position sensor  82 ; motor-ball screw coupling  93 ; ball screw bearings  77 ; and a block  75  connecting the ball screw  79  with the piston  80 . Advantageously, the PVCU apparatus  70  and the pump unit  71  are operable at high temperatures up to 200 degrees C. The section of the bypass flowline  35  with an inlet valve (not shown) is directly connected with the pump unit  71  to reduce the dead volume of the isolated formation fluid. In this, by situating the piston  80  of the pump unit  71  along the same axial direction as the bypass flowline  35 , the dead volume of the isolated fluids is reduced since the volume of fluids left in the bypass flowline  34  from previously sampled fluids affects the fluid properties of subsequently sampled fluids. 
   To decrease motor backlash a 1/160 reducer gear may be utilized and to precisely control position of the piston  80  a DC stepping motor with a 1.8 degree pulse may be utilized. The axis of the piston  80  may be off-set from the axis of the ball screw  79  and the motor  73  so that total tool length is minimized. 
   In operation, rotational movement of the motor  73  is transferred to the axial displacement of the piston  80  through the ball screw  79  with a guide key  91 . Change in volume may be determined by the displacement value of the piston  80 , which may be directly measured by an electrical potentiometer  82 , for example, while precisely and changeably controlling rotation of the motor  73 , with one pulse of 1.8 degrees, for example. The electrical DC pulse motor  73  can change the volume of formation fluids retained in the flowline by actuating the piston  80 , connected to the motor  73 , by way of control electronics using position sensor signals. Since one preferred embodiment of the invention includes a pulsed motor and a high-resolution position sensor, the operation of the PVCU can be controlled with a high level of accuracy. The volume change is calculated by a surface area of the piston times the traveling distance recorded by a displacement or linear position sensor, such as a potentiometer, which is operatively connected with the piston. During the volume change, several sensors, such as pressure, temperature, chemical and density sensors and optical sensors, may measure the properties of the captured fluid sample. 
   The electrical motor  73  may be actuated for changing the volume of the isolated fluids. The displacement position of the piston  80  may be directly measured by the position sensor  82 , fixed via a nut joint  95  and block  75  with the piston  80 , while pulse input to the motor  73  accurately control the traveling speed and distance of the piston  80 . The PVCU  70  is configured based on the desired motor performance required by the downhole environmental conditions, the operational time, the reducer and the pitch of the ball screw  79 . After fluid characterization measurements are completed by the sensors and measurement devices of the module  32 , the piston  80  is returned back to its initial position and the seal valves  52  and  54  are opened so that the PVCU  70  is ready for another operation. 
   An imager  72 , such as a CCD camera, may be provided on, the bypass flowline  35  for spectral imaging to characterize phase behavior of downhole fluids isolated therein, as disclosed in co-pending U.S. patent application Ser. No. 11/204,134, titled “Spectral Imaging for Downhole Fluid Characterization,” filed on Aug. 15, 2005. 
   A scattering detector system  76  may be provided on the bypass flowline  35  to detect particles, such as asphaktene, bubbles, oil mist from gas condensate, that come out of isolated fluids in the bypass flowline  35 . 
     FIG. 6  is a schematic representation of a scattering detector system of the apparatus  70  according to one embodiment of the present invention. Advantageously, the scattering detector  76  may be used for monitoring phase separation by bubble point detection as graphically represented in  FIG. 6 . 
   The scattering detector  76  includes a light source  84 , a first photodetector  86  and, optionally, a second photodetector  88 . The second photodetector  88  may be used to evaluate intensity fluctuation of the light source  84  to confirm that the variation or drop in intensity is due to formation of bubbles or solid particles in the formation fluids that are being examined. The light source  84  may be selected from a halogen source, an LED, a laser diode, among other known light sources suitable for the purposes of the present invention. 
   The scattering detector  76  also includes a high-temperature high-pressure sample cell  90  with windows so that light from the light source  84  passes through formation fluids flowing through or retained in the flowline  33  to the photodetector  86  on the other side of the flowline  33  from the light source  84 . Suitable collecting optics  92  may be provided between the light source  84  and the photodetector  86  so that light from the light source  84  is collected and directed to the photodetector  86 . Optionally, an optical filter  94  may be provided between the optics  92  and the photodetector  86 . In this, since the scattering effect is particle size dependent, i.e., maximum for wavelengths similar to or lower than the particle sizes, by selecting suitable wavelengths using the optical filter  94  it is possible to obtain suitable data on bubble/particle sizes. 
   Referring again to  FIG. 4 , a circulation pump  78  is provided on the circulation flowline  37 . Since the circulation flowline  37  is a loop flowline of the bypass flowline  35 , the circulation pump  78  may be used to circulate formation fluids that are isolated in the bypass flowline  35  in a loop formed by the bypass flowline  35  and the circulation flowline  37 . 
   The bypass flowline  35  is looped, via the circulation flowline  37 , and the circulation pump  78  is provided on the looped flowline  35  and  37  so that formation fluids isolated in the bypass flowline  35  may be circulated, for example, during phase behavior characterization. When the isolated fluid sample in the bypass flowline  35  is circulated in a closed loop line, accuracy of phase behavior measurements can be improved. 
     FIG. 7  schematically shows the structure of the fluid analysis module  32  with the PVCU apparatus  70  according to an exemplary embodiment in a simplified manner. 
   During the sampling job, the formation fluids are flowing inside the main flowline  33  while the seal valves  53  and  55  are closed and the seal valve  59  is open. At this time, other fluid analysis modules analyze the characteristics of the sample flowing inside the main flowline  33 . 
   When the sample flow becomes stable, the sample contamination is sufficiently low, and sample is single phase, the sample is collected inside the sampling chamber. After the sample is collected or the user decides to start phase behavior analysis, the seal valve  59  is closed and the seal valves  53  and  55  are opened. Then, the sample flows into the bypass flowline  35  and the circulation flowline  37 . After the sample is flowing in the bypass flowline  35  and the circulation flowline  37  for a few minutes, the seal valves  53  and  55  are closed and the seal valve  59  is opened to capture the sample inside the bypass flowline  35  and the circulation flowline  37 . 
   Next, the circulation pump  78  is started while the density and viscosity sensor  68  measures the sample density and the viscosity. The speed of the circulation pump  79  (sample flow rate) can be controlled by the surface positioned software based on the density and the viscosity measured by the density and viscosity sensor  68 . Then the PVCU pump unit  71  changes the pressure of the sample captured inside the bypass flowline  35  and the circulation flowline  37  while the pressure/temperature gauge  64  measures the pressure change and the temperature of the sample. The scattering detector  76  monitors the solid (solid precipitation from liquid or oil coming out from condensate) or gas (bubble from liquid) coming out. 
   The structure of the circulation pump  78  of one exemplary embodiment will be described with reference to  FIGS. 8 to 11 .  FIG. 8  shows an example of the structure of the circulation pump of the present embodiment. In this embodiment, the circulation pump  78  is an in-line type flow pump which shows low flow impedance at power off condition compared with the conventional linear stroke piston type pump or gear pump. In this embodiment, the circulation pump  78  is located on the circulation flowline  37 . 
   The circulation pump  78  includes an impeller assembly  100 , a cylindrical pump housing  101 , a magnetic coupler  120 , and a motor  124 . The impeller assembly  100  is provided in the pump housing  101 . The magnetic coupler  120  and the motor  124  are provided outside of the pump housing  101 . 
   The material for forming the pump housing  101  should have resistance for H2S corrosion and other downhole fluid chemical corrosion and erosion as the formation fluid directly contacts the pump housing  101 . In addition, the pump housing  101  may be formed of a non-magnetic alloy. The material for the pump housing  101  may be, for example, Ti6Al4V, K-MONEL® (an alloy of nickel, copper, and aluminum) or INCONEL® (a nickel based super alloy). In another case, the pump housing  101  may be formed of a plastic material provided that the material has a sufficient strength and high corrosion resistance. 
   The pump housing  101  defines part of the circulation flowline  37 . The pump housing  101  may be formed such that the section where the impeller assembly  100  is placed has a larger diameter than that of the rest of the circulation flowline  37 . The structure of the impeller assembly  100  is shown in  FIGS. 9 and 10 .  FIG. 10  schematically shows the structure of the impeller assembly  100  for purposes of the explanation herein. 
   The impeller assembly  100  includes a shaft  102 , a diffuser  104 , an impeller  106 , a straightener  108 , and a magnetic coupler pole piece  107 . The diffuser  104 , the impeller  106 , and the straightener  108  respectively have a central through hole for the shaft  102  to be inserted. The straightener  108  and the diffuser  104  are formed to secure the shaft  102  therein. The straightener  108  and the diffuser  104  are fixed within the pump housing  101  and therefore the shaft  102  is secured within the pump housing  101 . 
   The impeller  106  is formed to be capable of rotating around the shaft  102 . The magnetic coupler pole piece  107  is fixed to the impeller  106  such that the piece  107  also rotates around the shaft  102  with the impeller  106 . 
   The impeller  106  and the magnetic coupler pole piece  107  directly contact the formation fluids, and therefore should have high corrosion resistance. The magnetic coupler pole piece  107  may be made from a ferromagnetic material. The magnetic coupler pole piece  107  may be formed of nickel, or an alloy including nickel, or a ferromagnetic material, with a non-corrosive coating such as, for example, gold plating. With this structure, the magnetic coupler pole piece  107  can have high corrosion resistance under high pressure and high temperature. In one example, the impeller  106  and the magnetic coupler pole piece  107  may be separately formed. In such a case, the impeller  106  may be formed of a plastic material, such as, for example, polyetheretherketone (PEEK), or the like. In other examples, the impeller  106  and the magnetic coupler pole piece  107  may be made as one integral part. In such a case, the impeller  106  functions as a part of the magnetic coupler. Therefore, the impeller  106  and the magnetic coupler pole piece  107  may then be made from a ferromagnetic material. 
   The straightener  108  adjusts the flow of the fluids in the flowline  37 . The diffuser  74  also adjusts the flow of the fluids in the flowline  37 . The diffuser  74  has a tapered shape such that the fluids in the pump housing  101  having a larger diameter than that of the rest of the circulation flowline  37  are smoothly guided to the rest of the circulation flowline  37 . 
   The shaft  102 , the straightener  108  and the diffuser  104  also directly contact the formation fluids, and therefore should have high corrosion resistance. The shaft  102  may be formed of INCONEL® 718, INCONEL® 725, INCONEL® 750, Ti6Al4V, or MONEL® K500. The straightener  108  may be formed of INCONEL® 718, INCONEL® 725, INCONEL® 750, Ti6Al4V, or MONEL® K500, or a plastic material such as, for example, polyetheretherketone (PEEK), or the like. The diffuser  104  may be formed of INCONEL® 718, INCONEL®°725, INCONEL® 750, Ti6Al4V, or MONEL® K500, or a plastic material such as, for example, polyetheretherkctone (PEEK), or the like. 
   Referring also to  FIG. 8 , the circulation pump  78  of this exemplary embodiment is a direct drive type circulation pump. The pump  78  uses a hollow axle stepping motor to directly rotate the magnetic coupler  120 . The magnetic coupler  120  and the motor  124  respectively have a central hole through which the pump housing  101  is inserted. The pump housing  101  is inserted into the center hole of the magnetic coupler  120  and the motor  124 . The magnetic coupler  120  is connected with the rotor of the motor  124  via screws or the like. The magnetic coupler  120  includes a pair of magnets  122  (only one magnet is shown here), a cylindrical magnetic rotary transmitter  121 , and a fixing portion  123  that fixes the magnets  122  inside the rotary transmitter  121 . The fixing portion  123  is formed into a cylindrical shape with a central through hole through which the pump housing  101  is inserted. The cylindrical magnetic rotary transmitter  121  may be formed of ferromagnetic material in this embodiment. The transmitter  121  is formed with a window  125  that is provided for reducing the weight of the transmitter  121  and for attaching the transmitter  121  to the motor  124 . 
     FIG. 11  schematically shows the cross sectional view of the circulation pump  78  showing the pump housing  101 , the impeller  106 , the shaft  102 , and the magnetic coupler  120 . 
   The magnetic coupler  120  has a cylindrical shape and a central through hole. A pair of magnets  122  of the magnetic coupler  120  are shown. The fixing portion  123  fixes the magnets  122  inside the rotary transmitter  121  to form the through hole. The fixing portion  123  fixes the pair of magnets  122  to face each other with the through hole interposed therebetween. 
   The magnets  122  may be permanent magnets. These magnets  122  may be rare earth magnets such as samarium magnets or neodymium magnets, as typified by SmCo5, Nd2Fe14B, and Sm2Co17. In this embodiment, the magnets  122  may be SmCo5 type magnets. By using this material, the magnets  122  can tolerate high temperature conditions. 
   The cylindrical rotary transmitter  121  may be formed of steel. The transmitter  121  is connected to the motor  124  to be rotated by the motor  124 . The magnets  122  are fixed inside the transmitter  121  by the fixing portion  123 . The fixing portion  123  may be formed of a resin material such as PEEK™ (polyetheretherketone). The fixing portion  123  may be formed into a cylindrical shape having a through hole at its center with the magnets  122  fit therein to face each other. As for the structure of the present embodiment, as the magnets  122  are surrounded by the cylindrical ferromagnetic rotary transmitter  121 , the magnetic force is sealed within the transmitter  121  and the magnetic force is effectively transmitted from the magnets  122  to the magnetic coupler pole pieces  107 . Thus, a sufficient magnetic force can be obtained even when viscosity of the formation fluids is high. 
   The impeller assembly  100  may include a pair of the magnetic coupler pole pieces  107  such that the pieces  107  respectively face the pair of the magnets  122  with the pump housing  101  interposed therebetween when the pump housing  101  is inserted in the through hole of the magnetic coupler  120 . 
   Referring also to  FIGS. 8-10 , the rotator of the motor  124  can rotate the magnetic coupler  120  around the pump housing  101 . In this embodiment, the rotator of the motor  124  itself rotates around the pump housing  101 . This structure can minimize the size of the circulation pump  78 . The rotation speed of the motor  124  is selected to be more than 15,000 rpm to provide enough flow, as will be explained later. 
   When the magnetic coupler  120  rotates around the pump housing  101 , the impeller  106  also rotates around the shaft  102  as the pieces  107  fixed to the impeller  106  follow the movement of the magnets  122 , respectively. It means that the magnetic coupler  120  is magnetically coupled to the impeller  106 . The motor  124  can rotate the impeller  106  from outside the circulation Bowline  37  without being directly connected to the impeller  106 . Rotation force is generated by the motor  124  which has no electrical feedthrough connection between the inside and the outside of the pump housing  101 . Motor torque is transferred to the impeller  106  through the magnetic coupler  120 . Therefore, the motor  124  can be placed outside the circulation flowline  37 . Thus, the motor  124  does not need a dynamic pressure seal, and the pump size and dead volume can be reduced. Furthermore, even when the circulation pump  78  is not operated, fluids can pass through the circulation flowline  37 . Therefore, the circulation pump  78  (i.e., the components inside and outside the circulation flowline  37 ) can be cleaned and maintained easily. 
   The force of the magnetic coupler  120  has an exponential relation to the pole (pole pieces  107 ) to magnet (magnets  122 ) gap that is the thickness of the pump housing  101 . Therefore, the pump housing  101  should have minimum thickness that is required to support the internal pressure generated in the pump housing  101 . For example, the thickness of the pump housing  101  may be about 3 mm when the pump housing  101  is formed of Ti6Al4V. 
   The circulation pump  78  works as an agitator to mix the sample inside the circulation flowline  37  and to create bubbles or solids inside the circulation flowline  37 . With this function of the circulation pump  78 , bubbles and solids that are generated are carried to the scattering detector  76 . The pressure value is recorded when the scattering detector  76  detects the bubbles or solids. The flow speed in the circulation flowline  37  depends on the performance of the circulation pump  78  and the viscosity of the sample. The circulation pump  78  can generate enough flow to carry a sample having a high viscosity, as much as 10 cP, to the scattering detector  76 . 
     FIG. 12  shows a relation between the flow speed that is generated by the circulation pump  78  and the viscosity of the sample. The flow speed is strongly related with the rotation speed of the impeller  106  and the viscosity of the sample. It is considered that more than 4 cc/s of the flow speed is suitable to measure the bubble point of a sample having any viscosity in the apparatus  32  of the present embodiment. In order to provide 4 cc/s of the flow speed, the motor  124  may be selected so that the impeller  106  is rotated, via the magnetic coupling, by more than 15,000 rpm. In this embodiment, the impeller  106  is rotated at the same speed as the rotator of the motor  124  rotates. Therefore, the motor  124  whose rotation speed is more than 15,000 rpm may be utilized. 
   The distance between the circulation pump  78  and the scattering detector  76  needs to be selected so as to be very small so that pressure measurement error is minimized. Since the circulation pump  78  carries bubbles and solids to the scattering detector  76  for bubble point measurements, the distance between the circulation pump and the scattering detector should be set to be as small as possible so that the time delay is minimized in the response of the scattering detector for accurate measurements of bubble point. The PVCU pump unit  70  changes the volume of the captured sample in the flowlines  35  and  37  to change the pressure of the sample. The PVCU pump unit  70  needs to have enough stroke of the piston to change the pressure. By minimizing dead volume of the circulation pump  78 , it is possible to minimize the PVCU pump unit  70 . 
   The circulation pump  78  of the present embodiment may be configured to be small, with a small dead volume, and to be driven by the magnetically coupled motor  124 . 
     FIG. 13  shows another example of the structure of the circulation pump  78 . In this example, the circulation pump  78  is a timing belt drive circulation pump. In this example, the magnetic coupler  120  and the motor  130  are connected with a timing belt (not shown). This pump uses a high rotation speed brushless motor with the timing belt that functions as a rotary transmitter to rotate the magnetic coupler  120 . 
   The magnetic coupler  120  includes a pulley  123 . Another pulley  132  is fixed to the motor  130 . The timing belt is engaged in the grooves of the pulleys  123  and  132  such that the rotation of the pulley  132  is transmitted to the pulley  123  to rotate the magnetic coupler  120 . Additionally, the pump housing  101 , in which the impeller assembly  100  is placed, is inserted into the center hole of the magnetic coupler  120 . Thus, the impeller  106  can rotate around the shaft (not shown here). The brushless motor  130  can generate more than 15,000 rpm of rotation speed. With this structure, higher rotation speed can be provided to the pump, for example, by adjusting the diameters of the pulleys  123  and  132 , respectively. Further, one or more pulley (not shown) may be provided between the pulleys  123  and  132 . With this structure, the rotation speed of the pump can be selectively adjusted by adjusting the diameter of the pulleys. In this embodiment, instead of the pulleys  123  and  132 , gears, including cogged gears and friction gears, may be used as well (not shown). 
     FIG. 14  schematically represents yet another embodiment of a fluid analysis module  32  according to the present invention. The apparatus  70  depicted in  FIG. 14  is similar to the embodiment in  FIG. 4  with a bypass flowline  35  and a circulation flowline  37  in fluid communication, via main flowline  33 , with a formation surrounding a borehole. The apparatus  70  of  FIG. 14  includes two valves  53  and  55  operatively associated with the bypass flowline  35 . The valves  53  and  55  are situated so as to control the flow of formation fluids in the bypass flowline segment  35  of the main flowline  33  and to isolate formation fluids in the bypass flowline  35  between the two valves  53  and  55 . A valve  59  may be situated on the main flowline  33  to control fluid flow in the main flowline  33 . 
   The apparatus  70  depicted in  FIG. 14  is similar to the apparatus depicted in  FIG. 4  except that one or more optical sensors, such as a 36-channels optical spectrometer  56 , connected by an optical fiber bundle  57  with an optical cell or refractometer  60 , and/or a fluorescence/refraction detector  58 , may be arranged on the main flowline  33 , instead of the bypass flowline  35  as depicted in  FIG. 4 . The optical sensors may be used to characterize fluids that are flowing through the main flowline  33  since optical sensor measurements do not require an isolated, static fluid. Instead of the arrangement depicted in  FIG. 4 , a resistance sensor  74  and a chemical sensor  69  also may be provided on the main flowline  33  in the embodiment of  FIG. 14  to acquire fluid electrical resistance and chemical measurements with respect to fluids flowing in the main flowline  33 . 
   Although a single set of the impeller  106 , the magnetic coupler  120  and the motor  124  (or  130 ) is described in the above embodiments, the circulation pump  78  may include a plurality of sets of the impeller  106 , the magnetic coupler  120 , and the motor  124  (or  130 ). The plurality of magnetic couplers  120  are respectively provided around the plurality of impellers  106 . The circulation pump  78 , for example, may include one set of the diffuser  104  and the straightener  108 . In this example, the plurality of impellers  106  may be placed in series between the diffuser  104  and the straightener  108 . As for another example, the circulation pump  78  may further include a plurality of sets of the diffuser  104  and the straightener  108  in addition to the plurality of sets of the impeller  106 , the magnetic coupler  120 , and the motor  124  (or  130 ). It means that the circulation pump  78  includes the plurality of sets of the straightener  108 , the impellers  106 , and the diffuser  104 . In this example, each of the sets of the straightener  108 , the impellers  106 , and the diffuser  104 , placed in this order, is placed in series. With the structure where the plurality of sets of the impeller  106 , the magnetic coupler  120 , and the motor  124  (or  130 ) are provided, the circulation pump  78  can provide appropriate flow speed to the fluids in the flowlines  35  and  37 . 
   Although the impeller  106  and the shaft  102  are formed separately in the above embodiments, the impeller  106  and the shaft  102  may be formed as one part. 
   In addition, although the case where the magnetic coupler  120  includes a pair of magnets  122  is shown in the above embodiments, the magnetic coupler  120  may include a plurality of magnets fixed inside the cylindrical magnetic rotary transmitter  121 . In this case, the plurality of magnets may be provided around the central through hole of the magnetic coupler  120  with predetermined equal intervals. In addition, the magnetic coupler pole piece  107  provided to the impeller  106  may be formed of a plurality of magnetic members. Each of the plurality of magnetic members may be provided to face each of the plurality of magnets of the magnetic coupler  120 , respectively, when the pump housing  101  is inserted in the magnetic coupler  120 . 
   A density sensor may measure density of the isolated formation fluid. A MEMS, for example, may measure density and/or viscosity and a P/T gauge may measure pressure and temperature. A chemical sensor may detect various chemical properties of the isolated formation fluid, such as CO2, H2S, pH, among other chemical properties. 
   The preceding description has been presented only to illustrate and describe the invention and some examples of its implementation. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. The preferred aspects were chosen and described in order to best explain principles of the invention and its practical applications. The preceding description is intended to enable others skilled in the art to best utilize the invention in various embodiments and aspects and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.