Abstract:
A PVT system for evaluating foamy heavy and extra-heavy hydrocarbons, including a cell having a wall defining an inner space; a floating piston slidable in the inner space; a stirring mechanism slidable into the inner space and operative to mix a hydrocarbon sample in the inner space; a volumetric pump associated with the cell for selectively increasing and decreasing pressure on the floating piston, and communicated with the inner space to control pressure in the space; an oven for heating the cell, wherein the cell is mounted within the oven; an inversion mechanism for inverting the cell at least about 180°; and a frame supporting the oven, wherein the wall of the cell and the oven have corresponding elongated transparent sections arranged to allow visual inspection of fluid in the inner space along an entire longitudinal extent of the inner space.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a thermodynamic measurement device for evaluating properties of a fluid placed in the device. Such devices allow control of one or more of the pressure, volume and temperature of a space containing a sample of fluid such that properties of the fluid can be evaluated at different settings or values of the modified parameter. 
         [0002]    Conventional PVT cells typically use mercury in some measurements, which can pose a health risk to the user. In addition, such cells are typically blind, meaning that there is no possibility of visually monitoring of what is happening in the cell. Further, conventional PVT cells typically take a long time to complete an experiment, and measurements produced are somewhat unreliable. 
         [0003]    In the field of production and transportation of crude hydrocarbons such as heavy and extra-heavy hydrocarbons, hydrocarbon fluids are frequently encountered which have a tendency to foam under certain conditions. It is important to know these conditions, since the formation of foam in the reservoir near the well bore or in the production or transportation equipment, can be detrimental to the overall process. Further, a large quantity of hydrocarbons with tendency to foam are heavy or extra-heavy hydrocarbons, for example having an API gravity of less than about 10 or even 8° API. 
         [0004]    Conventional PVT cells are not well suited for use with hydrocarbons having a tendency to form foam, nor are they well suited to heavy and extra-heavy hydrocarbons. The need exists, therefore, for a safe and accurate PVT cell which can be used to accurately evaluate the thermodynamic properties of heavy and extra-heavy hydrocarbons, particularly those with a tendency to foam. The present invention provides such a device, and method, as well as further details and characteristics which will be further discussed below. 
       SUMMARY OF THE INVENTION 
       [0005]    In accordance with the present invention, a PVT system is provided for evaluating foamy heavy and extra-heavy hydrocarbons, which system comprises a cell having a wall defining an inner space; a floating piston slidable in the inner space; a stirring mechanism slidable into the inner space and operative to mix a hydrocarbon sample in the inner space; a volumetric pump associated with the cell for selectively increasing and decreasing pressure on the floating piston, and communicated with the inner space to control pressure in the space; an oven for heating the cell, wherein the cell is mounted within the oven; an inversion mechanism for inverting the cell at least about 180°; and a frame supporting the oven, wherein the wall of the cell and the oven have corresponding elongated transparent sections arranged to allow visual inspection of fluid in the inner space along an entire longitudinal extent of the inner space. 
         [0006]    In further accordance with the invention, a method is provided for conducting a PVT analysis of a foamy heavy or extra-heavy hydrocarbon, wherein the method comprises the steps of placing a sample of a foamy heavy or extra heavy hydrocarbon into a PVT system comprising a cell having a wall defining an inner space; a floating piston slidable in the inner space; a stirring mechanism slidable into the inner space and operative to mix a hydrocarbon sample in the inner space; a volumetric pump associated with the cell for selectively increasing and decreasing pressure on the floating piston, and communicated with the inner space to control pressure in the space; an oven for heating the cell, wherein the cell is mounted within the oven; an inversion mechanism for inverting the cell at least about 180°; and a frame supporting the oven, wherein the wall of the cell and the oven have corresponding elongated transparent sections arranged to allow visual inspection of fluid in the inner space along an entire longitudinal extent of the inner space; altering one property of pressure, volume or temperature in the inner space; mixing the sample while allowing the sample to reach equilibrium, and observing phases present in the cell through the transparent sections. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    A detailed description of preferred embodiments of the present invention follows, with reference to the attached drawings wherein: 
           [0008]      FIG. 1  schematically illustrates a system in accordance with the present invention; 
           [0009]      FIG. 2  is a partially sectioned schematic view of a system in accordance with the present invention; 
           [0010]      FIG. 3  is a partially sectioned schematic view of a system in accordance with the present invention; 
           [0011]      FIGS. 4 and 5  show specific details of a cell component in accordance with the present invention; 
           [0012]      FIG. 6  illustrates components of the cell of  FIGS. 4 and 5 ; 
           [0013]      FIG. 7  sequentially illustrates operation of the system of the present invention with a foamy fluid; 
           [0014]      FIGS. 8   a  and  8   b  show an example of constant composition expansion process and isolation of bubble points Pb′ and Pb; and 
           [0015]      FIG. 9  further illustrates transparent portions of components of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The invention relates to a PVT cell and system for use in evaluating heavy and extra heavy hydrocarbons, particularly such hydrocarbons which have a tendency to foam. 
         [0017]    As discussed above, conventional equipment is not well designed to handle highly viscous heavy and extra-heavy hydrocarbons with a tendency to foam, and existing equipment is unreliable and time-consuming in order to attempt analysis of such fluids. Further, a large number of conventional systems use mercury for some measurements, and this is a hazardous substance, particularly to those using the device. 
         [0018]    In addition, it is highly desirable to have a system which can accurately and safely produce thermodynamic analysis of heavy and extra-heavy hydrocarbon fluids, particularly those which have a tendency to foam. For example, such fluids are produced from extremely large reservoirs throughout Venezuela, and the tendency to foam can cause significant problems in production and transportation of the hydrocarbons. 
         [0019]      FIG. 1  illustrates a system in accordance with the present invention which is well suited to conducting thermodynamic analysis of such fluids. 
         [0020]      FIG. 1  shows system  10  including a PVT cell  12  which is mounted in an oven schematically illustrated at  14 , and supported on a frame  16  which allows a 180° inversion of cell  12 . A pressure source such as a volumetric pump  18  is communicated with cell  12 , preferably in two different paths to allow adjustment of pressure and volume within cell  12 , as will be discussed below. 
         [0021]    Further, a stirring mechanism  20  can be operated within cell  12  to mix fluids within cell  12 , and a motor  22  can be releasably engaged with mixing structure  20  to mix fluids when desired. Pressure and temperature measurements within cell  12  are taken using pressure and temperature sensors P 1 , T 1 . Further, a control unit  24  is shown which can be provided in the form of any computing device such as a desktop or laptop computer, dedicated mainframe or the like, and can be communicated with all components and sensors of system  10  to appropriately control system  10  and record all data and measurements collected thereby. 
         [0022]    Referring also to  FIGS. 2 and 3 , cell  12  can preferably have a wall  26  which defines an inner space  28  for holding a fluid sample to be evaluated. Wall  26  can advantageously be formed into a cylinder or tube-shape, as shown. 
         [0023]    A floating piston  30  is slidably positioned within inner space  28  and substantially sealingly interacts with an inner surface of wall  26 . As will be evident from a consideration of  FIG. 1 , piston  30  divides inner space  28  into two different sections. One section  32 , will typically contain the fluid to be evaluated, while the other section,  34 , receives pressure from pump  18  which is used to position floating piston  30  to a desired location so as to adjust volume of section  32 . Piston  30  advantageously has sufficient seals and/or engagement with the inner surface of wall  26  that leakage past piston  30  in either direction is substantially prevented. 
         [0024]    In order to control position of piston  34 , pressure can be conveyed from pump  18  through line  36  and inlet  38  which is communicated with section  34  of inner space  28 . Increasing the pressure in section  34  moves piston  30  so as to decrease the volume of section  32  in which the sample is located. It is with this structure that the volume of the sample can be adjusted during thermodynamic analysis, and such thermodynamic analysis is understood by a person of ordinary skill in the art. 
         [0025]    Pump  18  is also communicated with section  32  within cell  12  such that pressure within section  32  can be increased without moving piston  30 . Pump  18  is preferably connected, in this fashion, through a line  40  which can lead to one or more piston cylinders  42  which, through pistons  44 , can convey pressure through line  46  and into section  32  of cell  12  as desired. 
         [0026]    Still referring to  FIGS. 1-4  and  6 , the mixing structure  20  and motor  22  will be further discussed. As shown, mixing structure  20  can be provided in the form of a plurality of vanes  48  extending from a hub  50  which is connected to a shaft  52 . Shaft  52  is releasably connected to motor  22  such that, when system  10  is in the position of  FIG. 1 , shaft  52  can be rotated by motor  22  so as to rotate vanes  48  on hub  50  within section  32  where the sample of fluid to be analyzed is present. Vanes  48  can have any shape or structure which would be effective at mixing the heavy hydrocarbons toward which the invention is directed. These structures could include turbine or propeller type vanes, for example. 
         [0027]    As also illustrated in  FIGS. 1-3 , frame  16  supporting cell  12  is advantageously mounted for at least 180° rotation or inversion so as to allow a good mixing of the sample positioned therein.  FIGS. 1-3  show frame  16  mounted on a shaft  54  which can be rotated as schematically illustrated by the arrows shown in  FIGS. 1 and 3  to fully invert cell  12 . The motor or other structure for causing such rotation is not shown in the drawings, but would be readily known to a person skilled in the art. It should be appreciated that cell  12  configured as described can be rotated or inverted within oven  14  through rotation of frame  16  around shaft  54 . 
         [0028]    In order to rotate cell  12 , it should be appreciated that shaft  52  of mixing structure  20  should first be disconnected from motor  22 , and to facilitate this disconnection, motor  22  can be vertically movable relative to shaft  52  as shown by the arrows in  FIG. 1 . 
         [0029]    Turning now specifically to  FIGS. 2 and 3 , it should be appreciated that wall  26  of cell  12  can advantageously be provided as a transparent member, and this is also illustrated in  FIGS. 4 and 5 . This section being transparent is advantageous in accordance with the present invention as it allows observation of fluid within cell  12  as it is being analyzed, and further allows visual monitoring of foam formation within cell  12 . To this end, corresponding portions of frame  16  and oven  14  should also be transparent, and preferably should have at least transparent sections which extend the entire vertical height of cell  12  so that foam can be observed at either end of cell  12  or any position therebetween. 
         [0030]    It should be appreciated that either end of cell  12  is closed by an end cap  56 ,  58 , and that some flow lines and structures must past through said end caps to allow for proper functioning of the device. 
         [0031]    For example, end cap  56  which is opposite to mixing structure  20  is a substantially solid end cap but for being provided with flow channels (not shown) for conveying pressure from pump  18  through lines  36 ,  38  and into pressurized section  34  of inner space  28 . 
         [0032]    End cap  58  at the same side of mixing structure  20  is slightly more complex, as shaft  52  of mixing structure  20  must sealingly pass through end cap  58  as shown in  FIGS. 2 ,  3  and  6 . 
         [0033]    As shown in  FIGS. 4 and 5 , end caps  56 ,  58  can extend laterally beyond the perimeter of cell  12 , and this advantageously allows a plurality of rods  60  or other support structures to be connected between end caps  56 ,  58  and thereby provide additional strength and durability to cell  12 .  FIG. 6  further illustrates end cap  58  and shows a flow passage  62  which passes from a perimeter of end cap  58  inwardly to a surface which communicates with section  32 . Flow passage  62  is advantageously communicated with pressure from pump  18 , preferably conveyed through piston cylinders  42  and line  46 , and this structure is used to adjust pressure within section  32  as desired. 
         [0034]    Returning to  FIG. 1 , it should be appreciated that an outlet line  64  is connected to line  36  as shown and can be used to vent pressure from section  34  of cell  12  or allow release of pressure directly from pump  18  as desired. In addition, an outlet line  66  can also be provided, preferably from end cap  58 , and this line can be used to bleed released gas from within section  32  as desired.  FIG. 1  shows a number of valves along the various lines which can be used to control the different flows discussed, and these valves, it should be appreciated, would preferably be under control of controller  24  in order to produce a fully automated system, preferably which provides all data to an operator in digital form. 
         [0035]    Turning now to  FIG. 7 , a first view  68  shows cell  12  in accordance with the present invention in an empty condition. View  70  shows cell  12  containing a sample of fluid in section  32 . View  72  shows mixing structure  20  being used to stir the sample fluid within section  32  until a stabilization point is reached. View  74  shows cell  12  in an inverted position. View  76  illustrates cell  12  which now contains two phases, namely a liquid and foam phase, along with stirring through mixing structure  20  such that the two phases are stabilized. 
         [0036]    View  78  shows cell  12  with two phases in an inverted position. 
         [0037]    Views  72  and  74  of  FIG. 7  show how to obtain a good equilibrium with the device of the present invention by inverting the cell in order to find a best homogenization for each pressure value (one phase region in this case) and analogically in views  76  and  78  for two phases regimes. 
         [0038]      FIGS. 8   a  and  8   b  show an experimental constant composition expansion process at reservoir temperature using an extra heavy fluid with a foamy behavior. The different views provided are conditions observed at different equilibriums (after stirring and stabilizing at each point). Pressures P 1  through P 8  with corresponding sample volumes (V 1  through V 8 ) are shown. When plotting sample volume vs. pressure, it is possible to determine the pseudo bubble point (P b ′), in this case between P 3  and P 4 , when foam starts to be produced, and the bubble point (P b ), in this case between P 6  and P 7 , when free gas starts to be produced in a free gas cap in the top of the cell. The device of the present invention can be used to evaluate the foamy heavy hydrocarbon behavior during the constant composition expansion process. 
         [0039]      FIG. 9  shows an arrangement with cell  12  within oven  14  and showing a transparent portion  80  of a wall of oven  14  to allow visual inspection of cell  12  therein. It should be appreciated that frame  16 , in order to allow reliable mounting and rotation within oven  14 , may have wall sections which extend in ways that would obstruct view of cell  12 . Thus, in one aspect of the invention, the frame  16  may also have transparent sections for example aligned within oven  14  behind section  80 , or could be made having slots or other clear areas to allow good viewing of cell  12  within oven  14  as desired, that is, without opaque sections between section  80  and cell  12 . It should be appreciated that the support rods  60  of cell  12  allow for the cell to have transparent walls to provide complete visibility into same, while preserving structural strength of cell  12 . 
         [0040]    The above features of cell  12 , oven  14  and frame  16  combine to provide a system along with all control equipment, which can produce excellent experimental precision when foamy heavy hydrocarbon phases are studied, because the pseudo bubble point (P b ′) and the bubble point (P b ) can both be identified with precision. These parameters are very important to evaluate as they are highly relevant production mechanisms in reservoir studies. Also, this information allows identification of the volumetric behavior, density and compressibility of foamy phase during depletion studies at reservoir condition. 
         [0041]    It should be appreciated that the present disclosure has been given in terms of a preferred embodiment. The scope of the invention is not to be viewed as being limited by this embodiment, but rather as being defined by the scope of the appended claims.