Abstract:
A magnetic stirring system for pVT and condensate cell is described. The stirring detection is performed by a solenoid to achieve reliable monitoring of the rotation of the stirring impeller in high-viscous fluids, dark fluids or cells without window.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a system for stirring by a magnetic coupled impeller, for use in pressure, volume and temperature (pVT) studies, of reservoir fluids and their properties in the laboratory and in the field. 
         [0003]    In pVT cells and condensate cells, petroleum fluids can be studied at varying pressure and temperature simulating the conditions in oil reservoirs before and during production. Typically these fluids contain gas. The change in fluid density (compressibility) and the tendency for the gas to come out of solution at decreasing pressure are of particular interest. 
         [0004]    pVT cells are optimised to study oils with dissolved gas, while condensate cells are optimised to study light oils with high gas to oil ratio. In the following they are both denoted pVT cells. 
         [0005]    Until the late eighties, the method to control the pressure in these cells was to pump mercury in and out of the cells. Mercury was considered to be inert with the respect to the petroleum fluids. There were however some health risks involved in the handling of mercury at high pressure and temperature, and mercury pumping has to a large extent been replaced by other methods for changing the volume in pVT cells. 
         [0006]    Several of the new designs are based on cylindrical cells with a sealed piston that can be moved by either direct mechanical drive or hydraulic drive. 
         [0007]    By shaking the old pVT cells with mercury, the mercury would also provide good stirring, so that a fast equilibrium between the phases was obtained. Equilibrium is essential to achieve reliable and reproducible measurements. This feature is lost when the volume is controlled with a piston. 
         [0008]    2. Description of Related Art 
         [0009]    As described in more detail later, several pVT cells are equipped with a magnetically coupled stirrer for mixing of the fluids under test. 
         [0010]    The principle of a loose magnetic pulse driven stirrer and placement/stirrer speed/stirrer drag in a high pressure cells is among other places described in Norwegian patent no. 312.921. While patent 312.921 is an optical method to detect viscosity changes, the present invention is suitable for any fluid where optical methods are not suitable. 
         [0011]    The magnetic coupling between the solenoids and the stirrer is relatively weak, due to the geometry and because the magnetic field is supplied through the metal piston. 
         [0012]    Some petroleum fluids are very viscous, and the stirrer may therefore not rotate at the desired frequency. 
         [0013]    Some cells are not provided with a window, and it is therefore not possible to see the fluid (blind cells). In many cases it would not be possible to see the stirrer even if the cell has a window, because the fluids are so dark. 
         [0014]    A monitoring device to indicate whether the impeller is stirring or not, is therefore needed to achieve assurance of reliable operation. 
         [0015]    Several patents, e.g. U.S. Pat. No. 6,834,990, describe a shaft driven stirrer with various configurations, especially of the stirrer/impeller with the focus on providing special features, e.g. aeration with bubbles from air supplied through the shaft and high efficiency (low power) shear of the fluid etc. 
         [0016]    The shaft driven stirrers are not magnetically coupled, but directly driven, with related sealing problems. The magnetic coupling provides a possibility of a closed container with long time pressure stability. 
         [0017]    One patent, U.S. Pat. No. 6,007,227, describes a control system for a blender application. A control system typically involves a sensor for feedback of the controlled variable. However, in the present invention, the stirrer acceleration, speed or angular position is not a part of a control system. 
         [0018]    An alternative apparatus from those described above is needed to perform pVT studies with a simple and reliable assurance of stirring and therefore mixing of fluids or phase equilibrium under test in cells without a means for visual observation of the fluid, or for fluids with high level of opacity in cells with means for visual observation. The alternative for monitoring should also be very compact with no moving parts. 
       SUMMARY OF THE INVENTION 
       [0019]    Hence, in accordance with the present invention there is provided a novel magnetic stirring system for providing equilibrium prior to test measurement in a high pressure pVT cell. More precisely, the present invention is defined by the wording of the appended independent claim  1 . 
         [0020]    Favourable and preferable embodiments of the invention appear from the dependent claims attached to claim  1 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0021]    In the following, a more detailed description will be given of illustrative embodiments of the present invention, and at the same time it will be referred to the appended drawings, of which 
           [0022]      FIG. 1  shows a cross section through a piston in accordance with one preferable embodiment of the invention, and 
           [0023]      FIG. 2  is a perspective view from the underside of the piston appearing in  FIG. 1 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]    One solution of assuring good stirring is to place a magnetic driven stirring impeller on top of the piston. An impeller  10  with inserted permanent magnets is placed on a shaft  11  in a cavity  2  on the test fluid side of the piston body  1 . 
         [0025]    The stirrer is energised by an assembly of solenoids  3  placed inside the piston  1  in separate wells  4  in the piston body, but separated from the fluids. One of these wells  4  is indicated in  FIG. 1 , while three such wells appear in  FIG. 2 . 
         [0026]    The solenoids are placed parallel to the piston cylinder axis and with one end of the core pointing to the magnets in the impeller  10 , while the opposite sides are magnetically connected to increase field strength and reduce stray fields. 
         [0027]    The piston  1  must be made of a non-magnetic material, preferably an alloy (e.g. Hastelloy C, Inconel, 316 stainless steel etc.). Phased power pulses to the. solenoids  3  are supplied by thin Teflon insulated electric wires  7  which are drawn through a hollow rod  9 . The hollow rod  9  is mounted on a cover  14  closing the bottom of the piston  1 . The wires  7  are connected to an electric pulse generator  8  on the outside of the cell. Rod  9  is attached in the centre of the piston  1 , reaching out through the cell body through a dynamic seal, but is only shown as a very short stub in the schematic drawing of  FIG. 1 . 
         [0028]    To be assured of operation of the stirrer, a small extra solenoid  5  with separate wires  12  is placed in its own well  6  in the piston, but isolated from the fluids so that the permanent magnet in the impeller  10  passes close to the tip (at ref. num.  13 ) of the solenoid  5  if it is rotating. This induces so strong electric pulses that they can easily be separated from the induction due to the pulsing solenoids  3 . An electronic pulse indicator  8  informs the operator of the status of the stirrer. (For simplicity, we have entered both the pulse indicator and the pulse generator in one common unit  8 .) 
         [0029]    In a practical embodiment, the pVT cell piston consists of two parts. One part is the main body  1  with a recess  2  for an impeller  10  and fixing shaft  11  on the fluid-under-test side, an outer section with reduced diameter for piston-cylinder seals and guides, and wells  4 ,  6  for solenoids  3 ,  5  and wires  7 ,  12  extending from the other side. The other part is a cover  14  with a piston rod  9  attached. Between and in the cover  14  and the main body, there are seals so that the inside of the piston is sealed off from both hydraulic fluid and the fluid under test. (In  FIG. 2 , the cover  14  has been left out to show wells  4 ,  6 .) 
         [0030]    The stirring impeller  10 , that can rotate freely on the shaft  11  in the piston recess  2 , has preferably a symmetrical design and has two or more symmetrical located permanent magnets. 
         [0031]    The stirring operation is performed by energising the two or more driving solenoids  3  sequentially, thereby pushing and pulling the impeller permanent magnets. The driving solenoids  3  are designed as a magnetically soft core with cylindrically wound wire. The number of windings and wire thickness and material may vary with desired magnetic force and other features. The driving solenoids  3  can be arranged axially inside the piston  1  or radially outside the cylinder, resembling an ordinary electric motor or step motor. 
         [0032]    In most embodiments, the impeller is fixed axially on the shaft and can only rotate about the shaft axis. The shaft can be designed as a detachable unit either in the form of a single screw entered from the piston top, forming the shaft, or a disk with an extended shaft. The shaft can also be an integral part of the piston. The rotation of the impeller mixes and stirs the fluids under test. 
         [0033]    The monitoring of the stirring is performed by having a solenoid  5  with separate wires  12  in a separate well  6  inside the piston  1 . The wires  7 ,  12  can also be connected in such a way as to form a common reference for the driving and monitoring solenoids  3 ,  5 . 
         [0034]    The monitoring solenoid  5  gives pulses caused by induction from the passing permanent magnets in the rotating impeller  10 . 
         [0035]    The cavity or well  6  for the monitoring solenoid  5  is shown with space  13  for an extended core, while the well  4  for the driving solenoid  3  is shown without. 
         [0036]    While the foregoing preferred embodiments of the invention have been described and shown, it is understood that all alternatives and modifications, such as those suggested and other, may be made thereto and follow the scope of the invention.