Abstract:
The invention relates to a process and a device for measuring the gelling of paraffin petroleum products, in particular, crude oil. The process involves: measuring, with respect to the temperature and over a given thickness, the propagation velocity and the amplitude of an ultrasonic wave in the crude oil being analysed; then determining the transition temperature T t , at which a sudden change in the thermal variation in the inverse of the propagation velocity is observed; determining the ratio of the slopes for the propagation velocity inverse obtained between the linear parts of the thermal variation of said propagation velocity, around said brake point corresponding to T t , above and below, respectively, the determined temperature T t  ; determining the ultrasonic signal amplitude difference ΔA between the temperature T t  and a given lower reference temperature, for example T t  -5° C.

Description:
BACKGROUND OF THE INVENTION 
     The Patent Abstract of Japan, Vol. 6, No. 116 (P-125) (994), Jun. 29, 1982, 8 JP-A-57 44 852, Mar. 13, 1982 discloses a method and an apparatus for revealing the solid state of an intentionally-congealed fluid in the pipe of a nuclear reactor. This process is not adapted for measurement of petroleum products. 
     The present invention concerns a procedure and device for measurement of the gelling of paraffinic petroleum products, especially crude products. 
     It is known that paraffinic crudes can form gels at temperatures approaching, or less than, 40° C. This gelling phenomenon, when it occurs during production or transport, may lead to problems involving the restart-up of facilities. It is thus critically important to have available a method making it possible to determine what crudes are capable of gelling within a given temperature range and, as needed, to determine the minimum quantities of gelling-inhibitor or doping product needed to prevent the formation of gel. 
     Current conventional practice can measure the pour point of crudes (see Standard ASTM D-97 or AFNOR T60-103), this method consisting in cooling crude under determinate conditions in a special test tube equipped with a thermometer, and in looking to see whether, at each three-degree interval, the surface has or has not congealed. The temperature at which the surface of the crude congeals is the pour point. 
     It is also known how to measure, by differential caloric analysis, the incipient crystallization temperature of the crude, this latter corresponding, in fact, to a detectable exothermic phenomenon. 
     It is further known how to measure the rheological behavior of crudes, the flow properties of these latter ceasing newtonian behavior below a certain temperature, when paraffin crystals form within them. 
     However, none of these data, i.e., the measurement of the pour point, measurement of the incipient crystallization temperature, and the temperature at which rheological behavior changes, make it possible to know with certainty that operating difficulties will not occur in the event the crude is cooled below that temperature. 
     SUMMARY OF THE INVENTION 
     The present application attempts to meet this need for more reliable knowledge concerning gelling of paraffinic crudes during use and proposes a procedure for measurement of gelling of paraffinic crudes which is characterized mainly by the fact that, to determine if a crude is about to form a gel capable of hindering its use at a given temperature, this procedure comprises the following steps: 
     measuring, as a function of temperature and on a given thickness, the speed of propagation and the amplitude of an ultrasonic wave in the crude analyzed, 
     then determining the transition temperature T t  at which an abrupt change in the thermal variation of the reciprocal of the propagation speed is observed; 
     determining the value of the ratio of the slopes p of the reciprocal of the rate of propagation obtained between the linear portions of the thermal variation of this latter around said break point corresponding to T t , below and above, respectively, the determined temperature T t , and 
     determining the difference ΔA of the amplitude of the ultrasonic signal between said temperature T t  and a lower reference temperature equal, for example, to T t  -5° C., a significant break of slope (p&gt;1) at said temperature T and a high value of the difference A (A&gt;0.3 dB/cm at T t  -5° C.) signifying that gelling of the crude will very probably occur at temperatures below T t , but that, to the contrary, gelling is not very likely for a slight variation of one of these parameters. 
     Applicant has found that knowledge of p and ΔA characterizes the physical state of crude paraffinic products. The gelling state of a crude implies, for the temperature below T t  previously specified, the change of said thermal slope as well as a significant attenuation of the wave transmitted. Inversely, for crudes exhibiting no risks of gelling, at least one of these parameters remains more or less unchanged when the temperature T t  is reached. 
     The invention also concerns a device for measurement of the rate of propagation and the amplitude of the ultrasonic wave. Any conventional device is suitable within the scope of the invention. The range of practical frequencies used lies between 300 KHz and 10 MHz. For example, use may be made of a device which measures the time of travel, or &#34;flight time&#34; and the amplitude of an ultrasonic signal which is propagated at a frequency of approximately 600 kHz through a thickness of crude of approximately 1 cm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described with reference to the attached drawings, in which: 
     FIG. 1 is a schematic transverse cross-section of a measuring cell belonging to the invention device; 
     FIG. 2 represents the block diagram of the electronic circuit of this device; 
     FIG. 3 is a diagram representing, as a function of temperature, the variation of time of propagation, or &#34;flight time,&#34; (homogeneous with the reciprocal of the rate of propagation) and of the amplitude in dB of the ultrasonic wave in the crude, and 
     FIG. 4 is a graphic representation, expressed in reduced values T-T t  and ζ- T  ζ (where ζ t  is the value of the time of travel at T t ) of the variations of the &#34;flight time&#34; of the ultrasonic wave as a function of temperature, for various paraffinic crudes. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, the measuring apparatus according to the invention essentially comprises a central inner chamber 1 which holds the crude paraffinic product to be analyzed, this chamber being attached to a metal jacket 3 equipped with a thermostat covered with a thermal-insulation wall 5. The jacket 3 is kept thermostated by a water circuit which enters through an inlet orifice 7 and exists through an outlet orifice 9. The inner chamber 1 is cylindrical, its internal volume being delimited laterally by two disks 11 and 12 made of piezoelectric ceramic and which form transducer elements, i.e., an ultrasonic transmitter and receiver, respectively, belonging the apparatus, and by a ring-shaped part 13 attached to the jacket 3. In a special embodiment, the distance between the disks 11 and 12 is 14 mm, and the internal diameter of the chamber 1 is 26 mm. Consequently, the chamber volume is 7.5 cm 3 . The upper section of the ring-shaped part has a radial filling orifice 15 having a volume of approximately 0.5 cm 3 . The total product volume contained in the chamber is thus 8 cm 3 , the overflow of the orifice compensating for the withdrawal of the fluid contained in the chamber during cooling. A thermal probe 17, inserted in the thickness of the lower section of the ring-shaped part 13, makes it possible to raise the temperature of the product to be analyzed. The outer insulating wall 5 imparts to the entire assembly a thermal stability of several one-hundredths of a degree. The transducer components 11 and 12 are connected to the electronic measuring circuit by spring wires 19 in contact with the piezoelectric disks and by means of connector plugs 21. 
     FIG. 2 illustrates the diagram of the electronic circuit making it possible to measure the time of propagation, or flight time, of the ultrasonic wave in the fluid to be analyzed in the inner chamber of the apparatus. The principle underlying the processing of the signal as performed by this circuit is as follows: 
     Every two ms, a low-frequency clock 23 triggers the firing of an ultrasonic impulse transmitted by the transmitter transducer disk 11; simultaneously, a CLEAR signal at the clock outlet, having a duration of ζ o , is sent to the zero-reset input of a binary counter 25. This signal inhibits the functioning of the counter at a starting value. Another effect of the CLEAR signal is to actuate a trigger circuit 27 which generates a COUNT validation signal, authorizing the activation of impulses from a high-frequency clock 29 (50 MHz) whose function is to increase the counter 25 incrementally. This latter begins to count only at the end of the CLEAR signal, and thus after ζ o . 
     During this interval, the ultrasonic wave is propagated in the measuring cell, before reaching the receiver-transducer 12, which supplies an electric RECEIVE signal reflecting the ultrasonic energy received after passing through the crude. The RECEIVE signal, suitably amplified, triggers a comparator 31, which deactivates the COUNT trigger circuit 27; at this instant, the counter 25 stops. The duration of the COUNT signal equals the time of propagation of the ultrasonic wave in the crude. The digital indication of the counter corresponds to the value ζ-ζ o , ζ o  acting only as a starting value making it possible to make the measurement values conform to the permitted measurement scale. 
     The digital counter outlets thus indicate a number equal to the difference between the durations of the COUNT and CLEAR signals, multiplied by the clock frequency. This number is transmitted to a computation unit 33 which computes an average value. 
     The measured propagation times of the ultrasonic wave are approximately 10 μs (approximately 14 mm at 1400 m/s). The variations of speed in the cell over the range of temperatures as a function of the crude are from 10 to 20%, and indicate variations of flight time of from 1 to 2 μs, i.e., 50 to 100 beats of the clock. One need only adjust the duration of the CLEAR signal as a function of this variation to adjust the reading properly within the measurement scale of the counter. 
     The resolution in time allowed by the clock is 20 ns. The various instabilities of the electronics introduce interference of several ns. To provide satisfactory precision, the computation unit computes the average of 1,000 acquisitions, thus reducing parasitic noise by a factor of approximately 30. The precision thus obtained is approximately 1 ns, or 1/10,000 of the time of propagation. 
     The ultrasonic attenuation of the propagation wave in the crude is measured based on the variation of amplitude of the signal received, after the thermal variations of the coupling of the transducer disks with the medium studied have been taken into account. 
     FIG. 3 is a graphic representation of the variations of the propagation time and of the ultrasonic attenuation of a typical crude, i.e., no. 2 crude. This crude has a transition temperature below which it is capable of gelling, which is observed at the break point of the propagation-time curve at a value approaching 30° C. On either side of this point, the variations of propagation time are linear, the segment corresponding to T&lt;T t  having a more pronounced slope than that corresponding to T&gt;T t . The above-mentioned ratio p of the slopes for these segments is thus clearly superior to 1. 
     As regards ultrasonic attenuation corresponding to variations of the amplitude of the signal for which the dB scale is represented on the right, a rapid decrease in amplitude resulting from increased losses and corresponding to gelling of the crude is observed below a temperature approximating T t . It will thus be seen that the parameters p and A cited are correlated with the physical state of the crude. 
     FIG. 4 represents the variations of the propagation time (inversely proportional to the speed of propagation) as a function of temperature of the paraffinic crudes No.1:a, no.2: b, no. 3:c, no. 4:d, no. 5:e, and no. 6:f. It is found that the variations of propagation time as a function of temperature of all of these crudes are linear for T&lt;T t , with different slopes depending on the crudes. They are also linear for T&gt;T t  (liquid phase), but, in that case, they are also identical. 
     Crudes nos. 1, 2, 3, and 4 are characterized by a break of the slope at a characteristic temperature T t  and by a variation, once again linear, but of slope p, which is greater when T&lt;T t  (crystalline phase). 
     For crude no. 5, the break of slope is only slightly pronounced (p≃1), while for crude no. 6, it is not detectable. These two latter crudes do not, in practice, pose risks of obstructing pipes under operating conditions. This shows clearly that p is one of the parameters characterizing the formation of gel, and that, if it is approximately equal to 1, there is very little chance that a gel will form. 
     Table I below recapitulates the values of p and the difference ΔA between T t  and T t  -5° C., respectively, for the crudes mentioned. 
     
                       TABLE I______________________________________Crude         T.sub.t (°C.)                      p     A (dB)______________________________________Crude no. 1   37.5         1.6   0.5Crude no. 2   31           1.8   2.1Crude no. 3   36           1.5   1.0Crude no. 4   36           1.9   2.3Crude no. 5   22           1.1   0Crude no. 6   --           1.0   0no. 2 + 200 ppm         31           1.4   0no. 2 + 400 ppm         31           1.4   0______________________________________ 
    
     In the cases of crude nos. 5 and 6, the difference ΔA remains close to zero, thereby confirming the information obtained by p. 
     The Table II below summarizes the test results for the different crudes studied and for doped crudes, and, in particular, two specimens of no. 2 crude doped with 200 and 400 ppm, respectively, a gelling-point depressing product. 
     
                       TABLE II______________________________________Crudes     Remarks        Test Result______________________________________Crude no. 6      No transition  No gelling possible      Slight variation of                     within the      A              temperature rangeCrude no. 5      p ≅ 1                A ≅ 0                         studiedCrude no. 1      Pronounced transition                     Gelling possibleCrude no. 2      Strong variation of                     when T &lt; T.sub.t      ACrude no. 4      p &gt; 11Crude no. 3      ΔA high when T &lt; .sub.tCrude no. 2 +      Marked transition                     No gelling possibledoping agent      Slight variation of                     within the range of      A              temperature studied    p &gt; 1   A ≅ 0______________________________________ 
    
     The presence of the doping agent is characterized by the presence of attenuation (ΔA≃0) of the ultrasonic propagation wave. 
     Consequently, the method proves to be valid also for doped crudes for which gelling can be prevented by adjusting the quantities of the doping product until a value of ΔA of approximately zero is obtained. 
     This method could be applied to other materials besides paraffinic crudes, and, for example, to paraffinic petroleum cuts, and, in general, to petroleum-derived products such as natural gas containing condensate (natural gas hydrates which one wants to prevent from being deposited within ducts, in particular by adding specific inhibitors); some products inherent in hydrocarbons, e.g., asphaltenes whose precipitation one wishes to prevent in these ducts; paraffins; waxes, etc. Furthermore, in the example described, the procedure was carried out in the absence of any particular pressure condition, the investigation being performed under atmospheric pressure. However, the scope of the principle underlying the invention also encompasses variation of this parameter, particularly for the study of ways to prevent the above-mentioned deposits. In this case, the measurement cell is pressurized using suitable conventional means.