Abstract:
The invention relates to a method of characterizing an oil borehole effluent, formed by a multiphase fluid mixture typically containing water, oil, and gas. According to the invention, a gadolinium 153 source is used to emit gamma rays at a first energy level of about 100 keV and at a second energy level of about 40 keV, and the attenuation of the gamma rays at these two energy levels is measured after the rays have passed through the effluent.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to measurements relating to the composition of oil well effluents, constituted by multiphase fluids typically comprising three phases: two liquid phases, namely crude oil and water, plus a hydrocarbon gas phase, and more particularly to measurements of gamma ray attenuation by the fluid. The invention also relates to the association of such measurements with flow rate measurements in order to determine the flow rates of the various phases. 
     In the oil industry, the traditional practice is to separate the effluent into its component phases and to perform measurements on the phases separated in this way. However that technique requires separators to be installed on site, which separators are bulky and expensive items of equipment, and when testing wells, it also requires additional pipes to be put into place. 
     Numerous proposals have been put forward for developing techniques that would make it possible to avoid using such separators. A description of these developments is to be found in SPE publication 28515 (SPE Annual Technical Conference, New Orleans, Sep. 25-28, 1994) by J. Williams, “Status of multiphase flow measurement research”. 
     Amongst such proposals, U.S. Pat. No. 4,788,852 describes apparatus including a device for measuring gamma ray attenuation, that device being associated with a Venturi total flow rate sensor and being situated at the constriction of the Venturi. Apparatus of that type is also described in patent application WO 94/25859 and in SPE publication 36593 dated Oct. 6, 1996, “Multiphase flow measurement using multiple energy gamma ray absorption (MEGRA) composition measurement” by A. M. Scheers and W. F. J. Slijkerman. 
     SUMMARY OF THE INVENTION 
     The invention seeks to provide such gamma ray attenuation measurements in an advantageous manner, that is particularly well adapted for being associated with flow rate measurements using the Venturi effect. 
     In one aspect, the invention provides a method of characterizing an oil borehole effluent, formed by a multiphase fluid mixture which typically comprises water, oil, and gas, comprising the steps of emitting gamma rays at a first energy level of about 100 keV and at a second energy level of about 40 keV, and measuring the attenuation of the gamma rays at these two energy levels after transmission through the effluent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be well understood on reading the following description given with reference to the accompanying drawings. The list of drawings is as follows: 
     FIG. 1 is a diagrammatic view of a measurement device for characterizing an oil well effluent, including a Venturi section and a device for measuring gamma ray attenuation; 
     FIG. 2 is a longitudinal section through an embodiment of the FIG. 1 device; 
     FIG. 3 is a detail view showing a portion of the device shown in FIG. 2, but on a larger scale; and 
     FIG. 4 is a diagram of a Venturi section as shown in FIGS. 2 and 3, and fitted for semi-periodic use. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows in highly diagrammatic manner a flow meter device adapted to oil well effluent. 
     The device comprises a pipe section  10  comprising a converging Venturi  11  whose narrowest portion  12  is referred to as the throat. In the example shown, the section of the pipe  10  is disposed vertically and the effluent flows upwards, as symbolized by arrow F. 
     The constriction of the flow section in the Venturi induces a pressure drop Δp between the level  13  situated upstream from the Venturi at the inlet to the measurement section and the throat  12 . This pressure drop is associated with the total mass flow rate Q and with the density ρ m  by the following relationship:                Δ                 p     =         K   ·     Q   2         ρ   m       +       ρ   m          gh   V                 (   1   )                                
     where g is the acceleration due to gravity, h V  is the distance between the upstream level  13  and the throat  12 , and K is a constant associated essentially with the geometry of the Venturi, and which is given by:        K   =       1   -     β   4         2        C   2          A   2                                
     where β is the constriction ratio of the Venturi, i.e. the ratio between the diameter of the throat and the upstream diameter of the Venturi, C is the discharge coefficient, and A is the section of the throat. The term ρ m .g.h V  s generally small or negligible. By writingΔp*=Δp−ρ m .g.h V , relationship (1) becomes: 
     
       
           Q=k (Δ p*.ρ   m ) ½   (2) 
       
     
     where k=K  −½   
     The density ρ m  is measured at the throat of the Venturi. This is relevant to the validity of relationship (2) for the following reason. The acceleration, and consequently the pressure drop, to which the fluid in the Venturi is subject takes place in privileged manner in the region close to the throat, because velocity, which is proportional to the square of the diameter, increases considerably in this region. Relationship (2) normally assumes a single phase fluid. It remains suitably applicable to a multiphase fluid providing densityρ m  is measured at the throat. This is particularly true with increasing Venturi effect, and for this reason, an appropriate value for the constriction ratio is β=0.5. With a pipe having a diameter of 10 cm, the diameter in the throat is then 5 cm. 
     The discharge coefficient C is approximately 1. It depends to a small extent and in predictable manner on the properties of the fluid. Traditionally, this corrective effect is taken into account by the Reynolds number. 
     The pressure drop Δp is measured by means of a differential pressure sensor  15  connected to two pressure takeoffs  16  and  17  opening out into the measurement section respectively at the upstream level  13  and in the throat  12  of the Venturi. In a variant, the measurement may also be performed by means of two absolute pressure sensors connected to the pressure takeoffs  16  and  17 , respectively. 
     The density ρ m  of the fluid mixture is determined by means of a sensor which measures the attenuation of gamma rays, by using a source  20  and a detector  21  placed on opposite sides of the Venturi throat  12 . The throat is provided with “windows” of material of low photon absorption at the energies under consideration. The source  20  produces gamma rays at two different energy levels, referred to below as the “high energy” level and as the “low energy” level. The detector  21  which comprises in conventional manner a scintillator crystal such as Nal and a photomultiplier produces two series of signals W hi  and W lo  referred to as count rates, representative of the numbers of photons detected per sampling period in the energy ranges bracketing the above-mentioned levels respectively. 
     These energy levels are such that the high energy count rate W hi  is essentially sensitive to the density ρ m  of the fluid mixture, while the low energy count rate W lo  is also sensitive to the composition thereof, thus making it possible to determine the water content of the liquid phase. 
     It is suitable for the high energy level to lie in a range 85 keV to 150 keV. For characterizing oil effluent, this energy range presents the remarkable property whereby the mass attenuation coefficient of gamma rays therein is substantially the same for water, for sodium chloride, and for oil, being about 0.17 cm 2 /g. This means that the high energy attenuation makes it possible to determine the density ρ m  of the fluid mixture without that requiring auxiliary measurements to be performed to determine the properties of the individual phases of the fluid mixture (attenuation coefficients and densities). 
     The attenuation measured by the detector  21  is expressed by the following relationship: 
     
       
           A=D   V .ν m .ρ m   (3) 
       
     
     where D V  is the distance travelled through the fluid, i.e. in this case the diameter of the Venturi throat, and ν m  is the mass attenuation coefficient of the fluid mixture. 
     Since the mass attenuation coefficients of water and oil in the energy range above are substantially identical, and since the contribution of the gas is negligible because of its very low density, the mass attenuation coefficientμ m , and thus the product D V .ν m  that appears in equation (3) can be considered as being substantially constant and independent of the densitiesρ o  and ρ w  of the oil and water phases. 
     Under such conditions, the high energy attenuation A hi  is a very advantageous indicator of the density ρ m  of the mixture. 
     A material that is suitable for producing high energy gamma rays in the energy range under consideration and low energy rays is gadolinium 153. This radioisotope has an emission line at an energy that is approximately 100 keV and that is entirely suitable for use as the high energy source. Gadolinium 153 also has an emission line at about 40 keV, which is suitable for the low energy level that is used to determine water content. This level provides good contrast between water and oil, since the attenuation coefficients at this level are significantly different, typical values being 0.228 cm 2 /g for oil and 0.291 cm 2 /g for sea water. It is also well separated from the high energy level and well above the noise level of the detector. 
     On the topic of the above-described gamma ray attenuation sensor, it should be observed that uses can be envisaged therefor other than those described above. The sensor may be used on its own and thus only provide water content information, in which case the sensor may be mounted in a straight section of pipe, or it may be combined with a flow rate sensor of a type other than a Venturi sensor. As an example of such sensors, mention can be made in particular of devices in which, as for a Venturi, a change of flow velocity is induced and the resulting pressure drop is measured (perforated plates). 
     FIG. 1 also shows a pressure sensor  22  connected to a pressure takeoff  23  opening out into the throat  12  of the Venturi, which sensor produces signals representative of the pressure p V  in the throat of the Venturi, and a temperature sensor  24  producing signals T representative of the temperature of the fluid mixture. The data p V  and T is used in particular for deriving gas density ρ g  under line conditions and gas flow rate q g  under normal conditions of pressure and temperature from the value for the flow rate under line conditions, determined in a manner described below. In this respect, it is preferable for the pressure to be measured at the throat of the Venturi. In contrast, it does not matter where temperature is measured. 
     Finally, a block  30  represents a unit for acquiring and processing data, which unit receives the signals coming from the above-mentioned sensors. 
     Description with Reference to FIGS. 2 and 3 
     FIG. 2 is a section through an embodiment of the Venturi section  10  shown diagrammatically in FIG. 1, and FIG. 3 shows in greater detail how the elements  20  and  21  of the gamma attenuation measuring device are assembled at the throat of the Venturi. Elements corresponding to elements of FIG. 1 are designated herein with the same reference numeral plus  100 . The Venturi section  110  thus includes a converging portion  111  whose throat  112  is the narrowest portion thereof. 
     In FIG. 2, there can be seen pressure takeoff orifices  116  disposed at the upstream level  113  of the Venturi. Other pressure takeoff orifices  117  are to be found at the level of the throat  112 . Each of these orifices is provided with a bore  125  opening out into the throat  112 , and with a tapped hole  126  of larger diameter in which it is possible to fix the endpiece of a duct (not shown), in order to put the flow into communication, as appropriate, with a differential pressure sensor or with an absolute pressure sensor such as the sensors  15 ,  22  mentioned with reference to FIG.  1 . The number of orifices is optional. FIG. 2 shows two pairs of orifices  116  disposed at 90°, however an appropriate measurement can be obtained with a single orifice  116 . 
     FIG. 2 shows the gamma ray attenuation measurement device constituted by a source block  120  and a detector block  121 , both assembled to the Venturi section. These elements have a longitudinal axis  118  disposed perpendicularly to the axis  114  of the Venturi section at its throat  112  and they are shown in longitudinal section in FIG.  3 . FIG. 3 shows in greater detail firstly the housings  130  and  131  opening out into the outside wall of the Venturi and provided with threads to enable the source block  120  and the detector block  121  to be screwed respectively thereto, and also showing housing  132  and  133  opening out into the inside wall of the Venturi and receiving the “windows” (pieces of material presenting low gamma ray attenuation)  134  and  135 , together with the associated elements. Each window is held in place by a tubular element  136  screwed into a larger diameter threaded portion  137  of the corresponding housing, together with a gasket  138  held against a shoulder in the housing. This arrangement of the windows provides a sealing barrier and enables the source block  120  and the detector block  121  to be implemented in the form of removable elements that can be dismounted in complete safety. In an appropriate embodiment, the windows  134  and  135  are made of beryllium, a material that presents low gamma ray attenuation at the energies under consideration, coated with a protective layer of boron hydride, a material which is highly resistant to corrosion and wear and which also has low gamma ray attenuation. It should be observed that the windows  134  and  135  which are described above as being distinct elements could also be implemented in the form of diametrically opposite distinct portions of a single annularly-shaped part housed in a recess of complementary shape formed inside the Venturi section. Under such circumstances, the gasket would naturally have the appropriate annular shape. 
     The source block  120  as shown in FIG. 2 is adapted to using a radioisotope having a short lifetime, such as the above-mentioned gadolinium 153 whose half-life is about 7 months, since it enables declining activity of the source to be compensated by reducing the distance of the source from the throat  112  of the Venturi section. 
     In the embodiment shown, the source block comprises a body  140  of tubular shape having a central bore that is closed by a plug  141  at its end remote from the Venturi section  110 , and provided at its other end with a portion  14  of smaller diameter suitable for being screwed into the above-mentioned housing  130 , said portion  142  having an O-ring  143  on its front face. The central bore is closed at its end at the portion  142  by a path  144  of material of low gamma ray attenuation, and that bears against a ring  145 . The source proper  146 , preferably a gadolinium 153 source having activity of 100 millicuries, for example, is centered in the central bore, and is displaceable along the longitudinal axis  118  by means of an adjustment knob  147 . The knob  147  is mounted at the end of a wormscrew  148  that is eccentric relative to the axis  118 . The screw  148  is supported at one end by the ring  145  which is provided with a recess for this purpose, and it is fixed at its other end to a rod  149  passing through the plug  141 , and it forms the shaft of the adjustment knob  147 . The source  148  is fixed to a source carrier  150  whose shape on one side matches the inside wall of the tubular body  140  so as to be slidable inside the tubular body, and which is provided with teeth adapted to engage with the wormscrew  148 . It is thus possible to move the source  146  in translation inside the tubular body  140 , in particular to bring it closer to the Venturi section, by turning the adjustment knob  147 . 
     This maneuver can serve to compensate for the source&#39;s drop in activity over time, in an embodiment where the source is gadolinium 153. Another advantage of this arrangement, regardless of the type of source used, is that it makes it possible to adapt the photon flux emitted by the source  146  to the nature, and thus to the attenuation characteristics, of the multiphase fluid flowing through the Venturi section. 
     It should be observed that any displacement of the source  146  requires a new measurement of the “empty” count rate W hi,0  and W lo,0  since the photon flux reaching the detector is modified by displacing the source. 
     Like the source block  120 , the detector block  121  includes a generally tubular body  160  closed at one end by a plug  161  and including at its other end, for assembly to the Venturi section, a portion  162  of smaller outside diameter that is designed to be screwed into the above-mentioned housing  131 . A bore  164  passes through portion  162  and has two portions of different diameters so as to form a shoulder  165  against which there bears on one side a ring  167  of a material of low photon absorption at the energies emitted by source  146 , the ring  167  being in abutment on the other side against the shoulder of housing  131 . 
     A box  175  of cylindrical outside shape is located in the tubular body  160 , said box being axially slidable relative to the body  160 , a helical spring being mounted between box  175  and a recess formed in plug  161 . The box contains a detector unit  177 ,  178  comprising, as mentioned above, a scintillator crystal, e.g. of sodium iodide, together with a photomultiplier. The above-mentioned circuit for processing pulses (not shown) is integrated in this detector unit and is thus placed inside the box  175 . Conductors, generally referred to as numeral  179 , connect the detector unit to an external power supply and the detector unit  177 ,  178  to the data processing unit  30 . 
     Description with Reference to FIG. 4 
     The section view of FIG. 4 shows an advantageous way of using a device of the kind shown in FIG.  2 . The principle consists in connecting a section of pipe which is adapted to taking measurements as described above to the production installation situated in the vicinity of a well head but does not include any measurement means proper (sensors and associated processor means). Such measurement section is permanently mounted on the main pipe connected to the well head, and valves are provided in the main pipe between itself and the measurement section so that when a measurement is to be performed, the well effluent can be diverted through the measurement section. Measurement means are assembled to the measurement section, thereby making up the device as shown in FIG. 2, only when a measurement is to be performed. This form of implementation, that can be referred to as “semi-periodic”, makes it possible to perform periodic monitoring of production from a well under conditions that are advantageous for the user. Since the measurement section is already in place, the operations necessary to perform a measurement are restricted to assembling the sensors onto the measurement section and that can be done quickly and simply. In addition, the sensors which constitute the expensive and fragile portion of the device are not left exposed other than while performing monitoring operations. This is particularly advantageous for the gamma ray attenuation sensor which includes a radioisotope. Also, the sensors constitute equipment that is very lightweight (compared with the measurement section) and that does not require special transport means. Conversely, the measurement section is a piece of equipment that is robust and simple and that can be left permanently on a production site without great risk. 
     FIG. 4 is a diagram showing a measurement section of the type shown in FIG. 2, but while not performing measurement operations. Compared with FIGS. 2 and 3, corresponding elements are given the same reference numbers plus 100. 
     The measurement section is given overall reference  210 . It is mounted in parallel with a production pipe connected to a well head in such a manner that the effluent can flow through the measurement section in the direction represented by arrow F when it is desired to take measurements. The pipes are provided for this purpose with valves (not shown). In appropriate manner, the measurement section is disposed vertically and the flow through the measurement section is upward, as indicated by arrow F. 
     The measurement section includes recesses for receiving the components of a gamma ray attenuation measuring device, said recesses passing through the wall of the measurement section at two diametrically opposite positions. A first recess, for the source, is formed in a housing  230  opening out into the outside wall of the measurement section  210 , and of a smaller diameter housing  232  opening out in the inside wall, and similarly a second recess for the detector comprises a housing  231  opening out in the outside wall and a smaller diameter housing  233  opening out in the inside wall. The housings  230  to  233  are made in appropriate manner respectively like the housing  130  to  133  described in detail with reference to FIG.  3 . Similarly, the housings  232  and  233  receive windows  234  and  235  made in appropriate manner like the elements  134  and  135  shown in FIG. 3 so there is no need to describe them again. It is merely recalled that each of the elements  134  and  135  is designed to have low gamma ray attenuation, and has sealing means associated therewith, and in a variant the two elements may be formed by separate portions of a single annular part provided with appropriate sealing means. The elements  234  and  235  are also protected by respective plugs  270  and  271  screwed into the housings  230  and  231 . During a measurement operation, the plugs are unscrewed and the source block and the detector block elements described with reference to FIGS. 2 and 3 are screwed into the housings  230  and  231 . 
     For measuring pressure in the flow, the measurement section shown includes pressure takeoff orifices  216  and  217  analogous to the orifices  116  and  117  of FIG. 2. A threaded housing  226  connects each housing to the outside of the measurement section. A connector  274  is screwed into the housing and is itself connected, as shown diagrammatically in FIG. 4, to a hydraulic line  275  provided with an end coupling  276  and including a valve  277  that is closed except when performing measurements. To perform a measurement, it thus suffices to connect the coupling  276  to the pressure sensor and to open the valve  277 . 
     The principles explained above are also applicable to the case mentioned above in which the measurement section is used for gamma ray attenuation measurements only. Under such circumstances, there is no need to provide a Venturi in the measurement section, nor is there any need to provide pressure takeoff orifices for measuring the pressure drop due to the Venturi. Under such circumstances, the measurement section may be of constant diameter and need only comprise the “windows” presenting low gamma ray attenuation and the elements associated therewith, as described above.