Method And Apparatus For Monitoring The Sand Concentration In A Flowing Well

Abstract

The sand concentration in a flowing well is measured quantitatively by an apparatus and method wherein an acoustic noise detector of wide band frequency response is placed in the flow stream. The output signal of the detector is selectively amplified at two frequencies, the lower of which is relatively unaffected by sand concentration in comparison to the higher, and the resultant signals are combined electronically on an analog computer whose output is virtually a unique function of sand concentration and nearly independent of fluid velocity.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the detection of acoustic noise in the flow stream of a producing well and more particularly to a method and apparatus for quantitatively monitoring the sand concentration in such flow stream.

2. Description of the Prior Art

It is well-known in the oil industry that the fluids recovered from certain incompetent hydrocarbon producing formations acquire concentrations of sand which increase with the production rate or velocity of fluid flow. Because daily allowables have been increased or removed on some wells responsive to accelerated demands in recent months, it is desired to set the flow rate for these wells as high as permissible without producing sand. If sand is produced, the flow must be reduced until the sand discontinues, in order to prevent damage to the well and to the formation itself.

Techniques and equipment have been devised in order to detect the critical flow velocity marking the onset of sand production. Exemplary of such techniques and apparatus is the invention disclosed in U.S. Pat. No. 3,563,311. The basic hypothesis of such a technique is that a fluid carrying sand particles will impart more energy to its surroundings than the same fluid flowing without sand, and further that with increasing velocity the average amplitude of the noise will also increase. In this prior art invention, a sound detector is positioned within a well at the level of a producing formation of interest so that it monitors the sound produced by the flowing fluids. An electrical signal generated by the detector is amplified and recorded at different flow rates. The onset of sand production marks a point at which there is a change in the slope of a log-log plot of sound level versus flow rate.

One of the disadvantages of techniques such as that described above lies in the fact that the accuracy of the critical flow rate determination is dependent upon the number of flow rates at which the formation concerned is produced. In addition, the assumption is made in such a method that by producing the formation at a flow rate below the critical value, sand will not be produced. However, changes can occur that will invalidate such assumption. For example, if water should come into a producing formation, the well tends to sand up rapidly without any apparent increase in the flow velocity. Under such circumstances, it becomes necessary to continually re-establish the critical flow rate by means of a new series of flow rate measurements.

Attempts have also been made to continuously monitor the condition of a well by simply providing a warning signal when the level of an electrical signal generated by an acoustic detector exceeds a predetermined threshold value. This type of indicator is unreliable because there is no way of telling with certainty whether the increase in signal value is the result of a change in flow velocity, gas/oil ratio, or, more importantly, the concentration of sand.

A further disadvantage of all such techniques is that they are not able to give a quantitative determination of sand concentration, which may be of considerable importance in assessing the probability of severe damage depending upon the extent to which tolerable sand concentrations are being exceeded.

SUMMARY OF THE INVENTION

It is, therefore, a general object of this invention to provide an improved method and apparatus for detecting the presence of sand in the flow stream of a producing well.

It is a further object of this invention to provide a method and apparatus for quantitatively determining the concentration of sand in the flow stream of a producing well.

It is yet another object of this invention to provide a method and apparatus for measuring the sand production in the flow stream of a producing well independent of flow velocity.

The acoustic noise generated by the fluid stream in a producing well is known to have a complex frequency spectrum. However, the position of the spectrum peaks does not appear to change appreciably with velocity or gas/oil ratio. The present invention is based, at least in part, on the discovery that the introduction of sand in the flow stream and subsequent changes in the volume fraction thereof have relatively little effect on the amplitude of the noise at lower frequencies, say on the order of 100 kilohertz, but at higher frequencies, say on the order of 700 kilohertz, result in a significant increase in noise amplitude.

In summary, in accordance with a preferred embodiment of this invention, a method for detecting and monitoring the sand concentration in the produced fluids in an oil well comprises generally the steps of acoustically sensing the noise generated by the flow stream in the well, producing an electrical signal of a known frequency spectrum responsive to said noise, selectively amplifying the signal at a first higher frequency at which such amplitude is significantly dependent upon both flow velocity and sand concentration and at a second lower frequency at which the amplitude depends only on flow velocity and is substantially independent of sand concentration, and thereafter electronically combining said first and second frequency signal components in an analog computer to produce an output indicative only of sand concentration and substantially independent of flow velocity.

Additionally, in another preferred embodiment this invention comprehends apparatus for carrying out such method wherein an acoustic sensor of wide band frequency response is supported within an oil filled probe positioned at the wellhead in the flow stream so as to provide an electrical output signal continuously representative of the acoustic noise generated by the fluid flow. The electrical output of the transducer is passed through a filter network which selectively filters two predetermined low and high frequency components of the total frequency spectrum as determined by the response characteristics of the particular transducer. These two signals are further amplified, rectified, and electronically combined in an analog computer. The computer is adapted to perform a simultaneous solution of emperically determined functional expressions for the low and high frequency components defined in terms of flow velocity and sand concentration as independent variables, the solution being such as to eliminate flow velocity between these functions. Means are provided for directly reading or continuously plotting the output signal of the computer as a quantitative measure of sand concentration whose accuracy is relatively unaffected by changes in flow velocity.

Other objects and advantages of the method and apparatus of this invention will become apparent from consideration of the detailed description to follow taken in conjunction with the drawings appended hereto.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view partly in sectiona nd partly diagrammatic of the apparatus of this invention.

FIG. 2 is a block diagram illustrating the electronic components of the signal processing circuits of this invention, and

FIG. 3 is a graph of a typical frequency spectrum for an acoustic sensing element as employed in this invention plotted against output signal amplitude for both sand-free and sand-producing flow stream conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with a preferred embodiment of this invention an acoustic probe 10 as seen in FIG. 1 extends within a flow line 12 at the wellhead of a producing well, preferably downstream of a choke (not shown). For example, this may be accomplished by inserting the probe 10, which may be of hollow metallic construction, within a vertically extending T-joint 14 connected in the line 12 so that the upper end of the probe 10 is engaged thereby, as by threading, and its lower end extends within the flow stream. A sensing element 17 may be positioned within the probe 10 surrounded with a bath of oil 18 to insure suitable coupling with the acoustic noise of the flow stream impinging on the external surface of the probe 10. Alternatively, the element 17 may be cemented directly against the probe 10, which in that event may be solid rather than oil filled. As a further alternative, the probe 10 may be formed of thin flexible rubber supported by a rigid frame to provide improved coupling to the sensing element 17 and may have the collateral advantage of combating the corrosive effects of sand on a metallic surfaced probe.

The sensing element 17 may consist of a piezo electric disc or cylindrical shell having a wide range electro-acoustical response characteristic, preferably extending from 100 to 1000 kilohertz. A pair of insulated electrical leads 20 are suitably fastened to the opposite flat surfaces of the sensing element 17 and extended therefrom to the upper end of probe 10. This enables transmission of the signal output of the sensing element 17, responsive to the acoustic noise of the flow stream, to a group of signal processing circuits 22. In a manner to be described, the signal is there operated upon to provide a further output representative of the volume fraction of sand in the flow stream, and such output may be still further applied to a continuous recorder 24 and/or an alarm device 26.

The signal processing circuits 22 may be better understood by reference to FIG. 2. Since the alternating current signal produced by the sensing element 17 will typically be in the microvolt range, it is necessary to pass it through a preamplifier 28 before further processing. The preamplifier 28 may be housed in a fashion to provide suitable electrical shielding against stray currents and other unwanted noise, and may conveniently be fixed to the probe 10 in a downhole location or at the wellhead. The output of the preamplifier 28 may be applied with appropriate impedance matching to a further wide band amplifier 30 which serves to step up the signal to a range of several volts. At this point, the acoustic noise responsive signal is filtered by means of a pair of networks 32 and 33 designed to pass two preselected narrow band frequency components, generally at the higher and lower ends of the frequency spectrum of the signal. The outputs of the filters 32 and 33, after suitable amplification in amplifiers 34 and 36, are rectified by means of a-c/d-c converters 38 and 39 to provide a pair of high and low frequency d-c signals V.sub.1 and V.sub.2.

A d-c power supply 41 is adapted to provide necessary operating voltages for preamplifiers 28 and amplifiers 34 and 36. The power supply 41 may conveniently be located remote from the preamplifier 28 and incorporated in the same electronic package as the other processing circuit 22. In order to isolate the power supply 41 from the signal output of the preamplifier 28, an inductor 42 and capacitor 43 are introduced as shown and selected to present high and low impedances respectively to such signal output.

The selection of the two frequencies for V.sub.1 and V.sub.2 is based upon the observed fact that the effect of sand concentration on the noise amplitude of the flow stream is much more pronounced at high frequencies than it is at low frequencies. At the same time, the effect of flow velocity and gas/oil ratio on noise amplitude is relatively constant over the entire frequency spectrum. It has been hypothesized, based on a consideration of the kinetic energy of sand particles and liquid flow and verified by experimentation, that these effects are sufficiently regular and consistent that they may be approximated by mathematical expressions. The signals V.sub.1 and V.sub.2, therefore, represent particular functions of flow velocity and sand concentration. These signals may thus be operated upon, by analog or digital techniques, to provide a simultaneous solution of such functional expressions for said sand concentration independently of flow velocity. Thus, it is the purpose of analog computer 40 to electronically combine signals V.sub.1 and V.sub.2 and compute therefrom an output signal uniquely dependent upon sand concentration. While the presence of any signal output from the computer 40 at all is indicative of the onset of sand production, the level of such signal may be used to differentiate between acceptable and unacceptable levels of sand production in the oil well flow stream.

The frequency dependent effect of sand concentration on acoustic noise in the flow stream of a well can be better appreciated by an inspection of the chart of FIG. 3. Here there are plotted values of signal output such as are obtainable from the sensing element 17 with frequencies varying from 100 to 1000 kilohertz. It will be understood that these curves are illustrative of the general character of this frequency spectrum. In a particular installation, the location of specific spectral peaks and the amplitude of the signal output of the sensing element 17 are affected not only by its geometry, resonant frequencies, and method of mounting, but also by the construction and material of the probe 10. Thus, the chart comprises a family of curves, namely 30, 31, 32, and 33, which are representative of generalized frequency spectra in which the arbitrarily varied parameter is sand concentration. For example, in curve 30 no sand has been introduced in the flow stream. Curves 31, 32, and 33 represent the addition of increasing volume percentages of sand in the total flow.

It should be carefully noted that at the low frequency end of the spectra, for example in the neighborhood of 100 kilohertz, there is relatively little change in amplitude with increase in sand concentration as compared, for example, to the increase observed at a higher frequency spectral peak, such as at 780 kilohertz. This difference has been found sufficiently pronounced so that a preselected higher frequency spectral component may vary more than five times as rapidly with increased sand concentration as the preselected lower frequency component within the same spectrum. There is no intent, however, to assert a lower limit or establish a specific minimum frequency within the signal spectrum at which the effect of sand increases becomes pronounced. As shown in the illustration of FIG. 3, other intermediate spectral peaks, for example at approximately 350 kilohertz and 480 kilohertz, may be equally indicative of the desired effect. Yet, it can be observed that lower frequency components clearly exist at which the effect diminishes substantially. The important point to grasp is that there is a hitherto unsuspected and yet consistent frequency interdependence of sand concentration and acoustic noise level which runs counter to the uniform effects of flow velocity on such noise. This invention, therefore, takes specific advantage of this particular frequency dependence.

For the values of amplitudes of frequency components of the signal output of the sensing element 17 for which substantial dependence upon sand concentration exists, the following general function may be stated:

V.sub.1 = K.sub.1 v.sup.a + k.sub.2 v.sup.b S.sup.c

where V.sub.1 is the amplitude of a preselected higher frequency component; v is the flow velocity; S is sand concentration expressed as a volume fraction of the total fluid volume; and k.sub.1, k.sub.2, a, b, and c are experimentally determinable coefficients and exponential values respectively.

Conversely, for lower frequency components which are substantially or relatively independent of sand concentration, the corresponding general function may be expressed as:

V.sub.2 = k.sub.3 v.sup.d

where k.sub.3 and d represent respectively further experimentally determinable coefficient and exponential value.

If now the signals V.sub.1 and V.sub.2 are applied to the analog computer 40, a simultaneous solution may be provided for the functional expressions for V.sub.1 and V.sub.2 for sand concentration independent of flow velocity, to wit:

S = [V.sub.1 - k.sub.1 (V.sub.2 /k.sub.3).sup.a/d /k.sub.2 (V.sub.2 /k.sub.3).sup.b/d ].sup.1/c

The output S can then be conveniently applied to continuous recording means 24 which can be queried remotely or which can if desired provide a suitable warning signal from alarm 26 if the value of sand concentration S should exceed a certain predetermined threshold value.

Prior to the operation of this method and apparatus in conjunction with a particular well, an empirical determination may be made of the coefficients and exponential values in the above recited general formulas for V.sub.1 and V.sub.2. In order to do this, the frequency response characteristics of a particular sensing element 17, if not already known, is first established by techniques well-known in the art. Examination of this spectrum will determine the location of appropriate spectral peaks such as have been previously discussed in connection with FIG. 2. Ideally, the sensing element 17 is thereafter installed in the probe 10 positioned in the manner described for this invention. Filters 32 and 33 are then adjusted to select a high and low frequency component of the output signal of sensing element 17 as described, preferably at widely separated spectral peaks, to increase the signal strength. The well is produced over a range of several flow velocities and with small varying concentrations of sand. Component output values of V.sub.1 and V.sub.2 are read and graphical or numerical methods used to fit the data to the general functional expressions for V.sub.1 and V.sub.2 so as to obtain discrete values for the coefficients and exponentials in those general expressions. It is believed, however, that the values of these coefficients and exponents are predictable for a given type of installation within an acceptable margin of error. Thus, the described calibration steps may not be essential in all cases.

It will be understood that the analog computer 40 may be designed, by employing techniques well-known to the art, to perform the necessary simultaneous solution of the particular functional expressions now available for the quantities V.sub.1 and V.sub.2 so that the output represents the quantity S independent of variations in flow velocity. The computer 40 may conveniently incorporate a plurality of amplification stages whose gains are adjustable in accordance with the particular values assigned to the coefficients and exponentials in the general formulas. In this fashion, therefore, the computer 40 may be easily accommodated to the requirements of a particular installation.

In the manner described above, an apparatus may be constructed using a single acoustic probe 10 that will measure the sand concentration in the flowing stream of a well to an accuracy of .+-.25 percent. This is adequate as a monitoring device and will allow proper measures to be taken for protection of the well and its surface equipment.

If desired, a more accurate measurement of sand concentration may be made by means of a separate flow velocity sensor such as a turbine flow meter or through the pressure drop across an orifice plate (not shown). In such case the circuitry 22 incorporates only a single filter channel for the high frequency component signal V.sub.1 and the lower frequency component V.sub.2 is eliminated. In place of the lower frequency component an alternate signal V.sub.3 is then available such as from a flow meter proportional to the first power of flow velocity. This again enables analog computer 40 to compute from the signal V.sub.1 and the flow meter output signal V.sub.3 a solution for the quantity S independent of flow velocity. Clearly, appropriate changes would be made in the design of the computer 40 to perform the intermediate steps in the computation

If desired, the signals V.sub.1 and V.sub.2 or the alternative flow meter output signal V.sub.3 may be digitized and with suitable programming the computation for S performed digitally using a mini-computer or a micro-processor.

It will be understood that what has been described and illustrated herein is representative of a particular embodiment of this invention. Many variations therein will occur to those skilled in the art without departing from the spirit and scope of the invention, the features of which are more particularly set forth in the claims appended hereto.

Claims

What is claimed is:

1. The method of determining the sand concentration in the produced fluids of a well comprising the steps of:

a. deriving from the acoustic noise generated by the flow stream of said fluids a first broad band electrical signal;

b. filtering said first electrical signal to obtain therefrom at least one preselected narrow band frequency component whose amplitude approximates a first known function of the flow velocity of said fluids and the sand concentration therein;

c. deriving a second electrical signal responsive to the movement of said flow stream whose amplitude approximates a second known function of said flow velocity and said sand concentration; and

d. computing from said at least one frequency component and said second signal an output signal responsive thereto whose amplitude represents a simultaneous solution of said first and second functions for said sand concentration independent of said flow velocity.

2. A method as in claim 1 wherein the step of obtaining said second signal comprises further filtering said first electrical signal to obtain therefrom a second preselected narrow band frequency component lower in frequency than said first preselected frequency component, the amplitude of said second frequency component being less sensitive to sand concentration than the amplitude of said first frequency component.

3. A method as in claim 2 wherein the center frequencies of said first and second frequency components are selected to coincide with respective spectral peaks of said first electrical signal.

4. A method as in claim 3 wherein the sensitivity to sand concentration of said second frequency component is at least five times that of said first frequency component.

5. A method as in claim 1 wherein said step of obtaining said second signal comprises directly measuring said flow velocity by means of a flow meter.

6. A method as in claim 1 wherein said step of deriving said first electrical signal comprises positioning sound detection means in the flow stream of said produced fluids at a predetermined location.

7. A method as in claim 6 wherein said sound detection means is an electro-acoustical transducer.

8. A method as in claim 1 wherein said at least one frequency component and said second signal are combined in an analog computer.

9. A method as in claim 1 wherein said at least one frequency component and said second signal are combined in a digital computer.

10. A method as in claim 1 comprising the additional step of continuously recording the output signal of said computer means.

11. A method as in claim 10 comprising the further step of generating a warning signal when the value of the output of said computer means exceeds a preselected threshold value.

12. Apparatus for determining the sand concentration in the produced fluids of a well comprising:

a. sound detection means adapted to provide a first wide band electrical signal responsive to the acoustic noise generated by the flow stream of said fluids;

b. means for filtering said first electrical signal to obtain therefrom at least one preselected narrow band frequency component expressible as a first known function of the flow velocity of said fluids and the sand concentration therein;

c. means for deriving from the flow stream of said fluids a second electrical signal expressible as a second known function of said flow velocity and said sand concentration; and

d. means for computing from said at least one frequency component and said second electrical signal a simultaneous solution of said first and second known functions for said sand concentration independent of said flow velocity.

13. Apparatus as in claim 12 wherein said means for deriving a second electrical signal is a flow meter adapted to provide a direct measurement of the flow velocity of said flow stream.

14. Apparatus as in claim 12 wherein said means for deriving said second electrical signal comprises means for filtering said first electrical signal to obtain a second preselected narrow band frequency component thereof lower in frequency than said first frequency component and having an amplitude less sensitive to said sand concentration.

15. Apparatus as in claim 14 wherein the amplitude of said first frequency component is more sensitive to sand concentration than the amplitude of said second frequency component by a factor of at least five.

16. Apparatus as in claim 14 wherein said first known function comprises the general formula V.sub.1 = k.sub.1 v.sup.a + k.sub.2 S.sup.b v.sup.c and said second known function comprises the general formula V.sub.2 = k.sub.3 v.sup.d wherein V.sub.1 and V.sub.2 represent the respective amplitudes of said first and second frequency components, v represents flow velocity, S represents the volume fraction of sand in the fluids, and k.sub.1, k.sub.2, k.sub.3, a, b, c, and d represent experimentally determinable coefficients and exponential values respectively.

17. The apparatus of claim 16 wherein said computing means is an analog computer provided with a plurality of amplification stages whose gains are individually adjustable to accommodate specific values of k.sub.1 k.sub.2, k.sub.3, a, b, c, and d.

18. Apparatus as in claim 16 wherein said computing means is a digital computer.

19. A method for detecting and monitoring the sand concentration in the produced fluids in an oil well comprising:

a. acoustically sensing the noise generated by the flow stream of the well;

b. transducing said acoustic noise into a broad band electrical signal;

c. selectively filtering said broad band signal at a first higher frequency to provide a first component signal whose amplitude is dependent upon said sand concentration and on the flow velocity of said fluids and at a second lower frequency to provide a second component signal whose amplitude is dependent on flow velocity and is substantially independent of said sand concentration;

d. rectifying said first and second component signals; and

e. thereafter electronically combining said first and second component signals in an analog computer adapted to provide therefrom an output indicative only of sand concentration and substantially independent of flow velocity.