Abstract:
Current supplied to electrical submersible pumps in wells is monitored, and signal processing based on wavelet analysis and phase diagram analysis is performed on the data obtained from monitoring. An incipient malfunction of the electrical submersible pump, such as one due to scale build-up in and around the pump, can be detected at an early stage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application No. 61,570,030, filed Dec. 13, 2011. For purposes of United States patent practice, this application incorporates the contents of the Provisional Application by reference in entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to monitoring performance and to failure prediction of electrical submersible pumps in wells. 
         [0004]    2. Description of the Related Art 
         [0005]    Submersible pumps have been used in wells for oil production at various depths and flow rates. The pumps are typically electrically powered and referred to as Electrical Submersible Pumps (ESP&#39;s). ESP&#39;s were one of several forms of what is known as artificial lift. ESP&#39;s were located in tubing in the well and provided a relatively efficient form of production. 
         [0006]    An ESP system used in oil production included surface components at the production wellhead or platform and subsurface components located in production tubing or casing at the level of producing formations in the well. Surface components included a motor controller and surface cables and transformers for power transfer to the subsurface components downhole. Subsurface components in the well included a pump, pump motor, fluid seals and power supply cables. 
         [0007]    The downhole ESP pumps were immersed in the well fluids being pumped for production at the operating depths in the well and drove formation fluids to the surface with power supplied from the electrically powered pump motor which received operating power from the surface over the power supply cables. 
         [0008]    During production from the formation, mineral deposits from the formation fluid occurred in and around the ESP&#39;s, well tubing and other subsurface equipment, and have caused recurrent problems. The mineral deposits were known as scale. One of the common failure reasons in ESP assemblies resulted from scale build-up in the pump stages, where scale gradually formed around the impeller vanes and eventually blocked fluid flow. Scale deposits led to a gradual decrease of the pump efficiency until pump failure eventually occurred. 
         [0009]    Problems with scale and other subsurface conditions as well as extended service eventually led to failure of the downhole ESP components, usually the pump. The causes and reasons of ESP component failure were usually analyzed after the system had been pulled out or extracted from the well. The analysis commonly used after the ESP had been removed from the well was a detailed DIFA (Dismantle Inspection &amp; Failure Analysis) process where each component of the ESP assembly was carefully analyzed for an understanding of the nature of the failure. Experience has shown that generally more than 20% of failure causes were attributed to motor failure. 
         [0010]    As noted, however, this form of failure analysis could only be performed after the failure occurred, and after the downhole or subsurface ESP components had been extracted from the well. Both the ESP failure and its removal from the well caused production from the well to be stopped. Production from the well was only resumed when a replacement ESP subsurface system could be installed in the well. Production from the well was thus interrupted for the time required for scheduling a workover rig and its transport to the well, in addition to the time for installation of a replacement ESP subsurface system. 
       SUMMARY OF THE INVENTION 
       [0011]    Briefly, the present invention provides a new and improved apparatus for monitoring performance of an electrical submersible pump in a well based on analysis of pump electrical current. The apparatus according to the present invention includes an analyzer of the frequency spectrum of the energy in the pump electrical current, and a wavelet analyzer of the waveform of the pump electrical current identifying time variations of the pump electrical current. The apparatus also includes an analyzer of the pump electrical current to identify dynamic behavior of the pump during pumping, and a phase space analyzer forming a measure of the identified dynamic behavior of the pump based on fluctuations in the pump electrical current. A graphical interface of the apparatus forms indications from the analyzers for monitoring performance of the pump to detect disturbances in performance of the pump. 
         [0012]    The present invention also provides a new and improved method of monitoring performance of an electrical submersible pump in a well based on analysis of pump electrical current. The frequency spectrum of the energy in the pump electrical current is analyzed, and the waveform of the pump electrical current is analyzed to identify time variations of the pump electrical current. The pump electrical current is analyzed to identify dynamic behavior of the pump during pumping, and a measure of the identified dynamic behavior of the pump is formed based on fluctuations in the pump electrical current. Indications of the results of the analysis are formed for monitoring performance of the pump to detect disturbances in performance of the pump. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic diagram of an electrical submersible pump in a well. 
           [0014]      FIG. 2  is an plot of an example log over time of motor current to an electrical submersible pump. 
           [0015]      FIG. 3  is a schematic diagram of diagnostic signal processing components according to the present invention for an electrical submersible pump. 
           [0016]      FIG. 4A  is a plot of example Fourier Transform plots from motor current logs for an electrical submersible pump. 
           [0017]      FIG. 4B  is a plot of example wavelet current plots from motor current logs for an electrical submersible pump. 
           [0018]      FIG. 5  is an example phase space diagram obtained from processing according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Referring to  FIG. 1 , an electrical submersible pump assembly P is shown in a well  10  at the location of a number of perforations  12  formed in a casing  14  to allow entry through an inlet or intake section  15  of oil and other hydrocarbon fluids from a formation  16  in a subsurface reservoir. The casing  14  may also be a liner installed within larger diameter casing in the well  10 . A pump section  20  of the electrical submersible pump assembly P is immersed in the fluids in the casing  14 . The electrical submersible pump assembly P is suspended within the casing  14  on tubing  22  at the well depth of the perforations  12  so that the pump section  20  may drive or pump fluids in the casing  14  as indicated at  24  to a collection facility at the surface. 
         [0020]    The pump section  20  includes a suitable number of centrifugal pump stages which are driven by an alternating current pump motor  26 . The pump motor  26  receives operating electrical power over a cable  28  from a suitable power source  30  at the surface. The pump motor  26  drives a shaft that extends through suitable sealing for driving the centrifugal pump stages of pump section  20  of the electrical submersible pump assembly P. The pump section  20  is conventional and comprises a large number of stages of impellers and diffusers. 
         [0021]    The electrical submersible pump assembly P of  FIG. 1  is equipped with a monitoring or logging system  32  to continuously record different operating parameters regarding the electrical submersible pump assembly P to ensure the good functionality of both pumping system and associated sensors. As part of the control and monitoring protocols of the electrical submersible pump assembly P, the current waveform of the operating power provided to the pump motor  26  over the cable  28  is continuously recorded along with other operating parameters, such as the fluid rates, the pump speed, intake and discharge pressures. 
         [0022]    An example pump electrical current log of current amplitude as a function of time is shown at waveform  40  in  FIG. 2 . The current log waveform  40  illustrates an example of an ESP motor current over a period of several weeks before a pump assembly failure as indicated at  42 . It can be noticed from  FIG. 2  that for a period of time of several days before the failure, the pump current waveform  40  shows a substantial variation in pump performance trends. 
         [0023]    According to the present invention, a diagnostic processor  44  ( FIG. 3 ) performs advanced signal analysis of pump motor operations to monitor for the likely occurrence of a pump motor failure due to scale build-up. The signal analysis may be either as a computer-implemented method on a general purpose computer, or may be specifically configured digital signal processing circuitry or chips, or a combination of the two. The diagnostic processor  44  processes the pump electrical current signal logs received from the logging system  32  after conditioning and conversion to a format for digital processing by a conditioning circuit  45  ( FIG. 3 ). The diagnostic processor  44  analyzes the pump electrical current logs to identify the dynamical behavior and performance of the pump motor  26  ( FIG. 1 ). 
         [0024]    In accordance with the present invention, the diagnostic processor  44  includes modules to analyze several aspects of variations recorded in the motor current signals by logging system  32  to dynamically follow the pump operation and indicate events which indicate a likely occurrence of failure of the pump motor in advance of the actual failure. Time series of the motor current signals collected by the logging system  32  are analyzed by Fourier transform analyzer module  46 , a wavelet transform module  48  and phase attractor module  50 , as shown in  FIG. 3 . 
         [0025]    The Fourier transform analyzer module  46  is a processor which operates either as a programmed digital signal processor or special purpose processing circuitry to analyze the pump current signal waveform by Fourier analysis. The Fourier transform analyzer module  46  provide measures of the power or energy present in the pump electrical current as a function of frequency over its frequency spectrum. The Fourier transform analyzer module  46  provides an indication of signal properties in a defined time window for the pump electrical current waveform. Fourier analysis by the analyzer module  46  yields an energy density in individual frequency ranges of the power spectrum. The Fourier transform analyzer module  46  in the preferred embodiment preferably performs a signal processing technique known as a Fast Fourier Transform. The power spectrum obtained by a Fast Fourier Transform analysis allows determination of the range of frequencies present in the pump electrical current in the pump assembly P at times prior to a pump failure and identifies characteristic frequencies of pump electrical current when present. 
         [0026]      FIG. 4A  presents an example of the Fourier transform plots obtained from the module  46 . In actual practice, plots like that of  FIG. 4A  are in color to indicate in more detail, the data of interest. The Fourier transform plot of  FIG. 4A  shows the characteristic frequencies of the motor current logs and exhibits amplitude peaks dominating the frequency spectrum indicating a singular structure in the signal corresponding to the scale build-up as an additional load on the ESP motor shaft. 
         [0027]    The wavelet transform analyzer module  48  ( FIG. 3 ) performs a wavelet analysis of the pump electrical current logs provided by the logging system  32 . The wavelet transform analyzer module  48  may also be a processor which operates either as a programmed digital signal processor or special purpose processing circuitry to analyze the pump current signal waveform by wavelet transform analysis. 
         [0028]    The wavelet transform analyzer module  48  is a module determines signal characteristic variations of the pump electrical current waveform in the time domain, while the Fourier analyzer module  46 , as set forth above, analyzes signal characteristic variations of the pump electrical current waveform in the frequency domain Wavelet analysis by the wavelet transform analyzer module  48  permits the tracking of the spatio-temporal evolution of the signal in various time scales. 
         [0029]    The wavelet transform of a continuous signal s(t) representing the pump electrical current as a function time t is given by: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    where ψ is a mother wavelet, which is an absolutely integrable function. 
         [0030]    Wavelet analysis in module  48  is performed by the dilatation and translation of the mother wavelet ψ. The parameter a in Equation (1) is related to the dilatation and is inversely proportional to frequency. Varying the parameter a for wavelet analysis in analyzer  48  changes the center frequency of the mother wavelet ψ and also the wavelet time parameter. The parameter a is thus used rather than frequency to represent the results of wavelet analysis in wavelet analyzer  48 , as will be seen. 
         [0031]    The parameter τ is the translation or time-shift parameter. The parameter τ specifies the location of the wavelet in time, and adjustment of the parameter τ causes the wavelet to shift over the pump electrical current signal being analyzed. For instance, a wavelet known as the ‘Mexican hat function’ given by: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    may, for example, be chosen as the mother wavelet when the signal s(t) has high fluctuations. In the wavelet analyzer  48 , a compression of the parameter a being varied to a lower value allows analysis of high frequency components of the electrical pump current waveform, while stretching of the parameter a to an increased value is related to low frequency components. 
         [0032]      FIG. 4B  is an example plot of the wavelet transform output formed by the wavelet analyzer module  48  as function of time-scale diagrams of iso-correlation contours a/Δt of the parameter a for the parameter a for different time shifts plotted in different frequency levels. 
         [0033]    In the example of wavelet transform plot shown in  FIG. 4B  the iso-correlation contour plots are normalized against the highest correlation value to highlight the peak of the correlation values. The centers of contour zones corresponding to peak values clearly show periodic structures appearing at different time scales. This periodicity in the peak alignments against the time scale exhibits the presence of a regular structure in the signal corresponding to an anomaly in the signal pattern due to the scale build-up in the system resulting from additional load or torque on the ESP shaft. It can be seen also that these peaks have different a/Δt values indicating a progression in the dynamical behavior of the system, meaning progression of the scale load on the rotating shaft. 
         [0034]    The diagnostic processor  44  also includes a dynamic behavior analyzer module  50  in which a time series signal corresponding to a certain flow regime of fluid through the pump assembly P is dynamically embedded in order to determine the signal fractal dimensions that are used to build the dynamical attractor described in  FIG. 5 . Based on data from the motor current log obtained by the monitor  32 , pump performance changes can be identified. The pump electrical current waveform is marked in behavior analyzer module  50  by the superimposition in a module  51  of several characteristic frequencies added in time. The presence of the added time series signals allows, after return to steady state, the signal identification of a resultant associated attractor indicating pump performance For example, the signal processing method of method of mutual information can be used to estimate the time delay of each time series recorded to construct an attractor associated with each introduced time delay. The appropriate time delay is the one which corresponds to the first minimum value of the mutual information function calculated from the time series. 
         [0035]    The diagnostic processor  44  also includes a phase diagram reconstruction module  52 , in which the dynamical behavior of the pump as indicated by measurements furnished by monitor  32  is determined from the embedded signals by reconstruction of phase diagrams. The signal embedding module  51  and the phase diagram reconstruction module  52  may each also be a processor which operates either as a programmed digital signal processor or special purpose processing circuitry to identify dynamic behavior of the pump assembly P. 
         [0036]    An example display of a reconstructed phase diagram formed by the module  52  is shown in  FIG. 5 . Analysis of such data as that of  FIG. 5  collected from the motor current logs has shown that the fluctuations recorded had very low frequencies, which are repesentative of the gradual build-up of the scale in the different pump stages. The phase space reconstruction plot shown in  FIG. 5  constructed from the signal recorded indicates a clearly typical chaotic tendency of the current logs shortly before pump failure. The plot in  FIG. 5  exhibits clearly a typical attractor basin that reveals from dynamical definitions the existence of a regular structure in the signal linked to the additional load on the ESP motor shaft resulting from the gradual scale build-up. The combined plots from  FIGS. 4A ,  4 B, and  5  clearly highlight the changes in the ESP motor current trends and dynamically indicate the existence of an additional load on the motor shaft as the scale builds up in the different ESP stages. 
         [0037]    The diagnostic processor  44  includes graphical interface  54  which receives processed data from each of the Fourier analyzer module  46 , the wavelet analyzer module  48  and the phase diagram reconstruction module  52  to form displays of the processed pump electrical current data. The graphical interface  54  forms displays of frequency spectra obtained form the Fourier analyzer module  46  as shown in  FIG. 4A . The graphical interface  54  also forms displays such as those shown in  FIG. 4B  based on processing results from the wavelet analyzer  48  and phase space reconstruction diagrams shown in  FIG. 5  based on processing results from the phase space diagram reconstruction module  52 . The graphical interface is a user friendly environment that allows the user to display the plots desired from the above-mentioned methods and follow the evolution of the raw signal as well. 
         [0038]    The graphical interface  54  provides as separate outputs the results formed in the diagnostic processor  44  as separate displays or windows. The analysis from the three displays provides indications to show any disturbance present in the pump motor current logs and also provides advance indications of pump performance or behavior likely to result in pump failure. The diagnostic processor  44  allows monitoring the growth of the scale on the electrical submersible pump assembly P by identifying the magnitude of the disturbances in the motor current logs. 
         [0039]      100371  Diagnostic processing according to the present invention continuously monitors the performance of the electrical submersible pump assembly P and predicts potential failure due to scale build-up. From motor current recorded prior to the failure, weak fluctuations in such current can be recorded, indicating a change in the motor load due to the scale build-up on the pump motor shaft. This scale build-up affects localized shaft torque and therefore the total motor power draw. These changes can be identified through the motor current draw. The advanced signal analysis of the motor current data provided with the present invention can reveal the presence of a dynamical character changes of the pump current signal when scale starts rapidly building up in the pump stages. 
         [0040]    Accordingly, the present invention provides a real time diagnostic system that predicts the likelihood of failure of an ESP system several days or weeks before the actual event of failure. This leads to better control of the well production protocol. For instance, if failure due to scale build-up is predicted through the diagnostic tools provided with the present invention, several actions can be planned to prevent or delay the pump failure. Such actions include, for example, reducing the motor speed to increase the production periods even at lower volumes. Thus, production may continue while a work-over rig is being scheduled for replacement of the electrical submersible pump assembly, or while planning for an acidizing job to remove the scale. Such actions could result in avoidance of costly work-overs and minimizing lost production due to downtime. 
         [0041]    The present invention thus identifies in real time disturbances in the performance of the electrical submersible pump assembly. Detection can occur at the very early stages of a pump motor malfunction resulting from change in the power draw due to scale build-up in the pump stages. Pro-active control of the pump run time can thus take place and remedial action planned to prevent total pump failure. The present invention also minimizes down time for pump repair in particular. 
         [0042]    The invention has been sufficiently described so that a person with average knowledge in the matter may reproduce and obtain the results mentioned in the invention herein Nonetheless, any skilled person in the field of technique, subject of the invention herein, may carry out modifications not described in the request herein, to apply these modifications to a determined structure, or in the processing methodology, requires the claimed matter in the following claims; such structures shall be covered within the scope of the invention. 
         [0043]    It should be noted and understood that there can be improvements and modifications made of the present invention described in detail above without departing from the spirit or scope of the invention as set forth in the accompanying claims.