Method and device for measuring surface dynamometer cards and operation diagnosis in sucker-rod pumped oil wells

1. A method to perform measurements, diagnose operation, evaluate the performance and generate alarms, in a pumping system of an oil well comprising a pumping polished rod linked to a reciprocating unit, wherein the method comprises the following steps:

a) determining an operating characteristic of the pumping system at the surface;

b) determining said operating characteristic of the pumping system downhole based on the operating characteristic of the pumping system at the surface; and

c) obtaining an operation diagnosis of the pumping system based on the characteristics of the pumping system at the surface and downhole,

wherein step a) of determining an operating characteristic of the pumping system at the surface comprises the steps of:

aa) determining at least one force cycle on the polished rod;

ab) determining the polished rod oscillation period based on the force cycle determined in aa); and

ac) determining the polished rod position as a function of time by the following steps:

i) obtaining a polished rod acceleration signal and simultaneously obtaining a Fourier transform of the polished rod acceleration signal, from the time domain to the frequency domain, with at least a first order coefficient of the Fourier transform series expansion of the transformed signal of the polished rod acceleration from said acceleration signal and the polished rod oscillation period determined in step ab);

ii) dividing Fourier coefficient(s) determined in i) by the square of its/their index, which is equivalent to integrating twice the acceleration signal in the time domain, and obtaining a new transformed signal proportional to a position transform; and

iii) anti-transforming the new transformed signal obtained in ii) and obtaining the position of the polished rod as a function of time,

wherein in step ac) i), said acceleration signal is produced by an accelerometer and the calculation of the coefficient(s) of the expansion is performed for each acceleration value obtained from said acceleration signal and the calculation of the zero-order coefficient is omitted in the Fourier transform series expansion of the transformed signal of the polished rod acceleration.

2. The method according to claim 1 , wherein the Fourier coefficient(s) of acceleration and position are obtained in step ac) i) based on the polished rod acceleration signal and simultaneous obtaining of the Fourier transform, from the time domain to the frequency domain, of the polished rod acceleration signal, with at least the first order coefficient of the Fourier series expansion of the transformed signal of the rod acceleration from said acceleration signal and the rod oscillation period determined in step ab).

3. The method of claim 2 , wherein in step ac) i) between 5 and 7 coefficients of Fourier series expansion are used.

4. The method of claim 3 , wherein in step ac) i) 7 coefficients of Fourier series expansion are used.

5. The method according to claim 1 , wherein the operating characteristic is a dynamometer card.

6. The method according to claim 1 , wherein the position of the polished rod as a function of time is rescaled with the stroke thereof.

7. A device for diagnosing the operation of a pumping system of an oil well comprising pumping polished rod linked to a reciprocating unit, wherein said device comprises, in a single housing or enclosure, at least one data acquisition unit, at least one power source and at least one processing unit, wherein said processing unit:

a) determines a dynamometer card of the pumping system at the surface;

b) determines a dynamometer card of the pumping system downhole based on the dynamometer card of the pumping system at the surface; and

c) obtains a diagnosis of operation of the pumping system based on the dynamometer cards of the pumping system at the surface and downhole;

wherein to determine a dynamometer card of the pumping system at the surface the processing unit:

aa) determines at least one force cycle on the polished rod;

ab) determines the oscillation period of the polished rod from the force cycle determined in aa); and

ac) determines the position of the polished rod as a function of time, by the following steps:

i) obtaining a polished rod acceleration signal and simultaneously obtaining a Fourier transform of the polished rod acceleration signal, from the time domain to the frequency domain, with at least a first order coefficient of the Fourier series expansion of the transformed signal of the rod acceleration from said acceleration signal and the oscillation period of the rod determined in step ab);

ii) dividing Fourier coefficient(s) determined in i) by the square of its/their index, which is equivalent to integrating twice the acceleration signal in the time domain, and obtaining a new transformed signal proportional to a transform of the position; and

iii) anti-transforming the new transformed signal obtained in ii) and obtaining the polished rod position as a function of time, wherein in step ac) i) said acceleration signal is produced by an accelerometer and the calculation of the coefficient(s) of the expansion is performed for each acceleration value obtained from said acceleration signal and the calculation of the zero-order coefficient is omitted in the Fourier transform series expansion of the transformed signal of the polished rod acceleration.

8. The device of claim 7 , wherein the power source is selected from the group comprising non-rechargeable batteries, rechargeable batteries by solar panels and ultracapacitors with solar panels, piezoelectric generators or a wired power from an external power source.

9. The device according to claim 8 , wherein the device uses 7 coefficients of the Fourier series expansion.

10. The device according to claim 7 , wherein the device uses between 5 and 7 coefficients of the Fourier series expansion.

11. The device according to claim 7 , wherein the data acquisition unit comprises a force transducer or load cell.

12. The device according to claim 7 , wherein the data acquisition unit comprises a position transducer.

13. The device according to claim 7 , wherein said device further comprises a unit for wireless transmission of data to a remote unit.