Controller for a rod pumping unit and method of operation

1. A controller for operating a rod pumping unit, said controller comprising a processor configured to:

operate the rod pumping unit at a pumping speed profile;

compute a first downhole dynamometer card from surface measurements at said rod pumping unit;

compute a second downhole dynamometer card from the surface measurements;

determine if a difference between a current pumping speed profile for a current pump stroke and a previous pumping speed profile for a previous pump stroke exceeds a predetermined threshold;

designate the first downhole dynamometer card as accurate if the difference does not exceed the predetermined threshold;

designate the first downhole dynamometer card as inaccurate if the difference does exceed the predetermined threshold; and

replace the first downhole dynamometer card with the second downhole dynamometer card when the first downhole dynamometer card is designated as inaccurate.

2. The controller in accordance with claim 1 , wherein said processor is further configured to receive surface position and load measurements for the rod pumping unit for a current stroke.

3. The controller in accordance with claim 1 , wherein said processor is further configured to compute the first downhole dynamometer card in real-time using a Fourier Series solution of a wave equation representing a sucker rod string for the rod pumping unit.

4. The controller in accordance with claim 1 , wherein said processor is further configured to designate the second downhole dynamometer card as accurate if the first downhole dynamometer card is designated inaccurate.

5. The controller in accordance with claim 1 , wherein said processor is further configured to:

compute a difference between a previous downhole dynamometer card for a previous pump stroke and at least one of the first downhole dynamometer card and the second downhole dynamometer card for a current stroke;

designate the first downhole dynamometer card as accurate if the difference does not exceed a predetermined threshold; and

designate the first downhole dynamometer card as inaccurate if the difference exceeds the predetermined threshold.

6. The controller in accordance with claim 1 , wherein said processor is further configured to compute the second downhole dynamometer card using a Finite Difference solution to a wave equation representing a sucker rod string of the rod pumping unit for a current stroke.

7. A method of operating controller for a rod pumping unit, said method comprising:

operating the rod pumping unit at a pumping speed profile;

receiving surface measurements of rod pumping unit position and load for a pump stroke;

computing a first downhole dynamometer card from the surface measurements using a first technique;

computing a second downhole dynamometer card from the surface measurements using a second technique;

determining the first downhole dynamometer card is inaccurate based on a rod pumping unit condition, wherein the first downhole dynamometer card is inaccurate if a difference between a current pumping speed profile for a current pump stroke and a previous pumping speed profile for a previous pump stroke exceeds a predetermined threshold;

designating the second downhole dynamometer card as accurate; and

replacing the first downhole dynamometer card with the second downhole dynamometer card.

8. The method in accordance with claim 7 , wherein computing the first downhole dynamometer card includes computing a real-time downhole dynamometer card using partial differential equations to solve a wave equation representing a sucker rod string for the rod pumping unit.

9. The method in accordance with claim 8 , wherein determining the first downhole dynamometer card is inaccurate comprises:

comparing a current pumping speed profile for the pump stroke to a previous pumping speed profile for a previous stroke; and

designating the first downhole dynamometer card as inaccurate if a difference between the current pumping speed profile and the previous pumping speed profile exceeds a predetermined threshold.

10. The method in accordance with claim 8 , wherein determining the first downhole dynamometer card is inaccurate comprises:

computing a difference between a previous downhole dynamometer card for a previous stroke and at least one of the first downhole dynamometer card and the second downhole dynamometer card; and

designating the first downhole dynamometer card as inaccurate if the difference exceeds a predetermined threshold.

11. The method of claim 8 further comprising:

determining the first downhole dynamometer card is accurate based on the rod pumping unit condition being satisfied; and

designating the first downhole dynamometer card as accurate.

12. The method of claim 7 further comprising:

computing a third downhole dynamometer card from the surface measurements using a third technique; and

comparing the first downhole dynamometer card, the second downhole dynamometer card, and the third downhole dynamometer card to validate accuracy of at least two of the first downhole dynamometer card, the second downhole dynamometer card, and the third downhole dynamometer card.

13. The method in accordance with claim 7 , wherein computing the second downhole dynamometer card includes computing a finite difference solution to a wave equation representing a sucker rod string for the rod pumping unit.

14. A control system for a rod pumping unit having a polished rod coupled to a pump by a sucker rod string, comprising:

a position sensor configured to measure a surface position of the polished rod and generate a position signal indicative thereof;

a load sensor configured to measure a surface load on the polished rod and generate a load signal indicative thereof; and

a controller coupled to said position sensor and said load sensor, said controller configured to:

receive the position signal indicative of a current surface position for a current stroke;

receive the load signal indicative of a current load for the current stroke;

compute a first downhole dynamometer card from the current surface position and the current load;

compute a second downhole dynamometer card from the current surface position and the current load;

determine if a difference between a current pumping speed profile for a current pump stroke and a previous pumping speed profile for a previous pump stroke exceeds a predetermined threshold:

designate the first downhole dynamometer card as accurate if the difference does not exceed the predetermined threshold:

designate the first downhole dynamometer card as inaccurate if the difference does exceed the predetermined threshold: and

replace the first downhole dynamometer card with the second downhole dynamometer card when the first downhole dynamometer card is designated as inaccurate.

15. The control system in accordance with claim 14 , wherein said controller is further configured to compute the first downhole dynamometer card using a truncated Fourier Series approximation of a steady-state analytical wave equation solution representing the sucker rod string.

16. The control system in accordance with claim 14 , wherein said controller is further configured to compute the second downhole dynamometer card using a finite difference approximation of a wave equation representing the sucker rod string.

17. The control system in accordance with claim 14 , wherein the rod pumping unit condition comprises a current pumping speed profile that varies from a previous pumping speed profile by no more than a predetermined threshold.

18. The control system in accordance with claim 14 , wherein the rod pumping unit condition comprises a previous downhole dynamometer card varying from at least one of the first downhole dynamometer card and the second downhole dynamometer card by no more than a predetermined threshold.

19. The control system in accordance with claim 18 , wherein the predetermined threshold is defined as plus-or-minus 0.25 meters.

20. The control system in accordance with claim 18 , wherein the predetermined threshold is computed based on at least one of a pumping depth, a downhole well condition, and a structure of the sucker rod string.