Controller and method of controlling a rod pumping unit

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

determine a pump fillage level for the pump stroke based on a position signal and a load signal;

establish an average pump speed based on the pump fillage level;

operate a pump piston of the rod pumping unit on a downstroke of a pump stroke;

decelerate the pump piston on the downstroke as the pump piston approaches a fluid surface in a barrel of a pump of the rod pumping unit;

detect if an overstress caused by the deceleration of the pump piston is present, wherein if the overstress caused by the deceleration of the pump piston is detected during the downstroke, the deceleration of the pump piston is reduced to mitigate the overstress; and

increase the pump speed on the downstroke after the pump piston contacts the fluid surface in said barrel of said pump to achieve the established average pump speed.

2. The controller in accordance with claim 1 , wherein said processor is further configured to compute a real-time pump card based on the position signal and the load signal, the pump card including a downhole position of the pump piston represented by the position signal, and a downhole load of the pump piston represented by the load signal.

3. The controller in accordance with claim 1 , wherein said processor is further configured to determine the pump fillage level based on a fluid contact position during a previous pump stroke.

4. The controller in accordance with claim 3 , wherein said processor is further configured to determine the fluid contact position based on the position of the pump piston and the load of the pump piston for the previous pump stroke.

5. The controller in accordance with claim 1 , wherein said processor is further configured to compute real-time stresses on the rod pumping unit using a rod pumping unit dynamics model based on the position signal and the load signal.

6. The controller in accordance with claim 5 , wherein said processor is further configured to modulate the pump speed based on the computed real-time stresses to control peak stresses on the rod pumping unit and to maintain the average pump speed over the pump stroke.

7. A method of controlling a rod pumping unit, said method comprising determining a pump piston position and a pump piston load;

computing a pump fillage level based on the pump piston position and the pump piston load;

operating the rod pumping unit at a pump speed equal to a first speed on a downstroke of the rod pumping unit;

reducing the predetermined pump speed on the downstroke to a second speed based on the pump fillage level and the pump piston position as the pump piston approaches a fluid surface in a barrel of the rod pumping unit,

detecting if an overstress caused by the reduction in the predetermined pump speed is present, wherein if the overstress is detected during the downstroke, modulating the reduction of the predetermined pump speed on the downstroke to mitigate the overstress; and

increasing the pump speed to a third speed on the downstroke after the pump piston contacts the fluid surface within the barrel of the rod pumping unit.

8. The method in accordance with claim 7 further comprising computing the first speed based on the pump fillage level.

9. The method in accordance with claim 7 , wherein computing the pump fillage level comprises determining a previous pump piston position at which the pump piston contacted the fluid surface during a previous stroke.

10. The method in accordance with claim 7 further comprising receiving data indicative of the second speed from a user of the rod pumping unit.

11. The method in accordance with claim 7 further comprising computing real-time stresses on the rod pumping unit using a rod pumping unit dynamics model based on the pump piston position and the pump piston load.

12. The method in accordance with claim 11 further comprising modulating the pump speed based on the computed real-time stresses to control peak stresses on the rod pumping unit and to maintain the first speed on average.

13. A rod pumping unit, comprising:

a pump comprising a pump piston and a barrel, said pump piston operable within said barrel;

a rod coupled to a motor and said pump, said rod configured to operate said pump at a pump speed; and

a controller coupled to said motor and configured to:

set a target average pump speed for the stroke based on the pump fillage level;

drive said pump piston on a downstroke at the pump speed, the pump speed equal to a first speed;

decelerate said pump piston on the downstroke to make the pump speed equal to a second speed as the pump piston approaches a fluid surface within said barrel,

detect if an overstress is present while the pump piston is decelerated, wherein if the overstress is detected during the deceleration, the deceleration is reduced to mitigate the overstress; and

accelerate said pump piston on the downstroke after said pump piston contacts the fluid surface within said barrel, wherein the pump piston is accelerated to a speed to achieve the target average pump speed for the pump stroke.

14. The rod pumping unit in accordance with claim 13 further comprising a position sensor and a load sensor configured to measure a position and a load of said rod at a well head for the rod pumping unit.

15. The rod pumping unit in accordance with claim 14 , wherein said controller is coupled to said position sensor and said load sensor, said controller further configured to:

compute real-time stress on said rod pumping unit based on the position and the load using a rod pumping unit dynamics model; and

modulate the predetermined pump speed according to the real-time stress.

16. The rod pumping unit in accordance with claim 14 , wherein said controller is coupled to said position sensor and said load sensor, said controller further configured to compute a real-time pump card representing a pump piston position and a pump piston load.

17. The rod pumping unit in accordance with claim 13 , wherein said controller is further configured to compute a pump fillage level based on a previous position at which the fluid surface was contacted on a previous downstroke, the pump fillage level corresponding to a position at which the fluid surface will be contacted on the downstroke.

18. The rod pumping unit in accordance with claim 17 , wherein said controller is further configured to compute the first speed based on the pump fillage level.