Direct drive pumping unit

1. A direct drive pumping unit, comprising:

a reciprocator for reciprocating a sucker rod string and comprising:

a tower for surrounding a wellhead;

a polished rod connectable to the sucker rod string and having an inner thread open to a top thereof and extending along at least most of a length thereof;

a screw shaft for extending into the polished rod and interacting with the inner thread; and

a motor mounted to the tower, torsionally connected to the screw shaft, and operable to rotate the screw shaft relative to the polished rod; and

a sensor for detecting position of the polished rod.

2. The unit of claim 1 , wherein the reciprocator further comprises a thrust bearing supporting the screw shaft from the crown.

3. The unit of claim 1 , wherein the reciprocator further comprises a torsional arrestor mountable to the wellhead for engagement with the polished rod to allow longitudinal movement of the polished rod relative to the wellhead and to prevent rotation of the polished rod relative to the wellhead.

4. The unit of claim 3 , further comprising a controller in data communication with the sensor and operable to regularly briefly retract the torsional arrestor from the polished rod to allow rotation thereof by a fraction of a turn.

5. The unit of claim 1 , wherein the motor is an electric three phase motor.

6. The unit of claim 5 , further comprising:

a variable speed motor driver in electrical communication with the motor; and

a controller in data communication with the motor driver and the sensor and operable to control speed thereof.

7. The unit of claim 6 , further comprising:

a power converter in electrical communication with the motor driver; and

a battery in electrical communication with the power converter and operable to store electrical power generated by the motor during a downstroke of the pumping unit.

8. The unit of claim 1 , wherein the motor is a hydraulic motor.

9. The unit of claim 8 , further comprising:

a hydraulic power unit (HPU) for driving the hydraulic motor;

a variable choke valve connecting the HPU to the hydraulic motor; and

a controller in communication with the variable choke valve and the sensor and operable to control speed of the hydraulic motor.

10. The unit of claim 9 , further comprising:

a turbine-generator set;

a manifold for selectively providing fluid communication among the HPU, the turbine-generator set, and the hydraulic motor;

a power converter in electrical communication with the turbine-generator set; and

a battery in electrical communication with the power converter and operable to store electrical power generated by the turbine-generator set during a downstroke of the pumping unit.

11. The unit of claim 1 , wherein the screw shaft interacts with the inner thread by mating therewith.

12. The unit of claim 1 , wherein:

a raceway is formed between the inner thread and the screw shaft, and

the reciprocator further comprises threaded rollers for being disposed in the raceway.

13. The unit of claim 1 , wherein:

a raceway is formed between the inner thread and the screw shaft, and

the reciprocator further comprises balls for being disposed in the raceway.

14. The unit of claim 1 , wherein the reciprocator further comprises a rod rotator operable to intermittently rotate the polished rod a fraction of a turn.

15. A wellhead assembly for a direct drive pumping unit, comprising:

a direct drive pumping unit of claim 1 coupled to a stuffing box;

the stuffing box mounted on a production tree by a flanged connection; and

the production tree mounted on a wellhead by a flanged connection.