Method and apparatus for precise control of wellbore fluid flow

1. A method for controlling flow of fluid from an annular space in a wellbore, comprising:

changing an amount of flow restriction in a fluid flow discharge line from the wellbore annular space, the amount of flow restriction changed at a rate that increases as a difference increases, and decreases as the difference decreases, the difference being one of a first difference and a second difference, the first difference being between a selected fluid flow rate out of the wellbore and a measured fluid flow rate out of the wellbore, and the second difference being between a selected fluid pressure in the annular space and a measured pressure in the annular space.

2. The method of claim 1 wherein the changing the amount of flow restriction comprises changing an orifice size of a variable orifice choke.

3. The method of claim 2 wherein the changing orifice size comprises operating an actuator coupled to an orifice size control in the choke.

4. The method of claim 3 wherein the actuator is operated by applying hydraulic pressure to one side of a piston disposed in the actuator.

5. The method of claim 4 wherein the rate is controlled by applying a controllable restriction to flow of hydraulic fluid from the other side of the piston.

6. The method of claim 4 wherein the rate is selected in response to an actual position of the actuator with respect to a position thereof resulting in the selected fluid flow rate or the selected pressure.

7. The method of claim 6 wherein the actual position of the actuator is determined by measurements from a position sensor.

8. The method of claim 7 wherein the position sensor comprises a linear position sensor.

9. The method of claim 5 wherein the controllable restriction comprises a variable orifice.

10. A method for controlling flow of fluid through a conduit, comprising:

changing an amount of flow restriction in the conduit, the amount of flow restriction changed at a rate that decreases as a difference decreases and increases as the difference increases between one of a first difference and a second difference, the first difference being between a selected fluid flow rate out of the wellbore and a measured fluid flow rate out of the wellbore, and the second difference being between a selected fluid pressure in the annular space and a measured pressure in the annular space.

11. The method of claim 10 wherein the amount of the flow restriction is controlled by operating a three way valve in hydraulic communication with a source of hydraulic pressure.

12. The method of claim 10 wherein the changing the amount of flow restriction comprises changing an orifice size of a variable orifice valve.

13. The method of claim 12 wherein the changing orifice size comprises operating an actuator coupled to an orifice size control in the valve.

14. The method of claim 13 wherein the actuator is operated by applying hydraulic pressure to one side of a piston disposed in the actuator.

15. The method of claim 14 wherein the rate is controlled by applying a controllable restriction to flow of hydraulic fluid from the other side of the piston.

16. The method of claim 15 wherein the controllable restriction comprises a variable orifice.

17. The method of claim 13 wherein the rate is selected in response to an actual position of the actuator with respect to a position thereof resulting in the selected fluid flow rate or the selected pressure.

18. The method of claim 17 wherein the actual position of the actuator is determined by measurements from a position sensor.

19. The method of claim 18 wherein the position sensor comprises a linear position sensor.