Gas lift method and apparatus

1. A method for injection of a lift gas from an annulus of the wellbore into a wellbore production string, the method comprising:

determining production pressure within the production string; and

autonomously controlling a variable orifice gas lift valve disposed in the wellbore on the production string in accordance with the determined production pressure, wherein the variable orifice gas lift valve controls an injection flow rate of the lift gas from the annulus into the production string,

wherein autonomously controlling the variable orifice gas lift valve comprises:

operating the variable orifice gas lift valve in a learning mode of operation to determine a setting of the variable orifice gas lift valve which provides a target production pressure or pressure condition, the pressure condition comprising a substantially minimum production pressure; and

subsequently operating the variable orifice gas lift valve in an operational mode of operation in which the variable orifice gas lift valve remains at a position previously determined during the learning mode of operation; and

wherein determining the setting of the gas lift valve in the learning mode of operation comprises at least one of:

determining the production pressure with the gas lift valve set at multiple positions, and then selecting the setting of the gas lift valve which provides the target production pressure or pressure condition; and

setting the gas lift valve at different incremental positions between a fully closed state and a fully open state, and determining the valve position which provides a lowest or minimized production pressure.

2. The method of claim 1 , wherein the lift gas in the annulus of the wellbore is from: a source located at a surface position, a source located at a downhole position, or a gas-bearing subterranean formation.

3. The method according to claim 1 , wherein the variable orifice gas lift valve comprises:

a pressure sensor, and the method comprises determining the production pressure using said pressure sensor; and

a controller, and the method comprises controlling the variable orifice gas lift valve using said controller.

4. The method according to claim 1 , comprising switching between the learning and operational modes of operation to define an optimization cycle.

5. The method according to claim 1 , comprising:

determining a first production pressure value with the gas lift valve set to a first position;

determining a second production pressure value with the gas lift valve set to at least one further position; and

using the determined first and second production pressure values to control the gas lift valve to be set to an operational position which provides the target production pressure or condition.

6. The method according to claim 5 , wherein the operational position comprises the first position or the at least one further position.

7. The method according to claim 1 , comprising:

setting the variable orifice gas lift valve to a first position which provides a first injection flow rate value of the lift gas, and determining a first production pressure value;

setting the variable orifice gas lift valve to a second position which provides a second injection flow rate value of the lift gas which is different from the first injection flow rate value, and determining a second production pressure value; and

controlling the variable orifice gas lift valve to be set to an operational position in accordance with the first and second determined production pressure values.

8. The method according to claim 7 , comprising controlling the variable orifice gas lift valve to be set to one of the first and second positions which provides the lowest or minimized production pressure.

9. The method according to claim 7 , comprising:

controlling the variable orifice gas lift valve, in accordance with the first and second determined production pressure values, to be set to a third position which provides a third injection flow rate value of the lift gas which is different from the first and second injection flow rate values, and determining a third production pressure value; and

controlling the variable orifice gas lift valve in accordance with at least one of the first, second, and third determined production pressure values.

10. The method according to claim 9 , wherein the third position is determined in accordance with an increase or decrease in pressure between the first and second determined production pressure values.

11. The method according to claim 9 , wherein the variable orifice gas lift valve is controlled to be set to the first, second or third position which provides the lowest or minimized production pressure.

12. The method according to claim 1 , comprising:

setting the valve at a first position and recording a first production pressure value;

setting the valve at a second position and recording a second production pressure value;

determining a first pressure variation between the first and second production pressure values; and

controlling the valve in accordance with the first pressure variation.

13. The method according to claim 12 , comprising controlling the valve to at least one of return to the first position or remain at the second position when the first pressure variation falls below a threshold value, wherein the threshold value defines a first derivative threshold value.

14. The method according to claim 13 , wherein if the first pressure variation is above the first derivative threshold value, the method comprises:

controlling the valve to be set to a third position and recording a third production pressure value;

determining a second pressure variation between the second and third production pressure values; and

controlling the valve in accordance with the second pressure variation.

15. The method according to claim 13 , comprising sequentially controlling the valve to be set to further positions and determining further pressure variations until such time as the first derivative threshold is reached.

16. The method according to claim 1 , comprising controlling the variable orifice gas lift valve to close to prevent flow of the lift gas when it is determined that the production pressure is lower than a lower threshold level.

17. The method according to claim 16 , wherein when it is determined that the production pressure is above the lower threshold level, the method comprises controlling the gas lift valve to define a desired injection flow rate value of the lift gas in accordance with the production pressure.

18. The method according to claim 17 , comprising operating the variable orifice gas lift valve in the learning mode of operation when the production pressure is determined to be above the production pressure lower threshold level, such that the lower threshold level defines a lower learning limit.

19. The method according to claim 16 , comprising controlling the variable orifice gas lift valve to at least one of fully closed or fully open when it is determined that the production pressure is higher than an upper threshold level.

20. The method according to claim 19 , wherein when it is determined that the production pressure is lower than the production pressure upper threshold level, the method comprises controlling the gas lift valve to define a desired injection flow rate value of the lift gas in accordance with the production pressure.

21. The method according to claim 20 , comprising operating the variable orifice gas lift valve in the learning mode of operation when the production pressure is determined to be lower than the upper threshold level, such that the upper threshold level defines an upper learning limit.

22. The method according to claim 21 , comprising operating the variable orifice gas lift valve over a production pressure window defined between lower and upper production pressure threshold limits.

23. A gas lift valve to inject a lift gas from an annulus of a wellbore into a production string, the gas lift valve comprising:

a valve inlet for communicating with the lift gas in the annulus and a valve outlet for communicating with a production string;

a variable orifice positioned between the valve inlet and valve outlet; and

a controller configured to receive data associated with production pressure and control the variable orifice in accordance with said production pressure to control an injection flow rate of the lift gas from the annulus into the production string,

wherein the controller is configured to:

operate the variable orifice gas lift valve in a learning mode of operation to determine a setting of the variable orifice gas lift valve which provides a target production pressure or pressure condition, the pressure condition comprising a substantially minimum production pressure; and

operate the variable orifice gas lift valve in a subsequent operational mode of operation in which the variable orifice gas lift valve is set at a position previously determined during the learning mode of operation; and

wherein to determine the setting of the gas lift valve in the learning mode of operation, the controller is configured to at least one of:

determine the production pressure with the gas lift valve set at multiple positions, and then select the setting of the gas lift valve which provides the target production pressure or pressure condition; and

set the gas lift valve at different incremental positions between a fully closed state and a fully open state, and determine the valve position which provides a lowest or minimized production pressure.

24. The gas lift valve of claim 23 , wherein the lift gas in the annulus of the wellbore is from: a source located at a surface position, a source located at a downhole position, or a gas-bearing subterranean formation.