A method for injection of a lift gas into a wellbore production string, comprising: determining production pressure within the production string; and autonomously controlling a variable orifice gas lift valve in accordance with the determined production pressure, wherein the variable orifice gas lift valve controls the injection flow rate of the lift gas 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 or minimised 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 the setting previously determined during the learning mode of operation 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 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 the substantially minimum or minimised production pressure. determining production pressure within the production string; and autonomously controlling a variable orifice gas lift valve in accordance with the determined production pressure, wherein the variable orifice gas lift valve controls the injection flow rate of the lift gas 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 or minimised 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 the setting previously determined during the learning mode of operation 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 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 the substantially minimum or minimised production pressure. 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 or minimised 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 the setting previously determined during the learning mode of operation 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 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 the substantially minimum or minimised production pressure. determining the production pressure with the gas lift valve set at multiple positions, and then selecting the setting of the 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 the substantially minimum or minimised production pressure.