Mechanical downhole pressure maintenance system

Methods of providing pressure maintenance in a multi-zone production well, characterised by isolating first and second external regions of a completion string from wellbore hydrostatic pressure, and using pressure maintenance devices 140. The isolation of the regions may be provided using packers 95, 100, 105. The pressure maintenance devices may be valves including a gas-charged chamber (fig. 3, 320), and the valve operation may be dependent on a comparison of pressure differentials between the gas-charged chambers and the internal and external regions of respective first and second zones.