I think they do inject water.. or at least return water - from their web site:

Production

The gas arriving at the surface, at high pressure and temperature, is controlled by a system of valves fitted to the wellhead. Through these valves, the gas is passed to an installation where the temperature and pressure are lowered. This process also serves another purpose. By reducing the pressure and temperature the water vapour and condensate present in the gas liquefies and this enable the gas to be easily separated from the gas. The water is returned to the rock stratum from which it came and the valuable condensate is transported to the Shell and ExxonMobil refineries. The gas itself is now of the correct quality and at the correct supply pressure for delivery to Gasunie, which takes responsibility for onward transport.

below is a link to the page about gas production this company uses. (company web site)

Canie

• Nederlandse Aardolie Maatschappij