A device for continuous water pressure measurement in a hydrocarbon reservoir comprising a pressure sensor, a hydrophilic membrane positioned between a reservoir formation and the pressure sensor, the hydrophilic membrane having a surface area A, and a biasing device pushing the hydrophilic membrane against the reservoir formation with a force equal to, or larger than, the pressure difference between a hydrocarbon phase Po in the reservoir and the water (Po-Pw) multiplied with the probe membrane contact area. Methods of installing the device in a hydrocarbon reservoir are also disclosed.