A condition caused by an increase in water saturation in the near-wellbore area. Water block typically forms during the drilling phase of a well, when the near-wellbore area is exposed to a relatively high volume of filtrate from the drilling fluid. The increased presence of water causes fine clay crystals that may be present in the formation, such as illite, to swell and cause a reduction in permeability. Water-block treatments typically use surfactants to reduce the surface tension between the oil and water, helping to displace the water from the near-wellbore area.