1. n. 
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.
2. n. 
A production impairment that can occur when the formation matrix in the near-wellbore area becomes water-saturated, thereby decreasing the relative permeability to hydrocarbons. Water block may result from the invasion of water-base drilling or completion fluids or from fingering or coning of formation waters. The most extreme cases of water block occur in low-pressure, low-permeability gas formations, where alcoholic acid systems are recommended because they promote water vaporization in the produced gas. Alcoholic acid formulations are a mixture of acid and alcohol. The acids normally employed are usually either hydrochloric acid [HCl], mud acid [HF-HCl or HF-organic acid (formic or acetic)]. The alcohol is either methyl or isopropyl. Alcohol lowers the surface tension of acid and allows deeper penetration of the acid into the matrix of the rock. Alcohol is somewhat soluble in both acid and water, and penetration of low-surface-tension volatile alcohol into a water block will aid in its removal.