Abstract: A mixture of a liquid epoxy material and a hardener is selectively placed in a well traversing a subterranean formation adjacent to a zone in said formation which is to be plugged. The mixture is displaced into the zone and allowed to harden, thereby plugging the zone. The process is particularly applicable to sealing off water production in a gravel-packed well. A positive displacement bailer may be effectively used to deliver the mixture to the zone. The epoxy material used is immiscible with and is heavier than the fluid in the well. The epoxy material is further characterized as being essentially free of solids and having a low viscosity at downhole conditions of temperature and pressure. The hardener has an activation temperature which is lower than the downhole formation temperature.