Forhindring av stromming gjennom undergrunnsomrader

Compositions useful for preventing the flow of water or gas, or both, through a subterranean zone having a high temperature and a depth such that a long pumping time is required to place a sealing composition therein, comprise water, a cross-linking agent which comprises chitosan and a selected water-soluble polymer which reacts with the cross-linking agent and forms a sealing gel which is stable for a desired period of time at the temperature of the zone and has a pumping time before gelation in the presence of the cross-linking agent whereby the composition can be pumped to the depth of the zone and placed therein.