PERMEABILITY IMPAIRMENT AROUND BOREHOLES IN MICACEOUS AQUIFERS

A number of possible mechanisms, both physical and chemical, are postu lated as being potentially damaging to borehole performance in micaceo us aquifers. Laboratory studies have been undertaken to assess the deg ree to which these mechanisms are operative, and the extent of permeab ility damage which occurs as a result. Some of the processes studied a re unique to micaceous formations, while others are enhanced where mic a is present. Two key processes identified by the study are: (i) miner alogical and structural alteration of mica particles as a result of in creased groundwater salinity; and (ii) mixing of formation sands, eith er between layers, or with gravel pack material. The studies together with field evidence show that both of these processes can cause a sign ificant reduction in permeability in the vicinity of a borehole, leadi ng to a deterioration in borehole performance. The degree of deteriora tion which can result from each of the processes is assessed, and reco mmendations for operation, maintenance and rehabilitation, to minimize damage are made.