Geology and gradient wells

Abstract

Drilling of gradient wells is a method used in exploration of both high- and lowtemperature geothermal fields. The applicability of the method depends on the permeability of the subsurface rock and the depth to groundwater. Low permeability is a prerequisite for the method to give reliable results because it is very important to avoid disturbance from internal flow of water inside wells. The drilling depth is accordingly anywhere from a few tens of metres to a few hundred metres. The latter may be necessary in areas of young or unaltered volcanics and low ground water level (such rocks are also as a rule costly to drill and secure from caving in). In fracture controlled low-temperature fields shallow boreholes come out best. This method has been applied also in high-temperature geothermal fields both as a regional survey and as testing the validity of indirect evidence (from geology or resistivity) of a potential. It is clear that knowledge of the hydrological conditions of the prospect area is needed, and also familiarity with the basic principles of hydrogeology.