Published online by Cambridge University Press: 25 February 2011
Field heat experiments with kaolinite/smectite clay surrounding heaters in boreholes were conducted for 0.7 and 4 years with temperatures up to 170-180°C. The short test gave a high degree of water saturation even in the hottest part (> 75 %) and almost no change in physical properties and mineral composition. The long test gave a dry inner zone of claystone, indicating gas formation, and rich precipitation of silica/aluminum compounds and sulphate minerals. Brittleness characterized the hot parts and stiffening occurred also in the colder parts due to precipitation of silica and aluminum.