ROUGH SIZE ESTIMATION OF A THERMALLY FRACTURED ZONE IN AN INFINITE HOT ROCK MASS

Concerning the extraction of geothermal energy from a deep thermal res ervoir by the downhole coaxial heat exchanger with a thermally insulat ed inner pipe proposed by Morita et al, we obtained rough estimates of a size of the fractured zone induced by thermal stresses due to injec ting cold water into the hot rock mass through the pipe. We assumed co mplete spherical symmetry of the temperature and stress fields. At the rough estimation, we considered three typical or extreme cases. (1) T he fracturing affects neither the loading capacity of a fractured rock mass nor the temperature distribution within the formation. (2) The f ractured zone completely loses its loading capacity and is fully invad ed by the borehole water. No disturbance of the fracturing makes any d ifference in the temperature. (3) The rock formation is assumed to hav e an appropriately increased fictitious conduction substituted for the heat transfer enhanced by the expected convection within the fracture d zone in order to discuss the effects of an occurrence of heat convec tion within the fractured zone on the temperature and stress distribut ions and the fractured zone size. As a result, the size of the zone ha s been estimated to be about ten or more times the borehole radius.