Heat, fractures and fluid are the fundamental elements of geothermal resources. From a different viewpoint, existence of natural hydrothermal convection to form convective geothermal resources, and fluid production from wells, are the important factors for geothermal development. Under this circumstance, fractures play two different roles. One is contribution for the onset of the natural convection in geothermal systems, and the other is contribution as flow paths to connect wells to the reservoir for fluid production. Since the inter-fracture ascending velocity of natural convection in geothermal reservoirs is of the order of 10(-8) m/s, fractures from very small to very large permeabilities contribute to the first role. However, in-flow velocity of single-phase liquid within fractures in the vicinity of the well face ranges from 10(-1) to 10(1) m/s for example. Thus, only fractures of very high permeability can contribute to the second role. Therefore, one should always note the roles of fractures when discussing permeability in geothermal reservoirs. (C) 2003 Elsevier Ltd. All rights reserved.
