It is well established that most high-temperature geothermal reservoirs are extensively fractured. The fractures provide the principal conduits for fluid and heat flow. The rock matrix contains most of the fluid and heat reserves but it usually has a very low permeability, perhaps in the microdarcy-range.

Conventional approaches to geothermal reservoir modeling have employed a porous medium approximation, although the validity of this approximation for naturally fractured reservoirs has never been demonstrated in detail. It appears that most researchers expected a porous medium approximation to work in cases with "not too large" fracture spacing. Recently it was shown by Pruess and Narasimhan (1982a), that in two-phase geothermal reservoirs strong discontinuities in vapor saturation can arise at matrix/ fracture interfaces, due to an interplay between fluid convection and heat conduction. This suggests that fractured systems with two-phase fluid may behave quite differently than porous medium systems even in cases where fracture spacing is small in comparison to characteristic dimensions of the problem (e.g., reservoir size, well spacings, completion intervals).

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