In many geothermal fields around the world, it is observed that (1) the vertical distribution of fluid pressure in the undisturbed reservoir is approximately liquid-hydrostatic (7-9 kPa per meter of depth), but that (2) when a well is drilled into the reservoir and fluid is withdrawn, the enthalpy (or steam fraction) of the stable fluid discharge is often anomalously high-sometimes the well discharges steam alone. As will be seen, this apparent paradox cannot be explained unless the reservoir is highly heterogeneous on a local scale, with a sharp permeability contrast between the relatively impermeable matrix rock and the "fracture zones" that penetrate it and provide channels for fluid flow. Also, anomalous wellhead discharge enthalpies will not be observed unless two-phase flow of a water/steam mixture is taking place deep within the reservoir itself. Finally, it is noteworthy that this so-called "excess enthalpy effect" is inherently transient in character and discharge enthalpies will eventually decline, although several years may pass before the deterioration in system performance becomes noticeable. Examples of these phenomena as observed in two geothermal fields in Japan are presented.
