A large amount of ¯uid circulation and heat extraction (i.e., thermal power production) research and testing has been conducted on engineered geothermal reservoirs in the past 15 years. In con®ned reservoirs, which best represent the original Hot Dry Rock concept, the ¯ow distribution at any given time is primarily determined by three parameters: (1) the nature of the interconnected network of pressure-stimulated joints and open fractures within the ¯ow-accessible reservoir region, (2) the mean pressure in the reservoir, and (3) the cumulative amount of ¯uid circulationÐand therefore reservoir coolingÐthat has occurred. For an initial reservoir rock temperature distribution and mean ¯uid outlet temperature, the rate of heat extraction (i.e., thermal power) is at ®rst only a function of the production ¯ow rate, since the production temperature can be expected to remain essentially constant for some time (months, or even years). However, as reservoir circulation proceeds, the production temperature will eventually start to decline, as determined by the mean eective joint spacing and the total ¯ow-accessible (i.e., heattransfer) volume of the reservoir. The rate of heat extraction, which depends on the production ¯ow rate, can also vary with time as a result of continuing changes in the ¯ow distribution arising from reservoir cooling.
The thermal power of engineered reservoirs can most readily be increased by increasing the production ¯ow rate, as long as this does not lead to premature cooldown, the development of short-circuit ¯ow paths, or excessive water losses. Generally, an increase in