Fluid circulation and heat extraction from engineered geothermal reservoirs

Abstract

A large amount of fluid 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 confined reservoirs, which best represent the original Hot Dry Rock concept, the flow 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 flow-accessible reservoir region, (2) the mean pressure in the reservoir, and (3) the cumulative amount of fluid circulation—and therefore reservoir cooling—that has occurred. For an initial reservoir rock temperature distribution and mean fluid outlet temperature, the rate of heat extraction (i.e., thermal power) is at first only a function of the production flow 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 effective joint spacing and the total flow-accessible (i.e., heat-transfer) volume of the reservoir. The rate of heat extraction, which depends on the production flow rate, can also vary with time as a result of continuing changes in the flow distribution arising from reservoir cooling. The thermal power of engineered reservoirs can most readily be increased by increasing the production flow rate, as long as this does not lead to premature cooldown, the development of short-circuit flow paths, or excessive water losses. Generally, an increase in flow rate can be accomplished by increasing the injection pressure within limits. This strategy increases the driving pressure drop across the reservoir and the mean reservoir pressure, which in turn reduces the reservoir flow impedance by increasing the amount of joint dilation. However, the usefulness of this strategy is limited to reservoir operating pressures below the fracture extension pressure, and may lead to excessive water losses, particularly in less-confined reservoirs. Under such conditions, a downhole production-well pump may be employed to increase productivity by recovering more of the injected fluid at lower mean reservoir operating pressures.