Abstract:
A test facility has been developed and operated to determine the performance characteristics of downhole heat exchangers (DHEs) in small diameter shallow wells. A U-tube and an annular design were consecutively retro-fitted to an unused 4in (100mm) production bore in Rotorua. Up to 140 kW of heat was produced, enough to supply the peak demand of eight Rotorua homes, saving 20-80 tonnes per day of fluid withdrawal from the field. Extensive field testing, supported by physical and numerical modelling has shown that heat transfer to the DHE occurs primarily in the well's open hole, and that output depends strongly on inter-tube heat transfer; this explains why, of the two output enhancement methods tried, the convection promoter pipe performed best. Well bleeding gave only 2- 3 kW additional output of heat and uses two tonnes of reservoir fluid per day, whereas the promoter pipe gave over 10 kW extra with no fluid extraction. Potential heat gains are higher if an un-orthodox convector pipe is used. Physical modelling reveals that forward flow, down the convector pipe, is the preferred circulation direction for maximum heat transfer; a scheme to ensure forward flow is suggested. Development of several numerical models, using the heat and mass transfer package PHOENICS, is discussed. A one dimensional model was used for a parametric study of the two DHE designs, leading to a hybrid U-tube design, which has a 30% higher output under local conditions. An iterative method for estimating transient conduction is developed; results show a good correlation to measured data. Convective reservoir flow sustains the long-term heat output, with conduction from storage providing less than 2% of the heat load. Despite this, conduction is found to be an important part of the reservoir to well heat transfer process; at least 10% of the DHE's heat output is conducted through rock immediately surrounding the well, from hot fluid which does not pass through the drilled hole. Somewhat inconclusive experiments to measure interference between neighbouring wells in Rotorua, and at Klamath Falls, Oregon, are described; some suggestions for improvements are included. Quench and recovery tests were used to determine well characteristics, and led to a method of predicting DHE heat loads before installation. A new method of quantifying the utility of hot returned fluid, using exergy analysis, is suggested. This has also been used to predict DHE performance characteristics. Verification of these methods is, however, incomplete. DHEs, retro-fitted to low pressure production wells, are a technically viable alternative to surface production of geothermal fluid. Using DHEs might further ease the stress on the Rotorua Geothermal Field which has resulted from high volume fluid production.