Abstract
The first experiment for the Downhole Coaxial Heat Exchanger (DCHE) was carried out successfully at the HGP-A well on the Island of Hawaii using an interval from the surface down to a depth of 876.5m as a cooperative project between the Engineering Advancement Association of Japan and the Pacific International Center for High Technology Research (Morita et al., 1992). Analysis of the experimental results was carried out using a simulator developed by Morita and others (Morita et al., 1984; Morita and Matsubayashi, 1986), to investigate the insulation performance of the inner pipe used in the DCHE and the heat transfer characteristics in the formation. Another purpose of this analysis was to confirm the accuracy of the simulator which has been used in this series of studies. The equivalent thermal conductivity of the pipe was estimated to be 0.06W / m · k. This indicates that the performance of the insulated pipes used as the inner pipe is sufficiently high for DCHE application, hence the highly efficient DCHE is feasible. Analysis also indicated that the heat transfer mechanism during the experiment in the main heat extraction interval between 300m in depth and the bottom of the DCHE was almost pure conduction and that the thermal conductivity of the formation in this interval was 1.6W/m·K. The observed heat transfer mechanism is concordant with the physical conditions of the well. The estimated thermal conductivity of the formation is concordant with that of water saturated Hawaiian basalt (Horai, 1991). Also, excellent agreement between measured values and computed values was obtained. This indicates that the simulator used in this series of studies is sufficiently accurate for DCHE application. Hence the results which have been obtained using the simulator in these studies were confirmed to be correct.
