Since 1985, a research and development on the technique of heat extraction from hot dry rock has been performed by NEDO (the New Energy and Industrial Technology Development Organization) at Hijiori, Yamagata prefecture (NEDO, 1991). Heat extraction tests showed that heated water and steam were continually produced through the reservoir created artificially in a hot dry rock. To evaluate this artificial reservoir, well logging and production tests were also conducted. From information obtained in these tests, it was estimated that the artificial reservoir might consist of the system of rock matrix and fracture-network (including natural and artificial fractures) as shown in Fig. 1. A theoretical base on fluid flow and thermal energy transport for this system should be developed to understand the production behavior. Several works have been done for modeling on fluid and heat flow in the area of frac-tured rocks (Bodvarsson, 1972; Bodvarsson et al., 1982; Chida et al., 1990; Gringarten, et al., 1975; Masuda, Y., et al., 1991; Pruess, K., 1985). Because previous models were developed to analyze fluid and heat flow in the rock mass with a single fracture or some parallel fractures, it is difficult to apply them to the rock matrix/fracture network system mentioned above. This paper deals with the numerical simulation of thermal energy transport in the matrix block/fracture network system. In particular, attension was focused on the validity of the proposed algorithm for energy transport simulations by comparing with that of a fine meshed porous model.
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