Reinjection of geothermal waste waters has become an important topic of i n t e r e s t f o r industry as well as for research. The environmental concerns due to chemical composition of geothermal waste waters had urged the industry to dispose i t underground. In several field applications no interference due to thermal front breakthrough was observed on the other hand some cases are reported where reinjection had caused severe declines in energy production due to unexpected breakthrough of injected water.(l,2) Several analytical and numerical studies are available (3,4) where the effect of fractures on the movement of thermal front are discussed I t was shown that when the conduction heat transfer from matrix to fracture dominates, retardation of the thermal front movement wi be observed (3). Bodvarsson and Pruess (5) considered the above problem i n a five-spot well pattern. They observed as the amount of fluid injected reaches the amount produced, the long-term energy output of the system increases. Pruess (4) in his study compares t h e behavior o f porous medium and fractured 1 medium i n terms of pressure decline due to production. Temperature and pressure profiles are presented between an injector and a producer where heating of the injected water i n porous medium and i n fractured medium w i t h small fracture spacing was high compared to a larger fracture spacing. Such observations from the numerical studies were checked against some l i - mited field examples (5,6). However understanding of the injection effects i n fractured reservoirs is limited. This work presents the results of laboratory experiments where effects of reinjection on temperature and pressure behavior of a 'porous medium and a fractured medium were investigated. The porous medium was a crushed limestone pack, w i t h 10 mm average particle size, packed in a 3-D box model where injection and production ports are located on the diagonal ends simulating a five-spot pattern. The fractured medium was made from unifromly cut marble blocks packed i n such a way to permit uniform fracture geometry.

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