A model to analyze tracer test data is proposed. It consists of a fractured layer and a slightly-fractured layer, where tracers move between the layers in proportion to their concentration gradients. It is shown that the slightly-fractured layer plays an important role in the movement of fluids and transport of tracers in fractured rock masses.The concepts behind the proposed model for identifying geothermal reservoir structure from the analysis of tracer returns and its application to field data are described. The results demonstrate that one main flow path can explain tracer responses showing two peaks or a long tail. The model allows us to calculate the flow fraction and the fluid flow velocity through the slightly-fractured layer from the tracer returns observed in geothermal wells.NOMENCLATUREC dimensionless concentration of tracerc concentration of tracer (mol/m 3 )E dispersion coefficient (m 2 /s)k c mass transfer coefficient (m/s)k c 0 Kozeny constantp pressure (Pa)P e Peclet numberq e total volumetric flow rate produced from the two fractured layers (m 3 /s)S v specific surface area (m - 1 )T dimensionless timet time (s)t * fluid residence time (s)U dimensionless fluid flow velocityu fluid flow velocity (m/s)V total pore volume in the two fractured layers (m 3 )X dimensionless length in the flow directionx length in the flow direction (m)x 1 distance between injection and production points (m)y 1 width of the fractured layers (m)Z D layer thickness ratio (=z f /z T )z T half of the total thickness of the two fractured layers (m)z f half-thickness of the fractured layer (m)Greekμ viscosity (Pa s)φ porosityΞ dimensionless mass transfer coefficientSubscriptsf fractured layers slightly-fractured layerr rock matrix0 at the injection point1 at the production pointSuperscript average for the two fractured layers