Analysis of tracer recovery curves from injection-backflow testing at two geothermal reservoirs reveals large differences in response between the two. The East Mesa reservoir is in a layered sandstone matrix, and tracer behavior can be adequately described by porous media theory. As the volume of water injected into the reservoir increases and, consequently, the depth of penetration into the formation, the ratio of dispersive flux to advective flux decreases, indicating the increasing importance of advective transport. This effect an be seen in normalized tracer recovery curves that become more symmetrical with greater injection volume. At Raft River site the reservoir is dominated by a single major fracture zone. Injecting larger volumes of water into the fracture does not change the shape of the normalized tracer recovery curves. This indicates that.the dispersion coefficient increases proportionally to the distance traveled by the injection front. Differences in the shape of tracer recovery curves are related to fundamental differences in reservoir characteristics. the tracer recovery curves at Raft River suggest a dual porosity reservoir with a secondary fracture network connected to the major fracture. Such findings may considerably affect calculations of secondary heat recovery using injection wells.

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