AN INVESTIGATION OF THE DIXIE VALLEY GEOTHERMAL FIELD, NEVADA, USING TEMPORAL MOMENT ANALYSIS OF TRACER TESTS
The method of analysis of the first temporal moment (mean residence time) of tracer return data was utilized to investigate the swept volume and fluid velocity between pairs of injection and production wells in the Dixie Valley geothermal field. Calculations were made using a spreadsheet program available from the Idaho National Laboratory. Jointly sponsored industry–government research at the Dixie Valley field provided several tracer tests over many years, and a test of six, chemically similar tracers, injected between 1998 and 2001, provided the tracer concentration data used in this study. The calculated parameters of mean residence time, fracture volume in contact with the tracer, fluid velocity, and heterogeneity of the reservoir were compared to published descriptions of the reservoir to evaluate their usefulness in refining the conceptual model of the Dixie Valley geothermal reservoir. Temporal moment analysis provided more information about the fractured fluid path between wells and the volume of the reservoir contacted by the tracers. Tracer injection in the center of the Dixie Valley field and production of that tracer in the northeastern and southwestern production wells clearly demonstrated that no barrier to flow exists between the two areas of producing wells.
Inert and Adsorptive Tracer Tests for Field Measurement of Flow‐Wetted Surface Area
Field tests in a discrete rock fracture validated a combined inert/adsorbing tracer test method to estimate the contact area between fluids circulating through a fracture and the bulk rock matrix (i.e., flow‐wetted surface area, A). Tracer tests and heat injections occurred at a mesoscale well field in Altona, NY. A subhorizontal bedding plane fracture ∼7.6 m below ground surface connects two wells separated by 14.1 m. Recovery of the adsorbing tracer cesium was roughly 72% less than the inert tracer iodide. Using an advection‐dispersion‐reaction model in one‐dimension, the adsorbing/inert tracer method identified substantial flow channelization. These results are consistent with Ground Penetrating Radar (GPR) and thermal sensors. All characterization methods suggest circulating fluids were concentrated in a narrow, 1–2 m wide channel directly connecting the injection and production well. The inert/adsorbing tracer method identified two flow channels with areas of 28 and 80 m2. A one‐dimensional heat transport model predicted production well temperature rises 20.5°C in 6 days, whereas measured temperature rise was 17.6°C. For comparison, two‐dimensional heat transport through a fracture of uniform aperture (i.e., homogeneous permeability) predicted roughly 670 days until production well temperature would rise 17.6°C. This suggests that the use of a fracture of uniform aperture to predict heat transport may drastically overpredict the thermal performance of a geothermal system. In the context of commercial geothermal reservoirs, the results of this study suggest that combined inert/adsorbing tracer tests could predict production well thermal drawdown, leading to improved reservoir monitoring and management.
SCIENCE AND EMPIRICISM IN PILE FOUNDATION DESIGN
Scientific approaches to pile design have advanced enormously in recent decades and yet, still, the most fundamental aspect of pile design—that of estimating the axial capacity—relies heavily upon ...