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.
A Systematic Method For Tracer Test Analysis: An Example Using Beowawe Tracer Data
Quantitative analysis of tracer data using moment analysis requires a strict adherence to a set of rules which include data normalization, correction for thermal decay, deconvolution, extrapolation, and integration. If done correctly, the method yields specific information on swept pore volume, flow geometry and fluid velocity, and an understanding of the nature of reservoir boundaries. All calculations required for the interpretation can be done in a spreadsheet. The steps required for moment analysis are reviewed in this paper. Data taken from the literature is used in an example calculation.
Carbonate scaling characteristics in Dixie Valley, Nevada geothermal wellbores
Calcite scale deposits in seven wellbores were logged with Schlumberger's multifinger caliper. The bottom of the scale is found at the deepest flash point. It thickens to a maximum in 46 to 107 m and then diminishes over a total length of 290 to 518 m. Scale deposition rates range from 267 to 911 kg of fluid produced for each cm3 of scale deposited. 
 
The dominant variable controlling the deposition rate is the pre-flash calcium content which increases with decreasing fluid entry temperature. Data from one pair of wells indicate a 22.44 cm (I.D.) wellbore can have an initial scale deposition rate about 30 % greater than a 31.53 cm wellbore. Downward movement of the flash point lengthens the scaled interval. Estimated times between scale cleanouts should be 3 to 4 months for wells with 22.44 cm production casing and 6 to 12 months for wells with 31.53 cm casing.