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.
UPDATED METHODS FOR ESTIMATING RECOVERY FACTORS FOR GEOTHERMAL RESOURCES
An important aspect of geothermal resource assessment methodology is the development of geothermal resource models consistent with the production histories of exploited geothermal fields. The primary method applied in past United States Geological Survey (USGS) assessments was the volume method, in which the recoverable heat is estimated from the thermal energy available in a reservoir of uniformly porous and permeable rock using a constant recovery factor, Rg, of 0.25 for the producible fraction of a reservoir’s thermal energy. More recent analyses of data from the fractured reservoirs commonly exploited for geothermal energy indicate that Rg is closer to 0.1, with a range of approximately 0.05 to 0.2. In general this apparent discrepancy in Rg reflects the contrast in thermal energy recovery from complex, fracture-dominated reservoirs compared to the uniform, high-porosity reservoirs considered in the early models. Models for the recovery of heat from heterogeneous, self-similar, fractured reservoirs predict variations in Rg that provide a physically realistic basis for evaluating the production potential of both natural geothermal reservoirs and reservoirs that may be created through the application of EGS technology. ASSESSMENT METHODOLOGIES – HISTORY, DEVELOPMENT, CHALLENGES
Removal of urea in a wearable dialysis device: a reappraisal of electro-oxidation.
A major challenge for a wearable dialysis device is removal of urea, as urea is difficult to adsorb while daily production is very high. Electro-oxidation (EO) seems attractive because electrodes are durable, small, and inexpensive. We studied the efficacy of urea oxidation, generation of chlorine by-products, and their removal by activated carbon (AC). EO units were designed. Three electrode materials (platinum, ruthenium oxide, and graphite) were compared in single pass experiments using urea in saline solution. Chlorine removal by AC in series with EO by graphite electrodes was tested. Finally, urea-spiked bovine blood was dialyzed and dialysate was recirculated in a dialysate circuit with AC in series with an EO unit containing graphite electrodes. Platinum electrodes degraded more urea (21 ± 2 mmol/h) than ruthenium oxide (13 ± 2 mmol/h) or graphite electrodes (13 ± 1 mmol/h). Chlorine generation was much lower with graphite (13 ± 4 mg/h) than with platinum (231 ± 22 mg/h) or ruthenium oxide electrodes (129 ± 12 mg/h). Platinum and ruthenium oxide electrodes released platinum (4.1 [3.9-8.1] umol/h) and ruthenium (83 [77-107] nmol/h), respectively. AC potently reduced dialysate chlorine levels to < 0.10 mg/L. Urea was removed from blood by EO at constant rate (9.5 ± 1.0 mmol/h). EO by graphite electrodes combined with AC shows promising urea removal and chlorine release complying with Association for the Advancement of Medical Instrumentation standards, and may be worth further exploring for dialysate regeneration in a wearable system.