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.
The Basin and Range Dixie Valley Geothermal Wellfield, Nevada, USA—A test bed for developing an Enhanced Geothermal System exploration favorability methodology
Abstract The Dixie Valley Geothermal Wellfield (DVGW), a Basin and Range type system in central Nevada USA, is used as an exploration case history for Enhanced Geothermal Systems (EGS). It encompasses an area of approximately 170 km2. The wellfield produces 67 MW of geothermal power and it contains a number of high temperature non-productive wells. This wellfield was chosen as the calibration site for the development of an EGS exploration methodology due to its extensive body of geoscience data and information on the geothermal system and most importantly, well results in the public domain. This existing body of data (ca. 2011) was in part re-interpreted to produce a baseline conceptual model in terms of a number of serial cross-sections for the available data sets: geology, seismic reflection, resistivity, temperature, gravity-magnetic, and p-wave velocity (Vp). The exploration methodology was calibrated against available geothermal well results. Based on a comprehensive review of all available geoscience and well data, EGS favorability maps were generated, from +1 km above sea level (asl) to −4 km asl at 0.5 km intervals, for the three key EGS parameters of interest: rock type, temperature, and stress. Complimentary trust (“confidence-in-the-data-used”) maps were also created at the same scale to, among other things, indicate where additional data may be required. Quantitative geostatistical analysis of the geoscience data was conducted, among other factors, to address the question of whether the baseline geoscience data could be used to predict EGS favorability without the advantage of existing well temperature data. Classification and Regression Tree (CART) was one of a number of geostatistical methods applied to the baseline geoscience data and it provided the most promising results. In CART, the response variable (RV) is predicted while using explanatory variables (EVs). The geoscience parameters (EVs) considered in the CART analysis included temperature, Vp, resistivity from magnetotellurics, Coulomb Stress Change (CSC), dilatational strain (from CSC modeling), vertical stress, lithology based on geologic analysis, lithology based on gravity–magnetic modeling (G–M lithology) and the presence or absence of a fault. Temperature increases with depth in the DVGW. Vertical stress also increases with depth and it was deemed as a redundant EV. As such, a CART sensitivity analysis was applied to the baseline data set with and without vertical stress being considered as an EV to determine the effect of removing vertical stress and to evaluate with subsets of EVs that could be predictive of key EGS parameters. R2-values ranging from 0.611 to 0.841 were obtained for the RVs: temperature, lithology, productive vs. non-productive hydrothermal cells and expected EGS favorable cells (the response variables) using both cross-section and well data and not considering vertical stress. However, these CART results were not used in the generation of the favorability maps because this is the first analysis of its kind that the authors are aware of and more testing at other sites needs to be done; the raw total baseline data set described above was considered the most appropriate for this study.
The Inca Occupation of the South Coast of Peru
like the others, however; it forks into several nearly parallel ravines of which the largest are Huayuri, Rio Grande-Palpa, Ingenio (called Collao in ancient times), Nasca proper (the ancient Caxamarca), Taruga, and Poroma. Due to peculiarities of the local topography, the irrigated areas in Ica and the ravines of Nasca are located inland, at the foot of the mountains, rather than at the mouths of the rivers as is more usual in Peruvian coastal valleys. No sharp natural or cultural boundary separates the valley of Chincha from the valleys of Cafiete (ancient Huarco and Runahuana), Asia, and Mala, the next valleys going up the coast to the northwest. At the other end of the area we are considering, however, there is a sharp break; beyond Yauca the Andean foothills reach out to the sea, natural conditions are quite different, and there are important differences in the archaeological picture as well. The historical sources which mention the south coast are not particularly helpful, except in so far as they provide us with a few dates. To begin with, no chronicler gives an explicit account of the political organization of the south coast at the time it came into contact with the Incas. It can, perhaps, be inferred from Cabello Balboa's statements about common policy toward the Inca invaders that there was some sort of political connection between the valleys of Mala, Cafiete, and Chincha, but the connection need have been no more than a temporary military alliance. Cabello makes no mention at all of Pisco. The valleys of Ica, Nasca, and 125