Fluid Volume and Flow Constraints for a Hydrothermal System at Beowawe, Nevada

Abstract

Abstract Tracer testing in geothermal reservoirs can yield valuable information concerning reservoir fluid volume and fluid exchange rate. We have been conducting a tracer test at the hydrothermal system at Beowawe, Nevada, for the past three years. Return curve data reveals that tracer concentration has been dropping at a very constant rate since approximately 600 days after the start of the test. An analysis of the long tail portion of the return curves indicates a reservoir fluid volume of about 17 billion gal and a maximum fluid exchange rate of about 1.2 million gal/hr. We verified our analytical approach to tracer return-curve analysis using the numerical simulation code TETRAD. Introduction With the increased use of reinjection in geothermal reservoirs, tracers have become an important tool in developing reservoir management strategies. If injectors are positioned too close to producers, a risk of short circuiting develops, resulting in the possibility of premature thermal breakthrough. If injectors are placed too far away, the injected water will not provide sufficient pressure support to the reservoir. Since chemical breakthrough is more rapid than thermal breakthrough, a tracer test can provide important interwell flow data that can be used to optimize injection well placement and injection flow rates. In tracing the flow of geothermal water along injection-production flow paths, a chemical compound is typically injected as a pulse into a selected well. The tracer enters the reservoir and is diluted as it is convected through fractures and diffuses into the pore matrix. The surrounding production wells are then sampled over an appropriate duration in order to determine the arrival times and concentrations of the tracer produced at each well. From an analysis of the tracer-return curves, it is possible to derive valuable information concerning the potential for thermal breakthrough between injection and production wells. A number of tracer tests have been conducted in recent years in liquid-dominated hydrothermal environments. In general, however, these tests were done in order to gain qualitative information about flow processes along injection-production pathways in order to predict fluid short circuiting and thereby anticipate thermal breakthrough. In this paper, we will demonstrate the use of tracer testing to determine useful information about reservoir fluid volume and upper limits on fluid exchange rates. Geology of Beowawe Hydrothermal System Regional Setting. The Beowawe Geothermal Field is situated within the Great Basin (Fig. 1) which is part of the larger Basin and Range geologic province. Most of the valleys in the Basin and Range are characterized by internal drainage. They are separated by isolated, N- to NE-trending mountain ranges that are about 6-12 mi wide and 16–22 mi long. Precambrian to Middle Paleozoic rocks, deposited in shallow marine settings, are present in the south and east parts of the Great Basin. Late Paleozoic and Mesozoic rocks consisting of fine-grained, deep sea shales and siltstones form a series of east-verging thrust sheets over the older rocks. The deformation responsible for the thrust faults lasted from 370 Ma to about 170 Ma. Most of the western part of the Great Basin is underlain by Mesozoic granitic plutons, which formed during the subduction events along the western margin of North America. In the Great Basin, widespread bimodal volcanism began about 43 Ma (mid-Eocene), spreading west and south across the area. Steeply dipping, NW-trending normal faults were contemporaneous with the volcanism. P. 129^