Investigation of Optical Technologies for Measuring Geothermal Fluid Properties

The results of an investigation evaluating the feasibility of using optical measurements for the real-time monitoring of fluid properties in geothermal process streams is described. The measurements exploit new technologies that have been developed for the telecommunications industry and include new solid state laser devices, large-bandwidth, high-sensitivity detectors and low loss optical fiber components. In particular, the potential application of improved lightemitting diode technologies for measuring the moisture content in steam and the development of a particulate characterization system based upon new, compact diode-pumped laser technologies for monitoring steam purity or mineral precipitation in fluids are presented. Introduction The efficiency and lifetime of steam-powered equipment are affected by the quality, or wetness, of the steam used. Steam quality is defined as the mass fraction of the total fluid mass that is in the vapor phase. Dry steam or steam of 100% quality consists solely of water vapor, while qualities less than 100% indicate that water is present in the liquid phase. In operating systems, condensate may form due to temperature drops in some part of the system. Excessive steam washing to remove particulate or reduce chloride concentrations to levels that are not damaging to equipment can also introduce moisture. Wet steam contains less useable energy than dry steam. In turbine systems, steam tends to become “wetter” as it expands. The subsequent impingement of water droplets that form, as well as entrained droplets, can initiate corrosion on turbine blades. In addition, entrained droplets often contain solids that can deposit on turbine surfaces adversely affecting the flow stream and turbine efficiency, as well as potentially causing imbalance and necessitating cleaning operations. In brine-dominated resources, the deposition of silica and other minerals is a serious concern. The formation of silica scale in pipelines, heat exchangers, and reinjection wells places major constraints in fluid utilization in some geothermal operations, and can result in large maintenance costs for operators. Silica precipitation kinetics is generally not well understood and deposition on plant components can occur quickly without proper controls. Techniques for preventing the deposition of scale include restricting brine temperatures to above that at which silica supersaturation occurs and acidification of the brine phase to inhibit silica deposition. In other approaches, chemical inhibitors are used to sequester or complex with silica, preventing its precipitation (Thomas and Gudmundsson, 1989). This objective of this work is to investigate the feasibility of using optical technologies for the real-time monitoring of fluid properties in geothermal plants. The application of new techno-