Mineral Scaling in Geothermal Two-Phase Pipelines

Abstract

Mineral scaling is common in geothermal power facilities. This scale deposition is usually caused by changes in the geothermal fluid temperature or composition. Precipitation of these minerals can limit fluid flow within the pipelines reducing plant efficiency and increasing maintenance costs. A layer of scaling is commonly found in most parts of the geothermal power production facilities. However, significant amounts of deposited mineral scales (mainly silica) are usually observed in the pipelines and vessels that handle super-saturated brines. This work focuses on a less common scaling in geothermal two-phase pipelines, when fluids are usually high in temperature and under-saturated with respect to amorphous silica. This is mainly caused by fluid mixing at the two-phase pipelines, when several wells share the same pipeline to transport the fluid to the downstream facility. Two case studies are discussed in this work reporting mineral scaling due to the mixing of incompatible geothermal fluids within the two-phase headers. The first case tackles the silica deposition caused by the mixing of a steam-dominated well fluid with fluid from two water-dominated wells. The scale deposition is caused by the small amount of brine entrained in the steam-dominated well fluid which reacts upon mixing with the other water-dominated well fluids, causing massive localised scaling at the mixing points. The second case investigates iron mineral and silica deposition caused by the mixing of a low-pH, high-silica fluid with a neutral-pH fluid. Header blockage result in the increase in well head pressures causing production from some wells to collapse, at the same time decreasing the flow of geothermal fluids to the separators. The scaling requires regular cleaning to return the pipelines to full flow capacity. Recommendations are given for each case, for possible site implementation.