Impact of Amorphous Silica Scaling on the steam pipes utilization in the Olkaria Geothermal Power plant wells: A case of well 37b in the Olkaria East field production - Naivasha, Kenya

Abstract

Amorphous silica scaling in geothermal power plants has historically caused operational challenges such as obstructing fluid flow and reducing heat transfer efficiency, thus, limiting the overall power production. Similar problems are encountered in Kenya, and in particular Olkaria, therefore urgent attention is needed to mitigate this problem. This study therefore, aims at investigating sustainable ways of mitigating the effects caused by the amorphous silica scales deposition in the steam pipes in well 37B. The specific objectives were to determine the chemical composition of the brine, dry steam and steam condensate, perform chemical analysis on the scale deposits, evaluate parameters that influence scales formation in the steam pipes, and demonstrate the effect of scaling on plant power output. To achieve this, brine and steam samples were collected from the well using standard methods for sampling two phase geothermal fluids and were chemically analyzed using titrimetric, spectroscopy and chromatography. Scale deposits were also collected from the separator U-seal, silencer, and turbine rotor for identification and structural characterization using X-ray diffraction (XRD) technique. The speciation computer code WATCH was used to evaluate processes that could influence the fluid composition and predictions for scale forming minerals. X-ray diffraction analysis revealed a broad diffraction peak at 23 angstrom on the 2D-θ scale indicating that the scale was mainly amorphous silica. The temperature of the brine was found to control the scaling conditions, with pro grade solubility (solubility that decreases as temperature and pressure decreases and vice versa, such as for silicates) simulated from 350°C to 110°C. It was noted that amorphous silica deposition occurs in fluids characterized by a wide range of total silica concentrations (350-700 ppm), temperature below 100°C, pH (4.5-5.5), and total dissolved solids concentrations (30-50 ppm). Consequently, temperature and pressure decrease was a significant cause of scaling while pH decrease was the principal cause of scaling. It was noted that the deposition of total dissolved solids, silica and other solutes contained in the dry steam carryover on the turbine reduces number of revolutions per second of the blades, hence lowering efficiency of the turbine blades rotation and, as a result, reducing the power output of the plant. Furthermore, in order to increase the pH, there was a need of reinjecting the diluted mixture of the steam condensates of a pH< 2.5 and temperature >50°C from the power plants with the separated brine of a pH>8.0 and temperature >150°C into the wells, this was to increase the pH of the resultant mixture, and maintain temperature above the silica saturation temperature, thus, preventing deposition of the amorphous silica in the steam pipes. There was also a need to dispose of high temperature water (steam condensate) at temperatures above amorphous silica saturation (say above >100°C for Olkaria wells), n by reinjecting it back into the wells, thus preventing deposition of the scales in the steam pipes. The findings of this study revealed that pH decrease was the main cause of amorphous silica scales deposition while temperature and pressure decrease were responsible for scaling in the steam pipes.