TITLE:
Effect of Boiling and Cooling of Geothermal Fluids on Precipitation of Secondary Minerals: A Case Study of Olkaria Fields, Kenya
AUTHORS:
Emmanuel Onesimo Duku, Benson G. Ongarora, Paul Tanui
KEYWORDS:
Adiabatic Boiling, Aqueous Speciation, Clogging, Conductive Cooling, Depressurisation Boiling, Equilibrium Degassing, Phase Separation, Saturation Index
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.10 No.9,
September
30,
2022
ABSTRACT: The main drawback in the utilization of geothermal
resources arises from the precipitation of secondary minerals within wells,
pipelines, steam separators, turbines and other surface equipment in form of
scales. Scale formation is an outcome of the alteration of various rocks
dissolved in geothermal fluids that find their way into a reservoir. Once
geothermal fluids ascend to the surface, hydrostatic pressure decreases toward
a phase separation level that permits the dissolved gases such as CO2,
H2S and H2, and steam to separate from the liquid phase
by “boiling”. Stripping of these volatiles may increase fluid pH, leading to precipitation and deposition of
secondary minerals. The study sought to establish the relationship
between water-rock interaction and secondary mineral
precipitates at the surface and deep fluid at different temperatures during depressurisation boiling and cooling. Samples were collected from
selected Olkaria wells; OW-38A, OW-910 and OW-910A. The analysis of the results outlined deep fluid Alkali-Chloride waters
and surface steam-heated Alkali-Bicarbonate
and acidic Sulphate-Chloride waters. Various models suggested adiabatic
boiling, conductive cooling and possible mixing and dilution in the wells. Hydrothermal alteration minerals
were found to be in equilibrium with the geothermal fluids at varying
temperatures, and the secondary minerals controlled the chemistry of the
reservoir. Silica-saturated solutions precipitated silica in OW-910 and
OW-910A, which may have resulted from rapid cooling following mixing with cold
surface water.