AN ASSESSMENT OF THE GEOTHERMAL ... - Orkustofnun

Akbar 22 Report 5
• Oxygen isotopes;
• Carbon dioxide and methane isotope geothermometer;
• Methane-hydrogen gas isotope geothermometer;
• Water-hydrogen gas isotopic geothermometer;
• Sulphate-water oxygen isotope geothermometer;
• Sulphate-hydrogen sulphide geothermometer.
5.3 The log (Q/K) diagram
The Saturation Index (SI) is the degree of saturation of minerals in aqueous solutions. This method
involves a comparison between ionic activity products and the thermodynamic activity product. An
expression of the saturation index (SI) is:
= log ( )
where Q is the calculated ion activity product; and
K is the equilibrium constant.
Saturation Index:
• If SI is negative: Undersaturated, no potential to scale;
• If SI is positive: Supersaturated, scale form precipitation may occur;
• If SI is zero: Equilibrium, borderline scale potential.
The values for Q and K can be calculated using the WATCH program on the basis of the calculated
chemical speciation in natural waters (Arnórsson and Bjarnason, 1993). For a mineral in equilibrium
with the aqueous solution, the temperature corresponding to the intersection point of the mineral
equilibrium curve and the SI=0 line gives the theoretical equilibrium temperature. The characteristics
of convergence of log(Q/K) curves for the mineral assemblage to zero at the temperature of
equilibrium can be used to confirm the most probable temperature at which the mineral used as a
chemical geothermometer reaches or approaches the equilibrium line, SI=0 (Reed and Spycher, 1984).
5.4 Scaling and corrosion
Mineral scale formation or scaling is a very common phenomenon observed in almost all geothermal
energy exploration fields. All geothermal fluids contain dissolved minerals. These minerals are
deposited at different points of the operation, adversely affecting the operation by restricting fluid
flow. The formation of scales has always been a difficult challenge for the geothermal industry.
Changes in water temperature, pressure, pH and mineral saturation are unavoidable when fluid is
tapped from geothermal reservoirs by production wells drilled into a reservoir. As a consequence,
minerals may deposit in producing aquifers, within the wells, in pipelines, steam separators and other
surface equipment and in injection wells. The most common scales consist of calcium carbonate and
amorphous silica but scales of various oxides, silicates and sulphides are also known.
5.4.1 Calcite scaling
Calcite scaling largely occurs in response to cooling and degassing of geothermal water, as it boils by
depressurization. Calcite solubility is retrograde, i.e. it decreases with increasing temperature.
Degassing of water due to boiling leads to a sharp rise in the pH of water which, in turn, increases the
ionic activity of carbonate ions (CO 3 2- ) becoming over-saturated:
= +