Page 126 - Geothermal Energy Systems Exploration, Development, and Utilization

102 2 Exploration Methods
from high velocity geothermal fluid passing through them. It is possible to estimate
pitting corrosion by a method of determining uniform corrosion rate by weight
loss measurement, in which a clean sample of test metal is measured, weighed,
exposed to a corroding attack for a known time interval, removed, cleaned, and
reweighed. Stress corrosion can be measured by using test coupons of different
types (Bridges and Hobbs, 1987). High salinity and high gas content imply specific
equipments and production conditions of the EGS power plant.
The formation of scale can present challenging operating problems for geother-
mal plants. The major species of scale in geothermal brine typically include calcium,
silica, and sulfide compounds. Calcium compounds frequently encountered are
calcium carbonate and calcium silicate. Metal silicate and metal sulfide scales are
often observed in higher temperature resources. Typical metals associated with
silicate and sulfide scales include zinc, iron, lead, magnesium, antimony, and
cadmium. Silica can present even more difficulties, as it will form an amorphous
silica scale that is not associated with other cations.
There are three potential situations favoring scale accumulation (Bowen, 1989).
First, deposition may take place from a single-phase fluid saturated with respect to
the relevant solids (reinjection pipeline). Secondly, it may occur from flashing fluids
(wells, separators, two-phase pipelines). Flashing is caused by drops in pressure
or cavitation in turbulent flow and probably produces calcite scale. It enhances
supersaturation by steam loss from the liquid phase, increasing concentration of
the residual solutes, by temperature diminution during expansion, and by loss
of stable gases such as carbon dioxide and hydrogen sulfide promoting increase
in pH. Thirdly, scaling can result from steam carryover (separators, turbines,
and steam pipelines). This can affect turbines badly where they are exposed only
to steam. Nucleation and depositional kinetics are a function of the degree of
supersaturation, pressure, temperature, and catalytic or inhibitory effects due to
minor elements.
Chemical thermodynamic methodology should be used to quantitatively assess
scaling tendencies from geothermal waters (Vetter and Kandarpa, 1987). Such an
assessment should be routinely carried out as a part of any geothermal development
program to identify optimum conditions for injection of waste geothermal fluids
and, at the same time, minimize the need for using inhibitors. The rate of
scale formation depends on temperature, the aqueous concentrations of the scale
forming components, the degree of supersaturation and kinetics.
Because of conductive or adiabatic cooling, geothermal fluids may become
supersaturated with respect to amorphous silica, which precipitates relatively
fast. Occurrence of amorphous silica precipitation hinders the use of quartz and
chalcedony geothermometers(Arnorsson, 2000). More importantly, the consequent
deposition of amorphous silica in surface installation and in reinjection circuits
is a major problem in the use and disposal of geothermal liquids for electrical
production. For this reason, it is very important to evaluate the temperature at which
saturation with respect to amorphous silica is attained. In the case of adiabatic
cooling (boiling spring or discharge from a well), a pH increase is expected due
to CO 2 loss. Boiling will cause an increase in silica and a decrease in enthalpy of