Efficient and sustainable production of geothermal energy requires constant
monitoring of changes occurring in the reservoir. These changes, which may
result from mass extraction for production, waste fluid injection for disposal and
pressure support, and from natural geologic processes, are usually manifested
in the chemistry and physical characteristics of the wells. Experience has also
shown that these changes are related to the structure of the reservoir—the
faults that transect the field as well as smaller fractures contained in the
reservoir rocks. Identification and evaluation of chemical changes, and their
correlation with the structural features, require among others the constant
analysis of hot brine and gases discharged by the wells.
Changes in water and gas chemistry, for example, can indicate: 1) lowering in
the water level of the reservoir, 2) invasion of cold and degassed re-injection
fluids, 3) entry of shallow acidic steam condensates and deep corrosive volcanic-
related fluids, 4) precursor of an earthquake, etc. Any of these changes can
significantly alter the short- and long-term viability of the geothermal
operation. Hence, it is critical that up-to-date collection and analysis of water
and gases be undertaken. However, since almost all of the production wells
are connected to the power plant, it is rarely possible to disconnect the wells in
order to collect samples for analysis because such disconnection will result to
shortfall in power generation. In addition, the process is time-consuming and
significant lapse is achieved from sample collection to the availability of the
information. There is therefore a pressing need for a continuous and on-line
system of measuring chemical parameters in the field.
A progress in this field is presented in this paper with the development of a
new versatile measuring system called MEDAS (MEDAS – Modular Environmental
Data Acquisition System) based on experiences and recent results from
different research groups. MEDAS is an innovative multi-parameter station,
which can continuously record as a function of time up to more than 100
geochemical and physical parameters suitable to earthquake research and
other applications. A microcomputer system inside the MEDAS handles data
exchange, data management and control and it is connected to a modular
sensor system. The number of sensors and modules can be selected according
to the needs at the measuring sites.
A MEDAS has been installed in four production wells in the Mahanagdong
production sector of the Leyte Geothermal Production Field located in the
island of Leyte, central Philippines. This field is chosen because: 1) it is the
largest geothermal field in the Philippines with five separate power plants with
total installed capacity of about 700 MW, and 2) the area is bisected by the
Philippine Fault, a major left-lateral transcurrent fault similar to the San Andreas
fault.
The preliminary results of the three years measurement on radon and CO2
concentrations; gas flow, temperature and humidity; water temperature and
pH; Cl-concentration the Redox potential and conductivity will be presented.
These parameters will be correlated with the historical and current data from
the PNOC EDC established monitoring set-up for seismicity, micro-gravity and
precise levelling surveys, wellhead pressure trends and wellbore chemistry
changes from monthly production sampling.
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