Mak-Ban Geothermal Field, Philippines: 30 Years of Commercial Operation
Vilma T. Capuno, Roman B. Sta. Maria and Emy B. Minguez
14th Floor, 6750 Building, Ayala Avenue, Makati City, Philippines, 1226
Abstract
The
Makiling-Banahaw (Mak-Ban, also known as Bulalo) geothermal field
was the second geothermal resource (after the Tiwi Field) developed
by Chevron Geothermal Philippine Holdings, Inc. (CGPHI) under a
service contract executed in 1971 with the National Power
Corporation. The deep discovery well, Bul-1, was drilled in 1974.
Commercial production began in 1979 with the operation of the first
two 55MWe generating units. Capacity was increased to 220MWe in
1980 and then to 330MWe in 1984, with the six 55MWe units installed
in three power plants. Six binary bottoming-cycle plants totaling
15.73MWe were installed in early 1994 followed by two 20MWe steam
turbine units in 1995 and a further two 20MWe units in 1996,
bringing installed capacity to 425.73MWe. In 2004-05, four units
were rehabilitated bringing the current fieldwide installed capacity
to 458.53MWe. However, the present plan is to base-load Mak-Ban at
402MWe with 40MWe on stand-by, plus the binary plants.
To
date, 113 wells have been drilled to measured depths ranging from
655m to 3,625m to support the production and injection capacity
requirements. A total of 69 production wells presently provide
steam to eight separation stations, while 23 injection wells provide
the injection capacity for separated brine and power plant
condensate re-injection. Generation in recent years has been
affected by steam supply limitations, power plant availability and
the introduction of a competitive wholesale electricity spot market
(WESM) resulting in decreased generation and operational concerns
when the units were used as load following rather than base load
plants, but this situation has improved in 2008.
Mak-Ban
is a remarkable example of a stable, relatively problem-free
geothermal reservoir, based on its performance during these 30
years. During the first few years, reservoir pressures drew down
rapidly resulting in the expansion of the two-phase zone with
increased discharge enthalpy and steam flash as the main effect on
production. More recently, however, the reservoir has been affected
by marginal recharge, injectate and meteoric water. The main effect
of these fluids is seen in a decrease in average steam flash which
was greater than 50% up to 2001 and is about 43% in 2008. Technical
and operational measures have been taken to mitigate the effects of
these various fluids on the resource and on generation.
In
the last 30 years, the Mak-Ban field operation has generated a total
of 61,827 GWh of electricity saving the government from importing
114.67 million barrels of oil equivalent (MBOE). This high level of
generation has been supported by an area of only ~7 km2, for a
production density level of about ~57MWe/km2, with only modest
declines. This is a very high density when compared to geothermal
fields worldwide. The recent drilling program has accessed hotter,
high pressure reserves deeper into the reservoir and the results are
very encouraging. CGPHI therefore looks forward to many years of
continued operation of this excellent geothermal resource.
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