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Most large-scale geothermal developments in New
Zealand extract water and heat at a greater rate
than the natural recharge to the system , and
are therefore "heat mining". However,
where pressures have been reduced significantly
by exploitation, as at Wairakei, in some cases
the rate of replenishment from depth has increased
several-fold to match the discharge rate.  This,
combined with the ability to recycle fluids through
reinjection, means that it can be difficult to
define what is meant by renewability and sustainable
management of a geothermal resource.
Although some geothermal resources have been
over-exploited and had to reduce their generating
capacity, as at Ohaaki, no geothermal field has
ever been run to exhaustion. In practice, all
resource consents to take and reinject are framed
in terms of fluid mass withdrawal, and in some
cases there are limits on how much energy can
be extracted. The fact that Wairakei has been
operating for 50 years, resource consents for
a further 25 years at the same generation capacity
have been given, and reservoir modelling indicates
that the resource will remain viable for at least
another 25 years beyond that all demonstrate the
longevity of geothermal resources.
See Also Other Environmental
Aspects
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