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PERFORMANCE OF GEOTHERMAL RESERVOIRS 285
as Class II disposal wells by the US Environmental Protection Agency. Class II
disposal wells inject produced water from oil and gas operation into subsurface
sedimentary formations.
Wastewater disposal has been associated with earthquakes and is an example of
IIS. IIS is earthquake activity resulting from human activity. A significant increase in
low magnitude earthquake activity has been observed in regions where hydraulic
fracture operations in shale have been conducted. Microseismic events are associated
with hydraulic fractures. Larger magnitude earthquakes on the order of magnitude
3 appear to be correlated to wastewater injection into disposal wells at high flow rates
(Hornbach et al., 2015).
Gas emissions from shale gas production wells have been reported (King, 2012;
Jacobs, 2014). Gas leaks are associated with faulty well casing and poor cement jobs.
Cement may not uniformly fill the annulus of wells, especially directionally drilled
or horizontal wells. The result can be a flow path from the formation to shallower
formations, such as water‐bearing formations.

14.5 PERFORMANCE OF gEOTHERMAL RESERVOIRS

Exploration, drilling, and reservoir technology developed in the oil and gas industry
can be applied to the discovery, development, and management of geothermal
reservoirs. In this section we use reservoir management principles to understand the
relationship between a geothermal reservoir and a geothermal power plant on the Big
Island of Hawaii.
Geothermal reservoirs in the crust of the Earth are typically heated by magma
close to the surface. For example, the Hawaiian Islands were formed by the movement
of the Pacific plate over a fixed hotspot in the mantle. The hotspot is at the interface
between magma convection cells in the mantle. The Big Island of Hawaii is the
closest Hawaiian island to the hotspot and is the youngest of the Hawaiian Islands.
Kauai is farthest from the hotspot and is the oldest Hawaiian island. Fractures in the
crust carry magma from the hotspot to the surface and are sources of heat for water‐
bearing formations.
The Puna Geothermal Venture (PGV) uses geothermal power plants to extract
heat from geothermal reservoirs. PGV is located in the Puna district of the Big Island
(Figure 14.12). The town of Puna is southeast of the city of Hilo and east of the active
crater of the Kilauea Volcano. The dashed lines in Figure 14.12 bound the Kilauea
rim. Lava flows from Kilauea tend to move downstructure from the Kilauea crater
toward the coast along the Kilauea rim. PGV power plants are located at the easterly
base of the Kilauea rim.
PGV began commercial operation in 1993. Electrical power from the PGV power
plants is sold to Hawaii Electric Light Company. Two types of air‐cooled power
plants with a combined generating capacity of 38 MW are installed at PGV: a
combined cycle system and a binary system.
Figure 14.13 illustrates key components of one of the power plants: an air‐cooled
binary geothermal power plant. Binary refers to the use of two fluids and air‐cooled