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Thermodynamics and Geothermal Systems 49

1 meter in a cross-sectional area would exert a pressure of 810 bars (8.1e7 Pa). The pressure exerted by a column of
rock is called the lithostatic pressure, P L . Figure 3S.1 compares how P H and P L change with depth.
At shallow levels (<1000 meters) in the crust, fluid in pore spaces is primarily subject to hydrostatic pressure, and
therefore fluid pressure (P f ) is equal to P H . This fact results from the ability of rock to support an open network of pore
space that is not compressed by the overlying rock burden. The rock behaves as though it were a container in which
fluid is stored, and the pressure the fluid experiences is mainly a function of the overlying mass of water. However,
at increasing depth, the mass of the overlying rock burden becomes sufficient to exert a compressive stress on the
rock that is directed vertically. This stress leads to deformation of the supporting geological material, resulting in a
progressive reduction in the volume of pore space with depth.
Reduction in pore space volume exerts an increasingly significant pressure on the fluid in the pore space. As a
result, pressure exerted on water in pores progressively increases above P H until it becomes equivalent to the lithos-
tatic pressure. The depth interval over which this transition occurs is very dependent on the local geology and state
of stress in the crust. Generally, however, P f becomes equivalent to P L by the time depths of 10 km are reached.
It is important to realize that P f is a force acting on the enclosing rock framework, as well as being acted upon by
the rock. As a result, it must be viewed as part of the structural architecture of the rock system. If fluid is removed
from depth and not replenished, the P f that was contributing to support of the overlying rock is diminished. When
this happens, the rock will compress proportionately to the extent to which P f is reduced. The manifestation of this
effect is the phenomenon of land subsidence. Land subsidence can occur in regions where large volumes of fluid are
pumped from depth and not adequately replenished. This effect is discussed in detail in Chapter 12.