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84 Geothermal Energy: Renewable Energy and the Environment

highly variable in their mineralogy and permeability over distance scales of meters–kilometers,
the environment that fluid experiences as it migrates along a flow path is not likely to be constant
or uniform. The fluid, as a result, will evolve in response to competing and changing chemical
reactions that occur along a flow path. It is thus likely that a fluid flowing through a geothermal
system will, if sampled and analyzed at different locations along a flow path, possess detectably
different compositions.
Conceptually, it is relatively straightforward to imagine the evolutionary history recorded in the
chemistry of an aqueous fluid migrating through a uniform sequence of rock. As noted in Chapter 4,
the hydrology of geothermal systems is strongly influenced by the local meteoric plumbing system.
If recharge of an aquifer occurs primarily in response to precipitation in a nearby highland region,
the hydraulic head that results drives the fluid flow. If our geothermal system is located within
a basin, the fluid flow into the basin interacts with the thermal regime generated by a local heat
source at depth and the fluid is heated. The result will be an increase in reaction kinetics that will
cause the migrating fluid to more aggressively interact with the surrounding and enclosing mineral-
ogy. In general, the concentration of dissolved constituents will increase along the flow path. If the
mineralogy along the flow path is known, the rate of fluid movement is known and estimates can be
made of the exposed surface area of the minerals, a reasonably good approximation can be devel-
oped of how the fluid composition and mineralogy would evolve along the flow path. Mathematical
formalisms useful for describing such a simple system have been developed and are relatively easy
to apply (e.g., Johnson et al. 1998; Steefel and Lasaga 1994; Steefel and Yabusaki 1996).
To achieve realism, however, our conceptual model must allow for mixing of fluids from a
variety of sources, as well as changes in the porosity and permeability of the geological frame-
work as the solution interacts with it, dissolving the original minerals in the rock and precipitating
new ones. Fluid mixing reflects the situation that the thermal regime that supplies the geothermal
energy increases the temperature of deep fluids, thus causing them to become more buoyant. As a
result, they rise from deep levels and interact with meteoric fluids at shallower levels, resulting in
a complex mixing regime that modifies both the chemistry and flow pathways of the system.
The consequences of fluid mixing on the energy content (enthalpy) of geothermal fluids can be
evaluated using the energy and mass balance equation we previously considered in Chapter 3. For
example, imagine that a meteoric fluid at 15°C mixes with a geothermal fluid rising from depth that
is at 250°C. Assuming that the meteoric water has an enthalpy of 70 J/gm and the geothermal fluid
has an enthalpy of 1085 J/gm, if the fluids mix in the ratio of 9 to 1 (meteoric to geothermal), the
mixed fluid would have an enthalpy of

(0.9 × 70 J/gm) + (0.1 × 1085 J/gm) = 171.5 J/gm.
v
l
Such a fluid would have a temperature of about 25°C. Hence, although not useful for power gen-
eration, such a thermal perturbation could be readily distinguished from the background ground-
water temperature of 15°C and thus indicates the presence of a thermal anomaly, or heat source at
depth. For nonpower applications (see Chapters 10 and 11) such a temperature may, in fact, be quite
suitable.
Recent developments in reactive transport modeling allow rigorous description of the evolution
of such systems over time. In the following chapters we will describe how such simulators can be
useful for detecting and characterizing flow and mixing in geothermal systems, and how such capa-
bilities also allow better management of reservoirs.

sImUlaTInG reacTIVe TransporT

Rigorous representation of the time-dependent evolution of complex geological systems requires
the ability to account for changes in the physical geological framework as chemical processes
unfold in a flowing system that is experiencing changes in temperature and pressure. Within the