Page 140 - Geothermal Energy Renewable Energy and The Environment

126 Geothermal Energy: Renewable Energy and the Environment

feldspar, quartz and calcite, and potassium feldspar and calcite. (Note that the sensitivity of
these calculations to the assumed temperature is not large—if the temperature were increased or
decreased by 50°C the results would shift by less than 5%.) These binary combinations are approx-
imations of rock types indicated in the figure. The bulk of the volume of real rocks is generally
made up of three–five minerals, but for the types of rocks usually encountered in geothermal sys-
tems, the binary combinations provide an indication of the variability to be expected among the
main rock types.
There are several key points that are evident from Figure 7.3. First, the difference in heat capac-
ity between the end-member compositions is 10–15% of the total heat capacity of the rock. This
defines the magnitude of uncertainty that can be associated with an assessment of a geothermal
reservoir if the mineralogy of the reservoir is poorly known and can only be estimated. Second,
most rock types tend to occupy a relatively small range of heat capacity values, reflecting the
minerals that dominate the rock volume. Limestone and marble, which are usually more than 90
vol.% calcite or other carbonate minerals, in particular, have relatively high heat capacities. Finally,
sedimentary rocks composed of complex mixtures of minerals, which are often derived from a
variety of different rock types, have a broad range of possible heat capacities, but will tend to over-
lap the same values as those observed in granitic rocks. This reflects the simple fact that granitic
rocks are dominantly composed of various proportions of feldspars and quartz, which are also the
erosional remnants of many other rock types. This reflects the resistance to abrasion these particu-
lar minerals possess.
For highly porous or fractured rocks, the approach outlined above using Equation 7.3 will tend to
overestimate the heat capacity of the reservoir. For this reason it is important that both porosity and
permeability be characterized in a reservoir, as well as the mineralogy of the host rock.

eFFIcIency oF heaT exTracTIon

The value of Q computed using Equation 7.1 is the total amount of heat contained in the reservoir.
R
This value represents the maximum amount of heat that can be extracted. Realistically, however,
only a fraction of this heat is obtainable. There are several factors that contribute to this situation.
These factors include the ability of wells to access and extract heat and the flow properties of fluids
in the reservoir.
As discussed in detail in Chapter 9, the amount of heat that can be used to generate power can
be represented as
Q = m × (H WH − H ), (7.4)
0
WH
where Q is the extractable heat at the wellhead, m is the mass flow from the wellhead to the tur-
WH
bine, and H WH and H are the fluid enthalpy at the wellhead and at a reference end state temperature,
0
respectively. From this relationship, the recovery factor R is computed as
R
R = Q WH /Q . (7.5)
R
R
Values for R are important in determining the likely energy that can be obtained from a
R
reservoir. Experience has shown, however, that the recovery factor is difficult to predict. In res-
ervoirs where the permeability is exclusively controlled by fractures, recovery factors are likely
to be low, within the range of 0.05–0.2 (Lovekin 2004; Williams 2007; Williams et al. 2008a).
In homogeneous porous media recovery factors can be much higher, potentially achieving values
as high as 0.5 (Garg and Pritchett 1990; Nathenson 1975; Sanyal and Butler 2005; Williams et al.