Page 302 - Geothermal Energy Systems Exploration, Development, and Utilization

278 5 Geothermal Reservoir Simulation
The trigger mechanism to start the development of a preferential flow path was
numerical heterogeneities in the mesh describing the reservoir. Slight differences in
the size and orientation of the elements led eventually to the start of the preferential
flow paths, which due to a positive feedback mechanism then continued to develop.
The location of the preferential flow paths is not a coordinate description of the
location in reality, rather an indication that under certain coupling of processes,
such a development can be expected to occur. In nature, heterogeneities will
also significantly effect the development of potential preferential flow paths and
suggests the presence of ample trigger mechanisms.
Returning to the development of these preferential flow paths, there is a signif-
icant application in terms of the management of HDR geothermal systems. The
development of such flow paths is due to flow concentration caused by an increase
in hydraulic conductivity as a result of the build up of thermal or hydraulic stress.
As flow is concentrated in a certain area, the amount of cooling in these areas
is increased. This in turn leads to a further increase in the thermal stress, which
turns the system into a positive feedback of everincreasing hydraulic conductivity.
Significantly, the cooling of the fluids in these overcooled areas, and the consequent
increase in viscosity, results in a brake to the feedback. The increase in viscosity
of the fluids reduces the rate of fluid flow in the overcooled areas, and therefore
helps to divert flow to the warmer areas where the viscosity is still low. Such
considerations have a significant impact on understanding the operation of the
reservoir, and deriving effective heat extraction schemes. In such systems, the in-
clusion of a viscosity reducer to increase productivity may have short-term benefits,
but may be disastrous in the long term, encouraging the development of preferen-
tial flow paths. The use of lower viscosity fluids, such as supercritical CO 2 would be
particularly prone to this type of reservoir damage scenario (Pruess, 2008b; Pruess,
2008a). Reservoir damage as a consequence of the development of preferential flow
paths would prevent an efficient recovery of the heat energy present.

5.6.3
The Importance of Thermal Stress in the Rock Mass

One key issue resulting from the study by McDermott et al. (2006) was the
consideration and quantification of the effect of thermal stress in the rock mass.
The thermal stress induced in a bar due to thermal cooling can be approximated as
σ T = K r β T EI T

assuming no viscous flow in the rock, after (Nilsson, 2001). Applying this to the
rock mass environment, the coupling between the temperature change and the
thermal stress is influenced by the coefficient of restraint. In the rock mass this
can be understood as being related to the degree of fracturing in a rock, or the
typical size of volume of blocks of rock between fractures. Figure 5.23 illustrates
this concept. During cooling the thermal stress is assumed to work to open the
fractures, opposite to the tectonic normal stress keeping the fractures closed but
in alignment with the fluid pressure. For a typical quartz-rich rock, the elastic