The review activity of modelling approaches is performed in
the framework of ENGINE WP3. Combined imaging methods are
essential for 3D modelling of permeable systems. The issues
from WP3 will be integrated in the deliverables of WP6 for
defining innovative concepts and proposing generic studies
that will both be published in the Best Practice Handbook
for Unconventional Geothermal Resources and Enhanced
Geothermal Systems. The focus of this presentation is to
introduce the state of art for individual approaches for the
evaluation of various geothermal reservoirs. 
Modelling of geothermal reservoirs is mostly focused on pure
data interpretation of an experiment and more importantly on
the understanding of dominant reservoir processes. The
complexity of mass transport in a reservoir becomes more
visible with larger number of geophysical and
hydrogeological measurements conducted in a geothermal
reservoir. Moreover, the non-linearity of hydraulic,
thermal, mechanical or chemical processes is often only
visible in especially designed experiments. From a
theoretical point of view, geothermal modelling could offer
the possibility of an overall system evaluation. In practice
however, only conceptual models are derived and allow a
quantitative description of individual aspects. Herein, the
bandwidth from low-enthalpy, porous media simulators up to
high enthalpy multi-phase flow models in fractured
environment will be covered. Generally, the modelling of
different physical behaviour is required for individual
reservoir types.
High enthalpy systems or high temperature aquifers are often
treated as porous and/or fractured medium. Darcy and
non-Darcian hydraulic regimes are used to accommodate the
high fluid velocities in the aquifer. Advection represents
the major coupling from hydraulics to thermal calculation.
Possible thermal effects influence density (buoyancy) and
viscosity. Often two phase liquid and steam systems are treated.
EGS (Enhanced Geothermal Systems) have the most complex
physical processes in an essentially fractured medium.
Again, Darcy and non-Darcian flow at high fluid velocities
in fractures are used. The same coupling mechanisms as in
high enthalpy systems apply, but additionally, mechanical
processes play an important role in the reservoir
development and assessment. Therefore, fracture mechanics
with shear / tensile fracturing processes need to be
considered. Also the matrix elasticity can become important
through poro- and thermoelastic effects due to high-pressure
injections of cold fluids. There are also examples of
biphasic flow or multicomponent transport. Recent advances
on rock-water interaction set first steps towards a
geochemical model that will be able to optimize the
composition of the circulation fluid and to enhance chemical
injection.
There are several reservoir simulators used. They can be
globally characterized by continuum or discrete fracture
network approaches. The classical "porous modelling" and
"dual porosity models" (or MINC) represent rather
continuum-type simulators, whereas "unique fracture model"
(often used in geochemical approaches) or complete
"stochastic discrete network approach" represent the
discrete-type approaches.
New trends can also be identified mostly on inverse
modelling approaches, automated calibration and on models
enhancements that concentrate on the simplification of the
physical processes by excluding second order features. In
the past, the complex methodologies were often only for
academic use to investigate principal mechanisms. In future,
modelling may focus on more practical applications: the new
developments will lead to the optimization of different
injection strategies and to the design of utilization
strategies.

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