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Geothermal energy, thermal energy stored within the Earth’s crust, can be used to
sustainably produce both heat and electricity. Here, a model of a geothermal doublet is used
to demonstrate how heat transport and subsurface flow processes can be coupled together
in a multiphysics model.


In geothermal energy production, one of                                                  FIGURE 1. Hydrothermal
the greatest challenges is minimizing the                                                doublet system in a layered
prospecting risk. How can you predict the                                                subsurface domain, ranged
number of years that a desired site will be                                              by a fault zone. The edge is
appropriate for extraction?                                                              about 500 meters long (left
	 Usually, very little information is                                                    drilling is the injection well,
available about the local subsurface                                                     the production well is on
properties and what is known often                                                       the right). Lateral distance
contains a large degree of uncertainty.                                                  between the wells is about
	 Over the past few decades, numerical                                                   120 meters.
models have become an important
tool for estimating prospecting risks.
Numerical simulation can be used to
perform parametric studies to predict
the performance of hydro-geothermal
systems with accuracy. Typical geothermal
applications require that coupled
subsurface flow and heat transport
phenomena be solved, necessitating the
use of a multiphysics software.

èSIMULATING GEOTHERMAL                       subsurface flow regime may be situated in   	 A COMSOL Multiphysics® software
SYSTEMS                                      fully or partially saturated porous media,  model of a geothermal doublet that
                                             or fracture flow may dominate, such as in   includes long time predictions for a
Simulations of hydro-geothermal systems      Karst aquifer systems.                      hydro-geothermal application can be
usually include heat transfer, fracture      	 Although the different techniques         easily set up. An example model, shown
flow, and pressure changes due to            for geothermal heat production differ       in Figure 1, contains three geologic layers
water injection and extraction. Heat         fundamentally, multiphysics simulation      with different thermal and hydraulic
transport in the subsurface proceeds by      can be used to model the subsurface         properties). In this example, a geothermal
convection and conduction. Convective        thermal development by coupling heat        tactic is explored that uses a separate
heat transport, either naturally driven by   transport with the velocity field.          injection and production zone. The
buoyancy forces or artificially established                                              geometry shows a section of a geothermal
through the wells, can play a role in—and    èMODEL OF A                                 production site that is ranged by a large
even dominate—the processes.                 HYDROTHERMAL                                fault zone, as can be seen in the figure.
	 When temperature gradients are             APPLICATION: A                              The layer elevations are interpolation
high, the temperature dependencies           GEOTHERMAL DOUBLET                          functions that are taken from an external
of material properties (hydraulic                                                        data set. In such a model, the temperature
conductivity, for example) are no longer     Hydrothermal heat extraction systems        distribution is generally uncertain, but
negligible and must also be included.        usually consist of one or more injection    assuming that there is a geothermal
Additionally, poroelastic processes may      and production wells. There are many        gradient of around 0.03 °C/m is usually
play a role in some cases, especially when   cases where separate boreholes are used,    accurate, and this has been used in the
it comes to fracking.                        but the modern approach is to create one    model. The concerned aquifer is fully
	 Depending on the local geology, the        (or more) multilateral well.

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