The exploitation of geo-energies often leads to difficulties that rock mechanics helps to overcome. These essentially consist of instability problems caused by changes in the state of stress, associated pore pressures, and temperature. Besides the classical near field problem of wellbore stability, the changes can impact a larger scale, triggering earthquakes and inducing seismicity. In this paper the current approaches envisaged to tackle these problems are briefly reviewed and discussed. Some extensions that have been made possible thanks to the combination of new numerical techniques will be outlined.

1. Introduction

The term geo-energy could be defined as the set of energy sources that the Earth produces. As in physical science where all is Energy or Matter, this would include fossil fuels such as coal, oil, gas or organic matter (wood) which is nowadays classified as unconventional geo-resources.

In fact, the neologism "geo-energy" is mostly devoted to the energy that has been created from the temperature of the Earth and which is used to heat or to produce electricity. This is the case for geothermal energy [1]; the exploitation of geothermal energy was considered a long time ago [2], but it has only taken off in the last twenty years. The main reason is the urgent need to limit emissions of greenhouse gases and the lure of a renewable resource [3].

Despite the fact that many other issues could be related to Geo-energies, such as energy storage by compressed air for instance, this paper will be limited to rock mechanics problems associated with Enhanced Geothermal Systems.

2. Enhanced Geothermal System (EGS)
2.1. Principle and brief history

The principle of a geothermal installation can be schematically represented by two boreholes. From the first borehole the injected cold water circulates into the rock mass and is heated up in contact with the hot rock. This hot water is then extracted in the second borehole and treated to remove energy. Later, this cooled water is re-injected in the first borehole. The effectiveness and the efficiency of the system depends on the existence or the formation of a sufficient exchange surface in the hot dry rock formation, between the two boreholes.

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