The purpose of this study in the field of Hot Dry Rock geothermics is to assess the effect of circulation of a fluid colder than the rock on the extension of fractures acting as a heat exchanger. The approach consisted of reproducing site conditions as closely as possible in laboratory test samples. After determination of the parameters which caused thermomechanical failure of the rock in the laboratory. It is shown how experimental results can be transposed to the site. This involves identifying the fields of thermomechanical stress at the lip of the fracture after calculation of the stress intensity factor K, in non-steady state In the laboralory and at the site.

1.1 General framework of the study

The framework of the study Is the fracturing of rock substratum caused by thermal stresses. i.e. during cooling or heating of the rock. Little is known about this Question of heat fracturing of rock today although It Is encountered in important fields such as:

  • oil prospection;

  • storage of radioactive wastes:

  • deep geothermics.

1.2 Example of deep geothermics

Prevailing circumstances led to Investigation of deep in Hot Dry Rock. In HDR (IOO·C- 200·Cat 2000–3000m), heat is recovered by making water run between two or more boreholes in a fracture network which has been created or re-opened generally by hydraulic fracturing. These fractures act as a heat exchanger.

Circulation in this exchanger of water that is colder than the rock can favour the appearance of fresh cracks On rock faces or the extension of existing cracks. The stability of fracture faces has been examined in previous Publ1cations [3,4,6.8). Scope is limited here to the effect of thermal stresses on the stability of the tip of one single fracture which is already subjected to hydraulic and mechanical stresses.

1.3 Approach used

The method devised consisted in recreating in the laboratory conditions which were as close as possible to those encountered in situ. Local similitude was created on a scale of 1: I in a small domain around the fracture lip.

In a given rock, a fracture in a laboratory sample fulfills simulation conditions when the mechanical. hydraulic, thermal and even chemical conditions at the tip of the fracture are identical to in situ conditions. Theorical study of the extremity of the fracture lies within the scope of failure mechanics. The field of stresses at the fracture tip is characterised by factors K referred to as the stress Intensity factors. in failure mechanics, 3 modes of opening of the fracture are considered. In the present case, modes 1/ and 1/1 (shear perpendicular and parallel to the extremity of the fracture) were negligible. Only mode I associated with the enlarging of the crack and to which corresponds stress intensity factor KI is considered Identical thermomechanical stressfields at all times t at the fracture tip make stress intensity factors K,(t) equal in situ and in the laboratory fissured sample.

Study of the problem can be represented by the organization chart in Figure 1.

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