Objective
TO DETERMINE IN THE LABORATORY THE EQUIVALENCE OF COOLING AND HYDRAULIC PRESSURE ON THE FRACTURING OF DIFFERENT ROCK TYPES, AND TO RELATE THESE OBSERVATIONS TO FIELD CONDITIONS.
The purpose of this study in the field of hot dry rock (HDR) geothermics is to assess the effects of circulation of a fluid colder than the rock on the extension of fractures acting as heat exchanger.
The framework of the study is the fracturing of rock substratum caused by thermal stresses (ie, 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 prospectaion, storage of radioactive wastes and deep geothermics.
The devised method consists in recreating in the laboratory conditions which are as close as possible to those encountered in situ. Local similtude is created on a scale of 1:1 in a small domain around the fracture tip.
In a given rock, a fracture in a laboratory sample fulfils simulation conditions when the mechanical, hydraulic, thermal and even chemical conditions at the tip of the fracture are identical to in situ conditions.
Theoretical study of the extremity of the fracture lies within the scope of failure mechanics. The field of stresses at the fracture tip is characterized 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 II and III (shear perpendicular and parallel to the extremity of the fracture) are negligible. Only mode I associated with the enlarging of the crack and to which corresponds stress intensity factor K1 is considered. Identical thermomechanical stress fields at all times t at the fracture tip make stress intensity factors K1(t) equal in situ and in the laboratory fissured sample.
Laboratory tests show the effect of pore pressure on rock thermal fracturing. A rigorous transposition of laboratory results to the site is thus dependent on the development of failure mechanics in biphasic media, coupled with temperature.
SPECIMENS OF GRANITE AND LIMESTONE HAVE BEEN TESTED IN A LABORATORY CELL CAPABLE OF APPLYING A TRIAXIAL STRESS FIELD. THE SPECIMENS WERE DRILLED THROUGH, WITH SYMMETRICAL SLOTS CUT IN THE WALLS OF THE HOLE TO SIMULATE FRACTURES. END LOADING, INDEPENDENT OF THE CONFINING STRESS, COULD BE APPLIED TO THE SPECIMENS; THE FLUID WITHIN THE HOLE COULD BE PRESSURISED AND/OR COOLED SEPARATELY TO INITIATE FRACTURE. OBSERVED BEHAVIOUR WAS CORRELATED WITH A THEORETICAL DESCRIPTION OF THE FRACTURE PROCESS IN TERMS OF THE STRESS INTENSITY FACTOR AT THE CRACK TIP. DIFFERENCES IN BEHAVIOUR WERE OBSERVED BETWEEN DRY AND SATURATED SPECIMENS; AN IMPORTANT OBSERVATION WAS THE SPEED WITH WHICH THE PROPERTIES OF INITIALLY DRY SPECIMENS CONVERGED WITH THOSE OF WET SPECIMENS WHEN WATER WAS ADMITTED. THE IMPLICATION OF THIS IS THAT THE BEHAVIOUR OF ALL ROCKS IN THE FIELD, EVEN THOSE NORMALLY REGARDED AS IMPERMEABLE, SHOULD BE ANALYSED IN TERMS OF A FULLY SATURATED PORE PRESSURE.
Fields of science (EuroSciVoc)
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
- engineering and technology other engineering and technologies nuclear engineering nuclear waste management
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Coordinator
34095 Montpellier
France
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