Résumés
Résumé
Un site expérimental représentatif du milieu calcaire fissuré a été étudié dans le Jurassique supérieur de la vallée de la Loue. Un forage carotté de trente mètres a été réalisé et étudié par différentes techniques de laboratoire (porosité, perméabilité, vitesse ultrasonique, quantification de la fissuration) et de terrain (essais hydrauliques par injection entre obturateurs, diagraphies soniques, densité). Le massif rocheux concerné a été exploré en surface à partir de la sismique réfraction.
Les principaux résultats obtenus permettent de relier les paramètres hydrauliques à la fissuration du massif calcaire. On constate une mauvaise relation entre les mesures de laboratoire et de terrain par suite d'un effet d'échelle, les investigations étant faites sur des volumes variant de quelques centimètres cubes à plusieurs mètres cubes. En effet, la matrice homogène et isotrope est perturbée par des fissures (fermées ou ouvertes). Le milieu continu représenté par la matrice et observé au laboratoire s'avère discontinu sur le terrain. L'indice le obtenu à partir de la diagraphie sonique paraît intéressant, mais doit être utilisé avec prudence et après correction dans les zones à argilosité reconnue, en effet dans ces zones on constate que pour des valeurs supérieures à 10 de l'indice lc brut, aucun écoulement n'a été mis en évidence par essai hydraulique. Un abaque de correction C (lc) a donc été établi qui tient compte de la teneur en argile et qui permet de proposer un indice corrige le -C (lc) plus fiable.
Mots-clés:
- Calcaire,
- fissure,
- sismique réfraction,
- gamma-ray,
- sonique indice lc,
- perméabilité,
- porosité
Abstract
An experimental site in fissured calcareous media has been analysed in the upper jurassic of the Loue valley. A tore of some 30 metres has been drilled and analysed by different laboratory techniques (porosity, permeahility, ultrasonic speed, quantification ol the fissuration) and in the field (hydraulic injection tests between shutters, sonic and density diagraphics data). The rock volumes have been explored at the surface by the refraction sesmic method.
ln the tore the fissures are distributed according to four families :
- subvertical stylolitic fractures, the horizontal pealo; of which denote the maximum compressive direction of the original tectonic stresses,
- mineralized crevices with calcitic filling, closed fissures,
- opened fractures which induced the hydraulical quality of limestones.
The total porosity of the limestones is characterized by homogeneous values (front 2 to 6 %) and three differents familles of porosity o and the average diameter dm (0.00310 0.27µm) of the accessible apertures in the matrix :
- family 1 : 0.003 µm < dm < 0.018 µm, 1.05 % < ø < 2.14 % in relation with intergranular spacing in the cementic phase. The clay in the basal part of the tore indures a great reduction of this kind of porosity,
- family 2 : 0.044 µm < dm < 0.11 µm, 0.49 % < ø < 4.39 % in liaison with intergranular spacing between the carbonatic figurative elements (biociasts, Iithoclasts, oolithes and microorganisms),
- family 3 : 0.14 µm < dm < 0.27 µm, 0.28 % < ø < 3.13 % in relation with the increasing of the sizes of the figurative elements.
A good agreement between permeability k and porosity ø is observed (k=3.4 10-5 - ø4-96 in darcy units).
Three diagraphic investigations has been carried out with gamma-gamma measurements (bulk density), with gamma-ray measurements (clay content) and with acoustic compensated measurements (sonic waves speed). The bulk density log is uniform (2.5 to 2.7 g/cm3), the gamma-ray log is increasing down the hole (21 to 29 m) whereas the clay content rises.
The acoustic log records the total wave signal chat can be traduced by two computed logs :
- speed propagation of the S waves (Vs speed) and P waves (Vp speed) with ihe ratio Vp/Vs,
- Poisson coefficients and Young modulus.
The principal results suggest the existence of a liaison between hydraulic parameters and the fissuration of the calcareous formation where three levels of fracturation are observed (from 11 to 12 m, from 16 to 18 m and from 19 to 20m).
The permeability values of the limestone matrix cores ara relatively homogeneous (1 to 19 10-5 darcy) with a decrease in the 23 to 27 m section of the bore-hole where a clayed cernent is detected. The hydraulic conductivity of the fractured zone, as determined by means of hydraulic injection tests between shutters leads to values of 1.10-6 to 8.10-6 m/s (# 0.1 to 0.8 darcy). Five zones have been choses for the hydraulic tests in liaison with different parts of the bore hole. The high values ot permeability k are in accordance with the high le index values of the open fractured zones.
A tirst local calibration function is proposed :
lc=7.13 Log k + 51
A pour relation connects the laboratory and field results because the elementary volume is increasing from cubic centimeters to some cubic matera. The homogeneity and the isotropy of matrix is disturbed hy the fissures (shut or open).
The matrix medium is continuous on the laboratory scate and discontinuous in the field. The ultra-sonic investigation confirms the liaison between the high values of the sonic P waves speed and the principal regional direction ot tectonical stress. The total acoustic recording yields the knowledge of the S and P waves speeds of in correlation with subvertical fractures when the speed value variations are numerus.
Three fracture zones are distinguished between 11 to 12 m, 17 to 18 m and 19 to 20 m. The lc index obtained from the sonic log must be used cautiously aller correction in the true clayed zones. In these zones the values of the le index are above 10 without waler flowing in the bore hale alter hydraulic testing. We propose the use of a corrective chart C (lc) to correct for the clay effect and a new corrected index lc - C (lc) is more reliable.
An adjusted statistical graphical chart is proposed with the following expression : C (lc) = ß • Xα with α= 1.84 and ß = 0.088 (valid only for limestones) and X : lite clay index obtained from the gamma-ray measurements. The le index values histogram localizes the permeable zones of limestones with hydreulically active open fractures.
Measurement of the speed variation of P sesmic waves on the bottom of the quarry by the means of the refraction method leads to a good agreement between the maximum speed anisotropy direction and the major fracture direction (N 120°-N 140°) which is perpendicular to the minimum anisotropy sesmic speed direction (N 40° -N 50°). The open fractures direction measured on the core is adjoining about N 135°-N 145° when the direction of maximum anisotropy in the quarry is near N 140°.
Keywords:
- Limestone,
- fissure,
- refraction sesmic,
- gamma-ray,
- sonic,
- permeability,
- porosity,
- le index