ABSTRACT: Model testings are conducted for the verification of the mechanism of hydraulic fracturing by the laboratory hydraulic fracturing apparatus. The initiation and propagation of hydraulic fractures and the influence of rift in granite on fracturing patterns are investigated. The A.E. activities begin at the 85–95% of maximum pressure. The longitudinal or transverse fractures develop according to the orientation of rift in granite and stress conditions. Tensile strength obtained from hydraulic fracturing and fracture toughness from 3 point bending test show variations of about 10% each according to the fracture direction to the rift. This shows that rift in granite has an effect on fracturing patterns under low deviatoric stress conditions.
RÉSUMÉ: Les essais par modèle ont ete realises afin de verifier le mecanisme de la fracturation hydraulique à I"aide d"un appareillage de fracturation hydraulique dans Ie laboratoire. L"initiation et la propagation des fractures hydrauliues et l"influence de la fente dans le granite sur le mode de fracturation ont ete examinees. Les activites de l"emission acoustique commencent sous une pression equivalente à 85–95% de la pression maximale. Les fractures longitudinales ou transversales se developpent suivant l"orientation de la fente et de l"etat de contraintes. La resistance à la traction obtenue par la fracturation hydraulique et la resistance à l"extension d"une fracture par l"essai de flexion à 3 points montrent une variation près de 10% de chaque valeur suivant la direction de la fracture par rapport à la fente. Cela signifie que la fente aun effet sur Ie mode de fracturation sous un etat de basses contraintes deviatoriques.
ZUSAMMENFASSUNG: Die Modellversuche zur Ermittlung des hydraulischen Bruchverhaltens wurden auf der Versuchsanlage des Labors durchfuehrt. Einleitung und Fortschreiten der hydraul erzeugten Bruche und der Einfluβ von Spalten auf das Brechungsmuster von Granit wurden untersucht. Bei 85–95% des Maximaldrucks beginnen bereib die vorstehend genannten Reaktionen. Die longitudinalen und transversalen Bruche entwickeln sich in Richtung der Spalten in Granit und den Beanspruchungsbedingungen. Die zugfestigkeit aus hydraul Druckversuchen und die Druckzahigkeit aus 3 Punkte-Biegetests zeigten Variationen von jeweils 10% in Abhangigkeit der Spaltrichtung. Daraus ergibt sich, daβ die Spaltung im Granit auf das Brechungsmuster bereits bei geringen Druckunterschieden einwirkt.
1 INTRODUCTION Hydraulic fracturing has been used to stimulate the gas hole and to extract heat from the hot dry rock, and recently is used for in-situ stress measurement. It has been studied theoretically by Hubbert and Willis (1957) and improved by Scheidegger (1962), Kehle (1964). Fairhurst (1983), etc. Haimson (1968) has examined the effect of fluid infiltration and pore pressure, and proposed the method of stress measurement in deep wells. Lockner and Byerlee (1977) have studied the behaviour of hydraulic fracture by considering factors such as differential stresses, porosity and permeability of rock and viscosity of pressurizing fluid. Also, many computer programs have been developed for modelling the mechanism of hydraulic fracturing. However, there are still many tasks to be examined closely.
In this research, the mechanism of hydraulic fracturing is examined through model tests. Also, the initiation and propagation of hydraulic fractures with synchronous A.E. activity and source location and the influence of rift in granite on fracturing patterns are investigated under various stress states.
2 ROCK SAMPLE AND TEST MODELS Pocheon granite, being produced in Korea, is used as the test sample. Dimensions of test models are 15x 15x15 cm, with a center hole of 10 mm diameter.