ABSTRACT:

At a granitic intrusion which has scarce joints in China, about 40 hydraulic fracturing tests were done In a borehole of 300m long. Through the experiments, several interesting facts concerning the effect of pressurization rate on breakdown pressure and other phenomena reflecting the crack behavior were observed to be discussed.

RESUME:

En utilisant des puits de forage de 300 m de long realises dans les batholites de granite intrusif faiblement fractures de Chine, nous avons effectue une 40 de tests d'ecrasement hydraulique. Les faits interessants observes à celte occasion en ce qui concerne les phenomènes influancant Ie comportement des fissures et les effets de I'augmentation de pression sur la pression de dèsintègration, font I'objet de cette communication.

ZUSAMMENFASSUNG:

An chinesischcm lntrusivgranit mit wenigen Rissen wurden etwa 50 Wasserdruckbruchversuche unter Verwendung eines Bohrlochs mit einer Lange von 300 m durchgefuehrt. Als Ergebnis wurden interessante Tatsachen in bezug auf dir Wirkung der Druckein-bringungsgeschwindigkeit gegenueber dem Zusammenbruchdruck, das Rißverhaltcn widerspiegelnde Erscheinungen usw, beobachtet, und diese werden berichtet und diskutiert.

1. Introduction

Hydraulic fracturing for rock stress measurement is a widely used technique because of its economical advantage and applicability to deep wells. The principle is that characteristic pressure points in hydraulic fracturing procedure such as breakdown pressure Pb, reopening pressure Pr, shut-in pressure Ps, etc. are related to rock stress. But the mechanism of initiation, closing and reopening of the induced crack is not fully understood. Hence there are several formulae of criteria to be used for rock stress calculation. Many theoretical and laboratory-based experimental works have been done to investigate into the behavior of the fracture and the appropriate criteria[Mizuta, Y et al 1980]. The authors have tried to Investigate into these points through in-Situ hydraulic fracturing experiments. Test site of rock with uniformity and scarce joints was selected at the outskirts of Beijing, China. The fracturing borehole of 300m long was drilled and about 40 fracturing were done. Three kinds of fracturing, hydrofracturing with fast and slow pressurization rate and sleeve fracturing, were performed. Besides the usual shut-in operation on the surface, downhole shut-in and slow vent test were occasionally tried. Through these extensive hydro fracturing experiments, interesting facts about pressurization rate effect on Pb and about other characteristic pressure points, were observed.

2. Test Site Description
2. 1 Geology and site description

The test site is located at a granitic intrusion in Fangshan county, 40km southwest from the center of Beijing. The age of the granite is about 130 million years. The intrusion is roundly exposed on the surface with 37km2 area (Fig. I). The site is marked by double circle. Five kinds of rock phases make concentric zones. Fracturing borehole of 300m long was drilled on one of the gentle hills in the intrusion. The distribution of joints and veins observed on the surface is shown in Fig.2. Two sets of joint system are observed, with one in N10E to N20E and the other in N60W to N80W direction. The geologic column is shown in Fig.3. The number of joints and veins that physically separate the core is only 37 below the weathered zone from the surface down to 30m. A compass was used to keep the orientation of the core.

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