Hydraulic fracturing and hydraulic tests on preexisting fractures in a foliated rockA comparison of results and techniques Available to Purchase

INTRODUCTION The large need and interest for rock stress measurements has led to development of new techniques of measuring and evaluating the stress state, but also to improvements, technically and theoretically, of presently existing methods. One of the most recently developed methods to determine the in-situ stresses is the HTPF method (hydraulic tests on preexisting fractures), Cornet and Valette (1984). During stress measurements near Gide? this method was tested for the first time in Scandinavia, Ljunggren and Raillard (1987). These tests showed promising results and more comprehensive field experiments were therefore planned. During the winter 1988/89 hydraulic fracturing and the HTPF method were used to measure the horizontal stresses in a vertical, 508 m deep core drilled borehole in Luleå. The primary objectives of this study were; (i) to compare the methods with each other, both with regard to field work and obtained information on the far field stress situation, (ii) to obtain reliable information on the stress magnitudes and stress orientations in the area. The paper summarizes the results and presents the main conclusions that can be drawn from this study. LOCATION AND GEOLOGY The tests were conducted in a borehole drilled at the campus of the University in Luleå. The area is situated in the southeast part of the Norrbotten county at the coast of Gulf of Bothnia. The topography in the area is generally flat. The borehole was core drilled to the bottom at 508 m depth with a diameter of 56 mm. The borehole penetrate soil to a depth of 6.5 m and the solid bedrock was encountered at 8 m. The principal rock type in the borehole is migmatized gneiss with a steeply dipping foliation striking N-S, Röshoff and Norin (1982), Ludvig (1981).