It has been studied whether or not microcracks are present in cores subjected to overcoring, by conducting diametrical measurements of p-wave velocity and by mapping of microcracks on thin sections taken from the overcored cores. From the p-wave velocity measurements it was found that a majority of the cores were anisotropic, although only to a small extent. By characterizing the thin sections it was found that microcracks were present in the overcored core. A large amount of the microcracks had a dominant orientation that coincides with the result from the P-wave velocity measurements. This implies that anisotropy, caused by microcracks with a preferred orientation, can be detected by p-wave velocity measured diametrically. Furthermore, d was concluded that, from the used approach, it was not possible to determine the cause of the development of the microcracks that were present in the overcored core, and therefore, a new approach was suggested.
In a study conducted by the Swedish Nuclear Waste and Management Company (SKB), it was found that the stresses determined with Deep Doorstopper Gauge System (DDGS) were higher (magnitude of 1.4 - 2.0) than stresses determined with hydraulic fracturing and triaxial strain cell (Christiansson & Janson, 2002). It was observed that the measured strain was very high; a 50% reduction of the strain would be needed in order to reduce the stress magnitudes to gain results similar to those obtained with the other two methods. One possible factor, among others, was that microcracks might be present in the rock, thus allowing large strain and causing the rock to be anisotropic. To determine if microcracks were present in the overcored rock and if they influence the measurements performed with DDGS a study was initiated by SKB. The study employs diametrical measurements of p-wave velocity and a study of thin sections in an optical microscope. The objective of this study was to (1) determine if the cores are anisotropic, by performing diametrical measurements of p-wave velocity, and (2) determine if microcracks are present in the overcored Core. In this investigation diametrical measurements of the p-wave velocity were performed on the entire circumference at two locations along the axis of each overcored rock core. The anisotropy ratio at the locations was compared and if differences were observed it was interpreted as an implication that microcracks might be present in the rock. Thin sections were taken from four of the cores and studied in an optical microscope. Fracture orientation, fracture length and number of fractures were mapped and a simplified model of the crack distribution in the core was established.
In this study 13 rock cores received from the northern part of the Aspo HRL area has been used during the testing. Six cores were taken from a horizontal borehole (KF0093A01) and seven cores were taken from a vertical borehole (KA2599G01). The horizontal borehole is inclined 20 and the vertical is inclined 10° and both boreholes were located at a depth of 450 - 470 m.