Stress measurements were undertaken in a significantly inclined borehole (denominated OL-KR40) at the Olkiluoto site, Finland, resulting in en echelon fracturing of the borehole wall. The failure to successfully stimulate pre-existing fractures using the HTPF-technique, which was the core in the testing strategy, rendered us to use the information from the en echelon features in order to derive the state of stress. This study indicates that the en echelon features do not support failure by tension, except possibly at shallow depth, as this would imply unrealistic stress gradient at the site. Instead, we demonstrate borehole wall failure by shear. By employing the Coulomb criterion, a probable stress distribution with depth is derived.
The hydraulic fracturing methodology has had a vast application worldwide since it was first introduced in the 1960-ies (e.g. Scheidegger 1962). The primary limitation with the methodology is the requirement that the investigated borehole should be aligned with a principal stress direction. If this is not satisfied within some 20°, en echelon fracturing appears, making the analysis more complex. Of this reason, the amount of hydraulic stress measurements in inclined boreholes is indeed sparse. The nuclear waste repository sites in Finland (this paper, Ask et al. 2010), Sweden (Ask et al. 2007), and France (Wileveau et al. 2007) are a few exceptions. The failure criterion for en echelon fractures has, similar to hydraulic fractures, generally been considered a result of tensile failure. Yet, several authors have expressed that this may not always be true (e.g. Paulding 1968, Lockner&Byerlee 1977, Solberg et al. 1977, Roegiers & Detournay 1988) and fracturing could be a result of shearing. In this paper, the results from the most recent hydraulic stress measurements at the Olkiluoto site, Finland (Fig. 1), are presented.