ABSTRACT:

We analyze the displacement, fluid pressure and injection flowrate data measured during hydrofracturing (HF) tests conducted in intact foliated metamorphic rock at 1490.5 m depth in the Sanford Underground Research Facility (SURF, USA), and at 485 m depth in the central Scandinavian Caledonides (Sweden). Through the comparison with other methods also implemented on these sites, we find that one SIMFIP test may be "sufficient" to estimate the full stress tensor's orientation and magnitude. We also observe that when a natural discontinuity activated, its properties must be considered in the analyses otherwise stress magnitudes may be significantly over-estimated.

1. Introduction

Hydro-fracturing (HF and HTPF) stress measurements in fractured or anisotropic rocks are notoriously difficult, because opening of existing geological features tends to prevent the creation of a pure hydraulic fracture perpendicular to the least compressive principal stress. The classical HTPF method utilizes a combination of datasets obtained at multiple depths and relies on the homogeneity and linear increase of the in-situ stress with depth (Cornet and Valette, 1984), a condition unlikely to be met in fractured and faulted rocks.

A number of key factors create great uncertainties in the estimated stress field. The real fracture opening time is impossible to estimate from pressure data only (Rutqvist et al., 2000). Post-test borehole inspections often are inconclusive since no trace of reactivation is left. The complex stress state related to non-vertical maximum principal stress (e.g. a thrust fault regime) may induce complex HF behavior (Evans et al., 1988). The heterogeneity of hydromechanical and frictional properties of pre-existing heterogeneities intersecting the borehole may impact the HF propagation, and a post analysis of the HF trace might lead to inaccurate conclusions.

All these factors can introduce ambiguities in interpreting the pressure curves into a stress regime (Warpinski and Teufel, 1991). This can therefore result in large uncertainties in the estimation of stress in or near fracture zones.

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