Abstract
The hydraulic fracturing technique (also termed mini-frac test) is commonly used to estimate the in situ stress field. We recently conducted a mini-frac stress measurement campaign in the newly-established Bedretto Underground Laboratory (BedrettoLab) in the Swiss Alps. Four vertical boreholes, dedicated for stress characterization of the granitic rock mass, hosted a total of 19 mini-frac test intervals. Systematic pressure transient analysis was performed to carefully estimate the magnitude of the least principal stress (S_mathrm {3}). We compared five different methods (inflection point, bilinear pressure decay rate, tangent, fracture compliance, and jacking pressure) to identify an adequate approach best suited for our test scale and the host rock mass. We found that the methods used to determine the fracture closure pressure underestimate the magnitude of S_mathrm {3}, presumably due to the rapid closure of the hydraulic fracture after shut-in. The most consistent results were found using the inflection point and bilinear pressure decay rate method, which both determine the (instantaneous) shut-in pressure as the proxy for the S_mathrm {3} magnitude. The determined shut-in pressure, or S_mathrm {3} magnitude, is 14.6pm 1.4 MPa from the inflection point method. This allowed us to further estimate the stress environment around the BedrettoLab, which is transitional between normal and strike-slip faulting. The measured local pore pressures from extended shut-in periods are between 2.0 and 5.6 MPa, significantly below hydrostatic. A combination of drainage, cooling, and the excavation damage zone of the tunnel may have significantly perturbed the in situ stress field in the vicinity of the BedrettoLab.
Highlights
Many Earth science and subsurface engineering applications require information about the in situ stress state
We characterized the in situ stress state at the BedrettoLab using mini-frac tests in four short vertical boreholes, focusing on stress magnitudes
The tangent method underestimates the stress magnitude and yields estimates that are below the frictional limit of the rock mass
Summary
Many Earth science and subsurface engineering applications require information about the in situ stress state. Hydraulic fracturing (HF) is widely applied as a direct stress measurement technique in boreholes (Haimson and Cornet 2003). It can be applied without prior knowledge of the elastic rock properties (Haimson and Fairhurst 1967, 1969), and samples a more representative rock volume than other methods Stress measurements are important for Underground Research Laboratories (URL), which require the rock volume to be well characterized, usually on a scale from 10 to 100 m, to provide a high level of experimental control for in situ experiments. Some examples are: an URL for nuclear waste storage in Canada (Martin 1990; Martin et al 1990), the Jurong caverns in Singapore (Synn et al 2015), the Korean Underground Research Tunnel for nuclear waste storage in South Korea (Jo et al. Vol.:(0123456789)
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