Estimating the Least Principal Stress in a Granitic Rock Mass: Systematic Mini-Frac Tests and Elaborated Pressure Transient Analysis

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 ($$S_\mathrm {3}$$ S 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 $$S_\mathrm {3}$$ S 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 $$S_\mathrm {3}$$ S 3 magnitude. The determined shut-in pressure, or $$S_\mathrm {3}$$ S 3 magnitude, is $$14.6\pm 1.4$$ 14.6 ± 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.

Period‐ and Pressure‐Dependent Hydraulic Properties of a Fractured Granite Inferred From Periodic Pumping Tests

“…Using acoustic televiewer logs (Figures 2b–2d), we chose the three adjacent sections containing pre‐existing natural fractures intersecting the SB 1.1 borehole. Besides the natural fractures, no stress‐induced borehole damage, such as borehole breakouts or drilling induced tensile fractures were expected (Bröker & Ma, 2022) or observed in the selected intervals. The final lengths of the isolated intervals between packers were 0.44, 1.94, and 1.01 m for Intervals 2, 3, and 4, respectively (Figure 2a).…”

“…We performed 175 PHT experiments in Interval 3 over a period of 5 months, during which the testing system remained at a fixed position. During the campaign, we considered (a) two different injection protocols, namely harmonic or sinusoidal‐wave‐type injection (Figure 3a) and non‐harmonic or square‐wave‐type production (Figure 3b) of fluid; (b) oscillation periods T from 7.5 to 1,800 s, that is, covering around 3 orders‐of‐magnitude; (c) a range of sufficiently small flow rates to ensure that the mean interval pressure remained below the expected fracture reopening pressures (Bröker & Ma, 2022); and (d) complementary conventional testing. The summary of parameters employed and dates of the testing campaigns are listed in Table 1.…”

“…For a fluid‐saturated porous medium, including fractured rocks, a change in pore fluid pressure generally results in a change in its stress and strain states. This interaction between hydraulic and mechanical processes, commonly referred to as hydromechanical coupling, prevails in all kinds of geological environments due to the omnipresence of compliant fluid‐saturated pores and fractures (e.g., Bröker & Ma, 2022; Rutqvist & Stephansson, 2003; Witherspoon et al., 1980). A comprehensive review of hydromechanical coupling effects in fractured rocks, which can be classified into “direct” and “indirect” ones, is provided by Rutqvist and Stephansson (2003).…”

Mechanical Compliance of Individual Fractures in a Heterogeneous Rock Mass From Production‐Type Full‐Waveform Sonic Data

“…The considered borehole, referred to as SB1.1, is 28.6 m deep and penetrates several pre-existing natural fractures, which have been identified by acoustic televiewer (ATV) logging. The selected borehole was used for a hydraulic stimulation experiment to characterize the in situ stress by measuring the formation breakdown, fracture closure, and fracture reopening pressures (Bröker & Ma, 2022). This resulted in the creation of several new small fractures.…”

Interactions of Hydraulic Fractures With Grain Boundary Discontinuities in the Near Wellbore Region

“…However, a number of theoretical analyses and experiments demonstrated that the plane of hydraulic fracture propagation is orthogonal to the minimum principal stress (Harrison et al., 1954; Hubbert & Willis, 1957). Based on this notion, hydraulic fracturing has become an established means to estimate the minimum principal stress as the fracture planes open against the minimum principal stress (Abou‐Sayed et al., 1978; Baumgärtner & Zoback, 1989; Haimson & Fairhurst, 1969) or even the intermediate principal stress (Schmitt & Zoback, 1989), though debates on pressure interpretation are yet to be settled (Bröker & Ma, 2022; F. Guo et al., 1993; Jung et al., 2016; Kamali & Ghassemi, 2019). A thorough review on this subject is provided by Schmitt and Haimson (2017).…”