In situ stress states at KURT, an underground research laboratory in South Korea for the study of high-level radioactive waste disposal

Abstract

The Korea Atomic Energy Research Institute Underground Research Tunnel (KURT) is an underground research laboratory in South Korea, built for investigations into the geological disposal of high-level radioactive waste. We characterize in situ stress states at KURT using data from a series of hydraulic fracturing (HF) tests and borehole image logs to depths of ~700 m in two boreholes. The tensile fractures induced by HF tests, plus several borehole stress indicators (e.g., drilling-induced tensile fractures and borehole breakouts) measured from image logs, consistently indicate an ESE–WNW oriented maximum horizontal principal compressive stress (SHmax). This site-scale SHmax orientation varies slightly from the regional-scale SHmax orientation, likely reflecting a local stress perturbation resulting from a fault network that traverses the site. Estimated magnitudes of the minimum horizontal principal compressive stress (Shmin), determined either from the shut-in pressures recorded during HF tests or from stress indicators on image logs, are comparable to the vertical stress, indicating that the stress regime at the KURT site straddles the boundary between strike-slip and reverse faulting. The depth-dependent trend in estimated SHmax magnitudes deviates at ~500 m depth, which we attribute to variations in the distribution of natural fractures in the granitic rock mass. This depth-dependent variation in SHmax magnitudes has implications for the slip stability of pre-existing fractures at the site. That is, at shallow depths, SHmax lies within the Coulomb stress limit for optimally oriented fractures and faults with a frictional coefficient of 0.6, whereas at greater depths, SHmax exceeds this limit, meaning that pre-existing fractures at shallow depth are more susceptible to slip reactivation. Our stress estimation results suggest that the site-scale stress state is strongly coupled with characteristics of natural fractures, emphasizing the importance of detailed geologic and stress data for subsurface utilization and its stability evaluation.