In-situ stress and fracture characterization of a candidate repository for spent nuclear fuel in Gansu, northwestern China

Abstract

Hydraulic fracturing in-situ stress measurements and acoustic image logging were used to characterize the southern Beishan region of Gansu Province for China's first spent nuclear fuel repository. A total of 47 hydraulic fracturing measurements and 18 impression tests were performed in four boreholes at two sites to estimate magnitudes and orientations of horizontal principal stresses. The stress regime is mainly characterized by SHmax>Shmin≥Sv, indicating that the regional stress field is dominated by the maximum horizontal stress. This stress regime is prone to thrust and strike-slip faulting, according to Anderson's faulting theory. Measured results indicate that magnitude orders and stress gradients of horizontal principal stresses are larger than those previously obtained from other boreholes in the Beishan region, implying that the current stress field of candidate sites might be influenced by regional tectonic and geological controls. Using acoustic borehole logging data, geometric features of natural fractures of the four boreholes were mapped. We also found that wide fractures or faults penetrated in both boreholes at one site changed the orientations of the maximum horizontal principal stresses. Fractures with optimal orientations can be permeable owing to their narrow angles with in-situ principal stress orientations. Rock mass permeability is a priority for future investigation. Adopting the Coulomb frictional-failure criteria, we evaluated the possibility of reactivation of the wide fractures and faults and concluded that the crust in Jinta field should be stable.