Modelling of Geomechanical Stability of a Large-diameter, Deep Borehole for Disposal of Long-lived Intermediate Level Waste - 22025

Borehole stability is critically important for deep borehole disposal of radioactive waste. Deep boreholes drilled 1000 m - 2000 m into crystalline rocks in Australia for geothermal energy exploration are known to have experienced borehole breakouts. Large diameter boreholes are more likely to experience borehole breakouts than the smaller boreholes due to the inherited scale effect. This study is aimed at assessing the risk of borehole instability during drilling, waste emplacement and long-term containment for long-lived intermediate-level waste (ILW). The investigation is conducted with numerical simulations using CSIRO-developed unique geomechanics modelling tools FRACOD and FRCAOD3D, which are designed to predict explicit rock fracture propagation processes. The commercial code Irazu is also used for comparison and validation purposes. The investigation started with a reference case study using data from the Blanche-1 well, a 1935-m deep borehole drilled in 2005 by Green Rock Energy Ltd near Olympic Dam (South Australia) for geothermal energy. FRACOD models previously developed have been found to predict similar borehole breakouts observed in this reference study. To assist with the design and planning of a deep, large-diameter test borehole for disposal of long-lived ILW in Australia, vertical boreholes with diameter of 700 mm at depths of 1000 m and 2000 m have been modelled. The 2D models take a horizontal cross-section of the borehole with different in situ stresses and rock temperatures at different depths. The 3D models considered 3D borehole geometry and fracture patterns at the end of the hole for different bottom-hole depths. The critical input parameters, such as rock strength, in situ stress magnitudes, thermal cooling during drilling and thermal heating during waste disposal, pre-existing joints, and presence of a large fault, have been considered to investigate the possible geological variation and disposition scenarios. The modelling results showed that, with the most likely values of horizontal (σ_Hmax, σ_hmin) to vertical (σ_v) in situ stress ratios σ_Hmax/σ_v = 2.5 and σ_hmin/σ_v = 1.5, and rock uniaxial compressive strength (UCS)=180 MPa, only limited borehole breakouts will be expected at the depth of 1000 m. At the depth of 2000 m, however, more extensive borehole breakouts around the disposition borehole are predicted, with breakout depths of up to 0.5 m. (authors)