Combining petrophysics and mineralogy to infer containment potential of deep granites for borehole disposal of ILW in Australia - 22216

One of the largest feldspar-rich granites in the world (Hiltaba granite) in the Gawler craton, South Australia, is used as a test material to demonstrate a comprehensive rock characterization methodology to assess the containment capacity for borehole disposal of appropriately conditioned long-lived intermediate level waste. The Blanche-1 geothermal well was drilled in 2005, close to Olympic Dam, in the Cooper Basin. Seven intact granite core samples were recovered from different parts of the granite body at depths between 718 m and 1935 m total depth (TD). Along with core samples, the well also contained gamma-ray, density and neutron wireline logs as well as Hylogger data with mineral signatures. Samples were subjected to a suite of petrophysical and other characterization techniques, including gas porosity-permeability, Nuclear Magnetic Resonance (NMR) under a range of reservoir effective pressures, X-ray CT, X-ray diffraction for mineralogy, dielectric measurements and optical and electron microscopy. The integration of laboratory data from core analysis with wireline logs and continuous Hylogger data discovered three distinctly different facies across the granite body. Type-3 (718-820 m) is the top of the granite body with high-neutron signal and low gamma-ray. It is strongly altered with occurrence of illite clays from albite alterations and is highly fractured with fractures being stress sensitive. It has the highest permeability (> 70 μD) and porosity (> 2%). Such rock is therefore considered to provide rather poor potential containment for waste disposal. Type-2 (820-1100 m and 1600-1935 m TD) surrounds the type-1 facies, with a low-neutron and high-density signal plus a moderate gamma-ray response. Some fractures are visible in the quartz grains but most of the fractures occur in the other minerals and are filled by epidote, chlorite and carbonates. Though the existing fractures close at low pressure leading to very low porosity (< 0.5%) and permeability (< 1 μD), the weakly bound water from small pores (<< 1 μm) is particularly stress sensitive, which could impede the capacity to retain radioactive fluids and allow migration through existing fractures. Such facies are therefore considered to have moderate sealing ability. Type-1 (1100-1600 m) has a high-neutron and low-density signal, combined with a moderate gamma-ray response. A gradient of alteration is apparent from bottom to top based on alteration by-products such as epidote and carbonate, which gradually decrease as chlorite and some illite clays become more predominant. The alteration zone has fully disappeared in the top 50 m of the Type-1 facies. With no visible fractures in SEM/X-ray CT images towards the top, the section between 1100-1150 m presents the freshest granite of the entire drilled body. This section also displays the lowest porosity and permeability and is not stress sensitive (≤ 0.03 % and ≤ 0.5 μD respectively). The top 50 m of this facies is therefore considered to have good containment potential. This study demonstrated how the combination of a comprehensive set of petrophysical and imaging techniques contributed to quantitative information on fundamental rock properties that can be used to assess potential of a crystalline host rock to provide long-term containment of disposed wastes. This proof-of-concept study showed that the Hiltaba granite in Blanche-1 is characterised by various degrees of alteration by hydrothermal fluids, and weathering in some parts, generating clay minerals and micropores which are particularly stress sensitive, capable of expelling fluids that could fill existing and/or induced macro-fractures potentially connected over a long distance. Such rock features may lead to a suboptimal containment capacity of the host rock. Based on the same rock testing procedures, a smaller interval (1100-1150 m) was identified to have superior containment potential. Future studies will incorporate the diagenetic alterations from the contact between the sedimentary rock and the Type-3 granite to the Type-2 and then Type-1 facies in a post-closure safety assessment model where effects of microcracks and mineralogy on radionuclide containment will be quantified. This will provide an additional line of evidence regarding the overall contribution of such rocks to the long-term containment and thus to the safety of radioactive waste disposal. (authors)